Protecting Mosfets Switching Inductive Loads - Page 1
Forum section: electronics / projects

author: Etesla
author_rank: Regular Contributor
timestamp: January 02, 2019, 05:36:08 am
content: So I'm playing with flyback transformers out of old CRT displays again, and my pile of dead mosfets grows every day. I think that it is because of the switching transients caused my hard switching of the mosfet that drives the primary coil. The general outline of the circuit I am using, as well as simulation results highlighting the problem, are in the images below. In my application (audio modulation), it is required that I hard switch the mosfet in the circuit. I know that adding snubber type circuits will protect the mosfet, but they also kill the inductive spike in the primary, and thus the super awesome high voltage in the secondary. I am wondering if anyone knows of a way to protect the mosfet from the primary coil's inductive spiking, while maintaining the primary coil's inductive spiking...Solutions that are off of the table:resonant-type drivers, because they make audio modulation impossibleresistor diode snubbers, RC snubbers, varistors, because they kill the high voltage funhigher rated mosfets or IGBT's, that's just a cop outImages:The circuit is super simplified and the values are very much ballparkThe one image with around 400V peaks is the voltage at the drain of the mosfet, and it shows the inductive spikeThe one image with around 160 Kv peaks is the voltage across the secondary coil. The goal is to keep this voltage as high as possible. (In real life the secondary is just a coil, I wound a new one).Thank you so much for your help in advance!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 02, 2019, 08:13:41 am
content: Right, you can't go forever.The best you can, then, is a TVS (or stack of them) rated for about 70-80% of the transistor's max. Or something equivalent to this,Alternately, set up a resonant driver, so that the inverter output voltage is well defined, and the load current rises with output voltage, until breakdown. (Inverter current rises with output voltage because of the impedance inverting effect of the resonant network.) Now you have to protect against overcurrent, but that is relatively easy to do with a desat protection circuit and maybe a supply current limiter.Tim

author: duak
author_rank: Super Contributor
timestamp: January 02, 2019, 06:52:47 pm
content: Etesla, you're stuck between a rock and a hard place. Due to the inductive coupling of the two coils (transformer action) a higher secondary voltage implies a higher primary voltage and this means an unavoidably greater voltage across the MOSFET. The parasitic inductance of the circuit and its effects are something you do have some control over. I don't see in your circuit the various other parts that are in an Horizontal deflection system that are needed to get the flyback to work as designed, in particular the capacitor and the deflection yoke. Have a look at:https://www.repairfaq.org/sam/deflfaq.htm#dshdfHorizontal Deflection System Fundamentals. Also look into Inductive Discharge (or Kettering) Ignition Systems. There is a capacitor across the switch that is sized to resonate with the coil that both minimizes the peak primary voltage and maximizes the energy delivered in the secondary.Smarter people than me worked out all this stuff and I don't fully understand it myself so I can't make any specific recomendations. I rigged up a self oscillating color TV flyback back in the early 70's using the matching deflection tube but without a clamp diode. I got the requisite HV out of the flyback but in the dark the deflection tube was faintly glowing blue and red. I now realize the tube was probably being exposed to way too much plate voltage (and wonder if it was generating X-rays). Modern semiconductors would not take the abuse that thermionic devices can so I was able to build something that was truly insidiously dangerous. Good thing I didn't get the HV rectifier working or I would have had an Xray generator for sure.Best wishes for a safe & happy 2019.