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Texas Instruments Basics of Ideal Diodes Application notes
Application Report
SLVAE57 – May 2019
Basics of Ideal Diodes
Karikalan Selvaraj
ABSTRACT
Schottky diodes are widely used in power system designs to provide protection from various input supply
fault conditions and to provide system redundancy by paralleling power supplies. Power schottky diodes
are used in automotive power system design to provide protection from reverse battery conditions and
protect from various automotive electrical transients. Industrial systems traditionally have employed
schottky diodes to provide reverse polarity protection from field power supply mis-wiring and provide
immunity from lightning and industrial surges. Commonly used industrial systems, telecommunication
servers, storage, and infrastructure equipments employ schottky diodes to provide system redundancy or
increase power capacity by ORing two or more power sources. However, the forward voltage drop of the
schottky diodes results in significant power loss at high currents and increases the need for thermal
management using heat sinks and a larger PCB space. Forward conduction loss and associated thermal
management reduces efficiency and increases system cost and space. With increasing system power
levels and need for improved power density, schottky diodes are not preferred for newer high performance
system designs.
This application report highlights the limitations of conventional input battery protection solutions using
schottky diodes or P-Channel MOSFETs and discusses how Ideal Diode Controllers from Texas
Instruments (TI) can be used to improve efficiency and performance in battery input protection applications
and power supply ORing applications.
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Contents
Introduction ................................................................................................................... 2
Reverse Battery Protection ................................................................................................. 2
ORing Power Supplies ...................................................................................................... 4
Reverse Battery Protection using MOSFETs ............................................................................ 4
Reverse Polarity Protection vs Reverse Current Blocking ............................................................. 8
What is an Ideal Diode Controller? ..................................................................................... 8
Reverse Battery Protection with Ideal Diode Controllers ............................................................. 13
ORing Power Supplies with Ideal Diode Controllers................................................................... 16
Integrated Ideal Diode Solution........................................................................................... 19
Summary .................................................................................................................... 21
References .................................................................................................................. 21
List of Figures
1
Reversed Connected Battery: Damage to MCU or DC/DC converter ................................................ 2
2
Reversed Connected Battery: Damage to Polarized Capacitors ...................................................... 2
3
Reverse Battery Protection Using Schottky Diode ...................................................................... 3
4
Schottky Diode Response to Reverse Battery Condition............................................................... 3
5
Diode ORing .................................................................................................................. 4
6
Reverse Battery Protection using P-Channel MOSFET ................................................................ 5
7
Dynamic Reverse Polarity - Schottky Diode ............................................................................. 5
8
Dynamic Reverse Polarity - P-Channel MOSFET ....................................................................... 5
9
Input Short - Schottky Diode ............................................................................................... 6
10
Input Short - P-Channel MOSFET ......................................................................................... 6
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1
Introduction
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................................................................................ 6
AC Superimposed Test - P-Channel MOSFET .......................................................................... 6
Reverse Battery Protection Using N-Channel MOSFET ............................................................... 7
Ideal Diode Controller - Typical Application Schematic ................................................................ 9
Block Diagram of Ideal Diode Controller ................................................................................ 10
Forward Voltage Vs Load Current ....................................................................................... 11
Power Dissipation Vs Load Current ..................................................................................... 11
Input Short Circuit Response of LM74700-Q1 ......................................................................... 12
Fast Load Transient Response of LM74700-Q1 ....................................................................... 13
Reverse Battery Protection Using LM74700-Q1 ....................................................................... 13
LM74700-Q1 Response to Static Reverse Polarity .................................................................... 14
LM74700-Q1 Response to ISO 7637-2 Pulse 1 ....................................................................... 15
LM74700-Q1 Response to Input Micro-short ........................................................................... 15
LM74700-Q1 AC Superimposed Test ................................................................................... 16
Typical OR-ing Application ................................................................................................ 17
ORing VIN1 to VIN2 Switch Over ........................................................................................... 17
ORing VIN1 to VIN2 Switch Over ........................................................................................... 17
ORing VIN2 to VIN1 Switch Over ........................................................................................... 17
ORing VIN2 to VIN1 Switch Over ........................................................................................... 17
ORing - VIN2 Failure and Switch Over to VIN1 ........................................................................... 18
ORing - VIN2 Failure and Switch Over to VIN1 ........................................................................... 18
LM66100 Reverse Current Blocking Circuit ............................................................................ 19
Reverse Current Blocking Waveform .................................................................................... 19
LM66100 ORing Solution ................................................................................................. 20
LM66100 Switchover from IN1 to IN2 ................................................................................... 20
AC Superimposed Test - Schottky Diode
List of Tables
1
Linear Regulation Control Vs Hysteretic ON/OFF Control ............................................................. 9
2
LM66100 Comparison ..................................................................................................... 20
Trademarks
All trademarks are the property of their respective owners.
1
Introduction
This application report discusses traditional methods using schottky diodes or P-Channel MOSFETs to
provide front-end input protection such as reverse battery protection, reverse current blocking, and
protection during input micro-shorts. Next, the report discusses ORing power supplies to provide supply
redundancy and increase power capacity. The report discusses in detail the drawbacks of existing
methods and the benefits of using TI's Ideal Diode Controllers for input protection and ORing applications.
