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Murphy's friend

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Joined: 04/10/2019
Location: Australia
Posts: 582

Posted: 07:24am 04 Mar 2020

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I have a requirement to connect up to 5 HY4008 mosfets in parallel (for an inverter).

There seems to be two ways to parallel connect the gates of them, see drawing:

Any suggestions which way (right or left sketch) would be better are welcome. Also, if possible, some comments why that is so.

Lastly, what value gate resistor would be ideal?

Thanks.

Solar Mike
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Joined: 08/02/2015
Location: New Zealand
Posts: 1123

Posted: 10:44am 04 Mar 2020

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The experts recommend individual gate resistors (left sketch) when high speed switching and parallel devices are used. Reason parasitic source inductance of each device can result in high-frequency oscillations in the gate drive circuit, the resistor damps these usually to a manageable level. You can think of the gate circuit as a transmission line and you are dampening it, easier to do the shorter distance between the gate driver chip and the mosfet gate, source leads.

Those mosfets have a gate charge (Q) of 195 nC, 5 in parallel is close enough to 1 uC.
The time to charge up the gate cct in large mosfets generally = Qmax/Imax, so in order to turn on your 5 fets in something approaching 100nSec or better will require 10 amps drive current shared between the 5 devices.

The Fod3182 is only rated at 3 amps, this low current will limit the turn on/off speed to approx 300 nSec which will cause higher switching losses and heat. Driving 5 devices would be unreliable, you would have to use higher value gate resistors to limit the currents to 3 amps, making switching even slower.

Personally for reliability, I would use a higher spec device like UCC5390SC, the Fod3282 would work ok driving a single possibly 2 mosfets only.

For high drive/frequency switching in inverters, gate resistors would range between 1 - 6 ohms.

Hope that helps...

Cheers
Mike

Murphy's friend

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Joined: 04/10/2019
Location: Australia
Posts: 582

Posted: 12:41pm 04 Mar 2020

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Thank you Mike, that was very helpful. The 5 mosfets in parallel was something down the track, the project in hand could work with just two in parallel.

Interestingly, I had used the right sketch idea in an experimental inverter in the past, now I know why it kept failing...

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