[FAQ] Whey are D1 and D2 pins of SN650x push-pull transformer driver shorted to GND?

D1 and D2 pins of SN6501, SN6505A, SN6505B and SN6505D are the switching nodes of the push-pull power supply solution. These pins are internally connected to open-drain power MOSFETs which drive an external center-tapped push-pull transformer to generate an isolated supply voltage. D1/D2 pins have an absolute maximum rating of 14V for SN6501 and 16V for SN6505A, SN6505B and SN6505D.

The following are the most common ways why D1/D2 pins are damaged and shorted to GND.

1. Input supply is noisy and contains transients exceeding 16V
   1. Including sufficient bulk cap at the primary-side center-tap of push-pull transformer makes sure such transients are suppressed.
      
      
2. High leakage inductance of transformer
   1. High leakage inductance of transformer forms an LC resonance with the body capacitance of power MOSFETs at D1/D2 pins of SN650x devices.
   2. This causes ringing in the switching waveform thereby exceeding the absolute maximum rating of D1/D2 pins of SN650x devices and causing permanent damage.
   3. Such ringing can be avoided by keeping the leakage inductance to <2%.
   4. If ringing can't be avoided then use of RC snubber circuits at D1/D2 pins can suppress the ringing and keep the voltage to expected levels.
   5. Please refer to the application below for more details on suppressing ringing using RC snubber.
      1. How to Reduce Emissions in Push-Pull Isolated Power Supplies
         
         
3. Non-optimal PCB layout
   1. Long PCB traces between D1/D2 and transformer pins or multiple via between them can add considerable parasitic inductance in the path of transformer primary.
   2. This results into the same effect as having high leakage inductance in transformer causing ringing at D1/D2 pins and permanent damage to SN650x.
   3. The PCB layout can be optimized by keeping the PCB traces between D1/D2 and transformer pins short and free from via.
   4. If the ringing can't be avoided then using of RC snubber at D1/D2 pins can suppress the ringing and avoid permanent damage.