2
Reverse Battery Protection
In front-end power system designs, modules, or subsystems that directly run from battery power require
protection from reverse battery connection or dynamic reverse polarity conditions during a inductive load
disconnect from the battery. During maintenance of car battery or jump start of the vehicle, the battery can
be connected in reverse polarity during reinstallation and can cause damage to the connected
subsystems, circuits, and components. Figure 1 shows a battery that is reverse connected. When this
occurs, huge current flows through ESD diode of micro-controllers, DC/DC converters, or other integrated
circuits cause severe damage to battery connected subsystems. Polarized components such as
electrolytic capacitors can be damaged by reverse connected battery as shown in Figure 2.
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Reverse Battery Protection
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ESD Diode of MCU
or DC-DC Converter
Electrolytic
Capacitor at input
of DC-DC Converter
Load
Figure 1. Reversed Connected Battery: Damage to MCU
or DC/DC converter
Load
Figure 2. Reversed Connected Battery: Damage to
Polarized Capacitors
Passenger cars and commercial vehicles are fitted with 12-V or 24-V battery and the subsystems powered
through the 12-V or 24-V battery are subjected to various electrical transients on their power supply lines
during the operating life time of the vehicle. Automotive EMC testing standards such as ISO 7637-2 and
ISO 16750-2, among others, specify electrical transients, test methods, and classify functional
performance for immunity against the specified transients. Reverse battery protection solution is expected
to protect the electrical subsystems from the transients and meet the functional performance status
required for each subsystem. Traditionally, schottky diodes are used to provide reverse battery protection
and prevent damage to battery connected subsystems.
2.1
Reverse Battery Protection with Schottky Diode
The simplest method of reverse battery protection is to add a series diode at input of the system power
path. Figure 3 shows a reverse battery protection using a schottky diode. When the battery is installed
correctly, load current flows in the forward direction of the diode. If the battery is installed with the wrong
polarity, the diode is reverse biased and blocks reverse current, thereby protecting the load from negative
voltage.
Figure 4 shows the response to a reverse polarity condition at the input. When the 12 V input is quickly
reversed to -20 V, the output voltage remains without collapsing immediately or following the negative
input as the schottky diode gets reverse biased and isolates the output from negative voltage. A bulk
capacitor placed at the output holds the output from falling immediately and can supply the load for a short
time before the input supply recovers.
VBAT
VOUT
Load
Figure 3. Reverse Battery Protection Using Schottky
Diode
Figure 4. Schottky Diode Response to Reverse Battery
Condition
Drawbacks of using schottky diode for reverse battery protection include:
• Power dissipation: Forward conduction results in significant efficiency loss at higher load currents.
• Thermal management: Heat sink is needed to manage power dissipation, increasing cost and space.
• Reverse leakage current: Reverse leakage current of high voltage schottky diodes increase
dramatically with junction temperature, resulting in higher power dissipation during reverse conduction.
• Head room for downstream power converter: During a cold start of the car, the battery voltage drops
as low as 3 V or 4 V during a warm start. Forward voltage drop reduces subsequent power converter
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ORing Power Supplies
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head-room and a wider operating power converter is required to work during 3 V or 4 V cold crank
operation. This requires a wider VIN range DC/DC boost converter to be used after the diode.
On systems where large holdup capacitors are used, inrush current during startup can be huge and must
not exceed the maximum diode current. This needs to be considered when choosing thermal layout or
heat sink.
3
ORing Power Supplies
Schottky diodes are traditionally used to OR two or more power supplies to increase system redundancy
or increase power capacity in N+1 configuration. Typically more than one power supply units (PSU) are
paralleled using schottky diodes in a N+1 redundant configuration. Minimum 'N' supplies are required to
power the load and additional supply unit is provided for redundancy incase of a single point failure: one
power supply unit fails. Power supply with higher voltage provides most or all of the power required by the
load. To share loads almost equally among the power supplies, power supply DC set point is adjusted to
match other units closely.
Power Supply 1
Power Supply 2
Load
GND
Figure 5. Diode ORing
Figure 5 shows dual ORing scheme where two PSUs power the load through two schottky diodes. When
one of the power supply fails and its input is shorted, schottky diode in its path is reverse biased and
isolates the other power supply from the failure. Load remains fully powered from the working power
supply until the faulty unit is replaced.
Load Sharing: Load sharing between two power supplies is mainly dependent on the forward voltage
difference of the schottky diodes and voltage difference between two power supplies. Power supply with
higher voltage and lower forward voltage schottky diode carries most of the current. Forward voltage drop
of the schottky diode has a negative temperature co-efficient and it reduces with increasing temperature.
This can lead to situation where a single supply carries the entire load current though second supply is still
present and results in increased junction temperature TJ. This necessitates a careful heat sink design and
thermal management between two diodes.
