TPS54628EVM-052: Snubber circuit 6 Zener diode

Shuji Ishiwata 

Shuji Ishiwata said:

(1) Is a snubber circuit necessary? In what cases is it necessary?

It is no populated on the EVM because it is generally not necessary, but the provisions for one are left in the layout.

A snubber circuit helps limit resonant ringing at the switching node between the MOSFETs and the Inductor, a number of factors can contribute to when one is needed.

1) Input voltage, especially input voltages near the maximum SW voltage since ringing on the switching node tends to have a higher peak with higher input voltages

2) Layout and VIN bypassing.  One of the primary sources of ringing energy is the parasitic inductance sourcing charge from the input bypass capacitors as the switch node is rising.  Good high-frequency, low inductance VIN bypassing will generally mitigate the need for a snubber.

3) Design margin derating.  Some users design processes require them to derate maximum allowable voltage on the switching node to 75 or 80% of the rated voltage.  The more design margin a user's design guidance requires, the more likely a snubber would be needed.

4) Noise requirements.  Some applications are sensitive to the high-frequency noise created by the switching node, either conducted to the output voltage, or radiated from the switching node.  These applications may require a snubber beyond what is needed to keep the switching node within its safe operating voltage range.

Shuji Ishiwata said:

(2) Is a Zener diode necessary?

No, the Zener diode is not required, but like the snubber, can improve noise performance by limiting the negative ringing on the switching node when turning off the main control FET.