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It's my first post here so please do not hesitate to tell me if I'm not doing it right :-)

A bit of context : we need to switch an array of power resistors (1,3kW, 1ohms) in order to test under load a battery pack (37V nominal) following a certain load profile. All these resistors will be switched by µC outputs (possibly through a multiplexer). Basically we're building a poor man's DC load using MOSFETs instead of SSPC.

So we're talking about 37A per power resistor, in order to do this, we've made a small prototype using the astounding number of 2 components :

  1. VOM1271 (the MOSFET driver)
  2. IRFB7530PBF (the MOSFET)

The great thing is that the VOM1271 allow us to make do without an isolated DC/DC and have this very simple BoM. The bad thing is that the rise and fall times are out of this world (in the bad way, talking about mS) and thus we cannot do any PWM if we want to do some load profiles... We can of course still follow the profiles but with 1,3kW steps...

So here's my question : is there a way to speed-up these rise and fall times in order to have acceptable switching losses? I'm not very optimistic about the rise since that only depends on the photvoltaic cell current inside the VOM but what about fall time (which is also the worst)? Vgs vs. t below Vgs vs. t Vgs vs. t

And an another one for the road : what could I add to "ruggedize" this circuit? The 2-components circuit was just to validate the basic principle but I'm guessing there are a couple of basic safety features missing...

Thanks for your help!

Marc

Well it seems that any "integrated power" coupler/gate driver will not give any useable switching speed so we're gonna stick with using it as a solid state relay...

Any input regarding making the circuit bulletproof though? I was thinking of a 1,7V zener diode in // with the input (the Vf of the input diode is 1,7V). Would that be good practice?

Thanks,

Marc

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  • \$\begingroup\$ I am not sure why you need isolation? \$\endgroup\$
    – EE_socal
    Dec 21, 2017 at 21:47
  • \$\begingroup\$ Well, in this case we're using 36V but we're still not that comfortable with having medium to high voltage (we're planning a similar load at a 400V nominal voltage) / high currents on the same galvanic plane as signal. Would you reccomend just using a non-isolated gate driver for this 36V application? \$\endgroup\$
    – VoltsAndNuts
    Dec 21, 2017 at 21:53
  • \$\begingroup\$ I think that would be ok for 36V but I share your concern for higher voltages. If you want one design for both then isolated would be the way to go. You could use an opto or digital isolator to control the gate of the MOSFET and derive the gate voltage from the battery pack. That could get a little tricky at 400V. Or just use a small isolated DC-DC converter then it would be independent of your battery pack voltage. \$\endgroup\$
    – EE_socal
    Dec 21, 2017 at 23:20
  • \$\begingroup\$ There are digital isolators with power built in such as ISOW7840 \$\endgroup\$
    – EE_socal
    Dec 21, 2017 at 23:46
  • \$\begingroup\$ I don't think those two parts really go together. The opto part has very little current drive and that MOSFET has a huge input capacitance. That's pretty much a non starter. \$\endgroup\$
    – Trevor_G
    Dec 22, 2017 at 1:18

2 Answers 2

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You can't get blood from a stone, nor high current from a small photodiode.

You could use a DC-DC converter to produce an isolated voltage and then use an isolator of your favorite kind (optical to logic, magnetic-transformer, magnetic GMR, capacitive) to transfer the signal and drive a fast multi-ampere gate driver (or possibly there are a few opto-gate drivers on the market).

A couple of the technologies I mentioned above do not necessarily have a known state at power-up so might be worth avoiding.

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  • \$\begingroup\$ Hi Spehro, thanks for the answer. The classical DC/DC + gate driver is actually what we were trying to avoid in order to keep the costs the lowest (we have to have such a circuit for each power resistor... we're talking 50 to 200 units...). Now, the known state at power-up is something I've never given any thought about... I'm quite surprised since a lot of isolated gate drivers are used in very high power electronics. Which technology would be problematic according to you? \$\endgroup\$
    – VoltsAndNuts
    Dec 21, 2017 at 21:25
  • \$\begingroup\$ The GMR (giant magneto resistive) types (eg. NVE) definitely, and I think there are others. Too bad because they are fast and have other positive attributes. I guess you looked at all the PV isolators? IIRC some of them have a circuit that's supposed to snap them off but it isn't all that fast. \$\endgroup\$
    – Spehro Pefhany
    Dec 22, 2017 at 3:04
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I don't think you can improve turn on time without resorting to an isolated power supply but you can improve turn off by using another optocoupler to short the gate, discharging It faster.

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  • \$\begingroup\$ There are several tricks one can do without a separate supply, altough it does simplify the circuit. Take a look at the VOM family datasheets. \$\endgroup\$
    – winny
    Aug 15, 2018 at 6:52

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