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InPhase

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Joined: 15/12/2020
Location: United States
Posts: 178

Posted: 05:25pm 04 Jan 2021

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Besides using a gate driver, what are the other methods to prevent shoot Through in MOSFET/IGBT bridges? I've thought about methods that might use an RC network to delay a signal, or using logic gates to slow it all down, but I don't have any actual experience. What methods did designers use before gate drivers were available?

Warpspeed
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Joined: 09/08/2007
Location: Australia
Posts: 4406

Posted: 09:00pm 04 Jan 2021

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No other method possible.

The gate turns on the mosfet. If you do not want to turn on the mosfet only the gate driver can do that, so shoot through protection must incorporate both upper and lower gate drivers somehow.

Best method I know of is a little circuit I thought up myself using a pair of optically isolated gate drivers. Connect the two LEDS in inverse parallel.

If you think about it, it is impossible for both LEDS to be on simultaneously, one must always be reverse biased and off. When you reverse the polarity of the drive waveform, the voltage must pass through zero, and both LEDS will be off, and both mosfets will be off.

Cheers, �Tony.

RFburns

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Joined: 21/07/2017
Location: Australia
Posts: 43

Posted: 06:16am 06 Jan 2021

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I believe that in fact it is possible to protect the gates to some degree ;using the following method on my am transmitter which switch's much faster than any of the Mosfets used in the inverters on this forum (switching at RF Frequency)and not yet suffering a failure. As this was not my idea I will quote from the original design source.

"Gate voltage spikes are generally a result of gate overdrive. This is usually remedied by using a stable driver, and by adding protection devices to the gate circuit. You can easily protect the gates of MOSFETs by using TVS (transient voltage suppressor) devices (sometimes called TransZorbs) from the gate bus to the source bus or ground plane. Try to keep the TVS lead impedance as low as possible to afford maximum protection to the gates.The 1.5KE18CA TVS works very well in protecting the gates. This is a 1500 watt, 18 volt device. The device starts to clamp at around 19.5 volts, and effectively keeps the gate voltage at or below that point. The TVS devices are very effective at protecting the gates from any over voltages or spikes. The TVS is is used to clamp accidental high gate voltage. It is not designed to be clipping the gate waveform under normal, steady-state operation, and excessive TVS device heating and device failure may result if the gates are over driven for too long. "

obviously you need to select the TVS to suit the application. RF
Edited 2021-01-06 16:18 by RFburns

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Warpspeed
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Joined: 09/08/2007
Location: Australia
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Posted: 06:51am 06 Jan 2021

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RF is a very different problem to what us 23KHz pwm inverter guys are facing.

For us, we run half bridge circuits between dc and ground with an upper and a lower mosfet.
Due to all kinds of problems at inverter startup and shutdown, or brownout, with multiple different power rails, and perhaps some dodgy software, its possible for both upper and lower mosfets to sometimes simultaneously conduct, with disastrous effect.

Clamping the gate voltage is definitely a good thing to do, but it does not help with our particular problem.
Tony VK3ALY

Cheers, �Tony.

RFburns

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Joined: 21/07/2017
Location: Australia
Posts: 43

Posted: 07:54am 06 Jan 2021

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Not wishing to enter into a discussion that may detract from the original posters query .But this transmitter is very similar to an inverter in that The PWM output for drive maybe standard TTL or +12V, operating at approximately 160kHz through opto isolators to the Mosfet's .So could be thought of as a large modulated power supply and suffers the same issues with shoot through (in some way more so as switching is at a much faster rate ,the amplifier uses two groups of Mosfets in single ended push pull driven out of phase so one must be off before the other is on - mmmm sounds familiar ).
There is no perfect solution to this; however clamping the gate drive with TVS will offer some degree of protection beyond just using opto isolators or large dead time values/low drive cycle.
I believe the information provided has merit it is merely offered as another method to limit possible failure. RF

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