# Using MOSFET Body diodes on an H-Bridge

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#### fastcomponents

##### Newbie level 5
Hi,

we're currently designing a servo controller for these motors that we've started getting made up https://www.fastcomponents.co.uk/product_info.php?products_id=56. The idea is to make a 'servo beater' for robotics applications, retailing at $15 -$20. Basic specs to include a variety of gearboxes (and nominal RPM outputs 40 ~ 250RPM), quadranture encoding, RSXXX, I2C, SPI, PWM, analogue, arbitrary (absolute & relative) positioning commands, arbitrary torque output commands, temperature overload control / feedback... etc.

To keep the cost down, I wondered if we could use the body diodes of the MOSFETs instead of using schottky diodes. Currently in our prototype we're using 2 P-channel (topside) FDFS2P103 (fairchild), and 2 N-channel (lowside) FDFS6N303 (fairchild). But in a S08 package they take a lot of space on the miniature board. Therefore ideally we could use some S023 MOSFETS, and use the body diode.

Quick specs - motor stall current is about 1.5A. Supply voltage is 4-6v, pwm switching is variable between 2 and 8khz to minimise audio noise approaching discontinuous operation, and switching loss at higher power levels.

I wonder if the body diode would be fast enough at these switching speeds?

Any ideas for alternative MOSFETS? Unit will be fitted with miniature heatsink to aid dissipation, and the aim is to mount the board on top of the motor casing (yes it is small!!!)

Thanks, Tom.

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#### scottieman

##### Full Member level 2
fastcomponents said:
Hi,

Quick specs - motor stall current is about 1.5A. Supply voltage is 4-6v, pwm switching is variable between 2 and 8khz to minimise audio noise approaching discontinuous operation, and switching loss at higher power levels.

First, I haven't checked the specification of the MOSFET (both N and P) you are using. Based on the switching frequency 2 - 8kHz you mentioned, the body diode should be relative fast enough, since in power electronics, we are normally operating the switching power converter in 20 - 100kHz.

Scottie

#### fastcomponents

##### Newbie level 5
Ok thanks . I was also looking at using a FDC6401N https://www.fairchildsemi.com/ds/FD/FDC6401N.pdf, since it's dual mosfets in a tiny SuperSOT-6 package

The drain-source diode for the above is rated at a max Is of 0.8A. Any ideas how one goes about determining whether this would be able to handle the reverse EMF spike from the motor?

Tom.

#### scottieman

##### Full Member level 2
fastcomponents said:
Ok thanks . I was also looking at using a FDC6401N https://www.fairchildsemi.com/ds/FD/FDC6401N.pdf, since it's dual mosfets in a tiny SuperSOT-6 package

The drain-source diode for the above is rated at a max Is of 0.8A. Any ideas how one goes about determining whether this would be able to handle the reverse EMF spike from the motor?

Tom.

If the max. current flow through your motor is more than 0.8A, than the diode associated with that MOS will have problem.

Scottie

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