Mosfet driver problem

[deepakchikane]

Dear all,

i have been facing the problem of mosfet drivers.

pls check the attachment & feedback me so i can make the respective changes.

pls pls pls

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mosfet is FDD 8870

 

[cl10greg]

What is the system doing? What is your PWM frequency and duty cycle? There is no power input for the NPN transistors on the collector side? The way the circuit is shown, there is no power supply for the NPN so when you turn on the transistors there is no source for current flow.

 

[yassin.kraouch]

what is the problem ? can you explain ?

 

[deepakchikane]

Hi supply is 5 volts.

 

[nikhilmahasvar]

You need to give 12 V as supply and there has to be a low value resistor between the MOSFET gate and the driver transistor in series.

 

[goldsmith]

Hi supply is 5 volts.

Hi
Your mosfet requires 15 volt to be fully turned on , it's as the first hint !
As the 2nd hint , such a driver will not do it's best instead of the high frequencies . how much is your PWM frequency ?

Best Wishes
Goldsmith

 

[deepakchikane]

Hi goldsmith,

Nice to c u after very long time...

1.How would know that mosfet requires 15 vols is the first question..??

2.i m looking for low budget mosfet driver..

3 frequency is 100 khz with variable duty cycle(30 to 80 %)

4.purpose is to make boost converter from 2volts to 14 volts..

do u have hints..??

pls help me..

also the thing is that,

if i provide him a 15 volts then my gate requirements will be higher.

according to gate current
IG=Vgs*Ciss/Tr
vgs=15v
ciss=6125pF
Tr=50nS

SO Ig will be around 2 amps..

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check the attachements

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goldsmith ...

pls check this waveforms

 

[cl10greg]

1.How would know that mosfet requires 15 vols is the first question..??

There is usually a part in the datasheet that is either a graph that shows on resistance based on gate voltage or it says what the RDSon is at different voltages. The reason you want 15 volts is because that is when the conduction path is complete and the on resistance is the lowest for optimal conduction without heating.

2.i m looking for low budget mosfet driver

Your solution is feasible at low cost but a regular low side driver with more feedback and safety features can be found for pretty cheap as well.

3 frequency is 100 khz with variable duty cycle(30 to 80 %)

I would crunch the numbers on this. You want to make sure that your MOSFET has enough power to achieve the full on and off switching for the application. You can get the input capacitance from the datasheet and use one of the many different application notes for MOSFET drivers (**broken link removed**). If you don't have ample current from your driver to fully charge the gate, you will get heating and early failure.

purpose is to make boost converter from 2volts to 14 volts

You're on the right track. There are a handful of application notes and calculators for boost circuits that can be referenced.

if i provide him a 15 volts then my gate requirements will be higher.

according to gate current
IG=Vgs*Ciss/Tr
vgs=15v
ciss=6125pF
Tr=50nS

SO Ig will be around 2 amps..

I like seeing that you ran the numbers already. You are correct it is going to be a lot more power but that is kind of the price you pay when you use MOSFETs are higher switching frequencies. To reduce this you would need to reduce your PWM frequency.