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Texas Instruments Charging Super Capacitor With eFuse Application notes
Application Report
SLVA920 – August 2017
Charging Super Capacitor With eFuse
Rakesh Panguloori....................................................................................................... Power Switches
ABSTRACT
Backup power management solutions are typically used in end equipment, such as solid state drives
(SSDs), storage systems, data concentrators, and smart meters, where an unexpected power disruption
can cause malfunction or data loss. Super capacitors or large hold-up capacitors are used as storage
elements to provide enough backup power to maintain data communication prior to the whole system’s
shutdown.
This application report discusses how an eFuse can be used as simple charger circuit for the super
capacitor. This report also provides design and measurement results showing the performance benefits of
using eFuse as SuperCap charger.
1
2
3
4
5
6
7
Contents
Introduction ................................................................................................................... 2
Discrete Implementation of Super Capacitor Charger Circuit .......................................................... 3
eFuse Solution ............................................................................................................... 5
Test Results .................................................................................................................. 8
Comparison Summary ..................................................................................................... 12
Conclusion .................................................................................................................. 12
References .................................................................................................................. 12
List of Figures
............................................................... 2
........................ 3
Charging Profile of 5-F Super Capacitor Using (a) ...................................................................... 3
Charging Profile of 5-F Super Capacitor Using P-FET-based Discrete Circuit ...................................... 4
SuperCap Charger Circuit (a) eFuse Solution (b) Typical Operating Waveforms .................................. 5
Thermal Shutdown Time Plot for TPS25940A eFuse .................................................................. 6
Design Flow Chart to Determine tdVdT ..................................................................................... 7
Averaged Energy Versus Thermal Shutdown Time at TA = 85°C for ................................................ 8
Charging Profile of 0.5-F Super Capacitor Using TPS25940A eFuse ................................................ 9
Charging Profile of 1-F Super Capacitor Using TPS25940A eFuse .................................................. 9
Charging Profile of 5-F Super Capacitor Using TPS25940A eFuse ................................................. 10
Charging Profile of 10-F Super Capacitor Using TPS25940A eFuse ............................................... 10
Charging Profile of 5-F Super Capacitor, When CdVdT is OPEN and ILIM set to 2.1A .............................. 11
Charging Profile of 5-F Super Capacitor, When CdVdT is OPEN and ILIM is SHORT ............................... 11
1
Simplified Block Diagram of the Backup Power System
2
SuperCap Charger Circuit (a) Resistor-Diode Network (b) P-FET-based Discrete Circuit
3
4
5
6
7
8
9
10
11
12
13
14
List of Tables
..........................................................
1
Output Ramp Time Calculations and Measurement Results
2
Comparison Table .......................................................................................................... 12
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1
Introduction
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Trademarks
All trademarks are the property of their respective owners.
1
Introduction
VSYS
VIN
5V
System
Main Power
COUT
SuperCap
Charger
TPS61030
SuperCap
Backup Boost Operation
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Figure 1. Simplified Block Diagram of the Backup Power System
Figure 1 shows a typical block diagram of a backup power system. The main components are:
• a super capacitor
• a charger circuit for the super capacitor
• a boost converter TPS61030
• a Schottky diode
During normal operation, the main power directly connects to the system, and the backup super capacitor
charges through the SuperCap charger circuit. In case of main power loss, the system voltage VSYS drops
below the programmed output voltage level of boost converter. Then the boost converter immediately
starts regulating the system voltage and uses the stored energy of the super capacitor for backup
operation.
During backup operation, the Schottky diode prevents the current flow from VSYS to VIN to prevent any
additional load on the backup power system. Similarly, the SuperCap charger circuit must have reverse
current blocking capability to avoid draining the super capacitor. When the main power restores, it is
important to limit the charging (inrush) current for the discharged super capacitor to avoid disturbance on
the system voltage.
As the super capacitors are sensitive to overvoltages, protection against overvoltage is required especially
when the difference between the system voltage and the voltage rating of the super capacitor is wide. In
this application report, the system voltage and the maximum working voltage of the super capacitor are
close, so overvoltage protection is not considered.
