Question about H bridge catch diodes

Discussion in 'The Projects Forum' started by pityukecske, Jun 7, 2013.

  1. pityukecske

    Thread Starter New Member

    Nov 6, 2008
    Hi everyone,

    While working on a motor controller, i experienced a very nasty thing:
    I am using a buck-boost converter to stabilize the supply voltage for my boards logic part. The buck-boost can take a maximum of 12V on its input, i configured it to have 3.3V on its output. Now, i have a H bridge, that has separate power supply for the logic, and for the load, i connected the load power supply pin to a 760mAh & 9.6V battery, and the logic power supply pin to the output of the buck-boost.
    When i start up my motor, the buck-boost immediately dies off, and i can't figure out why. No components are damaged on the 3.3V side of the circuit, the buck-boost internal FET-s and flyback diodes are all ok, it's the logic that dies off.
    Since this happens at the moment, when i turn my motor ON, i suspected some sort of transient behavior causes the problem. Is it possible, that the voltage spikes snubbed by the diode charge my battery up bit by bit, and the voltage rises above the voltage limit of the dc-dc converter? When i charge my battery up, it has ~10.8V on its terminals.

    Any ideas on what's happening are welcome.


  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    What the... can't follow a thing you said.

    Got a schematic? Here even a block diagram may well do.
  3. pityukecske

    Thread Starter New Member

    Nov 6, 2008
    I attached a simple diagram i just created, i doubt it will be of help, but at least you see what i'm up to. The two modules marked with red dot get power from an internal regulator inside the TPS63030. If they don't, switching doesn't work anymore, and the output is somewhere at 0.03V.
    I realized, that the regulator is the part, that is causing me trouble, through my measurements. Since no components were damaged on the Vout side of the regulator, i suspect problems are on the Vin side. The datasheet states, that a maximum of 12V is allowed on the Vin pin. After i recharged the 9.6V battery, i measured ~11V across its terminals.
    I also know, that my h bridge has n mosfets inside, which have catch diodes on them. So, if there are voltage kickbacks from the motor, there might be spikes of 11V + forward voltage of these diodes ~0.7V. This is scary close to my 12V limit, and i was asking for opinions on this.
    Is it possible, that this 11.7V spike killed the internal regulator of the buck-boost?

    I hope it's clearer, if not, please ask. Thanks!