rew wrote:Where did you get that funny idea that the current will only be going one direction? You need a bidirectional current sensor even when it's in the low-side-fet-return-path.
I thought that as it was essentially connected to ground you'd only have current flowing one way? My modifications to the maths should be safe for bidirectional current so this is more of a learning point for me.
Anyway, I've been struggling with a massive problem for the last week. Phase C is weird.I would connect up the phase A drivers, phase B drivers,phase C drivers, phase A current sensor and phase C current sensor, power up he ESC and I'd hear a buzzing noise. Playing 'remove wires until it goes away' (and powering down when removing wires) I found it was entirely due to the phase C gate driver circuit. I could put any phase gate wires into the driver circuit and it'd would buzz. Probing the gate driver supply found it would not provide a stable +12V supply.
So as you can see what should be a +12V rail drops to +5V which is pretty bad. The weird thing is if I disconnected the the MOSFETs from the phase C driver then the rail would stabilise at +12V. The +12V switching power supply is rated for 500mA and the 3 gate driver chips theoretically only pull 2.5mA even when charging the boost capacitors so it shouldn't be an overload condition.
I continued on thinking it had to be a short. Of course without the MOSFETs attached I couldn't find a short between any of the pins or between the pins and ground or power. This confused me somewhat so I decided to measure the voltages on all the chips while they were powered but inactive. I found:
- VDD is the +12V rail, so is fine.
- HB has a difference of one diode drop so is also fine.
- HO is where things go different, phase C is ~4.00V higher than phase A & B.
- HS needs to be the same as HO to ensure the high side is off, but is ~4.00V higher than phase A & B.
De-soldered the offending UCC27211A a douple of times without any change, also tried changing the boost capacitor between HB & HS but that had no effect either. I could see any shorts that could have caused the problems before or after the soldering.
In short, I'm completely stumped.