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Texas Instruments Reliable Startup with Large and Unknown Capacitive Loads Application notes
Application Report
SLVAEB9 – May 2019
Reliable Startup with Large and Unknown Capactive
Loads
Lokesh Ghulyani
ABSTRACT
Industrial equipment such as a Programmable logic controller (PLC) need large capacitors for storing
energy to provide back-up time for storing critical information before equipment shutdown. In the case of
DIN power supplies, the output capacitance of the load is unknown and the power supply designer has to
design for a wide range of output capacitance. This application note describes how large and unknown
capacitors can be powered using the TPS2663 and TPS1663 devices.
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Contents
Introduction ...................................................................................................................
Powering Capacitive Loads with Constant Inrush Current and Output Slew Rate at Start-up ....................
Powering Capacitive Loads with Constant Power Dissipation in Power Switch at Start-up .......................
Powering Capacitive Loads with Thermal Regulation at Start up .....................................................
Conclusion ....................................................................................................................
References ...................................................................................................................
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3
5
7
9
9
List of Figures
1
Block Diagram of a PLC CPU .............................................................................................. 2
2
Block Diagram of a DIN Power Supply ................................................................................... 3
3
TPS26600 Application Circuit for Charging Capacitor with Constant Inrush Current............................... 3
4
Power Dissipation for COUT = 1 mF with IINRUSH = 115 mA ............................................................... 4
5
Hiccups in Start-up with Constant Inrush Current
6
TPS2471x Application Circuit for Start up with Constant Power Dissipation in Power Switch .................... 5
7
Clean Start-up with Constant Power Dissipation in Power Switch .................................................... 6
8
Hiccup in Start-up with Constant Power Dissipation in Power Switch ................................................ 7
9
TPS2663x Application Circuit .............................................................................................. 7
10
Clean Start-up with Thermal Regulation with COUT = 4.7 mF........................................................... 8
11
Clean Startup with COUT of 20 mF .......................................................................................... 9
......................................................................
5
List of Tables
...............................................................................
1
Power Dissipated with Load Capacitance
2
Charging Time and Capacitance with Temperature..................................................................... 9
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1
Introduction
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Trademarks
All trademarks are the property of their respective owners.
1
Introduction
The Programmable Logic Controllers (PLCs) are widely used for automation in industries. The PLC
collects data from sensors, analyses this data using CPU, and controls the industrial process through
actuators. The PLC CPU needs energy storage to provide a back-up for storing critical information in case
of loss of power. The energy storage is either provided by a battery or a large capacitor. A large capacitor
is preferred over a battery for energy storage due to its lower cost. Figure 1 provides a block diagram for
the PLC CPU.
24V
Field
Power
PLC CPU
E-Fuse &
Protection
DC/DC
ORing
Energy
Storage
MCU/
FPGA/
ASIC
RS-485/
Ethernet/
CAN
Data
Backplane
Figure 1. Block Diagram of a PLC CPU
The 24-V field power is the primary source of power for the PLC CPU. An eFUSE device with integrated
MOSFETs and protection features can be used at the power input of PLC CPU to protect from surges and
faults on a field power bus. The energy storage capacitor is used to power the DC/DC converter during an
event of failure of power on a field power bus. A capacitor of value typically more than 1 mF is used to
provide power to PLC CPU during failure. This capacitor draws large current during start-up and can
cause the eFUSE to go into shutdown due to overload or due to excessive thermal dissipation. Another
similar application of energy storage requiring large capacitance is for motor and servo drives. TI Design
Compact, Efficient, 24-V Input Auxiliary Power Supply Reference Design for Servo Drives provides the
complete design procedure and test results for power supply in servo drives.
In PLC systems, there is a 24-V power bus which provides power to modules in the PLC system. This
power bus is powered from DIN power supply. The number of modules connected on this power bus
varies with architecture of PLC systems. The output capacitance seen by DIN power supply is unknown.
Typically, a DIN power supply of less than 250 W is designed for a maximum capacitive load of 10 mF.
Figure 2 provides a block diagram of DIN power supply.
