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Do avalanching diodes "latch up"? Answered

While working and revising my flyback driver design, I had one of the 1N4148 diodes that I was using as protection get cooked!  (it seemed to fail short circuit) I have not tested to see if the MOSFET survived yet. Obviously the diode needs to react fast and handle avalanching energy well. Any suggestions? are some beefy schottky diodes suitable? Also, does making any ordinary diode go into that avalanche or zener reverse breakdown region cause it to potentially lock up and conduct for a period longer than the EMF spikes I'm dealing with? Or with any sort of hysteresis (like when using neon lamps)?

Originally my wimpy glass 1N4148 diodes was wired in parallel directly across the MOSFET drain and source reverse biased. However, carefully examining this tesla coil schematic,it appears that there is both a forward biased diode in series with the MOSFETs, and another reverse biased diode(s) in parallel across the MOSFET & diode. It does not look like a bad idea given one the reverse biased diodes can shunt EMF spikes that are too large while the other isolates them from the MOSFET. My only concern is how lossy that would be at 12V operation.

Discussions

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iceng

3 years ago

The mosfet diode is probably the very fastest diode BUT not good for a lot of current.

So you put a good low fwd diode in parallel, that lets the fet diode catch the fast event, while the slower device takes over the heavy fet damaging current...

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-max-iceng

Answer 3 years ago

I found contradicting evidence online. I guess it really depends on the MOSFET. I posted what I found from the talking electronics site, explaining that built in diode has really poor recovery times and can lead to shoot-through conditions, although honestly this is not even something I considered. My idea was that by adding an external diode would prevent the Vds voltage from getting too high.

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steveastrouk

3 years ago

One of the paradoxes of a flyback system is that you don't NEED a diode across the mosfet.

Have you actually tried a full-blown model in LTspice of what's going on ?

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-max-steveastrouk

Answer 3 years ago

Without it my MOSFETs die much easier. :/ (I do use LTspice to simulate my circuits, but I do not trust it. I am pretty sure I would never see 100's of MV to GV across my MOSFETs!!!)

Also isn't that internal parasitic body diode pretty lousy? The 1N4148 diodes I used should have a breakdown voltage of about 75V, vs my IRFP250's which are rated 200V. My thought is that I would prefer the cheap and plentiful diodes to fail rather than my more expensive MOSFETs. I switched over to 1N4007's, which seem to work fine so far.

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Downunder35m-max-

Answer 3 years ago

I did a lot of smoke testing my first induction heater, both from diodes and then from the failing mosfets.
One of the big problems when diodes get bigger in size is their capacitance as it can mess with the circuit too.
But after producing a lot of smoke I finally found some easy fix.
Check old TV's and similar for these big double shottky diodes.
They look like a big transistor of mosfet.
In many cases the datasheet for them will show you quite high amp levels and very fast switching times.
Another one that does a reall good job is found inside the flyback - the internal rectifying diodes.
Bit of a pain to use the heatgun to free them up but as a last resort measure in a hurry for a replacement...

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-max-steveastrouk

Answer 3 years ago

I found this article on talking electronics, and in does indeed mention as one of the killers of MOSFETs is relying on slow body diode's conduction, and that it should be prevented:


When switching current through any inductive load (such as a Tesla Coil) a back EMF is produced when the current is turned off. It is essential to provide a path for this current to free-wheel in the time when the switching device is not conducting the load current.
This current is usually directed through a free-wheel diode connected anti-parallel with the switching device. When a MOSFET is employed as the switching device, the designer gets the free-wheel diode "for free" in the form of the MOSFETs intrinsic body diode. This solves one problem, but creates a whole new one...

  • Slow reverse recovery of MOSFET body diode:

A high Q resonant circuit such as a Tesla Coil is capable of storing considerable energy in its inductance and self capacitance. Under certain tuning conditions, this causes the current to "free-wheel" through the internal body diodes of the MOSFET device. This behaviour is not a problem in itself, but a problem arises due to the slow turn-off (or reverse recovery) of the internal body diode.
MOSFET body diodes generally have a long reverse recovery time compared to the performance of the MOSFET itself.
This problem is usually eased by the addition of a high speed (fast recovery) diode. This ensures that the MOSFET body diode is never driven into conduction. The free-wheel current is handled by the fast recovery diode which presents less of a "shoot-through" problem.