Problems snubbing HF noise on flyback transistor 2

[Jdo300]

Hello,

I am working on the design of a flyback converter based on the LT3751 capacitor charging circuit. My design is similar to the 42 A cap charger example on pg. 25 of the LT3751 datasheet. In my case, the circuit runs on 12V and steps up the output to 430 V, but I am having some major noise issues with the MOSFET driving the primary of the transformer. The flyback powers some other high voltage circuitry and the noise from the FET during turn-off is radiating into the other parts of the design, both through the power and ground lines and through the air.

My first attack plan was to add an RC/RCD snubber across the primary of the transformer to snub the high frequency oscillations on the switch. But for some reason, none of the values I calculated for it, (or just plan experimented with) had any real impact on the waveform -- in particular, the first high voltage spike, which is rising to as high as 200 V. I have attached scope shots of the turn-off waveform, including a closeup of the first spike, which is around 12 ns wide. One challenge is that this waveform's spike is so narrow, and also the waveform seems to be rich in harmonics.

Has anyone run into problems like this before or seen a waveform such as this which seems to defy normal RC/RCD snubbers? I'm not sure what to try next to remove this noise.

MOSFET Drain turn-off Waveform

MOSFET Drain turn-off Waveform (Closeup)

- Jason O

 

[itsmoked]

Have you tried adding a gate resistor to slightly slow the dvdt? Something in the 1 to 5Ω range?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Jdo300]

Hi Keith,

Actually, I have not (yet). Unfortunately, when I designed the PCB, I did not add a footprint for a gate resistor. So I'm hoping I can try a few other things first to see if I can fix the problem before hacking the board up to patch in a resistor. That's a good suggestion though.

 

[zappedagain]

Check your layout. I had a similar problem where my snubber was having minimal effect. I found I had a poor path between the snubber and the primary (a single via instead of multiple). Attaching my snubber components directly to the transformer pins instead of the 'proper' PCB pads was the only way I could snub that revision of the PCB. It works much better on the new PCB revision.

I'm now working a similar issue and seeing large discrepancies between two different transformer manufacturers. The primary leakage inductance and the capacitance of the switching FET set up the resonance. You may need a different transformer. I found that even though transformer A has better performance than transformer B in every other aspect, the higher leakage inductance makes it a worse performer in the circuit because I have to waste so much power snubbing the resonance.

Z

 

[Jdo300]

@itsmoked,

I did finally try the Gate resistor and it worked very well to cut down the HV spike by 90%. The main issue is that the circuit is now taking an efficiency hit due to the slower switch transition times. This is a good fix for now, but I'm hoping to make some additional improvements to the layout and component selections to minimize the value that needs to be there.

@zappedagain,

Thanks for the advice on the snubber and transformer. I'm actually in contact with the manufacturer of the transformer we are using now to determine what can be done to minimize the leakage inductance and parasitic capacitance between the primary and secondary. As for the snubber, that's a good trick to keep in mind (soldering directly to the transformer pins).

- Jason O

 

[itsmoked]

Thanks Jdo300, we always appreciate feedback.

The gate resistor solution is always a trade-off game. Essentially you use as much as you can thermally stand to make it ring the least. The ringing has its own 'wear' issues too so it's not as bad a trade-off as it appears. Also sometimes only the turn-on needs to be slowed and not the turn-off or possibly vise-verse in which case you parallel the slowing resistor with a reversed diode which removes the slowing in one direction. That can remove the losses of a slower transition in that one direction.

Note also in the LT3751 data sheet (pg21) they discuss how to size a snubber. It's the transformer leakage inductance as Zapped mentioned that is usually the problem.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Jdo300]

That's a good idea. I was actually planning to provision the gate drive with separate diode and resistors to separately control the rise and fall times of the MOSFET. It looks like the problem is with the turn-off waveform so I should be able to gain some efficiency back by eliminating the slow-down during turn-on. Also, it is abundantly clear now that the transformer leakage inductance is the real culprit here. I'm currently working with the transformer manufacturer to see what can be done to reduce it, along with improving my current layout and component selections for HV bypassing, etc.