PMP12027 Rev A UCC28740 Demo board questions

Hello Simon,

I have requested a copy of the transformer datasheet from the PMP12027 design engineer, and will attach it to a subsequent reply if it can be made available.

There is a fast way to change the output voltage of the PMP12027, but it will not be an optimal design for 36 V/0.5 A. Referring to the schematic diagram at zone C5, R11 and R13 control the feedback divider which sets the output voltage (currently set for 48 V). For 36 V output, reduce the effective value of R11 to 73.6K by adding 280K in parallel with the existing 100K at R11. This is not enough, however.

Also reduce the value of D201 from 33 V to 20 V by removing the 33-V Zener diode and replacing it with a 20-V Zener of the same family or an equivalent family of zeners. (Actually, you can also simply add the 20-V Zener in parallel with the existing part at D201). This is necessary to ensure that the opto-coupler and TL431 receive the same bias current with a lower 36-V output as when Vout is 48 V.

Next, the existing secondary-to-auxiliary turns ratio (4:1) is design to provide ~12 V to the UCC28740 controller when Vout = 48 V. At 36 V, VDD may be less than 9 V which is very close to the UVLO threshold (7.8V) of the controller. Although it may be possible to modify the transformer to increase the auxiliary-winding turns, this is an operation that I do not recommend for a newbie to undertake. Instead, increase the amount of VDD capacitance at C200 (zone C2) by adding a 33uF 25~50-V aluminum electrolytic capacitor in parallel. This will slow down start-up time slightly, but the extra cap should provide lower ripple voltage and longer hold-up time during transient load steps to avoid tripping the UVLO shutdown threshold.

These three changes should give you 36 V at the output, but the PMP12027 design is based on delivering 30 W of output power and your application needs only 18 W. The primary-side current sensing is set for 30 W, so the simple voltage-setting changes will allow the board to deliver 30 W at 36 V, or 0.833 A before current limit. The transformer and output components may not be rated to carry this increased current indefinitely and may overheat if your load somehow exceeds 0.63A. To reduce this to ~0.5 A, the value of the current-sense resistor R7 (at zone C4) needs to be increased, which is not a simple parallel addition. Remove the existing R7 and replace it with a 0.68-ohm resistor of the same size. This should scale the input power limit down to the desired ~18-W level.

Output over-voltage protection is still set for a 48-V output, so to scale it down a 36-V level, you’ll need to change R5 (zone C2) from 25.5K up to 43.2K.

These modifications should result in 36 V at 0.5 A from the PMP12027 board, but the overall design is still excessive (based on 30 W, 48 V). The transformer inductance and turns-ratio are not appropriate for a 36-V output at 18-W maximum loading. A new transformer design will provide the best matching, size and cost for your application. The modified board can be used for evaluation purposes, but for production, I recommend to follow the design procedure in the UCC28740 datasheet application section to derive an optimal design for your voltage and power level.

Regards,
Ulrich

Hello Simon,

Here is the transformer datasheet that you requested.

G154069LF r 0.pdf

I hope the modifications work okay for you.  Performance may not be optimal, but you should be able to get reasonable functionality from it.

Regards,

Ulrich