High side switch base/gate drive question.

[hughmanoid]

I considering ways to drive a high side switch without using an isolated supply for a gate driver.

The first stage of my project is to chop 300 to 450 volts DC with a high current IGBT or Darlington emitter/source follower down to a maximum of 170 volts at several amps (15 or less). The switched side will feed a 200 to 500 millihenry toroid and a capacitor (TBD based on load and H bridge in stage 2). Details are evolving from my earlier posts so I won't elaborate here.

1: I have several VICOR isolated DC - DC converters (200 to 400 VDC in, 15 (or 24) VDC out) that I will need to protect from overvoltages. They will power the high side gate drivers with several amps if needed. I have solved that problem but it is complicated and messy.

2: I am wondering if anyone has used a dozen or more forward biased high current silicon diodes between the power input and the collector/drain of a NPN darlington or NFET or IGBT to provide 10 to 15 volts to drive the base/gate ? Since the diodes carry the full current of the rest of the circuit they will have to dissipate several watts and may be stud types bolted to a heat sink. Regulation is not critical. If I use overkill on the diodes, I hope to simplify this part at the expense of a dozen or more watts of wasted heat.


3: I may use a SMPS boost to to a specific voltage or even add ano3:ther inverted boost SMPS to get a relative minus 5 volts to turn the gate off hard.

4: if I use a Darlington, I assume I will need fewer volts (diodes *.7) but will have to account for current pulses into a base rather than current peaks as seen into a gate ?


I recently picked up a few isolation tranformers, an isolated variac, and a 500 VA isolated transformer that takes 120/240 in and puts out 240/480 depending how I connect the split primaries and secondaries so I can control the volts as I work things out.

Any thought would be appreciated.

Thanks,

Brian in Santa Cruz.

 

[FvM]

1. Isolated power supply is the classical means in high power electronics. The DC/DC converter is usually designed into the gate driver modules.
2. Due to the involved power losses, noone seriously considered this solution
3. Negative gate voltage is also standard in high-power IGBT applications, apparently small power IGBT circuits (home appliances and similar) are working acceptably without it.

I would try to supplement an industry standard bootstrap gate driver with an auxilary circuit that charges the bootstrap capacitor as long as the output isn't switching (with 0 and 100% duty cycle). Fortunately, the static quiescent current is only in a 100 µA range. Charge pump circuits have been suggested for this purpose.