Watch the supply to the Op-Amps - make sure it NEVER goes beyond the maximum device rating. For example, if your product is an idustrial device connected to 24vAC... many transformers have huge winding variations - I have seen transformers marked 24vAC producing 36v when only lightly loaded, you the recify this and have over 50vDC, way beyond the max rating limit of most Op-Amps, 78L05 and 7812 voltage regulators and the like. Many people forget this. Over-voltage on the supply is THE most common cause of Op-Amp fry-ups.

Another reason is short circuit outputs. If you have a PIC, then sample the output back from the op-amp thru a voltage divider chain to a spare ADC. If you have a short, then switch-off the PWM drive until the short has been removed.

Melanie