In many of the schematics of the DC-DC stage of SMPSs, there is an external capacitor connected across the MOSFET switch (across drain and source). Why is this capacitor required?

MOSFETs have an inherent Cds, right? I found this capacitor especially in the primary side of the DC-DC stage (in PSFB and LLC).

What I think is:

  • To cancel out the effect of stray inductance (to avoid ringing effect).
  • To avoid a sudden change in voltage (stress dv/dt) across the MOSFET switch node (as the capacitor doesn't allow a sudden change in voltage).

What are your thoughts? Please share your knowledge regarding this.

I have attached a examples for your reference.

Thank you.

This is in PSFB Topology

  • \$\begingroup\$ If really connected across drain-source, I would guess it’s someone’s last resort to damp EMI, or bad design. But there could be some exception. \$\endgroup\$
    – winny
    Jan 20, 2022 at 14:57

2 Answers 2


My guess is that with high voltages if they are being switched synchronously by a pair of FETs, the lumped caps in parallel with the FET Coss helps to balance the voltage changes when switched off with an inductive load.


The capacitor catches and changes the harmful Instantaneous Fleeting voltage by slowing it down in a controlled manner, also bringing it more closer in phase with the slower amps released from the inductive coil windings. In my mind, this would be a clever way to harvest the lost BEMF and design something that can use the "bad" back EMF for a more useful benefit. By selecting the correct capacitor, you can slow down the voltage by 90Degrees, bringing it 100 % in phase with the Amps , in which case it will work exactly like a power factor correcting capacitor used in generators , making real power. Bear in mind, the REAL power will be negative though.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.