Other Parts Discussed in Thread: TPS23754
Hi. I needed a solution for DC:DC converter with primary and secondary power supply source widh wide input voltage (from 8Vdc if was possible to ~55Vdc) and fixed isolated output 12V/ ~1.5A. I was researching and find the TPS23756. I studied the IC and built my circuit with the following requirements:
a) Primary (priority) power suply in VDD/Vss = 24VDC (from fixed Regulated or Switching power supply). I don't want use any PSE/POE 802.3at type 1/2.
b) Auxiliary(backup) variable power supply in VDD1/GND = 10.8VDC- 48VDC
c) Isolated Vout = 12VDC/1.25A.
I based my design from TIDU190 + SLVA306A. The changes that I introduced was:
- I used PPD pin for disable the classification. VDD/Vss is 24VDC. Initial PPD pin Resistor divider was Rppd1=30K / Rppd2=3K
- Although the PPD pin inhibits class current, I built the circuit with CLS = 63.4ohm and DEN=24.9K to VDD. Please, can you clarify this point because the datasheet don't says nothing about CLS/DEN in this case.
My circuit was working with primary 24VDC and 10.8VDC<->48VDC for auxiliary. I been testing the switches between the sources to obtain smooth changes without voltage drop in the changes of sources. I started to add more capacitance (until 470uF) to stabilize the output voltage... but the TPS23756 begin to fail the start-up sequence randomly, problably it required much current to charge the efective extra capacitance added... then I reduced to 220uf in my test with good results.
Additionaly I made a little changes :
- I added more capacitance (22uf/25VDC) to Vc .
- I added 47uf/100Vdc to the input Primary power to improve de input ripple.
With theses changes I was working for 1 month. After 3 months of inactivity I powered the circuit with primary power only and I observed that for a time the circuit is turning off and I could not turn on again after a while. So It was for several days...
I was checking the soldering and clean the circuit. I changed the divider to Rppd1=50K / Rppd2=6K and this work ok for 1 hour aprox.
After this, the converter shut down and did not turn on again.
Voltage Test:
Vdd/Vss =24VDC
PPD > 1.55VDC
Vc = 0VTPS23756_2017.pdf
If I power the auxiliary, it's works fine!
Questions:
1) Is posible that the internal hotswap mosfet is damaged although the arquitecture of this is designed to support thermal shutdown/short circuit protection/etc ...or exits any error in my circuit?
2) If I replace by a new TPS23756, How does it my guarantee that the IC works fine for a long time. What must be the caution to take in the future?
Thank you very much in advance.