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Discrete Implementation of Super Capacitor Charger Circuit
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2
Discrete Implementation of Super Capacitor Charger Circuit
VIN
VIN
RCH
VD
VCAP
SuperCap
VCAP
VD
CG
SuperCap
RG1
RG2
EN
(a)
(b)
Figure 2. SuperCap Charger Circuit (a) Resistor-Diode Network (b) P-FET-based Discrete Circuit
Figure 2 (a) shows discrete charging circuit for the super capacitor. The resistor RCH controls the charging
current, and the Schottky diode prevents reverse current flow when the main power VIN is at low voltage.
Figure 3 illustrates the charging profile of a 5-F super capacitor using resistor-diode network. The current
set resistor RCH is chosen as 25 Ω to limit maximum charging current ICH_max to 200 mA as per Equation 1.
ICH _ max =
(VIN - VD ) = (5 - 0.3 ) » 200 mA
RCH
25
(1)
As seen in Figure 3, the charging current decreases exponentially with the increase of super capacitor
voltage VCAP. Though this charger circuit is simple, the circuit results in:
• Long charging time due to exponential charging profile
• Lower storage usage as the super capacitor is charged to a voltage VIN- VD
• No controllability—the charging starts as soon as main power VIN restores. If the application demands,
this circuit realization requires additional series switch for controllability.
Figure 3. Charging Profile of 5-F Super Capacitor Using Figure 2 (a)
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Discrete Implementation of Super Capacitor Charger Circuit
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The circuit configuration, shown in Figure 2 (b), uses a P-channel MOSFET to manage the charging
current of the super capacitor. The active component P-FET provides handle to control the charge flow;
however, this discrete circuit also has several challenges:
• As shown in Figure 4, the charging current increases exponentially with the increase of super capacitor
voltage VCAP. The RC components around the gate of P-FET must be adjusted through several
iterations to restrict the inrush current within the desired maximum limit.
• Lower storage usage due to Schottky diode drop
• In case of short circuit on super capacitor side, there is no limit on short circuit current whereas in
discrete circuit (Figure 2 (a)), the current set resistor RCH limits the short circuit current to ICH_max.
Figure 4. Charging Profile of 5-F Super Capacitor Using P-FET-based Discrete Circuit
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eFuse Solution
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3
eFuse Solution
VIN
VIN
OUT
IN
EN
VCAP
CIN
SuperCap
EN
EN/UVLO
OVP
DEVSLP
dVdT
GND
CSUP
FLT
PGOOD
PGTH
IMON
ILIM
eFuse
turn ON
delay
VCAP
ICAP
TPS25940
RILIM
CdVdT
tdVdT
t
(a)
(b)
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Figure 5. SuperCap Charger Circuit (a) eFuse Solution (b) Typical Operating Waveforms
Figure 5 (a) shows SuperCap charger solution using TPS25940A eFuse. Figure 5 (b) illustrates the
operating waveforms during the charging of a super capacitor. When the eFuse is enabled, the super
capacitor voltage starts building up after the eFuse turn ON delay. The ramp rate of the super capacitor
voltage VCAP can be adjusted by an external capacitor CdVdT at the dVdT pin as per Equation 2. This in turn
defines the charging current ICAP of the super capacitor, as seen in Equation 3.
The eFuse-based SuperCap charger circuit overcomes the challenges of discrete solutions as follows:
• The integrated back-to-back FET configuration along with true reverse current blocking feature of
TPS25940A eFuse eliminates the necessity of series Schottky diode.
• As there is no Schottky diode drop, the super capacitor charges to full supply voltage VIN, thus using
the complete storage capacity of the super capacitor.
• The TPS25940A eFuse has an EN/UVLO pin to control the ON or OFF state of the internal FET and
hence provides full controllability on the charging path of the super capacitor.
• The constant charging current profile with eFuse significantly reduces the charging time over discrete
solutions.
• eFuse solution provides quick termination of transient short circuit currents and offers robust short
circuit protection.
The total ramp time tdVdT of VCAP from 0 to VIN for TPS25940A can be calculated using Equation 2.
t dVdT = 8.3 ´ 104 ´ VIN ´ CdVdT
(2)
The charging current of the super capacitor ICAP can be calculated as Equation 3.
æ V ö
ICAP = CSUP ´ ç IN ÷
è t dVdT ø
(3)
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eFuse Solution
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The constant charging current ICAP sets a linear output ramp profile throughout the charging interval tdVdT.