Power Dissipation and Thermal Management: Apart from the key concerns such as power dissipation
and the associated thermal management, reverse leakage current at a higher temperature can result in
efficiency loss and lead to thermal run away situations if thermal design is not done properly. Reverse
leakage current of high voltage schottky diodes increase drastically with temperature. For example, 60 V
rated schottky diode STPS20M60S has a 100 mA reverse leakage current at 150 °C, which amounts to 6
W of power dissipation at -60 V. Consider a case when only one power supply is fully supplying the load
current due to forward voltage difference of schottky diodes or offset in power supply DC set point. If this
first power supply fails, the second supply takes over and supplies the entire load, but the schottky diode
of the first one had a higher TJ before turning off and conducts large reverse leakage current. This can
lead to a thermal run-away situation where the schottky continues to conduct increased reverse current
and gets damaged. A damaged schottky diode and failed power supply can pull down the entire power
system leading to a system failure. Even if thermal run-away is avoided by careful heat sink design,
sustained power dissipation in the reverse conduction results in unwanted power loss.
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Reverse Battery Protection using MOSFETs
In this section, reverse battery protection using P-Channel MOSFETs and N-Channel MOSFETs are
discussed along with the benefits and drawbacks.
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Reverse Battery Protection using MOSFETs
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4.1
Reverse Battery Protection using P-Channel MOSFET
The schottky diode can be replaced with a P-Channel MOSFET to provide reverse battery protection as
shown in Figure 6. The forward drop of the diode can be reduced by replacing the schottky diode with a Pchannel MOSFET and its body diode oriented in the same direction as the schottky diode. During normal
operation of the battery, the body diode from MOSFET is forward biased and conducts for a very short
time until the MOSFET is turned ON when gate voltage is pulled below source. When the battery polarity
is reversed, gate-source voltage swings positive and the MOSFET is turned off, protecting the
downstream circuits from negative voltage.
VBAT
VOUT
Load
Figure 6. Reverse Battery Protection using P-Channel MOSFET
During dynamic reverse polarity where input quickly swings negative from positive, the P-Channel
MOSFET is turned OFF when the input starts going negative because the gate-source voltage swings
positive. Further note that the output also reaches close to or a diode drop lower than the system ground
and protects the downstream DC/DC converters from negative voltage. The holdup capacitors are
discharged as this P-Channel MOSFET protection does not block reverse current from flowing back into
the input. An additional circuit can be added to sense the voltage difference between the input and output
and turn off the MOSFET when the input goes lower than output, but requires increased cost and board
space.
In Figure 8, the P-Channel MOSFET circuit protects the output from dynamic reverse polarity condition
where the input quickly changes from 12 V to -20 V. Output remains protected from the negative voltage
after the initial reverse voltage for 50 µs. Note that the output is completely discharged due to lack of
reverse current blocking functionality and any hold up capacitors are discharged.
Figure 7. Dynamic Reverse Polarity - Schottky Diode
Figure 8. Dynamic Reverse Polarity - P-Channel
MOSFET
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Reverse Battery Protection using MOSFETs
4.2
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Input Short or supply interruption
Figure 9 and Figure 10 show the performance comparison of P-Channel MOSFET with a schottky diode
during an input short or interruption. During an input short, the schottky diode is quickly reverse biased
and blocks reverse current from flowing back into the shorted input. Output is isolated from the input short
and hold capacitors at the output provide back-up power to the load, leading to droop in the output
voltage. When the input short is removed, load is powered through the schottky diode.
In Figure 10, P-Channel MOSFET turns off when the gate-source voltage crosses VTH of MOSFET, close
to 0 V. Output voltage sees a huge drop due to lack of reverse current blocking functionality.
Figure 9. Input Short - Schottky Diode
4.3
Figure 10. Input Short - P-Channel MOSFET
Diode Rectification During Line Disturbance
In automotive systems, the battery line is subjected to various disturbances and transients during normal
operation of the vehicle. One of the key tests is the supply line disturbance called AC superimposed test
where AC disturbance with 2 V - 4 V peak to peak and 20 Hz to 30 KHz is applied on the battery supply
line and the subsystems are expected to run without any functionality loss.
The schottky diode rectifies the AC line disturbance by blocking the reverse current. Figure 11 shows the
rectified output and input current which has the AC component added due to the injected AC line
disturbance. Power dissipation on the schottky diode increases due to the increased RMS current
multiplied by the forward drop. This additional heat needs to be managed for reliable operation during the
AC superimposed test.
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Figure 11. AC Superimposed Test - Schottky Diode
Figure 12. AC Superimposed Test - P-Channel MOSFET
Figure 12 shows the performance of P-Channel MOSFET circuit. Since the P-Channel MOSFET does not
block reverse current, rectification of the line disturbance does not happen and this results in increased
RMS input current. Power dissipation on the MOSFET may not be a major concern due to the low forward
voltage drop as the MOSFET remains on, however, the RMS current of output electrolytic capacitors due
to its ESR produces additional heat on the capacitors. To prevent damage to the capacitors due to
overheating, required capacitance can be split into multiple parallel capacitors to reduce the ESR. This
adds to the system cost and space.
Rectification is possible by adding reverse current blocking functionality using an external comparator
based methods, but this increases cost and space.
