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Turn-off delay investigation

A project log for 10kW (30kW pulse) Electronic Load

All we need to do get some big resistors and connect them up in different combinations, right?

tinfevertinfever 07/13/2023 at 18:410 Comments

Let’s take a look at that turn-off delay issue. 

Yellow is FET_EN, Blue is gate drive output. Turn-off delay is 28-32us depending on test conditions.

If my goal is to be able to do 100us pulses, having a 30us turn-off delay is a problem. The timing is fairly critical because I need all of the load stages to switch at the same time, more or less. I’d also like the option to precisely sequence the load stages to either control the DUT current slew rate, or to compensate for the higher inductance load resistors which have a slower current slew rate by turning on some of the “faster” low inductance stages briefly. I think I’d like the load stages to respond within 5us, and ideally faster.

After more learning and research, I think I’ve determined the root cause. In short, I picked a cheap and slow optocoupler with phototransistor output, the slow kind. (PN: LTV-816S)

I didn’t read the datasheet closely enough and was relying on the listed 3-4us response time, but I didn’t pay attention to the test conditions. They actually specify that response time with a V_ce of 2V, an R_load of 100R, and I_c of 2mA, which actually means they are measuring a voltage swing of 200mV around a 2V signal. Definitely not the 0-600mV swing needed in my design to switch the following BJT inverter. Also, I was using an output pull-up of 3.3k which made things much worse.

I think the fact that I'm pulling up to a higher voltage (12.2V assumed from DUT) and then clamping the output with the following BJT inverter is actually helping me do better than these numbers.

I recreated the optocoupler and inverter section of the gate drive circuitry on a breadboard and used that to confirm the issue. Although, I could pretty clearly see how long it was taking the optocoupler output to rise measuring in-circuit on the load stage.

Schematic of breadboard demo setup

Well there’s your problem... Yellow is FET_EN, Blue is collector of optocoupler on breadboard setup.

I also tried to reproduce the datasheet specs under their test conditions, with 2.2V Vcc on the optocoupler output, a 100R pullup resistor, and then reducing the input drive current until the voltage when low was 2V, thus a 200mV swing or 2mA I_c. I was able to get 8.3us response time on input going low (Tr), and 14.5us response time on input going high (Tf). Technically within the max specifications I suppose but not good enough for me.

I found reducing the input drive current would reduce the turn-off delay, but it would also increase the turn-on delay so it’s not much of an improvement. Also, reducing the designed input drive current to keep the phototransistor from saturating would mean we are depending on the optocoupler CTR to stay the same, and that spec can vary wildly between parts and over temperature, I believe.

Yellow is FET_EN, Blue is gate driver output. Now driving the optocoupler with a 1.45V signal through the same 180R resistor, meaning lower input drive current. Turn-off delay is better (8us vs 30us before) but now turn-on delay is worse (11us vs 1.3us before).

I also didn’t want to reduce the value of the optocoupler output pullup resistor, because that resistor is always drawing current from the DUT, and so it directly affects the quiescent current of the load stage. This means even at 3.3k, assuming a DUT voltage of 12V, the load stage has a quiescent current of 3.6mA. With 53 load stages, that becomes 191mA drawn from the DUT even when all the load stages are off. That’s a lot more than I’d like on principle alone, even if it wouldn’t matter much in my application. This ties in with the question of “Why power the gate drivers from the DUT anyways?” which I need to address in the main project details section at some point.

Knowing that my optocoupler circuit is fundamentally flawed, that left a few solutions:

I settled on the last option: redesign the gate drive circuitry to use a better optocoupler, the 6N136.

In the next post I’ll go over the new design with the 6N136, the extra protection circuitry it requires, and its performance.

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