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Old 21st January 2012, 03:17 AM   #1
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Default Ringing on gate signal

What causes ringing on the upper and lower portions of a output gate signal? The output config is the same as Sinuslive 1500, opto-isolated half bridge.

A picture from each channel. Do I worry about this?
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Old 21st January 2012, 03:23 AM   #2
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Is this with or without the outputs in the circuit?
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Old 21st January 2012, 03:26 AM   #3
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I have two fets per side.
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Old 21st January 2012, 03:29 AM   #4
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I doubt that it's a problem. It's likely something caused by the output filter inductor. If you have two probes, you can use the scope in differential mode to see if that gives you a cleaner signal.
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Old 21st January 2012, 03:39 AM   #5
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It's actually me using the wrong gnd for the lower fets. It is referenced to -rail not the gnd point I was using. Isolation transformer on your scope is a must, for me at least.
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Old 21st January 2012, 03:48 AM   #6
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An isolation transformer is dangerous. If you had it grounded to a rail, the case of the scope was at rail. For some amps, that's more than 150v DC. You need to learn to use the differential setup. It's on the DVD. Tech tips 8, the last item on the page.
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Old 21st January 2012, 01:38 PM   #7
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I know how to setup differential using 2 probes. I guess never really thought about what happens when using a isolation transformer.

One other question on this amp. I set the supply voltage to 13.1Vdc coming in, idle current is ~2A. When I increase the voltage to 14.4V, idle current goes up to ~7A. Is this right? That seems high for something that should be in the operational voltage range.

The pwm frequency doesn't change both in the supply and output carrier. Just the amplitude of the signal.
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Old 21st January 2012, 04:16 PM   #8
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That does seem too high. It could be that the gate drive isn't quite right for the power supply FETs and with higher drive voltage, the driver transistors aren't able to get the FETs switched off properly.
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Old 23rd January 2012, 01:50 AM   #9
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I bought this amp used and it was apparently worked on by a previous person. It uses an IRFB31n20 output with 47ohm gate resistor. I found the diodes attached to the gates replaced with more 47 ohm resistors. I would assume this diode could be critical to get a faster turn off time of the fet. Is this correct?
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Old 23rd January 2012, 09:55 AM   #10
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Yes. The diodes connected in parallel with the gate resistors allow the FETs to be driven off more quickly.
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