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Old 18th February 2005, 07:15 AM   #1
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Default Paralleling large number of Mosfets.

Hello All,

Is any one able to provide a calculation / information on how many mosfets you are able to drive from the front end of a standard class AB amplifier?

For example: If mosfets require only a tiny amount of current to satisfy drive requirements, how do you determine the maximum number of mosfets you can safely drive from a given driver stage ?

Any help or information would be appreciated.

-Dan
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Old 18th February 2005, 08:33 AM   #2
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It depends on the design of the circuitry,hard to tell how many MOSFETs are able to be driven,at least in my knowledge level.But u can go to Accuphase's website for their amplifier manual.Hope this will help
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Old 18th February 2005, 08:56 AM   #3
AKSA is offline AKSA  Australia
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The big problem driving mosfets is the gate capacitance, which is typically 600pF on a largish hexfet, say an IRFP150. At 20KHz, 600pF represents a capacitive reactance of only 13K25; two together is half this. This is a tough load to drive at high frequencies where open loop gain starts to droop anyway, with resultant loss of linearity. So you must takes steps to minimize the effects of this heavy loading at HF.

If you are driving in source follower mode (which is most mosfet output stages), then for two pairs of mosfets, you need about 12mA in the driver circuit. For three pairs, 18mA would be needed, and for four - you guessed it, 24mA.

There are often difficulties coping with these high currents, particularly anything over about 15mA, because of voltage amplifier/CCS dissipation issues, and of course ensuring stability.

Sometime you will see common collector (emitter follower) bipolar or common source (source follower) mosfet drivers interposed between the VAS circuit and the mosfet gates. While this adds an additional stage within the global feedback loop, it does mean the driver stage on the VAS can be run at five or six mA, which is far more manageable.

Cheers,

Hugh
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Old 18th February 2005, 02:44 PM   #4
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Hi DAN,

Parallelling Mosfets requires extreme consideration towards the high frequency drivability interms of gate charging speed and current required for charging and of course discharging.
The gate capacitance of Mosfets acts as a high pass filter which requires lot of current drive to drive a heavy load with high frequency signal content.
The driver current is selected so that , it can accomodate both the SINK/SOURCE phenomena in a specific manner. Severe high frequency content at the gate would simply destroy the gate isoltaion layer which inturns damages the die of mosfet, therefore gate resistors are implemented to eliminate this overdrive as well as high frequency oscillations.

Here is the formula for the adequate driver bleeder resistor choosing.

"RDbleeder<=2*[Rgate/N]"

where n is total number of mosfets in parallel.

The current in RDbleeder is set accordingly to the threshold or biasing of mosfet.

hope this helps
kanwar
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Old 18th February 2005, 08:13 PM   #5
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The only limitation seems the idle current through driver stage (usually emitter followers).
Some recomend 10mA of idle current trough driver bjts per mosfet pair

Quote:
Originally posted by AKSA
The big problem driving mosfets is the gate capacitance, which is typically 600pF on a largish hexfet, say an IRFP150.
(...)
you mean Cgs?
I thought power hexfets have more or less 1500pF?

regards
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Old 18th February 2005, 08:51 PM   #6
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Well said Hugh. It never fails to suprise me how many people think MOSFETs are easy to drive because of the 'high impedance' input.

darkfenriz, Cgs is usually referred to as Ciss or input capacitance. Hugh may have been a little optimistic, IRFP240 for instance has 1300pF. The amount can vary, but is generally between 800-3000pF for a decent sized MOSFET.
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Old 18th February 2005, 09:48 PM   #7
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Oops! My mistake...... Sorry guys!

I've recalled so many 'facts' off the top of my head in recent times whole bits have gone missing......

It seems the situation is worse than I thought! Perhaps I'd better stick either to common source drive (CFP by another name) or use bipolars!

I've gone over completely to biplolars in the last ten years because they've become so good, SOARs are excellent, speed is fine, there's not parasitic oscillation, and rail efficiency is so much better.

Cheers,

Hugh
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Old 18th February 2005, 10:15 PM   #8
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Hi Guys

The driving requirements ,for driving Mos Fets in the usual source follower configuration are much less stringent.

And why?....because in the source follower this capacity is effectively bootstraped.

One example the popular 2SK135 and 2SJ49 have respectively a input capacitance of 500 pF and 900 pF.
Used in the source follower mode ,this input capacitance is reduced by the local feedback .
As the transconductance of the devices is fairly low (gm= 0,7 to 1,4 S) this reduction is moderate. It's grater in a higher transconductance device.
Calculations based in the published figures of the Hitachi (now Ranesas ) data sheets , show that we will end up with 100 to 200 pF input capacitance for each of the devices driving a 8 Ohms load.
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Old 19th February 2005, 07:08 AM   #9
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Thanks for the reply guys,

I appreciate the info hugh, thanks for the fomula kanwar.
I will take your advice into consideration.


-Dan
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Old 19th February 2005, 08:39 AM   #10
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Quote:
Originally posted by Tube_Dude


Used in the source follower mode ,this input capacitance is reduced by the local feedback .
As the transconductance of the devices is fairly low (gm= 0,7 to 1,4 S) this reduction is moderate. It's grater in a higher transconductance device.
.

Quite true, and if you parallel the fets the total transconductance increases and therefore so does the reduction ratio of the capacitance

Chris
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