H-Bridge Instant Death

I have a DRV8840 that I am trying to use. I have burned up about 10 of them trying to get it working.

For this application, I need the H-Bridge to Current Limit and Brake the motor. Max current draw can be no more than 2A and it needs to stop very quickly

I have gone through a couple revisions, but always the H-Bridge blows up.

Here are the motor details:

24V 2A drawn Brake enabled (called slow decay in the datasheet)

The voltage applied to this motor flips polarity, so I have a diode bridge in between imcoming power and the H-Bridge

Here is my schematic

Brown and white is incoming power to feed the H-Bridge. Brown and white switch polarity and the diodes prevent the H-Bridge itself from having a reverse polarity applied

Red and Yellow are the motor outputs

Some notes on the diodes:

(I'm new to eagle and didn't really know how to create new components)

D1-D4 = Schottky Diodes (I used these for less forward voltage drop)

I don't use the built in 3.3V voltage regulator but instead use a voltage divider and use the reference implentation on page 8 of the DRV8840 datasheet

Again, the only reason I need the H-Bridge is to brake the motor and to limit the current. Maybe I am going about this the wrong way, but I still do not understand why the H-Bridge dies so fast. Thanks for your help!

• What do you mean by "The voltage applied to this motor flips polarity"? An H-Bridge is by definition designed to be able to drive a motor in both directions.
– mng
Aug 26, 2014 at 18:12
• @mng What I mean is the inputs labeled brown and white (far right of schematic) change polarity. (Sometimes brown is +24V and white is 0V, sometimes white is +24V and brown is 0V)
– Dan
Aug 26, 2014 at 18:40
• I think you're going to have to describe your power waveform in more detail.
– mng
Aug 26, 2014 at 18:57

So this circuit sits between some power source and the motor; and all it has to do is limit the current to 2A and short-circuit the motor when the power source is turned off?

Your schematic is not very clear, most pins are not connected. The chip probably ceases to function properly the moment 24V goes away.

A circuit like posted in here http://matterwiring.blogspot.nl/2013/07/current-limiter-circuit-using-two.html can limit the current; combine it with the lower part of the circuit described here http://www.edn.com/design/analog/4342289/Circuit-forms-dc-motor-switch-with-brake

• I did not think about your second paragraph very well, thank for your input. I will go back to the drawing board. Thanks for your links, I am trying to read and understand them now
– Dan
Aug 28, 2014 at 6:27

You have to understand that when you brake a motor, that mechanical energy has to go somewhere. If you brake by shorting the terminals, it is dissipated in the winding resistance. If you brake by applying a voltage lower than the back emf voltage, or by applying a reverse voltage, that energy is forced back into the supply. If there is no way for the energy to return, the bus voltage (your Vm and Ground) will rise to the point where it destroys your bridge. Typically, in this kind of application, a circuit is used to switch a big resistor across the bus when the voltage exceeds a predetermined level.

• The back emf cannot generate a voltage greater than the supply that caused the motor to rotate -- that alone cannot damage the controller. Shorting the input supply can cause very large currents to flow backwards through the H-bridge as it brakes the motor. Dec 24, 2015 at 16:33

The driver chip might not be able to handle short high currents when ie breaking: the voltage created by the indution to mainatin the current might be to high for your driver when using may be a lower speed protection diode parallel to the motor

Have a look at this chip

http://www.st.com/st-web-ui/static/active/en/resource/technical/document/datasheet/CD00000240.pdf

and it might give you a couple of hints understanding the braking process.