MOSFET ferrite bead placement - Page 1
Forum section: electronics / projects

author: theleakydiode
author_rank: Regular Contributor
timestamp: February 04, 2018, 03:29:18 pm
content: Hi, which legs of a MOSFET should I put ferrite beads on? I need to get rid of some very fast spikes as they are coupling into the gate and current sense resistor via miller capacitance.Should I put one on all three legs or just two? This is for an educational flyback SMPS project (its not mains powered) and the MOSFET is the primary switch. I already have gate resistors but these spikes are very fast.So which MOSFET legs is it best practice to include them? I'm guessing I'll need them on gate and drain but just wanted some advice as its a bit of a pain to unmount the MOSFET, would like to get it right the first time.Thanks.

author: Zero999
author_rank: Super Contributor
timestamp: February 04, 2018, 03:42:38 pm
content: I don't see how ferrite beads will prevent this. The Miller capacitance is internal to the device and no amount of ferrite beads will mitigate it.

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: February 04, 2018, 03:45:56 pm
content: Usually ferrite beads go onto the gate terminal but their primary use is it to kill RF oscillations (often caused by sharp turn on.)Leo

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: February 04, 2018, 03:49:40 pm
content: Quote from: theleakydiode on February 04, 2018, 03:29:18 pmCould you re-phrase this? It's not very clear where the spikes are coming from if they are coupling to both gate and wherever current sense shunt is.Leo

author: theleakydiode
author_rank: Regular Contributor
timestamp: February 04, 2018, 04:15:10 pm
content: Quote from: Hero999 on February 04, 2018, 03:42:38 pmYes however the sharp current spikes caused by this charging up are finding their way onto the gate and through the current sense resistor, if I can squash these spikes a bit then that would be great. The miller capacitance still needs a loop to complete and that would mean a current path involving at-least two MOSFET leads.Quote from: Leo Bodnar on February 04, 2018, 03:45:56 pmThere is a bit of that during turn off well into the Mhz range, but just putting one on the gate would still leave the drain to source capacitance open to sharp pulses which goes through the current sense resistor.Quote from: Leo Bodnar on February 04, 2018, 03:49:40 pmThey come from the primary coil and LCD snubber, there is a small RCD snubber too but the main one is the LCD since several amps flow through the primary at low and its driving a LOPT so that big hv drain pulse is needed. Its well below the MOSFET breakdown voltage but the LCD snubber does make it very ringy as it cycles through its phases.My aim is to reduce or eliminate miller induced spikes on the gate turning it back on slightly as it reaches just above the turn on threshold, and also to reduce the big spikes on the current sense resistor ramp waveform. Right now they are very fast and cross the threshold where the UC3844 does its thing.The circuit actually works ok but just this funny business on the gate and current sense resistor is making me want to fix it.

author: daedalux
author_rank: Contributor
timestamp: February 04, 2018, 09:14:51 pm
content: Have you tried biasing the gate off to ground with a potentiometer to get experimentally a value that will prevent the oscillation?

author: theleakydiode
author_rank: Regular Contributor
timestamp: February 05, 2018, 08:33:43 am
content: Quote from: daedalux on February 04, 2018, 09:14:51 pmYes, I got down to sub 100 ohm and the Miller induced gate spike is still there. If the low impedance path through the chip doesn't keep it off then I'm not sure a pull-down resistor ever will.Also I traced it down the secondary rectifier diode, problem is the transformer is potted so I can't simply place an RC across it. Any other ways to snub the secondary diode without having access to the anode side? Its also at about 30kV so that adds another spanner to the works.

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: February 05, 2018, 08:52:31 am
content: Have you tried removing gate resistor and acceleration diode? It's not like 3844 has a mighty output driver. Especially since you are using FB on the gate.If you are somehow convinced that your problemismiller capacitance, remove miller capacitance (use lower current / Vds MOSFET.)Leo

author: theleakydiode
author_rank: Regular Contributor
timestamp: February 05, 2018, 09:00:19 am
content: I currently have it like this output>12 ohm resistor>reverse diode and 10 ohm resistor in parallel>gate>1k pull down resistor and 18v zener diode.Putting a small 10n capacitor across the gate resistors or from gate to source reduces the spike a bit so it must be miller induced.

author: fourtytwo42
author_rank: Super Contributor
timestamp: February 05, 2018, 09:05:00 am
content: Could you perhaps post some scope shots of these spikes ?