application of e-fuse in a dc/dc converter · application of e-fuse in a dc/dc converter no smoke,...
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Celestica Power Systems Innovative Solutions You Can Count On
Application of E-Fuse in a DC/DC converterApplication of E-Fuse in a DC/DC converter
No Smoke, No FireNo Smoke, No Fire
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Celestica Power Systems Innovative Solutions You Can Count On
Want to AvoidWant to Avoid
Burnt UnitsBurnt Units
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Celestica Power Systems Innovative Solutions You Can Count On
Want to AvoidWant to Avoid
Burnt Motherboards
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Celestica Power Systems Innovative Solutions You Can Count On
Output Over VoltageOutput Over Voltage
Common Output Over Voltage Protection SchemesPWM controller turns on low side FETs to limit Vout excursion.Separate reference and comparator used to trigger output clamp (SCR or interrupting PWM signal to low side driver)Separate reference and comparator used to trigger input clamp (SCR crowbars input fuse)Input disconnect with latch off feature for over voltage
Common Output Over Voltage Protection SchemesPWM controller turns on low side FETs to limit Vout excursion.Separate reference and comparator used to trigger output clamp (SCR or interrupting PWM signal to low side driver)Separate reference and comparator used to trigger input clamp (SCR crowbars input fuse)Input disconnect with latch off feature for over voltage
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Output Over VoltageOutput Over Voltage
PWM controlled OV limit
Pros:Natural feature of PWM controller, provides fast responseInexpensive
Cons:No protection against controller or feedback loop failureRelies on input source holding up long enough to blow input fuseIf fuse doesn’t blow, then no protection on subsequent power on.
PWM controlled OV limit
Pros:Natural feature of PWM controller, provides fast responseInexpensive
Cons:No protection against controller or feedback loop failureRelies on input source holding up long enough to blow input fuseIf fuse doesn’t blow, then no protection on subsequent power on.
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Output Over VoltageOutput Over Voltage
Output Clamp (Crowbar)
Pros:Protects against controller and control loop failuresFast response
Cons:If external components added, they must be very large (and expensive)Relies on input source holding up long enough to blow input fuse
Output Clamp (Crowbar)
Pros:Protects against controller and control loop failuresFast response
Cons:If external components added, they must be very large (and expensive)Relies on input source holding up long enough to blow input fuse
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Output Over VoltageOutput Over Voltage
Input Clamp (Crowbar)
Pros:Protects against controller and control loop failuresSize of input clamp doesn’t need to be as large as the output clamp
Cons:Slow response may not keep output voltage below acceptable limitsRelies on input current being sufficient to blow input fuse
Input Clamp (Crowbar)
Pros:Protects against controller and control loop failuresSize of input clamp doesn’t need to be as large as the output clamp
Cons:Slow response may not keep output voltage below acceptable limitsRelies on input current being sufficient to blow input fuse
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Output Over VoltageOutput Over Voltage
Input Disconnect (E-fuse)
Pros:Protects against controller and control loop failuresSize of disconnect FET is smaller than output clampProtects against subsequent power onFast responseInput power supply is not a factor
Cons:Disconnect FET is more expensive than a similarly rated fuse
Input Disconnect (E-fuse)
Pros:Protects against controller and control loop failuresSize of disconnect FET is smaller than output clampProtects against subsequent power onFast responseInput power supply is not a factor
Cons:Disconnect FET is more expensive than a similarly rated fuse
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Over CurrentOver Current
Common Over Current Measurement TechniquesLow side Rdson sensingInductor sensingInput current sense resistor
Common Over Current Protection SchemesFold BackConstant currentHiccup mode (PWM goes to zero duty and then restarts)Tri-state outputDisconnect input power
Common Over Current Measurement TechniquesLow side Rdson sensingInductor sensingInput current sense resistor