4.4
Reverse Battery Protection using N-Channel MOSFET
An alternate method of reverse battery protection is using an N-Channel MOSFET on the low side, such
as the ground return path. The operating principle is similar to the P-Channel MOSFET in Figure 6. During
normal operation, the body diode of the MOSFET is forward biased and conducts until the MOSFET is
turned ON. MOSFET is turned ON quickly as the battery input charges the gate through the current
limiting resistors. MOSFET turns OFF during static reverse battery or dynamic reverse battery conditions
after the battery input starts to swing negative, as the gate-source voltage starts to go below MOSFET Vth
and swings negative.
VBAT
VOUT
Load
Figure 13. Reverse Battery Protection Using N-Channel MOSFET
Section 4.1 describes the performance during the dynamic reverse polarity is similar to the P-Channel
MOSFET solution. However, a jump in the system ground voltage during turn ON/OFF or load current
transients may not be tolerated by all systems and needs to be considered during system designs.
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Reverse Polarity Protection vs Reverse Current Blocking
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Reverse Polarity Protection vs Reverse Current Blocking
Reverse battery protection involves two aspects of protection, commonly referred to as reverse polarity
protection (RPP) and reverse current blocking (RCB). Reverse polarity protection, also referred to as
reverse hookup protection (RHP), prevents the load from getting damaged due to negative voltage at the
input during a reversely connected battery or dynamic reverse polarity conditions during a inductive load
disconnect from battery. Reverse polarity protection does not necessarily block reverse current flowing
into the battery from the load or downstream DC/DC converters. In many automotive subsystems, large
holdup capacitors are used to provide sufficient back up power during a short interruption of battery line or
shorted battery input, so that the subsystem can function uninterrupted or perform maintenance
housekeeping tasks such as memory dump before turning off. Reverse current blocking prevents reverse
current from flowing back into the battery from the load and allows holdup capacitors to provide additional
back up time for the subsystem to function during various dynamic reverse battery conditions or short
interruptions.
One key difference between battery protection using schottky diode and protection using P-Channel
MOSFET is the schottky diode blocks reverse current flowing from the load back into the battery all the
time and inherently provides both reverse polarity protection and reverse current blocking. When the
battery is connected with its terminal reversed, the schottky diode gets reverse biased and blocks reverse
current from discharging the holdup capacitors connected to the load. This naturally isolates the load from
the negative input voltage and provides reverse polarity protection to the load.
The battery protection shown in Figure 6 or in Figure 13 does not block reverse current from flowing back
into the battery since the MOSFETs are turned off when the battery voltage is close the Vth of the
MOSFETs and not as soon as the battery voltage starts to drop. During a input micro-short at the battery,
holdup capacitors can be discharged to a voltage lower than the downstream DC/DC converters UVLO,
leading to reset of the subsystem.
6
What is an Ideal Diode Controller?
An ideal diode controller drives an external N-channel MOSFET to emulate an ideal diode with a very low
forward voltage drop and negligible reverse current. Key features such as low operating quiescent current,
very low shutdown current, regulated forward voltage, and fast reverse current response enable ideal
diode controllers to emulate an ideal diode in variety of applications. The power MOSFET is connected in
such way that its body diode blocks reverse current when the MOSFET is turned OFF. Forward voltage
drop and power dissipation are reduced significantly as the MOSFET is turned ON during forward
conduction. Ideal diode controllers sense the reverse current through MOSFET and turn it OFF, allowing
the body diode to block reverse current. Ideal diode controllers can be classified into two types based on
the gate control mechanism: Linear Regulation Control and Hysteretic ON/OFF Control.
6.1
Linear Regulation Control Vs Hysteretic ON/OFF Control
In linear regulation control, forward voltage of the MOSFET is regulated by controlling the gate voltage
based on the load current. Linear regulation is achieved by controlling the gate voltage and thereby
varying the RDS(ON) of the MOSFET based on load current. At nominal load currents, gate-source voltage is
maintained above the MOSFETs Vth and at lower load currents, gate-source voltage is maintained close to
MOSFETs Vth with increased RDS(ON). At higher load currents, gate-source voltage is parked close to the
maximum gate drive voltage, operating close to lowest possible RDS(ON). Choosing the MOSFET based on
the operating power requirements helps maintain the MOSFET under regulation during most of the load
conditions. Linear regulation of the forward voltage along with a fast reverse current blocking helps ensure
zero DC current flows back into the input. Further, it also minimizes the peak reverse current during a
input supply failure, input supply transients or input supply droop.
In Hysteretic ON/OFF control, MOSFET is fully turned ON when the forward turn ON comparator threshold
VFWD_ON is exceeded and turned OFF when the reverse comparator threshold VREV_OFF is reached. When
the MOSFET is ON, the gate is fully enhanced and the gate-source voltage is not controlled depending on
the load current. MOSFET is turned off when the reverse current reaches VREV_OFF / RDS(ON). Note that the
MOSFET cannot turn OFF if a reverse current less than VREV_OFF / RDS(ON) flows. The reverse turn off
threshold VREV_OFF is fixed negative value or programmable allowing small positive value. In ideal diode
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controllers where the reverse comparator threshold is programmable and can be set to a small positive
value, the DC reverse current can be completely blocked. Additionally, setting the reverse comparator
threshold to a small positive value can require a minimum forward load current VREV_OFF / RDS(ON) to turn ON
the MOSFET. If a minimum forward load current is not maintained, MOSFET can turn ON/OFF
continuously leading to sustained oscillation of the gate voltage.
