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400V DC, high side P-channel FET load switch with delay

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adnan012

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400V DC, high side P channel FET load switch with dealy

hi,

I want to control a load with high side P-channel Mosfet with 5 second delay. VCC is 400 Vdc . There is a schcmit trigger which senses VCC and when VCC reaches 400VDC it turn on high side P channel MOSFET after 5 seconds. I am using IXTH10P50P MOSFET. After turning on load (100 watt) few times the mosfet is destroyed (drain source short). I think slow rising gate volateg causes MOSFET damage. I need help to sort-out this issue.

regards
 

Re: 400V DC, high side P channel FET load switch with dealy

Hi,

We don´t know which circuit you refer to --> show your circuit, with all part values and informations like voltages...

Also give information about your load: Capacitive, inductive, purely resistive, motor, incandescent light bulb...all have different power_ON / power_OFF behaviour.

Klaus
 
Re: 400V DC, high side P channel FET load switch with dealy

Thanks for reply.
Load is resistive.
This is what i have tried to introduce delay (see capacitor at the base of the npn transistor).
 

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Re: 400V DC, high side P channel FET load switch with dealy

Hi,

There are three major reasans for a MOSFET to get killed.
* overcurrent --> not likely in your circuit
* overvoltage --> stabilize supply voltage, avoid inductance, add overoltage protection.
* power dissipation = overheat --> reduce switching time

***
To your circuit:

The circuit doesn´t show the 5s delay circuit and how the BJT is driven.

But there are two large capacitors slowing down the switch "ON". Why?
This causes a lot of dissipated energy (heat).
--> reduce capacitor values

****
Additionally I recommend to add a capacitor to stabilize the 400V
and add some overvoltage protection across the MOSFET.
Don´t use wirewound resistors.
Avoid stray inductance.

Klaus
 
Re: 400V DC, high side P channel FET load switch with dealy

Hi,

If the 5 seconds isn't a precise value, an option could be as simple as removing the 10k*10uF capacitor from the BJT base and placing them on a 5-second monostable 555 circuit block and the 555 output into an inverter and that into the BJT base. Maybe too convoluted a solution with too many parts for you, understandable.

I think Klaus said it already, if that's what I think it is, that isn't a useful time delay circuit there, it's only a very slow turn-on - the opposite to what the MOSFET should see. Look for gate drive circuits, they're usually about reducing actual switching time to the minimum possible.
 

Re: 400V DC, high side P channel FET load switch with dealy

Inrush current limiting is one reason to build in a slow
gate risetime. However this is probably better done
Miller Rgg-Cdg style (~ constant d(Vdg)/dt ) than by
making a slow gate risetime, as current will jump more
abruptly and less controlledly.

But inrush current control this way, has to respect Joule
heating / temp rise. And this may be why the thing fails
after some cycles, heat is "pumping" the baseplate temp
and what it can survive once, from ambient, is not what
it can take when the case temp has been raised.

A thermocouple and short experiment would show the
deal, there.
 
Re: 400V DC, high side P channel FET load switch with dealy

I once had a similar problem: starting a DC motor driving a high inertia load.

What I did, which is not very elegant, was to include a resistor in series with the Mosfet. Then after a short period, a second Mosfet shorts the series resistor.
 
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