Stabilising an ON/OFF controlled SMPS (TNY287 flyback) that has low output capacitance
Hello,
We designed and layed out a 10W, offline, isolated Flyback converter using the TNY287 controller as in the attached schematic. We were not allowed to use any electrolytic capacitors. We only have room for 30uF of ceramic capacitance on the output. The output has a 4V pkpk oscillation on it at some 4kHz…but this is solved when we use the RC network R4 and C3 as in the schematic.
TNY287 is an ON/OFF controller, and the manufacturers recommend high levels of electrolytic capacitance on the output…in our case, the manufacturers software program (PI Expert) recommends 250uF of electrolytic capacitance….but we are just not allowed to use that…and we can only fit 30uF on the board.
Anyway, I was wondering if we could alternatively avoid R4 and C3, and instead achieve “stability” by having a capacitor connected from cathode to anode of the TL431 (U17)?
(I use the term stability with some reserve, because I realise that true “stability” isn’t on the menu for an ON/OFF converter like this.)
Files
| Attachment | Size |
|---|---|
| TNY287_Flyback_SMPS.pdf | 48.76 KB |
Hi treez,
Thank you for using PI product on you design. The stability issue you saw was probably because of very low ESR characteristic of a ceramic cap. Putting a capacitor between anode and cathode will not have same effect as with the RC phase-lead network. Maybe you can try to add a series resistors (lower value) on each of the output capacitors to add an ESR. Also using only 30uF instead of the recommended 250uF would mean large overshoot and undershoot during load transient conditions. Thank you.
Best Regards,
PI-SevenofNine