Table 1. Linear Regulation Control Vs Hysteretic ON/OFF Control
IDEAL DIODE
CONTROLLER
OPERATING RANGE (ABSOLUTE
MAXIMUM RATINGS)
LINEAR REGULATION CONTROL
LM74700-Q1
±65 V
Yes
No
LM74610-Q1
±45 V
No
Yes
LM5050-1 and
LM5050-1-Q1
±100 V
Yes
No
LM5050-2
±100 V
Yes
No
TPS2410 and
TPS2412
±18 V
Yes
No
TPS2411 and
TPS2413
±18 V
No
Yes
TPS2419
±18 V
No
Yes
HYSTERETIC ON/OFF CONTROL
A typical application schematic in Figure 14 shows the LM74700-Q1 ideal diode controller driving an
external N-Channel MOSFET. MOSFET is connected with its source tied to the input so that body diode
blocks the reverse current when turned OFF. The charge pump capacitor is connected between Anode
and VCAP to provide sufficient gate drive voltage to turn on the MOSFET. EN pin is used to turn the
MOSFET ON, providing regulated low forward drop across anode to cathode during normal operation.
Pulling down the EN pin turns OFF the MOSFET and the controller goes into low shutdown current mode.
When the MOSFET is turned OFF, the load can still draw power through body diode of the MOSFET.
VOUT
VBATT
Voltage
Regulator
TVS
ANODE
VCAP
ON OFF
EN
GATE
CATHODE
LM74700
GND
Figure 14. Ideal Diode Controller - Typical Application Schematic
This section discusses the key performance features of the LM74700-Q1 using the functional block
diagram shown in Figure 15. Ideal diode controllers have an internal charge pump to drive the gate of
MOSFET sufficiently higher than anode during normal operation, forward comparator to turn ON and
reverse current comparator to turn OFF when reverse current is detected, allowing the MOSFET body
diode to block reverse DC current completely.
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ANODE
CATHODE
GATE
VANODE
VCAP
COMPARATOR
+
±
+
±
Bias Rails
50 mV
GM AMP
+
±
+
±
GATE DRIVER
ENABLE
LOGIC
20 mV
COMPARATOR
+
±
+
± -11 mV
S
Q
R
Q
VANODE
ENGATE
VANODE
VCAP_UV
VCAP_UV
VANODE
VANODE
Charge Pump
Enable Logic
Charge
Pump
VCAP
ENABLE LOGIC
REVERSE
PROTECTION
LOGIC
VCAP_UV
VCAP
VCAP
EN
GND
Copyright © 2017, Texas Instruments Incorporated
Figure 15. Block Diagram of Ideal Diode Controller
6.2
Low Forward Conduction Loss
Forward voltage drop of schottky diodes increases the forward conduction power loss and requires
thermal management using heat sink and requires PCB space leading to increased cost. Ideal diode
controllers use an external MOSFET to reduce the forward voltage to 20 mV or lower, depending on the
control scheme. Linear regulation control scheme maintains 20 mV forward voltage during most of the
operating current range. Hysteretic ON/OFF control fully enhances the MOSFET to reduce the forward
voltage and the forward drop is decided solely based on the MOSFET used.
Forward voltage of the MOSFET DMT6007LFG driven by ideal diode controller is compared against
forward voltage of schottky diode STPS20M60S in Figure 16. An ideal diode controller using linear
regulation scheme regulates the forward voltage to low 20 mV up to load current = 20 mV / RDS(MIN) and
load current higher than 20 mV / RDS(MIN) forward voltage solely depends on MOSFETs RRD(ON). In
Figure 16, MOSFET is regulated to 20 mV forward voltage up to 5.7 A and beyond 5.7, a MOSFET is fully
enhanced and forward voltage increases based on load current. At 10 A, low forward voltage drop is
reduced to 35 mV against 465 mV using a Schottky diode.
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Figure 16. Forward Voltage Vs Load Current
Figure 17. Power Dissipation Vs Load Current
Figure 17 shows the power dissipation comparison between schottky diode and ideal diode controller. At a
10 A load current, DMT6005LPS-13 MOSFET dissipates 0.35 W of power, whereas schottky diode
STPS20M60S dissipates 4.65 W of power leading to more than 10x power saving when using ideal diode
controller and MOSFET.
6.3
Fast Reverse Recovery
During input supply failure or micro-short conditions, huge reverse current can flow into the input,
discharging the load capacitors used for holdup. Ideal diode controllers feature a very fast reverse
comparator and strong gate drive to pull down the gate to source voltage to turn OFF the MOSFET. The
internal reverse comparator monitors the voltage across anode and cathode and if it exceeds the reverse
current threshold, external MOSFETs gate is shorted to anode (source) with strong pulldown current.
Reverse comparator delay and gate pulldown current determine how fast the MOSFET can be turned off.
