Any suggestions for what could blow the FETs up would be helpful (please leave in comments section).
You can get a larger image by clicking on the thumbnail above.
Links to the various parts are:
IRFP350
IRFP460
SG3525A
The failing conditions are when the 12V accessories are drawing a lot of current (25 amps) and the output voltage has sagged down to 12V (DC-DC tries to put out 13.9V).
I'm guessing that as the voltage sags, the output (pin 6) of the 741 op-amp goes low and turns off the LED in the opto-coupler. With the opto-coupler off, the voltage goes low on pin1 of the SG3525A (inverting input to error amp) and increases the pulse widths going to the drive FETs. If the pulse widths are too long, they cause the FETs to stay on too long and saturate the inductors, which causes a high-current spike of +144V that blows the FETs.
Supposedly, the current sense loop voltage that is adjusted by the R8 potentiometer should send a positive pulse into pin10 of the controller to shut down the FETs, but this may not be happening quickly enough.
Initially, I cringed because there were no kick-back diodes in the circuit, but I think that C7 and R16 (2 watt resistor) send any kick-back currents into the opposite inductor when the FETs let go.
Any ideas? Thanks for your input.
Cheers,
Tim
4 comments:
Tim, if you haven't done so already, why don't you post this on the EV discussion list. There are some pretty deep thinkers there.
/roger
What does the manufacturer say?
Hi Tim,
I remember that you said the Battery Pack voltage sags under load.
The DC-DC converter has an ideal ratio of Pack Voltage to Low Voltage of ~ 10 : 1
You also commented that you had to adjust the inverter parameters because of the Battery Pack voltage sag.
When you add the reduced Battery Pack voltage supplied to the DC-DC converter to a possible voltage sag at the Low voltage side then, when the headlights come on, it seems to me that you don't have much run-time left in your car. At least, not with the CC Power converter.
I would suggest contacting Viktor and see if his idea about putting an ultra capacitor bank in parallel with the Battery Pack might give you more run-time between charges.
Thanks for all your suggestions. I've given the schematic to several power-supply designers at work to look at and think about. They were most helpful in suggesting things to look at.
I also sent an e-mail to CCPower earlier today, so perhaps they'll respond by the morning (they're in the UK)
The standard-plate lead-acid batteries I'm using sag quite a bit under heavy acceleration and return to their initial voltage after coasting a bit. I'll have to contact Viktor about the ultra-capacitor. The car drives OK now, especially with the pack getting more broken in. I just don't like having a blown DC-DC. Ideally, I should be able to short the DC-DC out and it should current limit without dying...
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