UCC28740: Help review our customer's UCC28740 schematic

Vio

Here are some comments on the schematic. I assume that the customer will use the Excel design calculator (www.ti.com/.../sluc487) or the datasheet equations to calculate the required transformer parameter and component values.

Since I don't know the transformer inductance or turns ratio, I cannot comment on all values. I can only comment where I see specific issues, but all component values will have to be checked by the customer.


- What is the component labelled PTC1? Is this a fuse, or input inrush limiter? Usually an NTC is used for inrush limiting, so that it warms up when load is applied, and the drop in resistance will reduce the power dissipation.

- There does not seem to be a fuse. An input fuse or fusible resistor is normally used to pass safety testing (safety testing can short or open any component on the board, and it has to self-protect with no smoke or flame).

- What is the input voltage range? 220 V is shown, is this design for narrow-input high-line only, or does it also have to operate down to 100/110-V ac?

- An EMI filter will be needed.

- Bulk cap C4 seems to be much bigger than required for a 10-W design - especially if the input voltage is high-line only. Normally for 10-W design, 20-22 uF is sufficient.

- The snubber/clamp R2/D2/D1/R1/C5 seems to be an RCD clamp + TVS/Zener clamp together – only 1 should be used, either RCD or TVS. TVS clamp will give better eff at light load.

- What device is used for Q1? The symbol looks like an IGBT, but I cannot find any reference for the part number NDF6N60ZH.

- Resistors R10/R11 value seem low for a 10-W design, but the value depends on the turns ratio of T1.

- As shown in the datasheet and EVM525, a cap in parallel with R17 might be required for the loop compensation.

- The RC snubbers R3-C6 and R12-C11 will need to be tuned, depending on the diode capacitance and the winding leakage inductance.

- Pre-load resistors R4 & R14 are far too small, they will dissipate a lot of power. They should be much larger in value. R14 is probably not required since the feedback TL431 circuit will always take some load from the 5-V output.

- The feedback network across the TL431 REF-CATHODE is connected to the wrong place - the right side of C17 & C16 should be connected to the junction of R20/R21, not to the top of R21.

- What is the purpose of R18? It looks like this is directly across the TL431 REF-CATHODE, I think this is in the wrong place.


If this addresses the question/request, please click the "verify answer" button.

Thanks,
Bernard