Total reverse current turn off delay includes reverse comparator delay and MOSFET turn off delay.
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TI's ideal diode controller LM74700-Q1 features a very low reverse comparator delay of 0.75 µs maximum
and gate pulldown current of 2.37A. A MOSFET with 5 nF of gate capacitance can be turned off within
0.75 µs + 21 ns = 0.77 µs, with 21 ns required to discharge 5 nF gate capacitance by 2.37 A of gate
pulldown current. Figure 18 shows LM74700-Q1 reacting quickly within 0.77 µs to a short circuit at battery
input. This prevents hold capacitors at the output from discharged into the shorted battery input line.
VOUT
VGATE
VBATT
IBATT
Figure 18. Input Short Circuit Response of LM74700-Q1
6.4
Very Low Shutdown Current
In automotive applications, very low shutdown current is a requirement to meet the overall system
requirement of less than 100 µA where many battery powered subsystems co-exist. TI's ideal diode
controller LM74700-Q1 features a very low shutdown current of 1.5 µA maximum to meet the automotive
system requirements. This also helps many other battery powered or energy harvesting applications
where low shutdown current is preferred.
6.5
Fast Load Transient Response
Ideal diode controllers operating with linear regulation control scheme maintain low forward voltage by
controlling the gate-source voltage depending on load current. Gate-source voltage is lower at light load
conditions and increases as the load current increases. While linear regulation scheme helps achieve zero
DC reverse current, it can be disadvantageous to have low gate-source voltage during a sudden load
transient from light load to heavy load. At lighter loads, gate-source voltage is operated just above Vth of
MOSFET and the RDS(ON) is higher (than nominal) to meet to forward regulation, RDS(ON)_LIGHT_LOAD = 20 mV /
ILIGHT_LOAD. When the load suddenly changes from light load to higher loads, the gate of the MOSFET
needs to quickly charge from lower voltage to higher voltage to meet the sudden increased load demand.
If the gate is not quickly charged, output voltage sees a worst case voltage drop equal to IHEAVY_LOAD ×
RDS(ON)_LIGHT_LOAD, but not more than MOSFETs body diode drop. In many ORing applications this droop in
output voltage may not be acceptable as this reduces the headroom of power supply downstream.
TI's ideal diode controller LM74700-Q1 features 11 mA peak source current when forward drop exceeds
50 mV. This feature helps in quickly charging the gate during fast load transient minimizing supply voltage
droop. Figure 19 shows the load transient response of the LM74700-Q1 when load changes from 10 mA
light load to 5 A suddenly. Output voltage drop is minimized to <50 mV as the controller quickly reacts and
enhances the MOSFETs gate quickly.
12
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Reverse Battery Protection with Ideal Diode Controllers
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Figure 19. Fast Load Transient Response of LM74700-Q1
7
Reverse Battery Protection with Ideal Diode Controllers
Ideal diode controllers drive external N-channel MOSFETs to emulate ideal diodes. As described earlier,
ideal diode controllers have a built-in charge pump to drive the gate of MOSFET sufficiently higher than
anode during normal operation and reverse current comparator to turn off when reverse current is
detected, allowing the MOSFET body diode to block reverse current completely. This enables the ideal
diode controllers to be used in reverse battery protection.
7.1
LM74700-Q1 with N-Channel MOSFET
Figure 20 shows a typical 12 V reverse battery protection circuit using the LM74700-Q1. The MOSFET
body diode is oriented correctly to block reverse current flowing back in to the battery when the LM74700
is turned off. When battery power is applied through anode, body diode of the MOSFET initially conducts
before the MOSFET is turned on. During startup, when the anode voltage reaches POR threshold, the
internal charge pump turns on and starts to drive the gate of MOSFET higher than the anode (source of
MOSFET), thereby turning on the MOSFET. If the battery is installed with reverse polarity or wired
incorrectly during maintenance or repair, the MOSFET is off already and body diode of the MOSFET
blocks reverse current. This prevents negative voltage from appearing on the output and protects the
downstream circuit from damage.
VOUT
VBATT
Voltage
Regulator
TVS
ANODE
VCAP
ON OFF
EN
GATE
CATHODE
LM74700
GND
Figure 20. Reverse Battery Protection Using LM74700-Q1
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Reverse Battery Protection with Ideal Diode Controllers
7.2
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Static Reverse Polarity
During maintenance of the car battery or jump start of the vehicle, the battery can be connected in reverse
polarity during reinstallation. When the battery is connected with reversed polarity, the LM74700-Q1
remains turned off to protect the downstream circuits and components from damage. In Figure 21, output
voltage remains protected when a reverse battery connection of -12 V is applied at its input.
OUTPUT REMAINS PROTECTED
FROM NEGATIVE VOLTAGE
Figure 21. LM74700-Q1 Response to Static Reverse Polarity
7.3
Dynamic Reverse Polarity
During dynamic reverse polarity conditions specified in ISO 7637-2 Pulse 1, negative transient voltage as
low as -150 V is applied at the 12-V battery supply line with 10-Ω generator impedance for 2 ms and -600
V at the 24-V battery supply line with 50-Ω generator impedance for 1 ms. Figure 22 shows the response
of the LM74700-Q1 to ISO 7637-2 Pulse 1 applied at the input. Before the test pulse is applied, the
MOSFET is ON and allows the load current to pass through. When the ISO 7637-2 test pulse 1 is applied
at the battery input, the load current starts to reverse quickly and tries to pull the output voltage negative.
