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Grogster

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Joined: 31/12/2012
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Posted: 01:07am 09 May 2014

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Just about every circuit I see for PWM motor control, all use FET's as the driving transistor, but rarely bi-polar NPN or PNP type. Is this just for power dissipation reasons, or is there something special about a FET which makes this kind of transistor the in-thing for motor control?

The IFR520 seems to be a popular choice in most designs I have seen on the net...

EDIT: I found this on the net, and specifically, sections 7 and 8 helped my understanding.

I've never really used FET's, as I was more of a bi-polar transistor user.

Edited by Grogster 2014-05-10

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DoodaDay

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Joined: 24/02/2014
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Posted: 04:21am 09 May 2014

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I am certainly no expert, but I believe that the main advantage of power FETs in motor control circuits is their ability to switch large currents rapidly with very low loss of power (heat).

Modern power FETs (there are many types) often offer 'on' resistances of 0.5 - 0.05 ohms. This greatly reduces I^2*R losses when switching high currents, allowing more power to the motor and less wasted heat.

Particularly useful are the 'ttl' level switching power FETS that such as the IRFL-XXXX Devices (and others). These make it very simple to drive the FET gate directly with a typical microprocesser pin.

I am not aware of any situation where using bipolar transistors remains a better choice.

Grogster said Just about every circuit I see for PWM motor control, all use FET's as the driving transistor, but rarely bi-polar NPN or PNP type. Is this just for power dissipation reasons, or is there something special about a FET which makes this kind of transistor the in-thing for motor control?

The IFR520 seems to be a popular choice in most designs I have seen on the net...

EDIT: I found this on the net, and specifically, sections 7 and 8 helped my understanding.

I've never really used FET's, as I was more of a bi-polar transistor user.

vasi

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Joined: 23/03/2007
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Posted: 04:51am 09 May 2014

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"... with a typical (5Vcc) microprocessor pin."

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DoodaDay

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Posted: 06:26am 09 May 2014

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vasi said "... with a typical (5Vcc) microprocessor pin."

True... Often more convenient than some of the others, in any case.

hitsware
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Joined: 23/11/2012
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Posted: 07:22am 09 May 2014

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Or 3.3 V

crez

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Joined: 24/10/2012
Location: Australia
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Posted: 06:40pm 10 May 2014

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Typical power transistor gain is 100 or less so to drive a 5 amp load you need at least 5000/100 ie 50mA of drive current. From a microcontroller you would need another transistor to get this level of drive. Also most mosfets have built-in clamping diodes which makes driving inductive loads like motors simpler.

David

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