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Discrete gate drive circuit

Project description

Recently, Steve Ward’s been experimenting with a new approach for providing gate drive to pover devices in his DRSSTC’s. It was based on a P+N channel double mosfet bridge, which he used to drive by a small gate drive chip using an ingenious level shifting technique, which allows the output section to run at higher voltage than the pre-driver. This makes it easy to produce currents required to rapidly charge and discharge gate capacitance even of largest IGBT modules.  Steve described his work on this page: http://www.stevehv.4hv.org/new_driver.html

I realized the potential of his design, but I also wanted to completely get rid of expensive gate drive IC which he used as pre-driver. Since the P+N channel pairs are generally logic level mosfets, they can be driven from 5V as well, and I thought it would be worth to try using some of the CMOS logic driver gates that run from 5V. My chip of choice was 74HC245, a line driver IC which can source lots of output peak current, and has 8 gates in total – I used 4 for each output mosfet pair. A 74ACT245 for example is even faster and could provide more drive current if necessary.

Prototype circuit schematic

Prototype circuit schematic

 

Prototype PCB, made entirely on a single side!

Prototype PCB, made entirely on a single side!

I put together a simple PCB, that also included fiber optic receivers so that I can drive the circuit safely from my FPGA development board.

Prototype date driver implemented on a PCB

Prototype date driver implemented on a PCB

Results

The experiment was quite a success, resulting in drive waveforms faster than I’ve ever seen before. The level of shoot-through in the P+N mosfets was also minimal due to low 5V drive voltage, which was known to be a problem before. When I tried testing with 12V drive, the P+N device quickly overheated and blew due to shoot-through!

So far I’ve only done limited testing of this circuit – I proved that it can drive a full bridge of HGTG30N60a4D IGBTs (notorious for their high input capacitance) well at frequencies above 1Mhz, with little heating in the MOSFET chips. The 5V regulator on the board was by far the hottest part!

I’ve however suggested this approach to Steve, and he considered it for some of his future Tesla coils as well.

 

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