LM74700-Q1 detects the reverse current and turns OFF the MOSFET within 0.75 µs to block reverse
current and prevents the output from going negative. Generally, bulk holdup capacitors are used after the
ideal diode circuit provides energy to rest of the module during such transients. The LM74700-Q1 turns off
the MOSFET quickly within 0.75 µs to prevent discharge of the bulk holdup capacitors. Note that the input
TVS is required to clamp the voltage from exceeding the absolute maximum ratings of LM74700-Q1 and
MOSFET.
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VOUT
VGATE
GATE TURNS OFF QUICKLY WITHIN 1…s
VIN
IIN
TVS CLAMPING AT -42V
Figure 22. LM74700-Q1 Response to ISO 7637-2 Pulse 1
7.4
Input Micro-Short
Many power systems are required to withstand short interruption in supply line or input micro-short and
continue to function uninterrupted. In Figure 23, when the input micro-short is applied at the input, the
LM74700-Q1 reacts quickly to turn off the MOSFET to block reverse current from flowing back into the
shorted supply. Output remains ON during the input short as the output hold-up capacitors are isolated
from input short and supply load current until the input recovers from micro-short. Note that after the
MOSFET is turned off, input current rings due to the parasitic inductance in the supply path and does not
contribute to reverse current.
Output remains ON during input short
MOSFET turned off quickly
within 1 …s: VGS = 0V
MOSFET is turned back ON after
the input short is removed
Reverse Current is
blocked within 1 …s
Ringing due to parasitic source
inductance, not reverse current
Figure 23. LM74700-Q1 Response to Input Micro-short
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Reverse Battery Protection with Ideal Diode Controllers
7.5
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Diode Rectification of Supply Line disturbance
Figure 24 shows the performance of LM74700-Q1 during supply line disturbance or the AC superimposed
test. Fast reverse current blocking and fast reverse recovery of the LM74700-Q1 rectify the AC
disturbance by turning on and off every cycle. Zero reverse current flows back into the supply during the
test thereby reducing the RMS value of input current by half. Power dissipation in the MOSFET during the
test is reduced due to low forward voltage drop.
Gate turns off every cycle to block reverse current
Reverse Current is
blocked every cycle
RMS Current is
reduced by half due
to rectification
Figure 24. LM74700-Q1 AC Superimposed Test
8
ORing Power Supplies with Ideal Diode Controllers
The LM74700-Q1 combined with external N-Channel MOSFETs can be used in OR-ing Solution as shown
in Figure 25. The forward diode drop is reduced as the external N-Channel MOSFET is turned ON during
normal operation. The LM74700-Q1 quickly detects the reverse current and quickly pulls down the
MOSFET gate, leaving the body diode of the MOSFET to block the reverse current flow.
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ORing Power Supplies with Ideal Diode Controllers
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VIN1
EN
ANODE
GATE
CATHODE
LM74700
VOUT
VCAP
GND
LOAD
COUT
VIN2
EN
ANODE
GATE
CATHODE
LM74700
VCAP
GND
Figure 25. Typical OR-ing Application
An effective OR-ing solution needs to be extremely fast to limit the reverse current amount and duration.
The LM74700-Q1 devices in an OR-ing configuration constantly sense the voltage difference between
anode and cathode pins, which are the voltage levels at the power sources (VIN1, VIN2) and the common
load point respectively. The source to drain voltage VDS of the MOSFET is monitored by the anode and
cathode pins of the LM74700-Q1. A fast comparator shuts down the gate drive through a fast pulldown
within 0.75 μs (typical) as soon as V(IN) – V(OUT) falls below –11 mV. It turns on the gate with 11 mA gate
charge current once the differential forward voltage V(IN) – V(OUT) exceeds 50 mV.
VIN1
VIN1
VOUT
VOUT SWITCHES to VIN2 15 V
VOUT
VOUT SWITCHES to VIN2 15V
VGATE1
VGATE1
IIN1
IIN2
VIN2 SUPPLIES LOAD CURRENT
Time (5
ms/DIV)
Time (5
ms/DIV)
Figure 26. ORing VIN1 to VIN2 Switch Over
Figure 27. ORing VIN1 to VIN2 Switch Over
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ORing Power Supplies with Ideal Diode Controllers
VIN1
VIN1
VOUT
www.ti.com
VOUT SWITCHES to VIN1: 12V
VOUT SWITCHES to VIN1: 12V
VOUT
VGATE1
VGATE1
IIN1
IIN2
VIN1 SUPPLIES LOAD CURRENT
Time (5
ms/DIV)
Time (5
ms/DIV)
Figure 28. ORing VIN2 to VIN1 Switch Over
Figure 29. ORing VIN2 to VIN1 Switch Over
VOUT
VIN1
VOUT
VIN1
VIN2
VGATE1
IIN1
IIN2
Time (10
ms/DIV)
Time (5
ms/DIV)
Figure 30. ORing - VIN2 Failure and Switch Over to VIN1
Figure 31. ORing - VIN2 Failure and Switch Over to VIN1
Figure 26 to Figure 29 show the smooth switch over between two power supply rails VIN1 at 12 V and VIN2
at 15 V. Figure 30 and Figure 31 illustrate the performance when VIN2 fails. The LM74700-Q1 controlling
VIN2 power rail turns off quickly, so that the output remains uninterrupted and VIN1 is protected from VIN2
failure.