The eFuse also has current limit function to limit output current to a value programmed by a resistor at the
ILIM pin. Equation 4 gives the relationship between ILIM and RILIM. To ensure safe operation, the eFuse
control logic limits the charging current to the lower of the two currents as determined in Equation 3 and
Equation 4.
89
ILIM =
RILIM
(4)
Where:
• ILIM is overload current limit in Amp
• RILIM is the current limit resistor in kΩ
While charging the super capacitor, the eFuse experiences significant power stress. The instantaneous
power dissipation across the device is (VIN-VCAP) × ICAP. As the super capacitor charges, the voltage
difference across the device decreases, and the power dissipated decreases as well. For a successful
design, the power dissipation during charging interval tdVdT should not exceed the shutdown limits as
shown in Figure 6. The average power dissipated in the device during charging interval tdVdT is given by
Equation 5.
PD _ STARTUP = 0.5 ´ VIN ´ ICAP
(5)
It is important to determine the correct ramp time tdVdT and hence charging current for a given super
capacitor (capacitance) such that the eFuse monotonically charges the super capacitor without reaching
thermal shutdown.
Thermal Shutdown Time (ms)
100000
TA = -40qC
TA = 25qC
TA = 85qC
TA = 125qC
10000
1000
100
10
1
0.1
1
10
Power Dissipation (W)
100
D002
Figure 6. Thermal Shutdown Time Plot for TPS25940A eFuse
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eFuse Solution
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3.1
3.1.1
Setting Output Voltage Ramp Time (tdVdT)
Using System Equations
This approach is iterative and requires several calculations to arrive at appropriate tdVdT for a particular
output capacitance. As shown in Figure 7, the design starts with a certain value of soft start capacitor
CdVdT and then influences it in the right direction, so the design does not violate startup power stress
considerations.
Select
soft start
capacitor CdVdT
Calculate
ramp time tdVdT
using Equation 2
Calculate
charging current ICAP
using Equation 3
Increase CdVdT
NO
Check on Thermal shutdown time
plot whether the design can
withstand PD_STARTUP
for tdVdT interval
Using Equation 5,
calculate PD_STARTUP
YES
Exit
Figure 7. Design Flow Chart to Determine tdVdT
3.1.2
Using Online Design Calculation Tool
The TPS25940A eFuse has online design calculator[2], which uses the same design equations explained
above. The designer must feed the supply voltage VIN and the super capacitance value CSUP in row 5 and
8 respectively. Then the designer must modify CdVdT value in row 55 until the startup power dissipation falls
within the acceptable limits and satisfies the system to have glitch-free startup.
3.1.3
Using Storage Capacity of the Super Capacitor
Substituting charging current ICAP from Equation 3 into Equation 5 gives Equation 6.
æ V 2
PD _ STARTUP = 0.5 ´ CSUP ´ ç IN
çt
è dVdT
ö
÷÷
ø
(6)
Equation 7 shows the energy required to fully charge the super capacitor.
ESUP = 0.5 ´ CSUP ´ VIN2 = PD _ STARTUP ´ t dVdT
(7)
To charge a super capacitor to its rated capacity without thermal shutdown, the area under the curve at a
particular operating point in Figure 6 should be equal to the energy capacity of the super capacitor ESUP.
Figure 8 shows the relation between energy ESUP and ramp time at TA = 85°C, which derives from
Figure 6. As super capacitors are restricted to operate less than 85°C for better life, maximum ambient
temperature of 85°C is considered.
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Test Results
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In this design approach, first the energy required to charge super capacitor is calculated and then the
corresponding ramp time (thermal shutdown time) is inferred from Figure 8. With the obtained ramp time,
soft start capacitor CdVdT is calculated using Equation 2. The result is then approximated to a standard
capacitor value.
1000000
Energy (w.ms)
100000
10000
1000
100
10
0.1
1
10
100
1000
Thermal Shutdown Time (ms)
10000
100000
D001
Figure 8. Averaged Energy Versus Thermal Shutdown Time at TA = 85°C for Figure 6
4
Test Results
In this section, measured charging profiles of various super capacitors are shown from Figure 9 to
Figure 12. Table 1 shows the calculations based on the design approach discussed in Section 3.1.3. It is
clear that any capacity of super capacitor can be charged easily using eFuse through proper selection of
soft-start capacitor CdVdT.
