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Parallel Mosfet for Hi Power

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MCMC

MCMC

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how to parallel mosfets

Hi all

I need some help,

when you need hi power, in aplications like Hi power-current dc-dc converter, it's necesary to parallel the mosfet transistors to get more current and power, I need to know how is the formula or calculations, to know how quantity of mosfet that I need to specific current and power ????

in the example, it's use 4 75amp mosfet's
 

an918 mosfet

What is very good about MOSFETs is that they share the current almost equally when placed in parallel.

You should make sure of two things.

1. The total power loss turned to heat is less than the sum of the FET maximum dissipations as calculated when they are on the heat sink you use. The reliability of the supply is improved if the FETs are run below their maximum safe temperature.

2. The total current is less than the sum of the rated currents of the FETs.
 

mosfet+parallel

In this case however, the bottleneck is going to be the magnetics. You can barely find a transformer that can handle more power than this circuit is designed for.

Bert
 

parallel mosfet

Hi partners

Thanks for reply

another ? is how to calculate the gate current for specific switching frecuency? to determine the value of r3-6 in this example is 100 ohm
 

Hi MCMC, to answer all your questions take a look on my post
about Unitrode Seminars, I have attempt some of them, they are
very good.

There is everything you want to know about SMPS, mosfets, igbts
it is worth to DL all, you will learn a lot.

**broken link removed**

Regards
Tornado
 

How many Mosfets can be paralled without problems
 

You can parallel as many Mosfets you like, you have to provide
enough drive energy, got to the link on Unitrode and DL the
Seminars. They explain how to parallel Mosfets and what drive
requirements you have to provide.

Tornado
 

Think about it this way. Say your mosfet has Rds- On about .5 Ohms. This means that it will pull about 20A from a 10v supply. Is this too little? Use two, they'll have RdsON=0.25 Ohm and pull 40A from a 10v supply. Or use 4 for 80A.

I know this is starting to sound silly, but you should see the new generation DC-DC for next year's pentium. They give about 1.3v out, and do not sag at all when hit with 120A load step. 8x si4420 mosfets as the drivers.


As for gate drive - say each mosfet has gate capacitance of about 1000p. You want to drive this 0-10v in 100ns. I=C(dV/dT)

One mosfet needs 100mA driver to achieve 100ns turn-on, two mosfets need 200mA, and so on and so on. Look at some DC-DC converter spec sheets, those guys source 2A or more to the mosfet gates to really bang them on in nanoseconds.
 

oh, the reason for the 100ohm is to stop ringing. If you use a 2 amp driver on a single mosfet, the response can ring, which lowers the efficiency of the driver (turns on-off-on-off-on-off as it rings)
 

For your reference!



*********************************************************
This post has been moderated by Cluricaun.
Action: Your attachment has been removed.
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123.analog.com/UploadedFiles/Data_Sheets/88878064ADP3168_a.pdf
 

see application notes an-944.pdf and an-941.pdf from International Rectifier
 

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