DRV8847: DRV8847 break down due to induced EMF

Part Number:DRV8847

Hi Team,

My customer has a issue as below: 

1. DRV8847 is used to drive a BDC and two air valve with independent H bridge as below.

2. When the system was power-off, using hand to rotate the BDC, it got 35V peak induced EMFand the device broke down (VM-GND impedance is 0) . test points were OUT1/OUT2 - GND, waveform as below.

3. so the question is does OUT1/OUT2 can only sustain maximum voltage @ 18V (which is the max stress of the power supply)? How we can solve this issue? 

Thanks a lot

Yang