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Snubber capacitor to safeguard HiperPFS B(Vdss) of 530V?

Posted by: treez on Mon, 04/05/2021

Hi,
Page 18 of the HiperPFS Boost PFC controller datasheet says that a 33-100pF capacitor should be connected across the Boost PFC diode in order to prevent the internal FET’s B(Vdss) of 530V from being breached.

Page 18 says that the 530V level is most likely to be breached during heavy loading of the Boost output to almost the overload level.

How can such a capacitor be of any use? Surely it would just create more circuit noise due to high di/dt as such a snubber capacitor gets rapidly charged/discharged?

How could the drain of the internal FET go up to 530V?....As soon as the drain voltage goes above Vout then the Boost diode will clamp the drain voltage to vout…..which wouldn’t get anywhere near 530V.

The only way the drain could get up to 530V would be due to PCB trace stray inductance…but this should be minimised with tight layout and having a small value ceramic cap right at the internal boost diode cathode going down to the controller’s star ground point with the power ground.

HiperPFS datasheet:
https://ac-dc.power.com/sites/default/files/product-docs/hiperpfs-3_family_datasheet.pdf

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Comments

Submitted by PI-csbabu on Thu, 04/08/2021

Hi Teez,

I have answered this question previously. Incase you missed, please refer below.

Thank you for choosing PFS3 for your product. The overvoltage depends on how tight the high frequency loop. The patristic inductances at switching node ( Mosfet drain and diode Anode), at diode cathode to bulk capacitor positive and ground loop connecting Mosfet source to bulk capacitor negative influences the voltage spike at switching node. The integration of mosfet and diode into single package greatly reduces the parasitic but doesn't eliminate completely. So when current transfers from mosfet to diode, you will have high frequency ringing and the ringing spike goes up with increasing load current. As you described you need to add a high frequency capacitor to minimize loop parasitic inductance.
The integrated package has 530V rated Mosfet. The Mosfet turn-off spike due to transients can be snubbed by adding a capacitor across the diode.
If you are looking for higher voltage rated Mosfet, please consider using PFS4 which has 600V rated Mosfet.

Submitted by treez on Sat, 04/10/2021

Thanks, if you've got stray inductance in the circuit then adding a cap across the diode surely isnt going to help? The diode itself will clamp its anode voltage to 0.7v above its cathode voltage, so having a cap across the diode will not be of any use?

Submitted by PI-csbabu on Sat, 04/10/2021

Hi Treez,

As I mentioned layout parasitics play critical role along with PFC diode characteristics. If you have good layout, where you can minimize the loop area with bulk capacitor, no need of high frequency capacitor. Let assume you have a long trace from diode cathode to bulk positive and bulk negative to source node of mosfet, this contributes to additional inductance. During commutation period, the current is large, stores energy in the loop inductance and create a stress. If you have a capacitance closing the loop tightly, the current during short overlap period returns through the high frequency capacitor and avoids energy storage in stray inductance.

Submitted by treez on Mon, 05/03/2021

Thankyou for clarifying. I realise that you know that in reality, many of your customers are going to do poor layout, and so you suggest the capacitor across the diode...so as to help these customers who do poor layout practice. With good layout, i am sure you would agree, the capacitor across the diode is not wanted. But anyway, we would add an RC snubber across the diode. The "R" being needed to prevent sharp di/dt as the capacitor is charged/discharged every switching cycle.