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Design >> Analog Design >> How to design the freewheeling diode for a H-bridge motor driver?
https://designers-guide.org/forum/YaBB.pl?num=1357482670

Message started by Yutao Liu on Jan 6^th, 2013, 6:31am

Title: How to design the freewheeling diode for a H-bridge motor driver?
Post by Yutao Liu on Jan 6^th, 2013, 6:31am

Hello everyone,
It is said in some documents that the freewheeling diode (also called flyback diode) is useful to alleviate the damage from reverse pulse when a circuit is driving a inductive load.
I am designing a H-bridge motor driver, in which high-side devices are pmos and the low-side are nmos. I think the forward-biased parasitic diode in the pmos is able to conduct the pulse at the output of the driver. So the freewheeling diode is unnecessary. Do you agree with me?
If no, why not? And how to design a robust freewheeling diode for the H-bridge motor driver?
Thank you very much.
Yutao Liu

Title: Re: How to design the freewheeling diode for a H-bridge motor driver?
Post by loose-electron on Jan 12^th, 2013, 4:08pm

4 diodes are needed (across each of the 4 driver transistors)

If its a PCB level design use schottky devices so the path of current is thru the diodes and not thru the transistors. (slightly lower forward biased on voltage)

The 4 diodes limit your range of operation to a diode junction above-below the power-ground rails.

Plus a series RC snubber across the motor itself. (R equals motor winding resistance, C depends on application need)

H bridge drivers, since you reverse polarity, doesn't allow the easy use of a flyback diode.

Word of caution - watch the control of your top-bottom drivers to avoid shoot-thru, when top and bottom drivers are both on - on one side of the H-bridge. Great potential for a smoke show.

Title: Re: How to design the freewheeling diode for a H-bridge motor driver?
Post by sheldon on Jan 14^th, 2013, 3:25am

Yutao,

One additional comment, the reason for using Schottky diodes and
not the junction diodes of the devices is performance, the saturation
recovery time of the integrated pn junctions limits their ability to turn
off quickly.

Sheldon

Title: Re: How to design the freewheeling diode for a H-bridge motor driver?
Post by loose-electron on Jan 16^th, 2013, 8:21pm

sheldon wrote on Jan 14^th, 2013, 3:25am:

Good point - motor drivers tend to be pretty slow things however.

One other reason for using external diodes is that often the characteristics of the transistor body diodes is often not well specified.