2
Reliable Startup with Large and Unknown Capactive Loads
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Powering Capacitive Loads with Constant Inrush Current and Output Slew Rate at Start-up
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AC
Mains
Supply
Offline
AC/DC
24V DC IN
E-Fuse &
Protection
24V DC OUT
24V
DC Bus
C1
Cn
Module 1
Module n
DC/DC
MCU
RS-485/
Ethernet/
CAN
DIN Power
Supply
Data
Backplane
Digital
Output
ALARM
Figure 2. Block Diagram of a DIN Power Supply
2
Powering Capacitive Loads with Constant Inrush Current and Output Slew Rate at
Start-up
Capacitors draw large currents from the power source at start-up, which can lead to tripping of the power
source due to overload. To limit the inrush current into capacitors, power switches implement constant
current charging of capacitors at start-up. To charge the capacitors with inrush current, the output voltage
is increased linearly with time. As an example, the TPS2660 device has a capacitor on the dVdT pin to
control the output slew rate and limit the inrush current for output capacitor. Figure 3 provides the
application circuit with the TPS2660 for charging capacitors with constant inrush current.
VIN: 4.2V-55V
CIN
R1
150PŸ
UVLO
OVP
R2
FLT
RTN
COUT
Health Monitor
ON/OFF Control
SHDN
IMON
MODE
CdVdT
RFLTb
TPS26600
dVdT
R3
VOUT
OUT
IN
Load Monitor
ILIM
GND
RILIM
RIMON
Copyright © 2016, Texas Instruments Incorporated
Figure 3. TPS26600 Application Circuit for Charging Capacitor with Constant Inrush Current
At power up, the output capacitor has zero voltage and there is power dissipation of [VIN X IINRUSH]. As the
capacitor gets charged, the voltage drop across the power device and the power dissipation decreases.
For charging the output capacitor to VIN voltage, an average power of [0.5 × VIN X IINRUSH] is dissipated in
the power switch during the start-up. Figure 4 provides power dissipation for COUT of 1 mF and IINRUSH of
115 mA.
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Powering Capacitive Loads with Constant Inrush Current and Output Slew Rate at Start-up
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Figure 4. Power Dissipation for COUT = 1 mF with IINRUSH = 115 mA
For lower voltages and lower output capacitance, the capacitors can be charged with constant inrush
current and constant output slew rate. But as the output capacitance and input voltage increases, the
power dissipation in the power switch at power up increases and could lead to thermal shutdown and
hiccup in start-up. Table 1 provides the power dissipated for a start-up time of 209 ms and constant output
slew rate of 115 V/s.
Table 1. Power Dissipated with Load Capacitance
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VIN
IINRUSH
COUT
AVERAGE POWER DISSIPATED
24 V
115 mA
1 mF
1.38 W
24 V
250 mA
2.2 mF
3W
24 V
540 mA
4.7 mF
6.5 W
24 V
1725 mA
15 mF
20.7 W
Reliable Startup with Large and Unknown Capactive Loads
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Powering Capacitive Loads with Constant Power Dissipation in Power Switch at Start-up
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With increased power dissipation at higher voltage and increased output capacitance, the power switch
goes into thermal shutdown and leads to hiccups in start-up. Figure 5 shows the hiccups in start-up with
output capacitance of 15 mF and VIN of 24 V due to thermal shutdown of power switch.
See the TPS26600 Design Calculator to design with TPS2660x devices with clean start-up.
Figure 5. Hiccups in Start-up with Constant Inrush Current
3
Powering Capacitive Loads with Constant Power Dissipation in Power Switch at
Start-up
Hot-swap controllers like the TPS2471x can provide the charging of output capacitors with constant power
dissipation in power switch. As illustrated in Figure 6, resistor RPROG sets the power dissipation limit for
power switch M1 and capacitor CT sets the maximum time for power limiting and overcurrent faults.
RSENSE
2 mΩ
VIN
M1
CSD16403Q5
VOUT
COUT
470 μF
C1
0.1 μF
RGATE
10 Ω
R1
130 kΩ
VCC
SENSE
3V
GATE
EN
R2
18.7 kΩ
OUT
R4
3.01 kΩ
R5
3.01 kΩ
PGb (PG)
TPS2471x
FLTb (FLT)
TIMER
PROG
CT
56 nF
RPROG
44.2 kΩ
GND
VUVLO = 10.8 V
ILMT = 12 A
tFAULT = 7.56 ms
Figure 6. TPS2471x Application Circuit for Start up with Constant Power Dissipation in Power Switch
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Powering Capacitive Loads with Constant Power Dissipation in Power Switch at Start-up
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Figure 7 shows start-up with capacitance of 1 mF and VIN of 12 V. The inrush current is at its lowest
value initially but increases as the drop across the power switch reduces with charging of output capacitor.
