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 200V. 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.

Scope Shot showing Voltage across MOSFET Drain and Source showing turn-off ringing

Closeup of noise spike


Here's some additional information about the circuit, along with a schematic of the power stage showing the control input and component values. The transformer was an off-the-shelf design so I did not wind it myself.

Flyback Converter Schematic

Edit 2

Here's a copy of the datasheet with information on the specific transformer I'm using:

Transformer Datasheet

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    \$\begingroup\$ That is a 100MHz waveform... what type of capacitance is on the diode your using? Is it high speed? What kind of parasitic inductance does the RC, RCD circuit have? What are he package sizes? \$\endgroup\$ – MadHatter Apr 2 '18 at 15:00
  • \$\begingroup\$ I just updated the post to show the part numbers for the diodes. According to the datasheet, the junction capacitance ranges from 47-720 pF depending on the bias voltage. Also Note that I have been playing with a couple different diodes for the snubber itself, so the one listed in the schematic isn't final. As for components, all components (except the diode, HV cap, and 68MΩ resistor are all SMT parts. The snubber circuit uses through-hole components plugged onto a tiny perfboard that I soldered to the PCB to test out different values. \$\endgroup\$ – Jason O Apr 2 '18 at 18:04

From what I observe in your waveforms, when the transistor turns off you are settling down to a peak voltage of about 60 volts and this is plainly not right for a flyback design running from 12 volts because the peak voltage seen (ignoring the initial glitches due to leakage inductance) should be about 24 volts i.e. about double 12 volts.

Because you are seeing about 60 volts this tells me that your transformer has the wrong turns ratio to suit the load on the secondary. Or, put another way, I think you are slipping into CCM (continuous conduction mode) due to the light load.

The glitch at the beginning of the 2nd half of the switching cycle could be so big due to the transformer problem I've alluded to. To resolve this any further you need to post your exact schematic with the exact values, operating frequency and transformer design.

Added section

The transformer is a 10:1 step up device producing an output of 430 volts and this means the flyback at the primary is 12 volts + 43 volts = 55 volts. OK you are seeing 60 volts so it's not too far off. You are probably generating a bit more than 430 volts.

The leakage inductance is 300 nH and this is what gives you the big pulse at the start of the 2nd half of the switching cycle. I would recommend a zener diode tied to the positive supply to catch this but you need to get the power rating sufficient AND you need to state what maximum output voltage you expect to produce because if it is 850 volts (as per your edited schematic), the natural flyback voltage will be 97 volts and you cannot do anything about that so you might consider zener clamping at 100 volts above 12 volts.

Anyway, with 169 pF of output capacitance when off and 300 nH of leakage inductance you will get somewhat distorted ringing at a frequency of 22 MHz and this doesn't look like a million miles away from what you are seeing.

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  • \$\begingroup\$ Thank you for your response, I just posted some additional information including the schematic for the power stage. \$\endgroup\$ – Jason O Apr 2 '18 at 18:00
  • \$\begingroup\$ Transformer design please or a link to the data sheet. \$\endgroup\$ – Andy aka Apr 3 '18 at 7:57
  • \$\begingroup\$ I just added the datasheet for the transformer. Please let me know if there is anything else I can provide \$\endgroup\$ – Jason O Apr 3 '18 at 12:50
  • \$\begingroup\$ Thank you for your insightful response. I was actually wondering why the voltage went as high as 60V during the turn-off, but didn't consider the back-coupled voltage from the secondary. As for the Zener suggestion, this too is a good point as well. Thank you for pointing out the minimum output voltage required to snub the spikes without killing the output voltage. I will do some experimenting along these lines as well. \$\endgroup\$ – Jason O Apr 6 '18 at 17:14

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