Hi,
I have copied your TPS23754EVM-383 / slvu304c.pdf reference design for the TPS23754. Two boards have been built this far, and they behave the same way, they powered up -once- but never again.
The first time I powered up the boards I got 12V out. That initial test took perhaps 8s to establish the DC output voltage after plugging in injector input power. I left it on for just enough time to measure the DC voltage, then turned the injector off. The 12V output is NOT LOADED by anything but an LED in series with a 10k resistor.
While the board was on I didn't measure VC voltage, but did see the VC LED turning on. Then I repeated the test with the other board with the same result. After that I have not been able to power them up at all, not even after letting the boards rest over night. Has anybody had the same issue?
There are no obvious signs of component damage.
In my test setup I use the Injector "Planet POE-171A-60". I have previously used the same type of injector successfully with an unloaded TPS23754EVM-383 kit. The injector has a "Standard" / "Legacy" mode switch. Both work with the reference design.
Compared to the reference design I have made the following changes in my design:
- VDD = VDD1, there is no externa supply like the reference design's J1
- R9 = 63.4k, slightly different switching frequency saves a BOM line
- APD = grounded through 10R resistor
- PPD = floating
- C9 at rectifier DC replicated 4 times for symmetry in layout
- C14 = 39uF, up from 22uF due to availability of vibration proof caps
I will be able to compare my implementation and the reference kit side-by-side. I'd appreciate your input on which measurements or modifications to do in order to have my board start up each time.
Best regards,
Børge