Home > Circuits > MOSFET Bridge Driver
Modified:21:37, 22 October 2013
CIRCUIT DESCRIPTION

CIRCUIT DIAGRAM

CIRCUIT EXPLANATION

The circuit is a very common arrangement enabling bi-directional control of a motor without using any relays.The actual direction will depend on the way round the wires are connected to the motor, but below is an expanation of how the circuit works.

Basically the MOSFETs act like switches. Q3 and Q7 are P-types and connect the motor to the positive supply, while Q4 and Q8 are N-types connect the motor to 0v. Q1,2,5 and 6 buffer and drive the MOSFETs. D1-4 suppress any EMF from the motor and don't have to be as heavy duty as the ones shown - I just had a tube of them from somewhere.

There is an issue of "shoot-through" where both MOSFETs in a branch are partially on while the signal is changing. However it is very brief and both transistors are not fully on. For a couple of applications where this is fixed, see the pages below:

This circuit is not suitable for PWM since when both inputs are the same, braking effect is applied to the motor.

The MOSFETs used Q3,4,7,8 need to be high-current types 30A up with higher surge current of about 65A upwards. Also they should have a low "on" resistance, typically less than 0.02 Ohms, such as STP75NF75 for the N-type. The trouble is P-types tend to have higher "on" resistance so a modification I will publish soon will be to use all N-channel types.

IN DETAIL...

Forward
Reverse
Action
Low
Low
Stop - with braking
High
Low
Forward
Low
High
Reverse
High
High
Stop - with braking

With both inputs low, Q4 and Q8 are on (via R4 and R5 respectively) connecting both ends of the motor to 0v, effectively braking the motor and opposing any EMF generated by it. This is known as "Regenerative Braking"

When a signal greater than about 1 volt is appled to "Forward" input, Q1 and Q2 conduct switching Q3 on and Q4 off. This connects the left side of the motor to positive. Since the "Reverse" input is low, Q8 is still conducting, connecting the other side of the motor to 0v thus turning it in one direction (we will assume this is forwards).

When a signal is applied to "Reverse", the opposite happens. Q5 and A6 conduct, switching Q7 on and Q8 off connecting the other side of the motor to positive. With "Forward" low, Q4 is connecting the left side to 0v thus turning the motor in the opposite direction.

If both inputs are on, then Q1 to Q4 are conducting turning Q3 and Q7 to positive, which shorts out the motor, applying a braking effect again.

It is a good idea to always suppress the motor with 3, 100nF-470nF capacitors. One across the connections and then one from each connector to the case. Motors create some horrendous interferance!
WARNING - The heatsink tabs of the TO220 MOSFETs and Diodes are NOT isolated and are connected to one of the terminals of each device. Take care adding any heatsink to them and ensure that any metal heatsink applied is insulated from all the tabs.
SIMPLIFIED EXAMPLE

Below is a simplified explanation of how the bridge driver works.
NOTE: The inputs are opposite of above as the inverters have been removed.

FORWARD CURRENT FLOW

REVERSE CURRENT FLOW

STOP CONDUCTION

Written by Phil Townshend 2010
Back to Circuits page...

www.edutek.ltd.uk - Working Electronics For Students & Teachers