Flyback Converter - RC Snubber Circuit at Primary Side Of Transformer - Page 1
Forum section: electronics / beginners

author: 2X
author_rank: Frequent Contributor
timestamp: January 26, 2025, 12:13:05 am
content: Hello,what is the purpose of adding an R//C in series with a diode at the primary side of a transformer on a flyback converter? Just a diode itself as I know it supress the backEMF/overvoltage where the inductor produces when the mosfet goes OFF and inductor's current go to zero instantaneously (VL=L*di/dt). For the driving of a relay with a mosfet or a BJT as I have seen in PCBs the designers use only just a diode parallel with the coil of the relay.Designing Isolated Flyback Converter Circuits: Selecting Critical Components ? CIN and Snubberhttps://techweb.rohm.com/product/power-ic/acdc/1311/Isolated Flyback Converter Basics: Flyback Converter Operation and Snubberhttps://techweb.rohm.com/product/power-ic/acdc/1150/

author: Konkedout
author_rank: Frequent Contributor
timestamp: January 26, 2025, 02:25:52 am
content: I have designed and built a lot of similar, in fact I have also added another resistor in series with the diode. Your snubber circuit is often called an RCD snubber.If you connect the diode cathode to +Vin then you will clamp the flyback voltage close to zero. Two problems with that:1) You will similarly be clamping your desired output voltage close to zero, at least if you have a significant load applied. Volts per turn are the same on both primary and secondary windings, neglecting the leakage inductance.2) The transformer needs volt-seconds to reset. If the volts are near zero then the seconds become very long.Connecting the diode cathode to +Vin will not work for a flyback transformer!The circuit you show provides what I might call a two-step waveform with ringing in between. This is caused by transformer leakage inductance which is the whole reason why you need the drain snubber. This is often OK but I was building self-oscillating "ringing choke converters" or "critical conduction converters" where the gate drive waveform was derived from the transformer. I wanted to clean up my gate drive work waveform. By properly engineering my RRCD snubber I can eliminate most of the ringing and get an exponential decay instead. It worked much better in my circuit design, and may be slightly better for EMI.When driving a relay, there is a lot of winding resistance and you are not trying to transfer power through a transformer. Due to voltage drop in the winding resistance, the relay will reset in a reasonable amount of time...whatever that is. The RCD snubber might be useful with a relay if you wanted to absolutely let the relay open-reset as fast as possible. But relays are mechanical, and doing that to speed a relay might only give you some microseconds of speed, but the mechanical motion of the relay is likely to take milliseconds. So there would not be much benefit.

author: Andy Chee
author_rank: Super Contributor
timestamp: January 26, 2025, 03:12:00 am
content: FWIW, a book about snubbers;https://rudys.typepad.com/files/snubber-e-book-complete.pdf

author: Konkedout
author_rank: Frequent Contributor
timestamp: January 26, 2025, 03:55:48 am
content: Quote from: Andy Chee on January 26, 2025, 03:12:00 amGosh anything by Rudy is a Keeper! He was a main contributor at PowerCon which preceded IEEE APEC.

author: MrAl
author_rank: Super Contributor
timestamp: January 26, 2025, 09:04:03 am
content: Quote from: 2X on January 26, 2025, 12:13:05 amHello,The short answer is that when the primary voltage reverses, you cannot clamp that to zero volts. It has to be allowed to go up to whatever is needed in order to get the output voltage to go much higher. You have to allow that voltage to go high, you can't stop it or else the converter will now work right.When the transistor turns off, the voltage at the drain goes up very high, and it will go up to a level higher than the input supply voltage. That is the way it has to work. However, you can't let it go up too high, and that is where the snubber comes in. The snubber keeps the spikes from going too high and blowing out the transistor. So it allows the necessary increase in reverse voltage on the primary, but it does not allow it to go too high and that protects the transistor.