MOSFET gate fast ringing

[Neukyhm]

So I have two IRFP250N transistors in a device that I think you will find interesting and I will post when it's fully finished.

The thing is that I have some noise at the gates, fast ringing. The MOSFETs stay cold and nice though, and I think that a part of that noise is being generated by the probe itself.

What do you think? The transistors seem good.

[fourtytwo42]

It is most likely that what you are seeing on the gate is in fact present in the main conduction path, frankly without providing a schematic and/or layout there isnt really enough information to be able to comment.

[SiliconWizard]

Yeah, without seeing what is actually driving the gates and the routing, it's hard to say much if it's ever related to the design itself.

To rule out a probe setup issue, you may want to check how you connected the probe ground relative to your circuit's ground. One common cause of ringing on fast slew rate signals when probing them is a ground loop. Try connecting the probe's ground to a ground on the pcb as close as the gate signal as you can.

[Neukyhm]

Thank you for the replies. It's seems I was right, I had my probe set to x10. When I changed it to x1, the noise was almost gone.

I bought that probe for ~25€ 

[Zero999]

A 1x probe has a lower bandwidth, so the ringing might still be there, but the 'scope no longer shows it.

[diyaudio]

Your waveform tells a story, the MOSFET gate doesn't look like its being shut down correctly what drive circuity are you using?.

[Neukyhm]

A 1x probe has a lower bandwidth, so the ringing might still be there, but the 'scope no longer shows it.

Really? I thought that probes show more noise when not set to x1.

Your waveform tells a story, the MOSFET gate doesn't look like its being shut down correctly what drive circuity are you using?.

I'm using the TPS2814.

[diyaudio]

Try dropping the frequency 350Khz fsw is very high for such an old school MOSFET with large capacitance .

[Neukyhm]

Try dropping the frequency 350Khz fsw is very high for such an old school MOSFET with large capacitance .

But it's working at 114kHz   

If my oscilloscope shows a higher frequency is because the noise as I asked here: https://www.eevblog.com/forum/testgear/why-does-my-oscilloscope-display-two-frequencies/

But the pulse is ~114kHz.

[Zero999]

A 1x probe has a lower bandwidth, so the ringing might still be there, but the 'scope no longer shows it.

Really? I thought that probes show more noise when not set to x1.

Because, with a x10 probe, the bandwidth is higher so you'll see more noise. Another possibility, in this case is the x1 probe is loading the circuit more, thus damping the oscillation. Try measuring it with two channels and probes simultaneously: one set to x1 and the other to x10. That will tell you whether the probe is damping the oscillation, or it's still there and the x1 probe doesn't have the bandwidth to see it.

[diyaudio]

what does your pcb layout look like? 100KHz with and a bad layout is enough to produce ugly overshot and perturbation.   

[T3sl4co1l]

I think you will find you have a combination of poor layout, high gate drive impedance, and poorly damped load.

The oscillation looks to be characteristic of a nonlinear capacitance, like a diode junction or MOSFET, with too much stray inductance in series with it.

Tim

[Neukyhm]

Well, after viewing the video where Dave explains the 1x probes, I feel embarrased 
I don't know why I thought that the more you attenuate, the more noise you get but not because of the bandwidth, but because of the attenuation itself.

So we are at the beginning again, I have those transistors with a noisy gate voltage, but they are working great without getting hot.

There is no PCB here because this is a prototype with  (short) cables, so It would be understandable that the noise would be gone if using a well designed PCB layout.

I suppose that those transients at the gate are so fast that the transistors are operating in linear mode for very short time.

[iMo]

This is with 10cm long wires (100nH/100mm) and a gate driver w/ 25ns rising falling/edges and 10V amplidute (like yours one).

[nuno]

Show us a photo of your probe connected to your measurement point.

[capt bullshot]

This looks like a serious GND layout related problem.

[Neukyhm]

I have detected noise at the driver input, so maybe the problem is not the path from driver to mosfets, but the path from pulse generator to driver. How can I clean it? Using a low pass filter?

[T3sl4co1l]

There are many ways to solve it, but not answering our questions is not one of them.

Tim

[capt bullshot]

I have detected noise at the driver input, so maybe the problem is not the path from driver to mosfets, but the path from pulse generator to driver. How can I clean it? Using a low pass filter?

That looks pretty much like a ground layout problem ...
Yes, you'd need to improve your ground connections from the pulse source to the driver to the MOSFET itself. A gate resistor of appropriate value (usually in the 1R to 10R range) often helps a lot. If haven't done that yet, make your ground a plane including MOSFET source, driver GND pin and pulse source GND pin. Keep the driver as close as possible to the MOSFET (think in millimetres here). If the driver picks up noise at its input and you cannot improve your layout, consider opto-isolating the pulse source from the driver, and place these three parts (optocoupler, driver and MOSFET) as close as possible together.

Often such oscillation results from ground bouncing (the drivers GND pin gets "lifted" against the pulse source output GND). Proper grounding helps here, so I'm repeating: Put everything as close as possible and make use of a ground plane.

A gate resistor that is too high won't work too,

Edit: you mentioned "short cables" somewhere, is this a breadboard like prototype?
If so, forget about it. It just wont work. This kind of circuit requires a proper layout or at least a veroboard (perf board) construction with real short (millimetres) connections.

See this example of a perfboard circuit I've made some time ago, this is a MOSFET pulser creating 800V / 5ns fall time pulses - works quite well. Located bottom left is an optocoupler and next to it the SO8 MOSFET driver. The driven mosfet is located directly above the driver chip (but on the other side of the board). Note the solder wick that I used as GND in this area, and some parallel wires to lower the ground impedance to the SMA connector.

See also here, this is a very unfinished (indeed not yet started) page about it:
http://wunderkis.de/pulser/

[Neukyhm]

(...)

Thanks for the reply. Sadly I'm not allowed to show the circuit because this is a piece of a bigger project that will be my final college degree project (here in Spain we have to do a final project in order to get the degree)

The circuit is smaller than a hand and mixed veroboard/breadboard. I will try to place a small capacitor in parallel with a 1K resistor that was already there at the driver input. The fun fact is that the circuit was already working quite well.

I will ask my teachers if I can post the circuit. This is serious business since I have to write and defend the project just like if it were a doc thesis.

[Zero999]

(...)

Thanks for the reply. Sadly I'm not allowed to show the circuit because this is a piece of a bigger project that will be my final college degree project (here in Spain we have to do a final project in order to get the degree)

The circuit is smaller than a hand and mixed veroboard/breadboard.

We don't need to see the schematic, only the layout.

Strip board and breadboard are generally poor choices for MOSFET drivers.

[Siwastaja]

Try:

Dead bug construction style (google it) over a solid copper (GND) plane, with minimized wire lengths, and some series resistance (a few ohms) between the gate driver and the gate.


You are using a gate driver IC right? No reason to put it more than a few millimeters away from the FET. Also remember to decouple the power to the gate driver properly - I suggest a combination of a cheap electrolytic cap and a small ceramic cap, such as 100uF and 1uF. It's important that especially the latter is only millimeters from the gate driver supply pins. Using a solid GND plane makes these kind of things extremely easy to implement. Just bend the gate driver ground pins so they can be soldered to the plane.

[rbola35618]

Here is a video I did a while ago about why the ringing occurs. Some have already explained here on the post. I am just adding more material.