Author Topic: H-bridge MOSFET question  (Read 552 times)

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Offline ErikYTopic starter

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H-bridge MOSFET question
« on: January 13, 2013, 09:05:21 AM »
I am trying to understand how H-bridges work and have been looking at a lot of different designs.

Most designs use p-ch MOSFETS for the high side and n-ch for the low side.

From what I understand, n-ch current flows from D to S when switched on, and p-ch current flows from S to D when switched on.

I get how each one is switched on, makes sense.

What I am struggling to understand is, when I look at H-bridge schematics, the motor terminals are connected between the D of the p and n mosfets.

That makes sense to me on the p-ch side, where current flows from S to D, but not on the n-ch side where current flows from D to S.

I would think that the motor would be between the D on the p ch and the S on the n ch.

Can anyone tell me what I am missing?

Offline jwatte

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Re: H-bridge MOSFET question
« Reply #1 on: January 13, 2013, 12:15:54 PM »
When switched on, current can flow *both ways* in a MOSFET. When switched off, the MOSFET has a body diode that prevents flow one way.

For the MOSFET to block current when not switched on, the Drain should point towards the load when a P-channel is a high-end switch and an N-channel is a low-end switch. This means that the diode is reversed (not conducting) for each of the devices.


Offline ErikYTopic starter

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Re: H-bridge MOSFET question
« Reply #2 on: January 13, 2013, 12:35:16 PM »
When switched on, current can flow *both ways* in a MOSFET. When switched off, the MOSFET has a body diode that prevents flow one way.

For the MOSFET to block current when not switched on, the Drain should point towards the load when a P-channel is a high-end switch and an N-channel is a low-end switch. This means that the diode is reversed (not conducting) for each of the devices.

Got it!  Thanks! Very helpful.

 


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