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I'm in the midst of prototyping my circuit. However during testing, the psu led light keep flickering and there was no output observed. Subsequent investigation revealed that the mosfet have actually short circuited. I have short circuited 3 pieces of mosfet so far. Please refer to the image for reference.

You might be wondering why I did not implement an ac/dc flyback. The area that I live in do not have customable flyback transformer and the epcos tdk was the best I could find. Is there any advice on how to modify my circuit. I am using an Arduino max to act as my pwm which output less than 5v. I have not connected any sorts of load as I would like to observe the no load voltage first.

Thanks in advance.enter image description here

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  • \$\begingroup\$ Show gate-source oscillogram. \$\endgroup\$
    – winny
    Commented Sep 22, 2017 at 7:24
  • \$\begingroup\$ @winny I didn't have an oscilloscope during the test. The flickering of the psu was like a sign for me to shut it down. \$\endgroup\$
    – Iskandar
    Commented Sep 22, 2017 at 8:21
  • \$\begingroup\$ Test without load and capture said waveform. \$\endgroup\$
    – winny
    Commented Sep 22, 2017 at 12:36

2 Answers 2

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MOSFETs fail shorted due to electrical overstress.

  1. Check your gate drive. It should be fast enough so you don't have excessive switching losses and high enough amplitude that you get good low RDSon. The threshold voltage on your FET can be as high as 4V, so you would want to drive it with a 10V driver. Not sure your Arduino is up to this task. Consider using a FET driver IC to provide better gate drive.

  2. Check your drain voltage during operation. You have an RCD snubber, which is good, but be sure your voltage does not exceed the VDS rating of the FET.

  3. Give 1 & 2, monitor the temperature of your FET during operation. It's probably getting way too hot.

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  • \$\begingroup\$ thank you for replying. Will try to source an external pwm first. \$\endgroup\$
    – Iskandar
    Commented Sep 22, 2017 at 8:24
  • \$\begingroup\$ @Iskandar , next to good John's advice above, note that MOS BVds=60V is a way too low for such a flyback topology driven by 48V. Vd peak is going to over 90V or so. \$\endgroup\$
    – carloc
    Commented Sep 22, 2017 at 8:34
  • \$\begingroup\$ @carloc hi there. I'm a bit confused. Are you able to share with me how does the voltage is able to rise up to 90v ? Appreciate your help on this. Thank you \$\endgroup\$
    – Iskandar
    Commented Sep 22, 2017 at 8:39
  • \$\begingroup\$ @Iskandar sorry, I am busy working cannot enter much of details but basically: Flyback works such as when MOS switches OFF its Vd rises to Vcc power supply plus output voltage scaled as transformer ratio. In your case from TDK site I see 7.5/1 ratio between primary and 5V output. So Vd peak can be ballparked to be around 48V+7.5×5V=85.5V without accounting any non ideality. \$\endgroup\$
    – carloc
    Commented Sep 22, 2017 at 8:47
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The MOSFET needs 10V to 15V Vgs, not 5V. The gate resistor is too big (220 ohm). To be able to push your circuit at 100 kHz it has to be well balanced in terms of gate driver, gate resistance,...

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  • \$\begingroup\$ Hi there. I was looking through the data sheet. It stated the Vgs th is from 1V -2.5V. Hence a 5V should be good enough to drive the mosfet? Please correct me if I'm wrong. I'm new to prototyping. Thanks in advance \$\endgroup\$
    – Iskandar
    Commented Sep 22, 2017 at 8:24
  • \$\begingroup\$ @Iskandar Link the datasheet of the MOSFET type you are using. I have this one: mouser.com/ds/2/149/FQP30N06L-244344.pdf \$\endgroup\$
    – Marko Buršič
    Commented Sep 22, 2017 at 11:19
  • \$\begingroup\$ I'm having the same datasheet as the one you have attached. I was looking at page 2 under the on characteristics. From my understanding, so Long as it beyond, 2.5v it will on? Correct me if I'm wrong. My external pwm from china is on the way and it will output 5v at 100khz. Hence based from the explanation from the kind users here , I should just switch up the mosfet to at least Vds = 100v and I'm all good ? \$\endgroup\$
    – Iskandar
    Commented Sep 22, 2017 at 11:49
  • \$\begingroup\$ @Iskandar No way, you look at threshold voltage, this is the minimum voltage when it begins conduct few micro amps. You can use your circuit to turn on/off, but not to do fast switching. Also your snubber is weird 47k, IMO it doesn't do anything, so you have might burned the MOSFET by HV spike. \$\endgroup\$
    – Marko Buršič
    Commented Sep 22, 2017 at 12:02
  • \$\begingroup\$ are you able to advice the components that I have to change in order for the circuit to work? Will me stepping up the output from the pwm from china through a external boost converter able to help? Your help is greatly appreciated \$\endgroup\$
    – Iskandar
    Commented Sep 22, 2017 at 12:08

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