Common Over Current Protection SchemesFold BackConstant currentHiccup mode (PWM goes to zero duty and then restarts)Tri-state outputDisconnect input power
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Over CurrentOver Current
Rdson Sensing
Pros:Lossless (no added loss)Inexpensive
Cons:Wide variation over FET vendors, or even lot to lotVariation over temperatureSampled measurement may not accurately represent current delivered to load (e.g. high side MOSFET short)Doesn’t protect against input over current
Rdson Sensing
Pros:Lossless (no added loss)Inexpensive
Cons:Wide variation over FET vendors, or even lot to lotVariation over temperatureSampled measurement may not accurately represent current delivered to load (e.g. high side MOSFET short)Doesn’t protect against input over current
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Over CurrentOver Current
Inductor Sensing
Pros:Lossless (no added loss)InexpensiveAccurately represents current delivered to load
Cons:Small signal creates signal to noise issuesVariation over temperatureDoesn’t protect against input over current
Inductor Sensing
Pros:Lossless (no added loss)InexpensiveAccurately represents current delivered to load
Cons:Small signal creates signal to noise issuesVariation over temperatureDoesn’t protect against input over current
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Over CurrentOver Current
Input Current Sense Resistor
Pros:Accurately represents input currentNo temperature driftLarger signal for increased sensing accuracy
Cons:Added costAdded power lossDoesn’t accurately reflect output current (especially on DC/DC converters with variable output voltages)
Input Current Sense Resistor
Pros:Accurately represents input currentNo temperature driftLarger signal for increased sensing accuracy
Cons:Added costAdded power lossDoesn’t accurately reflect output current (especially on DC/DC converters with variable output voltages)
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Over CurrentOver Current
Fold Back and Constant Current
Pros:Fault condition recovery without user interventionEasy and inexpensive to implement
Cons:May cause overheating of whatever is causing the fault
Fold Back and Constant Current
Pros:Fault condition recovery without user interventionEasy and inexpensive to implement
Cons:May cause overheating of whatever is causing the fault
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Over CurrentOver Current
Hiccup mode
Pros:Fault condition recovery without user interventionEasy and inexpensive to implementSignificantly reduces power delivered to short
Cons:May cause under-voltage ringing on output when PWM goes to zero duty cycle (low side MOSFETs turn on)
Hiccup mode
Pros:Fault condition recovery without user interventionEasy and inexpensive to implementSignificantly reduces power delivered to short
Cons:May cause under-voltage ringing on output when PWM goes to zero duty cycle (low side MOSFETs turn on)
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Over CurrentOver Current
Tri-state Output
Pros:Easy and inexpensive to implementEliminates power delivered to short
Cons:Requires user intervention for recovery
Tri-state Output
Pros:Easy and inexpensive to implementEliminates power delivered to short
Cons:Requires user intervention for recovery
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Over CurrentOver Current
Disconnect Input Power
Pros:Eliminates power delivered to short
Cons:Requires user intervention for recoveryMay increase cost
Disconnect Input Power
Pros:Eliminates power delivered to short
Cons:Requires user intervention for recoveryMay increase cost
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Ideal SolutionIdeal Solution
An Ideal Solution would protect both the module and the down stream components:
Eliminate output over voltageFast response to input over currentReduced power into output short circuitSafe post failure power on
An Ideal Solution would protect both the module and the down stream components:
Eliminate output over voltageFast response to input over currentReduced power into output short circuitSafe post failure power on
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Input Disconnect ImplementationInput Disconnect Implementation
The ideal solution would be an input disconnect (E-fuse) with reset-able latch.
The latch is set for input over current and output over voltage.
Output over current is handled by the PWM.
