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1: Need for large number of capacitors in the power circuit (store the charge and dissipate to which part of the component mosfet or diodes ??) ?
2: what are diodes parallel to the mosfets for ? ( other than preventing the back emf coming from the motor ) and is it have anything to do with firing angle ?
3:advantage of using many mosfets in parallel than using a single mosfet and on what basis must that mosfet chosen?

I am from India a college student pursuing my electronics engineering and making motor controller,i am able to get all the circuit details and how the motor controller works but i have above few doubts

7 Replies

iceng (author)2014-02-21

OK inductance problems are what happens when you interrupt current flow through an inductive element like a DC motor armature controlled by PWM

or better a permanent magnet inside-out brushless machine.

Any inductor does not like to suddenly have its current interrupted and will generate a microsecond time increasing voltage across its terminals as the stored magnetic field begins to collapse in an effort to maintain current flow.

If this condition is allowed to develop the reactive voltage can raise to hundreds of volts, causing arc overs and damaging the switching semiconductor due to secondary breakdown.

Adding a parallel inverse diode to the switching semiconductor allows

the voltage to push a reactive current back to the capacitors of the DC supply

safely dissipating the stored magnetic field energy.

Placing an inverse diode across the field would also dissipate the reactive current through the diode, but that would slow the inductive motor's ability to respond properly to Pulss-Width-Modulation PWM motor speed control.


I hope this addressed your inductance query :-)


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doraditya (author)2014-02-20

thank you i was expecting answers related to inductance problems.

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doraditya (author)2013-12-26

so you mean no need of connecting extras fast switching diodes across the mosfets ?
i have seen in many controller circuit they have connected a diode across mosfet which has internal diode.

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iceng (author)doraditya2014-02-18

The way I have that, is the Intrinsic MOSFET diode is as fast as the FET but not capable of long heavy current.

So a designer places a moderate speed parallel diode

to carry the long heavy current that would damage the FET

while the body diode is allowed to catch the first fast reactive flow.

This is for high speed switching situations

where the inductive reactance of a 6" about 15cm length of

straight wire can generate a couple of hundred volts at MOSFET turn-off !!!

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steveastrouk (author)doraditya2013-12-27

You should have posted circuit diagrams. Yes, sometimes the internal diode is shunted by a faster one, it depends on the requirements and the topology

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steveastrouk (author)2013-12-26

Mosfets ALL have a "parallel diode", by their very nature.

The advantage of many mosfets are reduces single point of failure, and 10 x 100A mosfets are generally still cheaper than 1x 1000A Mosfet. And you can heatsink them better

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caitlinsdad (author)2013-12-25

You might start out with the specs or design requirements of the load (rpm, torque, power usage, runtime, hot/cold environment, etc). From there you can think about what you need to control and what part can do what and how to configure it best. Good luck.

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