After Weber removed the dead battery-side MOSFETs and cleaned the soot off the PCB, we were ready to try testing the gate drivers. We've been trying to devise a way of testing the gate drivers, other than reassembling the main board in a case, attaching a 48 V battery, and powering up. After all, that's what we did last time, and the MOSFETs blew up. There has to be a way to check the gate drivers without having things blow up.
Having traced some schematics lately, I had a plan. It involves testing the drivers after the old battery-side MOSFETs are removed, and before they are reinstalled. Firstly, we use a current limited power supply. Weber's is limited to 31 V, but it seems to be good enough. We set the current limit to one amp (half an amp would be plenty). Secondly, there needs to be power for the driver electronics, which means powering up the main power supply. Shorting the wires that lead to the main power switch does that. There is a capacitor, C7, that will charge up and cause Q9 and hence Q10 to stop conducting, but once Q36 and TX9 are generating pulses, there are power from diodes D57 and D49 to keep U10 powered.
Thirdly, something needs to enable the PWM chip that generates the square waves for the DC-DC gates, both the MOSFETs on the battery-side, and the IGBTs on the high voltage (~400-450 V) side. Usually the processor would do that, but we'd prefer not to plug in the processor daughter board at this stage. Shorting pins 3 and 4 of opto U19 would do this.
The red oval shows the two pins to be shorted. Using a long pigtail as shown enables a DSO earth lead to be clipped to it, should the need arise to debug the PWM chip. You may wish to use a clip lead instead of soldering to the pins.
[ Edit: U9 is near the middle of the board, near the end with the battery terminals. ]
We didn't want to remove any IGBTs, since they seem to be working fine, and we'd prefer that we didn't have hazardous voltages running about, not to mention IGBTs that could blow up if not driven correctly. But the transformer won't be providing energy with the battery-side MOSFETs not yet installed. We know now that the bus soft start circuit is usually enabled by the processor, so with the processor removed, that won't be charging the big bus capacitors either. So the IGBTs will have no voltage across them, so turning them on, with good signals or bad, won't hurt anything.
So we powered it up, and attached the DSO leads to the gate and source of the four sets of battery-side MOSFET holes. The result:
I don't know the reason for the step change part way through the square wave to the gates, but it looks pretty good to me. These two waveforms are from diagonally opposite pairs, hence the difference in phase. Note the ~1.7 uS dead time when neither diagonal of the full bridge (i.e. none of the MOSFETs) is turned on. The frequency of the gate drive is about 37 kHz.
We also checked the drive to the IGBTs on the high voltage side of the DC-DC converter. These are the four TO-247 devices that are nearest U9. The signals were similar, except for some overshoot at the start of the square wave drive.
It all looks good, so we may be able to get this PIP working again. And nothing blew up! [ Edit: so far... ]
If you try this, make sure you remember to unsolder the short on opto U19 when finished, if you used solder to short the pins.
[ Edit: clarified "neither diagonal of the full bridge" with "(i.e. none of the MOSFETs). ]
[ Edit: "battery side MOSFETs" -> "battery-side MOSFETs". ]
Learning how to patch and repair PIP-4048 inverter-chargers and Elcon chargers.