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Registered Member #2939
Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615
In these situations its also worth checking that all the signal you are seeing is 'real'. Connect the probe ground clip to the tip to form a loop and see how much signal is being radiated directly into the probe.
Registered Member #61905
Joined: Sun Nov 12 2017, 03:27AM
Location:
Posts: 23
the_anomaly wrote ... I took out both gate resistors and put a ferrite bead in their place.
Thanks for that link to the AN. Do you have your scope hooked up like they suggest in that AN?
Also, with the bead: How do you have it hooked up? I've noticed a difference in a bead with a single straight wire through it, and the same bead with a loop. i.e. take a short length of magnet wire, pass it through the bead, then loop it over and pass it through again. This will increase the inductance and will tend to block somewhat lower frequencies. Some beads have 2 holes and you can pass the wire once through each hole (in a U shape) and this can fit in places with less clearance in some cases.
Registered Member #19
Joined: Thu Feb 02 2006, 03:19PM
Location: Jacksonville, FL
Posts: 168
In these situations its also worth checking that all the signal you are seeing is 'real'. Connect the probe ground clip to the tip to form a loop and see how much signal is being radiated directly into the probe.
Thanks for that link to the AN. Do you have your scope hooked up like they suggest in that AN?
I do not but my gut feeling was I had a lot of work to do before the probe would be the dominant factor. Also physically it would be very difficult for me to do so with my current PCB design. Still, I've kept the probe and ground clip routed as efficiently as possible and if I could not get better signals after trying these suggestions that was my next step.
Here is my concluding waveform:
I am much happier with this. After a bunch of trials, the final values I chose are a 33ohm resistor in series with a ferrite bead PN: BK2125LM252 (which seemed to be the most lossy one offered by Taiyo Yuden). You will notice a bit of a discontinuity in the gate wave form (blue); there is a small spike / plateau at about the middle on both the rising and falling edges. I can completely eliminate this by using a smaller resistance such as 18ohms but then the over/undershoot increases a few volts. Truthfully I would like to try a 27ohm resistor but I don't have one on hand.
Using a ferrite bead alone produced the fastest turn on/off speed but also had the greatest amount of ringing. I tried several ferrite beads out and the most lossy one performed the best. I think there are more bead options that could work even better but for the moment I limited myself to just surface mount components. Interestingly, using two ferrite beads in series only offered marginal improvements to using just one.
My conclusion is a ferrite bead can help reduce ringing and allow one to use a lower gate resistance for faster turn on / turn off when hard switching.
Registered Member #11591
Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556
The small plateau seen midway on your gate waveform is called the miller plateau and the length of it represents the time the MOSFET actually spends switching (it's caused by the voltage on the drain rising / falling, effecting the voltage difference on the parasitic capacitance that the gate driver has to charge and discharge). You would like that miller plateau as small as possible without too much ringing. A spike of a few volts isn't too bad as long as it dies off.
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FET not turning fully on with AC on drain
