Application of E-Fuse in a DC/DC converter. No Smoke, No Fire

Transcription

1 Application of E-Fuse in a DC/DC converter No Smoke, No Fire 1

2 Want to Avoid Burnt Units 2

3 Want to Avoid Burnt Motherboards 3

4 Output Over Voltage Common Output Over Voltage Protection Schemes PWM 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 4

5 Output Over Voltage PWM controlled OV limit Pros: Natural feature of PWM controller, provides fast response Inexpensive Cons: No protection against controller or feedback loop failure Relies on input source holding up long enough to blow input fuse If fuse doesn t blow, then no protection on subsequent power on. 5

6 Output Over Voltage Output Clamp (Crowbar) Pros: Protects against controller and control loop failures Fast 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 6

7 Output Over Voltage Input Clamp (Crowbar) Pros: Protects against controller and control loop failures Size of input clamp doesn t need to be as large as the output clamp Cons: Slow response may not keep output voltage below acceptable limits Relies on input current being sufficient to blow input fuse 7

8 Output Over Voltage Input Disconnect (E-fuse) Pros: Protects against controller and control loop failures Size of disconnect FET is smaller than output clamp Protects against subsequent power on Fast response Input power supply is not a factor Cons: Disconnect FET is more expensive than a similarly rated fuse 8

9 Over Current Common Over Current Measurement Techniques Low side Rdson sensing Inductor sensing Input current sense resistor Common Over Current Protection Schemes Fold Back Constant current Hiccup mode (PWM goes to zero duty and then restarts) Tri-state output Disconnect input power 9

10 Over Current Rdson Sensing Pros: Lossless (no added loss) Inexpensive Cons: Wide variation over FET vendors, or even lot to lot Variation over temperature Sampled measurement may not accurately represent current delivered to load (e.g. high side MOSFET short) Doesn t protect against input over current 10

11 Over Current Inductor Sensing Pros: Lossless (no added loss) Inexpensive Accurately represents current delivered to load Cons: Small signal creates signal to noise issues Variation over temperature Doesn t protect against input over current 11

12 Over Current Input Current Sense Resistor Pros: Accurately represents input current No temperature drift Larger signal for increased sensing accuracy Cons: Added cost Added power loss Doesn t accurately reflect output current (especially on DC/DC converters with variable output voltages) 12

13 Over Current Fold Back and Constant Current Pros: Fault condition recovery without user intervention Easy and inexpensive to implement Cons: May cause overheating of whatever is causing the fault 13

14 Over Current Hiccup mode Pros: Fault condition recovery without user intervention Easy and inexpensive to implement Significantly 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) 14

15 Over Current Tri-state Output Pros: Easy and inexpensive to implement Eliminates power delivered to short Cons: Requires user intervention for recovery 15

16 Over Current Disconnect Input Power Pros: Eliminates power delivered to short Cons: Requires user intervention for recovery May increase cost 16

17 Ideal Solution An Ideal Solution would protect both the module and the down stream components: Eliminate output over voltage Fast response to input over current Reduced power into output short circuit Safe post failure power on 17

18 Input 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. 18

19 Disconnect FET Actual implementation Use N-channel FET for best cost vs. performance Include protection diodes to limit gate voltage Include ceramic capacitor on input side to reduce voltage spike under heavy load turn off Use a charge pump from one of the phases to enhance the disconnect FET VDG Charge_Pump 10k R56 4.7u 5m R9 Input_decoupling irf7463 Input_Disconnect_FET D26 D27 BZX84-10 BAT54W VIN_F VIN_F_SW 19

20 Disconnect Latch Actual implementation VDG VCC Use discrete transistor latch to turn off the disconnect FET if Latch is pulled low Guaranteed no latch state until Vcc reaches a predetermined value. Use open collector signals so various protection mechanisms can be orred together easily MMBT3904LT1 Q12 MMBT3906LT1 Q11 R61 3k 5.11k R57 25k R k R47 25k R48 MMBT3904LT1 Q15 Latch 20

21 Reference Voltage Actual implementation Use 2% zener diode for reference generation Least expensive reference for cases where exact over voltage or over current threshold isn t necessary Could easily use a TL431 or similar device V_ref 100p C1 8.45k R k R50 D30 BZX79-6V2 VCC 500 R51 21

22 Over Voltage Detect Actual implementation Use LM339 to compare Vout to reference voltage Hysteresis isn t needed since when Latch goes low the input disconnect FET is latched off Small amount of filtering on Vout for noise immunity, while still allowing fast response Vout is the output voltage local on the module, not the remote sense point (in case of open, or shorted remote sense) Latch VCC X9 LM339 47p C18 R55 10k 10k R59 V_ref Vout 22

23 Input Over Current Detect Actual implementation VCC Sense input current across input fuse or sense resistor Create proportional current through a resistor to ground (R6) Compare the voltage developed across R6 to the reference Filter the signal only as much as necessary for noise immunity, to guarantee fast response InputFuse 3.5m Latch VIN VIN_F 1k R3 1k R4 VCC X1 LM339 X2 LMC7101B/NS R7 10k R8 10k 100p C3 V_ref 12.7k R6 MMBT3904LT1 Q1 47p C2 23

24 Test 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 phases 300kHz switching per phase 1.3V Vout (no load voltage, unit has AVP) 12V Vin One high side FET was wired for normal gate pulses, or +20V Shorted high side FET while running 24

25 Test Results 12Vin, 1.3Vout, 110A load induced high side FET short Ch1 offset by 1V = Vout Ch2 = current sense signal Ch3 = VIN_F (12V fused) Ch4 = VDG (gate of FET) 25

26 Test Results 12Vin, 1.3Vout, 0A load induced high side FET short Ch1 offset by 1V = Vout Ch2 = current sense signal Ch3 = VIN_F (12V fused) Ch4 = VDG (gate of FET) 26

27 Test 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 rise At no load the PWM can t overcome the high side FET short and the output voltage does rise In 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 27

28 Why Input Current Limit Why input current limit and not just a fuse Not all systems have enough current capability to blow a fuse Fuses take a long time to blow (10ms to many seconds) and an output over voltage may still occur Some components may draw large surge current and emit smoke without drawing enough sustained current to blow a fuse (e.g. ceramic capacitors) 28

29 Open 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 sense Both converters have OVP threshold of 2V Both were run with a minimal load of 5A 29

30 Final 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) 30

31 Looking Forward Discrete implementation is inexpensive but space consumptive High side current monitors are available in one small package Disconnect 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 31