Table 1. Output Ramp Time Calculations and Measurement Results
SUPER
CAPACITOR
(F)
8
CdVdT CALCULATIONS
MEASURED RESULTS
REQUIRED
ENERGY TO
CHARGE TO
5V
tdVdT (ms)
FROM
Figure 8
CdVdT (uF)
USING
Equation 2
STANDARD
CdVdT (uF)
tdVdT (ms)
CHARGING
CURRENT (A)
0.5
6250
2000
4.82
4.7
1800
1.3
Figure 9
1
12500
4000
9.64
10
4200
1
Figure 10
5
62500
22000
53.02
68
30000
0.8
Figure 11
10
125000
46000
110.84
100
41000
1.1
Figure 12
Charging Super Capacitor With eFuse
TEST
WAVEFORM
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Test Results
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As shown in Figure 9, the super capacitor fully charges to supply voltage 5 V. In case of discrete circuits,
the super capacitor reaches maximum of 4.7 V (refer Figure 3 and Figure 4).
Figure 9. Charging Profile of 0.5-F Super Capacitor Using TPS25940A eFuse
From Figure 9 and Figure 10, it is clear that higher charging currents can be allowed at lower output
capacitance without reaching thermal shutdown.
Figure 10. Charging Profile of 1-F Super Capacitor Using TPS25940A eFuse
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Test Results
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The time to charge 5-F super capacitor with discrete circuits is greater than 400 s whereas the time is just
30 s with eFuse as illustrated in Figure 11.
Figure 11. Charging Profile of 5-F Super Capacitor Using TPS25940A eFuse
Figure 12. Charging Profile of 10-F Super Capacitor Using TPS25940A eFuse
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Test Results
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Figure 13 highlights the key advantage of using eFuse. If the designer places inadequate value of CdVdT
(here OPEN), the charging current gets safely limited to a value set by RILIM. For Figure 13 test case, ILIM is
set to 2.1 A. Because the charging current is higher, the device enters into thermal shutdown and attempts
to charge again after auto-retry delay of 128 ms. The built-in over temperature cutoff of eFuse protects the
internal FET and any further system damage.
Figure 13. Charging Profile of 5-F Super Capacitor, When CdVdT is OPEN and ILIM set to 2.1A
Figure 14 demonstrates another key feature Safe during Single Point Failure of TPS25940A eFuse. For
example, during a single failure, such as SHORT to an external current limit resistor and inadequate value
of CdVdT (here OPEN), the device safely limits the charging current to 670 mA (Typical).
Figure 14. Charging Profile of 5-F Super Capacitor, When CdVdT is OPEN and ILIM is SHORT
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Comparison Summary
5
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Comparison Summary
In this section, comparison between discrete and eFuse charger solutions is shown for a 5F super
capacitor charger application. As shown in Table 2, the eFuse solution offers full controllability on the
charging path and better storage capacity usuage. Additionally, several integrated protection features of
eFuse provides design freedom in the selection of external components for the charger circuit.
Table 2. Comparison Table
PARAMETER
6
SCHOTTKY DIODE BASED
DISCRETE CHARGING CIRCUIT
P-FET-BASED DISCRETE
CHARGING CIRCUIT
eFUSE CHARGING CIRCUIT
Controllability
No
Yes
Yes
Storage capacity
utilization
87%
87%
100%
Charging time for 5F super capacitor
≈ 400 s
≈ 325 s
≈ 30 s
Short circuit
protection
Yes, RCH limits the current
No
Yes
Overvoltage
protection
Requires additional circuitry
Requires additional circuitry
Yes
Thermal protection
RCH limits the current and provides
protection
No
Yes
Conclusion
The programmable charging current, true reverse current blocking capability, and the monotonous voltage
ramp-up profile with eFuse makes it preferable choice for charging larger capacity super capacitors.
Further, built-in thermal protection feature and Safe during Single Point Failure feature avoids damage to
the eFuse and any disturbance to the system voltage. In the similar way, eFuse can be used to charge
large holdup capacitor in backup power systems.
7
References
1. TPS25940, 18V, 5A, 42mΩ eFuse With Integrated Reverse Current Protection and DevSleep Support
(SLVSCF3)
2. TPS2594x Design Calculation Tool (http://www.ti.com/product/TPS25940/toolssoftware)
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