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nirmit27

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Mosfet in series connection for High voltge operation

hey
i want to make Switch circuit to drive 1800 voltage load
for that i am using 500 volt MOSFET, but for making above 1800 volt shall i connect 4 MOSFET in series to devide voltage across mosfet for 1800 volt. ???

second Q is how to use gate driver for switch on MOSFET in seriec connetcion of MOSFET ?
and how much voltage capacity GAte driver i have to use ? 1800 volt ya 500 volt for my circuit ?

is my demo circuit is correct for series ? View attachment UNTITLED.BMP

how to connect driver to this series circuit ? is this correct way ya have to connect driver to each mosfet ?

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Re: Mosfet in series connection for High voltge operation

Hello,
That circuit will not work because you will require isolated gate drives for each of your mosfets. In addition to that you will also require balancing resistors across each drain-source in order to ensure that they will share voltage properly. I would simulate this to understand the ramifications of switching delays between each mosfet.
 

Re: Mosfet in series connection for High voltge operation

Hello,
That circuit will not work because you will require isolated gate drives for each of your mosfets. In addition to that you will also require balancing resistors across each drain-source in order to ensure that they will share voltage properly. I would simulate this to understand the ramifications of switching delays between each mosfet.

Hi brushhead sir
ya i knw i need to isolate gate drivers for mosfet....my Q is should i have to use gate drivers for each MOSFET ya only 1 MOSFET like i mention in circuit snapshot . like i put in on condition 3 MOSFET and drive only last mosfet as switch,,,,can i do like...ya i have to Use Gate drive for Each MOSFET ?
and what is " ramifications of switching delays " i dint get this ?
 

Re: Mosfet in series connection for High voltge operation

yes you need a separate driver (isol) for each fet.
You should bewary of say three fets turning fully on when the other is still off........then it will kop the big voltage by itself.
You can put a capacitive totem pole in there to mitigate...and some bleed res across each cap.
But you can use HV silicon carbide fets to handle it too.
 

Re: Mosfet in series connection for High voltge operation

yes you need a separate driver (isol) for each fet.
You should bewary of say three fets turning fully on when the other is still off........then it will kop the big voltage by itself.
You can put a capacitive totem pole in there to mitigate...and some bleed res across each cap.
But you can use HV silicon carbide fets to handle it too.

hey treez,
shouldn't i use one driver for bottom FET and keep on top fet ON ...as i mention in test circuit
cant it possible ?
means for four mosfet 1st 3 mosfet i keep in on condition and only bottom mosfet i connect driver and switch on of by pwm throgh driver ? ya i have to connect driver for each mosfet ?
if yes than is there all driver will work at same time ...if there may be delay in any modfer than my circuit will not work
this only my Q
 

Re: Mosfet in series connection for High voltge operation

You can use pulse drivers using transformers for the gates. But how to tackle the division of the total voltage...
 

Re: Mosfet in series connection for High voltge operation

You can use pulse drivers using transformers for the gates. But how to tackle the division of the total voltage...

in series each mosfet will take equal voltage through series rule , means voltage equally divided to each MOSFET ,
wil u help me in driver
 

Re: Mosfet in series connection for High voltge operation

Pulse transformers can be easily used for gate driving but if you have very skewed on-off ratio, then you may get into problems. You can also use optical isolators for gate drives. They can take care of the floating gate without fuss. You need to make a rough plan of the project you will be doing...
 

Re: Mosfet in series connection for High voltge operation

Pulse transformers can be easily used for gate driving but if you have very skewed on-off ratio, then you may get into problems. You can also use optical isolators for gate drives. They can take care of the floating gate without fuss. You need to make a rough plan of the project you will be doing...

tht i knw tht i hav to use isolator gate driver but Can u help in my Q ? for series connected mosfet
 

Re: Mosfet in series connection for High voltge operation

for series connected mosfet

I really do not know how to ensure fair division of the applied external potential so that one is not overburdened. What happens if three transistors are conducting and one is off?

Each gate must reference to source/ drain potentials and the secondary of the pulse transformer must connect to the gate and source/ drain of each transistor.

MOSFETs have been connected in parallel and there are small tricks to ensure a fair share of the current. It appears that a fair share of the potential is quite different.
 
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Re: Mosfet in series connection for High voltge operation

in series each mosfet will take equal voltage through series rule , means voltage equally divided to each MOSFET ,
wil u help me in driver

Series FETs are never used in practice because this is not true. Slight differences in the FETs' leakage currents will cause their voltages to be different. And during switching it's practically impossible to ensure that they share voltage. You will certainly exceed their voltage ratings at some point while switching.
 

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Re: Mosfet in series connection for High voltge operation

any idea ?

Yes, I think the circuit is untested and, in simple language, bogus.

Don't waste your time. It simply won't work.
 
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