TPS23730: PoE PD Schematic Review Guidelines

Part Number: TPS23730
Other Parts Discussed in Thread: TIDA-050045

Hi
According to the PoE PD Schematic Review Guidelines, I have some questions I cannot understand.

1.) according to this figure, why need to add the C33 capacitor, and how to calculate this value?

2.) What the function of C39 capacitor, I cannot understand

3.) Do you have some material talking about how to design the snubber and clamp, when I reference this guideline, I don't know how to calculate the value.

over 3 years ago

0 Michael Pahl over 3 years ago

TI__Mastermind 21320 points

Hello,

1. Yes this capacitor is required for active clamp topologies. Because the switching frequency is the same, you do not need to change it. More information on how to select the clamp capacitor is in Section 4.3.2 of the following application note: https://www.ti.com/lit/an/slua535a/slua535a.pdf?ts=1626197351758&ref_url=https%253A%252F%252Fwww.google.com%252F

2. C39 is not in the second picture you showed, but it is on the PFET of the active clamp MOSFET. It helps with the timing of the PFET.

3. This guide is not a design guide but a schematic review guide. The paper above is a paper on how to design an active clamp forward. The following blog explains how to design RC snubbers. https://e2e.ti.com/blogs_/b/powerhouse/posts/calculate-an-r-c-snubber-in-seven-steps

My advice is to directly copy the circuit if it is applicable to your design. Please note we also have a 5V/10A version of this design called TIDA-050045

If this post answers your question, please indicate so by marking this thread as resolved. Thank you.

Regards,

Michael P.

Applications Engineer

Texas Instruments

0 J over 3 years ago in reply to Michael Pahl

Expert 1035 points

Hi
1. If I want to know the ACF topologies principle where I can refer.
2. Sorry for the Q2, the figure is the same as Question 1, the C39 is series connect to the Q10, my question is what's the function of C39 and how it works to drive the MOSFET.

+1 Michael Pahl over 3 years ago in reply to J

TI__Mastermind 21320 points

Hey Jack,

1. The paper from my last post: https://www.ti.com/lit/an/slua535a/slua535a.pdf?ts=1626197351758&ref_url=https%253A%252F%252Fwww.google.com%252F

2. The following paper is extensive guide to gate drive circuits: https://www.ti.com/lit/ml/slua618a/slua618a.pdf?ts=1618348067606&ref_url=https%253A%252F%252Fwww.google.com%252F

This cap is also discussed in the first paper as "Cuax". Check equation 48.