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I want to control a brushed DC motor of the below specification:

  • Motor: Permanent magnet DC brushed motor
  • Voltage: 180V
  • Current: 5 Amps
  • RPM: 1500

The control method is PWM switching through a microcontroller, with speed as a feedback.

This is the basic control block diagram:

enter image description here

How to select the MOSFET for controlling the above motor?

What should be the MOSFET continuous current Id, Drain-Source VDS, RDS, VGS (for the given motor rating)?

Switching Frequency 10 KHz

MOSFET Package: Through hole will be apt

Based on the ratings I have designed the circuit below is the same with component specification:

enter image description here

The control circuit is working fine, except I have encountered once MOSFET failure, that is why I was wondering if I have to increase the MOSFET rating.

Mosfet Details:

  • IRFP460
  • VDS = 500V
  • RDS = 0.27ohm
  • Id = 20 A (at 25°C)

And I have put considerable heat sink to dissipate heat from the MOSFET.

So I thought if there is a standard way of choosing the MOSFET, then I could compare with the one I have already chosen.

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  • \$\begingroup\$ do you want surface mount or through-hole package? What switching frequency will you be using? \$\endgroup\$ – KyranF Oct 9 '14 at 12:35
  • \$\begingroup\$ @KyranF I have updated the frequency and package. \$\endgroup\$ – siva Oct 9 '14 at 12:40
  • \$\begingroup\$ I have provided an answer, with some of my thoughts \$\endgroup\$ – KyranF Oct 9 '14 at 12:47
  • \$\begingroup\$ Measure the DC resistance of the motor - several times, turning the shaft between measurements, and post the lowest measurement. If it's 3 ohms, that suggests 60A starting current, or with the motor stalled... \$\endgroup\$ – Brian Drummond Oct 9 '14 at 12:54
  • \$\begingroup\$ @BrianDrummond Sure I will measure and update the same, but how are you saying the starting current will be 60A . \$\endgroup\$ – siva Oct 9 '14 at 13:24
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I would go for 1.5x voltage rating just to be safe, meaning 270-300V. Most of the N Channel FETs I see on Digikey with those specs, and are cheap/plentiful, are already more than enough in terms of current handling capability.

One of your regular looking TO-220 package FETs such as the FDP14N30 from Fairchild Semiconductor, which is a 300V 14A rated N Channel MOSFET, is plenty enough. It will dissipate 7.5 Watts with 300mOhm On resistance and 5 Amps continuous. It can do pulsed currents up to 56 Amps so i'm sure it will handle start-up current surges. Here is the datasheet from the manufacturer

Basically, try to select a component which is rated at MORE than your given parameters, with reasonable and logical room for less than ideal conditions, such as after temperature has increased or if the component happens to be on the low end of tolerance during manufacture.

If you over-rate components too much though it can cost quite a lot more in terms of production cost and PCB space, but if you have a project for university for example, over-rating a component just means your project will fail less during the desperate times you are trying to do final tests and write your reports etc.

If you expect your motor will be turned without powering it, look out for generated voltages that may actually exceed your 300V rated FET. I suggest you get some heavy duty (300-400V) rated diodes to clamp the motor + and - connections to VCC and GND. This is for "Back EMF" protection, and sometimes the diodes are referred to as flyback or freewheeling diodes I believe (this may help you research the topic, and their use). You can also put a big blocking diode parallel over the + and - connection to the motor, which helps with/does the same thing. These are usually used for any type of inductive load.

Also double check the voltage that your N channel low side gate driver uses, the IC I suggested that you use has +-30V gate voltage ratings, so you should be okay - but there ARE components which have much lower (12V, or 20V) gate voltage max ratings.

Because you will be using a proper gate driver IC, I suspect you will not have problems with 10kHz switching, but when not using a gate driver you may have gate capacitance issues causing higher switching loss, as the MOSFET takes more current to discharge/charge than for example a small micrcontroller output pin can provide. The MOSFET would then be in the "linear resistance" region much longer than if a proper gate driver had been used.

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    \$\begingroup\$ You can also put FETs in parallel for more throughput, but be aware that this also parallels the gate capacitors, making it even harder to turn them on quickly. \$\endgroup\$ – KyranF Oct 9 '14 at 12:55
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    \$\begingroup\$ If you wanted to go for a higher current handling FET you can go for that FDP's big brother, the FDP24N40 or similar, which is 400V 24A rated, with the same package. \$\endgroup\$ – KyranF Oct 9 '14 at 12:58
  • \$\begingroup\$ I have already designed the circuit based on the ratings, and its running okay. \$\endgroup\$ – siva Oct 9 '14 at 13:00
  • \$\begingroup\$ @siva It only takes one microsecond of things not being okay, to blow up ;) \$\endgroup\$ – KyranF Oct 9 '14 at 13:00
  • \$\begingroup\$ for example if you chose a 200V 6A MOSFET, you will very quickly have a broken device. \$\endgroup\$ – KyranF Oct 9 '14 at 13:01

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