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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

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  • \$\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

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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.

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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.

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