Half H-Bridge Design

ertansuluagac · Joined: 13 Jul 2017 Posts: 135 Location: IZMIR

Hello everbody
I want to get an idea about a topic. The subject is;
I want to drive using mosfet instead of relay. Do you think it makes sense?
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temtronic · Posted: Sun Apr 26, 2020 4:45 am

re: MOSFET vs relay

Implies a very low speed control signal as relays are 'slow' devices. Just on, off, usually less than 5Hz. So NOT PWM.

1/2 H-bridge requires a split (center tapped) power supply. $$$ !! especially at high current.

Depending on the 'device' a relay could be a LOT cheaper than a MOSFET (and support parts). A 40 Amp relay might be $5, MOSFET +parts $10-20 ?

MOSFET will need a 'driver', so added parts, PCB design, etc. Power layout on PCB is critical.

We need to know more 'specs' about the intended 'load' or 'device' that the PIC will control.

edit. rethinking about this and the 1/2 bridge power supply HAS to have IDENTICAL + and ++ supplies, probably within 1/2%, so that say CW and CCW rotations of the motor is within a reasonable design specification.
it could be one reason why full H-bridges are used. Again this goes back to 'what' the device is that has to be controlled.

elcrcp · Joined: 11 Mar 2016 Posts: 62 Location: izmir / Turkey

It depends on purpose... If you are not dynamically controlling the motors, not applying pwm, not changing course in miliseconds then relays are the right choice. But if you have a very dynamic motor control system then the mosfet drivers are the right thing.

As Jay mentioned, 10-40Amps mosfet motor drivers comes pricy and designing one might come tricky. But there are many H bridge IC's If you are working on low power.
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dluu13 · Joined: 28 Sep 2018 Posts: 395 Location: Toronto, ON

If you're using a small motor then I would consider MOSFETs because they take less power to run than relays if that's a concern.

newguy · Joined: 24 Jun 2004 Posts: 1909

In either case ensure you have good quality/properly rated flyback diodes. A snubber might be required as well.

dluu13 · Joined: 28 Sep 2018 Posts: 395 Location: Toronto, ON

Ttelmah · Joined: 11 Mar 2010 Posts: 19541

Lets get this clear.
A half H bridge, only allows you to turn a motor on, disconnect it, or
apply braking to it.
With a relay, that is all.
With mosfets, and PWM, you can also do speed control.
Nothing else.
Is this all you need?.

Now with mosfets, first key thing is there is no isolation. The drive voltage
for the bottom fet has to share it's Vss rail with that of the control voltage.
For the upper fet, with a 5v motor, a P-channel type can be used, and
the same then applies with the Vdd. However for higher voltages, and
better efficiency, and N-channel fet can also be used, but then a voltage
rail above the supply has to be generated for the gate drive, and the drive
voltage then has to be isolated using an opto-coupler typically.
Also, because mosfets have extremely high input capacitances, switching
them at speed requires a lot of drive current, so high current drivers have
to be used, not just a direct output from the logic.
Properly done a mosfet bridge is a great way to go, but use a proper
driver IC designed for this. It saves a lot of problems....