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# [FAQ] TPS23731: Vout setting on diode rectification and primary-side regulation (affect by VFCP)

Part Number: TPS23731

I considering TPS23731 with diode rectification and primary-side regulation, referenced by answered below E2E thread.

https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1169390/tps23731-cp-pin-operation-on-diode-rectification-and-primary-side-regulation

About Vout setting, normally calculated results become a few gap.

I guess these reason is body diode (~0.7 V) could influence the feedback loop by above thread.

Please let me confirm about Vout setting, is below equation method correct?

Based on above schematic (Name for VFCP changed to VCP on this condition),

・Va setting is;

・In this case when VCP (=VFCP on datasheet) determine 0.7V, AUX voltage: Vx is equation (3);

・VFB will be regulated by VREFC (=1.75V), VREFC is equation (4);

・Vout equation is below by equation (1)~(4), is these method of calculation (how to use VCP) correct?

Best regards,

Satoshi

• Hi Satoshi,

Thanks for your calculation. Seems you missed the Vf. Vf could be 0.3 - 0. 7 V normally according to the diode's I-V curve.

There are two main ideas to use the CP pin:

1. As your calculation with PSR + DIODE, for eliminating the forward voltage drop of the secondary side diode by involving the body diode at the primary side feedback loop calculation.

2. Second difference of PSR + SYNC MOSFET and PSR + DIODE is at the discontinuous mode (DCM). The coupled inductance’s current can be reversed with PSR + SYNC MOSFET. PSR SYNC + MOSFET does not have DCM, the CP’s MOSFET can be turned on anytime when PRI’s MOSFET is off. But PSR + DIODE has DCM, the CP’s MOSFET cannot be turned on during discontinues period when PRI’s MOSFET is off since that will discharge the feedback capacitor (C_fb).

Best regards,

Diang

• Hi Diang,

Thank you for kind reply,

In this case, I think that equation (1) is already involving Vf and Vout1 calculation is not need Vf, is it correct?

(Also Vx and VREFC are involving Vf, too.)

Or is these case need to add "-Vf" on equation for Vout1?

Best regards,

Satoshi

• Hi Satoshi,

Please see the derivation below:

Best regards,

Diang

• Hi Satoshi,

I will close this thread for now. Please open a new thread if you have any further questions.

Best regards,

Diang

• Hi Diang

Sorry for my reply delay,

Our question was cleared by your answer.

Thank you very much.

Best regards,

Satoshi