Transformer DC offset

[bowman1710]

Hi Guys,

I'm looking at a design from TI, I noticed that they have used a cap and resistor on the transformer primary side, to eradicate any DC offset seen by the transformer.



Documents
https://www.ti.com/tool/PMP4435#Technical Documents

Schematic
https://www.ti.com/lit/df/tidrhw3/tidrhw3.pdf

My question is, the resistor is rated to 1/4W, with the currents seen on the primary, how can you have the resistor rated so low in power? Also whats the best way for calculating the optimum values for this?

Cheers

 

[Easy peasy]

how much voltage do you think the caps will have across them?

 

[dick_freebird]

The resistor is just a Q-killer (snubber) and the average
power it sees will be low at low-ish switching frequencies.
You do want to figure the power at some worst case load
and line condition and frequency, and satisfy yourself about
the rating. I'm pretty sure TI has done a thorough job of
disclaiming any and all responsibility for anything.

 

[kathmandu]

At the switching frequency, those two caps in paralel (20uF + 20uF) have a much lower impedance than the resistor itself thus all the tranformer primary current passes through capacitors not the resistor (during normal operation).

As @dick_freebird has mentioned above, the resistor's role is to act as a snubber, to discharge the capacitors and to create a DC path at startup.

 

[treez]

as above allure, those caps are dc blockers, and are designed to have as low as possible voltage drop, as you dont want to loose your precious primary voltage to the caps....the resistor stops them overly ringing.

 

[bowman1710]

Thanks for the reply guys it makes sense now looking at it