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Disconnect FETDisconnect FET
Actual implementation
Use N-channel FET for best cost vs. performanceInclude protection diodes to limit gate voltageInclude ceramic capacitor on input side to reduce voltage spike under heavy load turn offUse a charge pump from one of the phases to enhance the disconnect FET
Actual implementation
Use N-channel FET for best cost vs. performanceInclude protection diodes to limit gate voltageInclude ceramic capacitor on input side to reduce voltage spike under heavy load turn offUse a charge pump from one of the phases to enhance the disconnect FET
5m
R910kR56
Input_Disconnect_FET
irf7463
D26
BZX84-10
D27
BAT54W
4.7u
Input_decoupling
VIN_F
VIN_F_SW
Charge_Pump
VDG
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Disconnect LatchDisconnect Latch
Actual implementation
Use discrete transistor latch to turn off the disconnect FET if “Latch” is pulled lowGuaranteed no latch state until Vcc reaches a pre-determined value.Use open collector signals so various protection mechanisms can be orredtogether easily
Actual implementation
Use discrete transistor latch to turn off the disconnect FET if “Latch” is pulled lowGuaranteed no latch state until Vcc reaches a pre-determined value.Use open collector signals so various protection mechanisms can be orredtogether easily
Latch
MMBT3904LT1Q12
VCCVDG
3kR61
MMBT3906LT1Q11
5.11kR47
MMBT3904LT1Q15
25kR48
25kR54
5.11kR57
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Reference VoltageReference Voltage
Actual implementation
Use 2% zener diode for reference generationLeast expensive reference for cases where exact over voltage or over current threshold isn’t necessaryCould easily use a TL431 or similar device
Actual implementation
Use 2% zener diode for reference generationLeast expensive reference for cases where exact over voltage or over current threshold isn’t necessaryCould easily use a TL431 or similar device
8.45kR49
20.5k
R50
VCC
500 R51
D30BZX79-6V2100p
C1
V_ref
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Over Voltage DetectOver Voltage Detect
Actual implementation
Use LM339 to compare Vout to reference voltageHysteresis isn’t needed since when Latch goes low the input disconnect FET is latched offSmall amount of filtering on Vout for noise immunity, while still allowing fast responseVout is the output voltage local on the module, not the remote sense point (in case of open, or shorted remote sense)
Actual implementation
Use LM339 to compare Vout to reference voltageHysteresis isn’t needed since when Latch goes low the input disconnect FET is latched offSmall amount of filtering on Vout for noise immunity, while still allowing fast responseVout is the output voltage local on the module, not the remote sense point (in case of open, or shorted remote sense)
V_refLatch
VCC
10k
R59
X9
LM33947pC18
10kR55
Vout
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Input Over Current DetectInput Over Current Detect
Actual implementation
Sense input current across input fuse or sense resistorCreate proportional current through a resistor to ground (R6)Compare the voltage developed across R6 to the referenceFilter the signal only as much as necessary for noise immunity, to guarantee fast response
Actual implementation
Sense input current across input fuse or sense resistorCreate proportional current through a resistor to ground (R6)Compare the voltage developed across R6 to the referenceFilter the signal only as much as necessary for noise immunity, to guarantee fast response
VIN
VIN_F
X2
LMC7101B/NS12.7k
R6
1k
R3
V_ref
1k
R4
VCC
X1
LM339
10kR7
10kR8
VCC
MMBT3904LT1Q1
100p
C3
47pC2
Latch
InputFuse3.5m
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Celestica Power Systems Innovative Solutions You Can Count On
Test ResultsTest Results
1 Unit was wired up to test a multiphase converter for the case of a high side MOSFET short
Test setup consisted of:4 phases300kHz switching per phase1.3V Vout (no load voltage, unit has AVP)12V VinOne high side FET was wired for normal gate pulses, or +20VShorted high side FET while running
1 Unit was wired up to test a multiphase converter for the case of a high side MOSFET short
Test setup consisted of:4 phases300kHz switching per phase1.3V Vout (no load voltage, unit has AVP)12V VinOne high side FET was wired for normal gate pulses, or +20VShorted high side FET while running
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Test ResultsTest Results
12Vin, 1.3Vout, 110A load induced high side FET short
