Ringing on gate signal

[brandes.cm]

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?

 

[Perry Babin]

Is this with or without the outputs in the circuit?

 

[brandes.cm]

I have two fets per side.

 

[Perry Babin]

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.

 

[brandes.cm]

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.

 

[Perry Babin]

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.

 

[brandes.cm]

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.

 

[Perry Babin]

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.

 

[brandes.cm]

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?

 

[Perry Babin]

Yes. The diodes connected in parallel with the gate resistors allow the FETs to be driven off more quickly.

 

[brandes.cm]

I will replace those with the correct part and see what happens.

 

[brandes.cm]

That did it. What amazes me is the fact some of the solder joints were factory. So those diodes were replaced with resistors intentionally when manufactured. Maybe that is why the company went out of business. Diablo Audio Technologies, model Hellfire 2500.1 I heard these production model amps would burn up and my guess this is why.

It's to bad it is a nice amp.

 

[brandes.cm]

I'm back working on this, the output fails when driven hard. Only thing I notice is the noisy gate signal on the upper fets. Is this a product of the output filter?

 

[Perry Babin]

Does it look noisy in differential mode?

 

[brandes.cm]

Yes, scope set to add, channel 2 inverted. Here is a pic.

An externally hosted image should be here but it was not working when we last tested it.

 

[Perry Babin]

That's definitely not right. Where does the signal become noisy?

input of opto
output of opto
base of driver transistors
emitter of driver transistors

 

[brandes.cm]

I see it on the +12V supply and the output of the opto.

 

[Perry Babin]

When you captured the last scope image, did you have one probe on the source leg and one on the gate leg?

By +12v do you mean the B+ 12v feeding the amp?

 

[brandes.cm]

If you reference the sinuslive a1500 schematic you sent me, the last measurement was from the drain to gate. The +12v I mentioned is the n_mos12 supply from a secondary winding of the transformer.

 

[Perry Babin]

If you look at the voltage across pins 5 and 8 of the opto-coupler in differential mode, is the DC noisy?

If so, check the capacitor connected across that supply.