After studying the oscilloscope waveforms, I believed that the current sense circuit was way too spikey. This is the "amp adjust" potentiometer output in the C400 schematic. I found that the inductive spikes were shutting off the DC-DC converter through the nShutdown pin on the SG3525A and causing craziness in the gate-drive pulse. Soooo, I picked a reasonable (.01uF) capacitor to filter out the current sense waveform and remove the inductive spikes.
Here's the .01uF capacitor soldered to the board across the "Amp Adjust" potentiometer output.
If you compare the voltage spikes in this scope view to the one in the previous post, the inductive spikes are still there, but not as prominent.
Here's the same view under a 7 amp load. Note the gate drive pulse at the bottom of the screen has widened out to 8usec. The inductive spikes are much smaller and don't trigger the nShutdown circuit.
At this point when I apply just over a 14 amp load, the current sense circuit pulses are high enough to trigger the nShutdown pin on the SG3525A. When this happens, I note that the output voltage drops because we are in current limiting mode (good). However, the FET gate pulse is all over the place and the DC-DC converter whines loudly. This is important to remember after you read the next section...
I turned down the current limit to 14 amps and re-installed it in the 914 EV. After powering up the car, the unit didn't blow. I turned on the headlights and the system kept working. I then turned on the fog lights, tail lights, turn-signals and reverse lights. The DC-DC started whining loudly as stated in the previous paragraph. I thought this was good because I figured it was in current limiting mode. However, after a few seconds, the system went silent and, as I feared, I had blown the FETs again.... (sigh).
After taking a break, I read some textbooks and analyzed the SG3525A circuit diagrams once more. I'm going to take a wild guess that this is what happened (for all you advanced EE geeks):
When the unit goes into overcurrent mode, it pulses the nShutdown signal which immediately turns off the output gate drive. This causes the output voltage to lower and the inverting input from the opto-coupler to fall, increasing the duty cycle of the gate-drive. What I find on the oscilloscope in over-current mode is that the PWM output to the gate drive jumps wildly from 0-100%. This clearly puts the primary coils of the transformer into saturation mode and stresses the drive FETs.
After much textbook research, I believe that the overcurrent circuit should quickly bleed voltage off the soft-start capacitor without quickly turning off the output PWM pulse to prevent the PWM pulse from going above a certain limit. What we want is to have the gate-drive pulse change its state smoothly instead of wildly jumping all over the place.
A good example of how this is done is shown at:
If you look at the schematic halfway down the page, the overcurrent sensing circuit draws down the soft-start capacitor and doesn't drive the nShutdown pin as in the CCPower schematic.
I'm going to try and alter this circuit with a pull-down transistor and put it into overcurrent to see if the gate-drive oscillates or simply maxes out.
This is one hell of a learning experience, especially for a digital engineer like myself.