Ch1 offset by 1V = VoutCh2 = current sense signalCh3 = VIN_F (12V fused)Ch4 = VDG (gate of FET)
12Vin, 1.3Vout, 110A load induced high side FET short
Ch1 offset by 1V = VoutCh2 = current sense signalCh3 = VIN_F (12V fused)Ch4 = VDG (gate of FET)
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Celestica Power Systems Innovative Solutions You Can Count On
Test ResultsTest Results
12Vin, 1.3Vout, 0A load induced high side FET short
Ch1 offset by 1V = VoutCh2 = current sense signalCh3 = VIN_F (12V fused)Ch4 = VDG (gate of FET)
12Vin, 1.3Vout, 0A load induced high side FET short
Ch1 offset by 1V = VoutCh2 = current sense signalCh3 = VIN_F (12V fused)Ch4 = VDG (gate of FET)
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Celestica Power Systems Innovative Solutions You Can Count On
Test Result ConclusionsTest Result Conclusions
Some interesting results from this test:
At high load the load and PWM overcome the high side FET short and the output voltage doesn’t riseAt no load the PWM can’t overcome the high side FET short and the output voltage does riseIn both cases, even with the trip threshold set at 3x the normal input current, the input over current circuit tripped the latch before the output voltage rose to the OVP limit, and before the fuses opened
Some interesting results from this test:
At high load the load and PWM overcome the high side FET short and the output voltage doesn’t riseAt no load the PWM can’t overcome the high side FET short and the output voltage does riseIn both cases, even with the trip threshold set at 3x the normal input current, the input over current circuit tripped the latch before the output voltage rose to the OVP limit, and before the fuses opened
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Why Input Current LimitWhy Input Current Limit
Why input current limit and not just a fuse
Not all systems have enough current capability to blow a fuseFuses take a long time to blow (10ms to many seconds) and an output over voltage may still occurSome components may draw large surge current and emit smoke without drawing enough sustained current to blow a fuse (e.g. ceramic capacitors)
Why input current limit and not just a fuse
Not all systems have enough current capability to blow a fuseFuses take a long time to blow (10ms to many seconds) and an output over voltage may still occurSome components may draw large surge current and emit smoke without drawing enough sustained current to blow a fuse (e.g. ceramic capacitors)
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Open Feedback LoopOpen Feedback Loop
Ch1 = Vout Ch2 = remote sense+
Comparing speed of over voltage detection and response of SCR crowbar (left) to input disconnect (right) when shorting remote senseBoth converters have OVP threshold of 2VBoth were run with a minimal load of 5A
Ch1 = Vout Ch2 = remote sense+
Comparing speed of over voltage detection and response of SCR crowbar (left) to input disconnect (right) when shorting remote senseBoth converters have OVP threshold of 2VBoth were run with a minimal load of 5A
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Final NotesFinal Notes
Input disconnect FET eliminates catastrophic over voltage on the output of a converter
Including input over current detection adds additional protection for systems with less current capability than needed to open a fuse
Input over current detection is faster than a fuse and will respond to very short pulses of high current (as may be present in failed ceramic capacitors or high side FETs)
Input disconnect FET eliminates catastrophic over voltage on the output of a converter
Including input over current detection adds additional protection for systems with less current capability than needed to open a fuse
Input over current detection is faster than a fuse and will respond to very short pulses of high current (as may be present in failed ceramic capacitors or high side FETs)
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Looking ForwardLooking Forward
Discrete implementation is inexpensive but space consumptiveHigh side current monitors are available in one small packageDisconnect FET latch, OVP, OCP and charge pump may all be integrated into one IC.
The problem becomes finding the IC with the needed features and not too many expensive extras
Discrete implementation is inexpensive but space consumptiveHigh side current monitors are available in one small packageDisconnect FET latch, OVP, OCP and charge pump may all be integrated into one IC.
The problem becomes finding the IC with the needed features and not too many expensive extras