mosfet break down voltage

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veladimir

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hello
I use a 2N80 (fairchild) mosfet in a switching power supply.
it should break down in 800v but it breaks down in 370 volt and
over heat and finally short three pins.
I repeat several times and i use gate drive IC(7667) .
Is there application note about this or can one help me?
 
You may have a very short voltage spike that is greater than 800 volts.
A real Fairchild MOSFET will hold, with out heat, at 800V and maybe 850V.
 
I use an other winding that when voltage of first winding reaches to 600v the secend winding permit current to flow .
Is it posible during diode(FR107) turn on time a spike occurs ?
 
Last edited:
I read it but maybe i dont act carefully .
I dont regard out damping issue.
can out damping have this effect ?
 
Last edited:
veladimir said:
I use an other winding that when voltage of first winding reaches to 600v the secend winding permit current to flow .
Is it posible during diode(FR107) turn on time a spike occurs ?
Click to expand...
againlease show a schematic.
You are getting a spike on the MOSFET not because of the diode but because of the leakage inductance in the transformer.
 
in theory when transistor is on diode withstand 600v and when transistor switched off diode should warranty transs magneticing current and transistor should withstand 600v.
but when mosfet forced to switch off drain voltage dont cross from 370v.
 

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Thanks for the attached thumbnail. If the primary and "catch" winding are wound very close together then there should be little spike. It is still possible for a spike to be present! Have you looked with a good scope? The pulse may be only 50 to 100nS long. What diode are you using? Is the diode hot? Does the diode fail when the FET fails?
 
Anytime you are switching magnetics with a transistor you should have a spike suppressor to protect it. There is always some small leakage inductance that will generate a spike. I suggest a high voltage zener connected directly across the transistor drain and source with as short a connection as possible.
 
thanks my friend ronsimpson for your response.
If the primary and "catch" winding are wound very close together then there should be little spike. --> is your idea that some things between windings like paper suppress spikes
i use a pc based digi oscilloscope with 80 Msps and i didnt see a spike.
i use fr106 and after each mosfet fail and it appears normal.
i didnt check diode temprature.
thanks crutschow
I dont Know any thing about spike suppressor please introduce a source about it or explain it.
zener is good idea but is there a zener with break down voltage of 600v.
 
I've seen then as high as 200V. If that's the highest available, you can just connect three in series.
 
veladimir said:
is it practical?
what is a spike suppressor?
Click to expand...
As the phrase implies, it suppress voltage spikes. It can be made with various components such as a transient suppression diode (zener), a capacitor, a MOV (Metal Oxide Varistor), etc. Which is the best depends upon the application. For relatively precise clamping of the spike voltage such as needed with a transistor, zeners are a good choice.
 
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