Power dissipation and its associated thermal management issues of using a schottky diode are minimized
due to the low forward voltage drop of ideal diode controllers. MOSFETs do not have leakage currents as
high as a schottky diode at high temperatures and using MOSFETs reduces the reverse leakage loss.
This improves overall efficiency and reliability of the system.
Load sharing concerns due to schottky diode difference in forward voltage and its negative temperature
co-efficient are not present when using ideal diode controllers. Further, the linear regulation of forward
voltage drop enhances load sharing between power supplies.
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Integrated Ideal Diode Solution
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9
Integrated Ideal Diode Solution
For lower voltage applications, such as backup battery solutions, an integrated ideal diode solution can be
used. The LM66100 uses a P-channel MOSFET and integrates the functionality of an ideal diode into a
single device.
A typical application schematic in Figure 32 shows the LM66100 ideal diode in a reverse current blocking
(RCB) circuit. The chip enable works by comparing the CE pin voltage to the input voltage. When the CE
pin voltage is higher than VIN, the device is disabled and the PMOS is turned off. When the CE pin
voltage is lower than VIN, then the MOSFET is on and the device operates with low forward voltage drop.
By connecting the CE pin to VOUT in this configuration, this ensures that the MOSFET is disabled
whenever the output voltage is forced higher than the input voltage. The LM66100 integrated ideal diode
also integrates reverse polarity/battery protection, which helps to prevent damage to the upstream battery
if it is wired incorrectly.
Figure 32. LM66100 Reverse Current Blocking Circuit
Figure 33. Reverse Current Blocking Waveform
Similar to the ideal diode controllers, the LM66100 can also be used in redundant power architectures for
ORing between power supplies. By using two LM66100s with the CE pins tied to the other input voltage
channel. This ensures that the highest input supply voltage is selected as the output. Since the highest
supply is always be selected, the solution allows for a make-before-break configuration, which prevents
any reverse current flow between the input supplies.
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Integrated Ideal Diode Solution
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Figure 34. LM66100 ORing Solution
Figure 35 shows a typical switchover event from VIN1 to VIN2. During this switchover event, VIN1 decays
causing the LM66100 to switch to VIN2, while blocking reverse current from entering VIN1.
Figure 35. LM66100 Switchover from IN1 to IN2
The LM66100 integrated ideal diode also contains the same advantages as the ideal diode controllers,
when compared to a discrete diode or FET solution. During normal operation, the LM66100 has a lower
forward conduction loss when compared to a discrete diode. Instead of operating with the normal 0.3
V–0.4 V drop of a discrete diode, the power loss across the LM66100 is minimized across the MOSFET
instead. This results in a lower power dissipation, leading to a higher power savings for applications.
The LM66100 also has a fast reverse voltage recovery time when compared to a discrete FET. While a
discrete FET does not turn off until the voltage drops below the VTH of the FET, the LM66100 stops
reverse current within tOFF once the voltage on the output rises above the input. This helps prevent
output capacitors from discharging current back into the upstream supply, which can damage components
such as input batteries or PSUs.
Table 2. LM66100 Comparison
20
FEATURE
IDEAL DIODE CONTROLLER
LM66100
Low Power
Dissipation
✓
✓
Low Reverse
Leakage
Current
✓
✓
Basics of Ideal Diodes
DISCRETE
DIODE
DISCRETE FET
✓
✓
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Summary
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Table 2. LM66100 Comparison (continued)
10
FEATURE
IDEAL DIODE CONTROLLER
LM66100
DISCRETE
DIODE
DISCRETE FET
Reverse
Polarity
Protection
✓
✓
✓
*(additional components)*
Summary
This application report discusses the benefits of using Texas Instruments ideal diode controllers in reverse
battery protection and ORing applications. The report discusses performance of the LM74700-Q1 Ideal
diode controller during front-end input protection tests such as dynamic reverse polarity, input micro-short,
and power line disturbance and compares it to existing methods. Key features such as low forward
conduction loss, fast reverse recovery, true reverse current blocking, and fast load transient response
enables the ideal diode controller LM74700-Q1 to provide more efficient and robust reverse battery
protection.
Key benefits such as low forward conduction loss, reduced leakage, and simplified load sharing enables
the ideal diode controller to OR power supplies more efficiently and reliably.
11
References
•
•
•
Texas Instruments, LM74700-Q1 Low IQ Reverse Battery Protection Ideal Diode Controller Data Sheet
Texas Instruments, Reverse Current / Battery Protection Circuits Application Note
Texas Instruments, Full Featured N+1 and ORing Power Rail Controller Data Sheet
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