PMP30700: PMP30700

Hi Samant,

We don't have a detailed design guide with all of the calculations. I have asked Roberto to help with questions you might have. Can you let us know what equations you are looking for? Also, I might suggest to try PowerStage Designer to help with your calculations.

Thanks,

Robert 

Hi Robert,

In addition to your notes, I would recommend Samant to deep dive into the internal structure of Flyback transformers, and specifically to our previous PSDS topic (which I believe being very interesting) here:

https://training.ti.com/flyback-transformer-design-considerations-efficiency-and-emi?keyMatch=FLYBACK%20TRANSFORMER%20DESIGN

Thanks,

Roberto

Thank you, Roberto, and Robert for reverting back.

1. I have issue with output capacitor selection design in PMP30700 design. 

2. How is crossover frequency is selected in this design for each output. Thumb rule is Fcross= (Frhpz/5).

3. Selection of secondary diodes 

These are some of the basic doubts. 

Samant,

The output capacitor selection of a multi-output Flyback converter follows the same rules of a single output Flyback: normally I calculate it by dividing the power over all outputs. Then calculate the capacitors based on voltage ripple, transient response and RMS current rating.

Regarding the crossover frequency, for an isolated DCM Flyback the limit comes from the optocoupler; typically I don't push Fco > 3 - 5 kHz, so maximum 5 kHz is fine. If it works in CCM (like in this case), then you are right and you have to consider the right half plane zero; you can calculate the position of this zero with the following formula:

RHPZ = R*(1-D)^2 / (2π*D*Lsec), where R is load resistance, D is duty cycle, Lsec is the secondary side transformer's inductance; this formula is mentioned in the following application report, page 19:

https://www.ti.com/lit/an/slva059b/slva059b.pdf?ts=1673530520939&ref_url=https%253A%252F%252Fwww.ti.com%252Fsitesearch%252Fen-us%252Fdocs%252Funiversalsearch.tsp%253FlangPref%253Den-US%2526searchTerm%253DUnderstanding%2BBuck-Boost%2BPower%2BStages%2526nr%253D1077

Now the rule of thumb is Frhpz / 5, but for more safety I would consider Frhpz / 10.

Diode selection:

Here you have to take into account voltage stress (Vin_max * Ns / Np + Vout + spike), average output current, selection of package (impacting thermal dissipation) and maximum peak current rating.

Thanks,

Roberto