Hello,
I have implemented a TPS23754 circuit according to SLUA593 and SLUA469 (Emi). The circuit is modified to perform 25W at 70 degress celsius, using auxillary input voltage as low as 18V. To lower peak currents at this particular scenario I am not using the POE 3000F transformer, but the Coilcraft JA4456-DL used in other TPS23754 application notes. Due to peak currents and E10 requirements additional capacitans and common mode filter is also added to the primary side and auxillary input.
Overall the circuit performs very well, stability is very good and ripple + noise is well within limits. The only issue is that at 18V auxillary input, I hear the transformer buzzing. I have traced this to be caused by switching cycles being cut off by the TPS controller (I believe the controller is current limiting the primiary switch), which occurs at a frequency which is audiable. Disabling the active clamp circuit fixes the problem (Either removing clamp capacitors or gate signal capacitor, or connecting DT to ground - equally good).
When in the failed situation I can meassure a 100mV drop on the output, this drop stays the same if I lower the input voltage to 16.5V or raise it to about 40V. Once the input voltage is raised to about 44V or above the controller resumes normal operation - output regulation is again 100% and audiable noise from transformer disappears. I have in addition tried the following, with no result:
- Filter the CS input
- Alter value of deadtime resistor
- Alter value of blanking resistor
- Change the TPS23754 to TPS23756
- Scope signals for noise, appears clean.
- Shield the heatspreading copper planes to verify against magnetic noise.
What is causing this behaviour and is there any way of working around it ?
I can send schematics and layout on request.
Thanks,
Rasmus