Low voltage testing of IGBT and opto-coupler switching times

I was keen to know how quickly the TC426 can switch the IGBT in my pump driver. As it’s slightly more difficult and dangerous to test at mains voltage, I decided to postpone that by carrying out some basic low voltage tests with the IGBT first, simply using an LED and 300R resistor as load from 5V to the IGBT collector.

First, as expected, there’s a small delay introduced by the opto-couplers and TC426. Using a dual trace ‘scope, I found that the gate voltage rises 3.2us after the rising edge of the PWM input, and falls 20.4us after the falling edge of the PWM input. This looks fit for purpose: the pulse would be delayed by about 3us and would be about 17us longer compared to the original pulse fed to the opto-coupler.

So, how does an IGBT rated at 14A and 400V perform at just under 10mA and 5V supply? Although the IGBT was perhaps a little disappointed at being asked to switch a paltry 10mA, it seems to perform well. Here’s a plot of the gate and collector voltage at 500us/div:

lv_igbt_cycle_500uS

This appears inverted, because when the gate voltage is high, the IGBT switches on and quickly pulls the load (close) to ground. When the gate voltage drops, the IGBT switches off and the collector voltage rises again.

The gate voltage rise and fall is extremely fast (<200ns) and I didn’t measure further. The collector voltage also falls very quickly as the IGBT switches on. The plot above is at 500us/div, but here’s a closer view of the collector voltage rising as the IGBT switches off, plotted at 10us/div (and inverted):

lv_igbt_off_10us

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