MOSFET gate fast ringing - Page 1
Forum section: electronics / projects

author: Neukyhm
author_rank: Contributor
timestamp: March 17, 2018, 11:25:20 am
content: 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.

author: fourtytwo42
author_rank: Super Contributor
timestamp: March 17, 2018, 04:40:09 pm
content: 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.

author: SiliconWizard
author_rank: Super Contributor
timestamp: March 17, 2018, 06:16:44 pm
content: 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.

author: Zero999
author_rank: Super Contributor
timestamp: March 17, 2018, 06:58:38 pm
content: A 1x probe has a lower bandwidth, so the ringing might still be there, but the 'scope no longer shows it.

author: diyaudio
author_rank: Frequent Contributor
timestamp: March 17, 2018, 07:29:37 pm
content: Your waveform tells a story, the MOSFET gate doesn't look like its being shut down correctly what drive circuity are you using?.

author: diyaudio
author_rank: Frequent Contributor
timestamp: March 17, 2018, 08:24:11 pm
content: Try dropping the frequency 350Khz fsw is very high for such an old school MOSFET with large capacitance .

author: Zero999
author_rank: Super Contributor
timestamp: March 17, 2018, 08:56:39 pm
content: Quote from: Neukyhm on March 17, 2018, 07:48:30 pmBecause, 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.

author: diyaudio
author_rank: Frequent Contributor
timestamp: March 17, 2018, 09:07:36 pm
content: what does your pcb layout look like? 100KHz with and a bad layout is enough to produce ugly overshot and perturbation.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 17, 2018, 09:20:09 pm
content: 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

author: iMo
author_rank: Super Contributor
timestamp: March 19, 2018, 12:01:38 am
content: This is with 10cm long wires (100nH/100mm) and a gate driver w/ 25ns rising falling/edges and 10V amplidute (like yours one).

author: nuno
author_rank: Frequent Contributor
timestamp: March 19, 2018, 12:12:24 am
content: Show us a photo of your probe connected to your measurement point.

author: capt bullshot
author_rank: Super Contributor
timestamp: March 19, 2018, 10:22:24 am
content: This looks like a serious GND layout related problem.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 19, 2018, 03:07:40 pm
content: There are many ways to solve it, but not answering our questions is not one of them.Tim

author: capt bullshot
author_rank: Super Contributor
timestamp: March 19, 2018, 03:16:43 pm
content: Quote from: Neukyhm on March 19, 2018, 03:03:47 pmThat 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/

author: Zero999
author_rank: Super Contributor
timestamp: March 19, 2018, 10:53:16 pm
content: Quote from: Neukyhm on March 19, 2018, 03:49:41 pmWe don't need to see the schematic, only the layout.Strip board and breadboard are generally poor choices for MOSFET drivers.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 20, 2018, 05:42:28 am
content: 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 fewmillimetersaway 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.

author: rbola35618
author_rank: Frequent Contributor
timestamp: March 23, 2018, 03:02:16 pm
content: 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.