Application of E-Fuse in a DC/DC converter · Application of E-Fuse in a DC/DC converter No Smoke, No Fire. 2 Celestica Power Systems Innovative Solutions You Can Count On Want to

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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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Want to AvoidWant to Avoid

Burnt UnitsBurnt Units

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Want to AvoidWant to Avoid

Burnt Motherboards

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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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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 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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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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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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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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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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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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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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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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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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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


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


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


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


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


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


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)


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


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


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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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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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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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

Documents

Application of E-Fuse in a DC/DC converter · Application of E-Fuse in a DC/DC converter No Smoke, No Fire. 2 Celestica Power Systems Innovative Solutions You Can Count On Want to