These devices can provide clean start-up with higher output capacitance and higher input voltage but
these devices require external MOSFET to handle the power dissipation during start-up. External
MOSFET needs more area on the board and leads to increased solution size.
Figure 7. Clean Start-up with Constant Power Dissipation in Power Switch
With increased input voltage and load capacitance, the power dissipation in the power switch can go
beyond SOA limits of the power switch and can lead to hiccups in start-up. Figure 8 shows the hiccup in
start-up with the TPS24710 device for VIN of 15 V and COUT of 2.2 mF. The device tries to charge load
capacitance up-to a time of 8 ms and output reaches up-to 7.5 V.
See TPS24710 Design Calculator to design with the TPS24710 device.
6
Reliable Startup with Large and Unknown Capactive Loads
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Powering Capacitive Loads with Thermal Regulation at Start up
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Figure 8. Hiccup in Start-up with Constant Power Dissipation in Power Switch
4
Powering Capacitive Loads with Thermal Regulation at Start up
To power large capacitive loads without hiccups, a thermal regulation is required at start-up to prevent the
shutdown of the power switch. With thermal regulation at start-up, the power switch regulates the junction
temperature less than the thermal shutdown temperature and allows clean start-up with large capacitive
loads. The TPS2663 and TPS1663 devices include the thermal regulation at start-up to provide clean
start-up with large capacitive loads. Figure 9 shows the application circuit with the TPS2663x device.
Optional components
for RCB and RPP
4.5V-60V
IN
OUT
B_GATE
D1*
COUT
R5
Protected supply
To Load
PGTH
DRV
IN_SYS
R4
TPS26630/31
PGOOD
FLT
R1
SHDN
UVLO
IMON
R2
Load Monitor
ILIM
OVP
R3
ON/OFF Control
MODE
dVdT
GND
CdVdT
RILIM
RILON
* TVS for Surge Suppression Only
Figure 9. TPS2663x Application Circuit
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Powering Capacitive Loads with Thermal Regulation at Start up
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Figure 10. Clean Start-up with Thermal Regulation with COUT = 4.7 mF
Figure 10 shows the clean start-up with 4.7 mF capacitive load for VIN of 32 V. The device starts with
constant inrush current and then goes into thermal regulation to prevent thermal shutdown and powers up
in 65 ms. Without thermal regulation, the startup time increases even in the order of minutes for charging
large capacitive loads. In case of unknown capacitive loads, it becomes difficult to select the appropriate
value of inrush current and slew rate for charging output capacitors. With thermal regulation, the designers
can achieve fast and reliable startup without selecting the appropriate value of inrush current and output
slew rate.
With higher capacitive load, the power switch heats up even faster and thermal regulation is initiated
earlier. Figure 11 illustrates clean start-up with a capacitive load of 20 mF and VIN of 32 V.
See TPS2663 Design Calculator to design with the TPS2663 and TPS1663 devices.
8
Reliable Startup with Large and Unknown Capactive Loads
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Conclusion
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Figure 11. Clean Startup with COUT of 20 mF
Table 2 lists the charging time with different capacitive loads and ambient temperatures for the TPS2663
and TPS1663 devices.
Table 2. Charging Time and Capacitance with Temperature
TEMPERATURE
VIN
4.5V
18V
32V
5
Charging Time
Capacitance Value
Charging Time
Capacitance Value
Charging Time
Capacitance Value
-40°C
0°C
25°C
105°C
20.5 ms
19.9 ms
20.9 ms
21 ms
32 mF
32 mF
32 mF
32 mF
287.9 ms
312.2 ms
408 ms
1390 ms
32 mF
32 mF
32 mF
32 mF
1625 ms
1757 ms
2144 ms
2622 ms
32 mF
32 mF
32 mF
16 mF
Conclusion
Powering large capacitive loads through constant inrush current can lead to hiccups in start-up due to
thermal shutdown whereas powering large capacitive load with constant power dissipation in power switch
need an external MOSFET to withstand the power dissipation during start-up. The TPS2663 and TPS1663
devices with integrated thermal regulation and MOSFET provide an optimized solution for achieving fast
and reliable startup with large and unknown capacitive loads.
6
References
•
•
•
•
Compact, efficient, 24-V input auxiliary power supply reference design for servo drives
TPS2663 Design Calculator
TPS26600 Design Calculator
TPS24710 Design Calculator
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