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TPS23756: Internal hotswap MOSFET damaged? Primary 24VDC /Auxiliary source with PPD pin

Part Number: TPS23756
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.

  • Hi Juan,

    As far as the DEN pin, it should be pulled up like you currently have it since the DEN pin has other functionalities.

    In terms of the design, this is currently being looked into. An answer will be posted when available. Thanks!
  • Ok dear Darwin...

    I'll hope your analysis..

    PD.

    P2 is the input for auxiliary power supply

  • Hi Juan,

    I haven't forgotten about your issue! I am still looking at it & will get back to you soon.

    - Tom
  • Hi dear Tom...
    Thank you for your attention..
    I'll be waiting...
  • Hi,

    Today I power the primary source (24VDC) without load and It can output 12VDC for any secods,,, After this, the output was 0VDC again....

    If I power from auxiliary, I'ts start very good.

    What's happening?

  • My bet is that the internal hot-swap mosfet was damaged during this:
    ---------
    I started to add more capacitance (until 470uF) to stabilize the output voltage... but the TPS23756 begin to fail the start-up sequence randomly
    ---------

    I took a really quick look at your schematic, and I don't see anything jumping off the page at me that would cause a failure. The fact that it switches when powered by AUX almost certainly means that the internal fet is damaged.
    I would suggest replacing the part and doing further evaluation. My bet is that with the now reduced capacitance, you'll be fine.

    Ben
  • Hi be,

    Thank you very much for your repply.

    I'll replace it and after comment you, but, What is the guarantee about the safe circuit as current limit or thermal shutdown?

  • Hi Juan,

    I second Benjamin's suggestion.

    Can you provide further clarification to your last question?

    - Tom
  • Hi Dear Tom,

    Datasheet says:

    7.1 Overview

    ..."The TPS23754 and TPS23756 devices contain several protection features such as thermal shutdown, current
    limit foldback, and a robust 100-V internal switch."

    7.4.1.7 Start-Up and Converter Operation

    ... TPS23754 device enables the hotswap MOSFET with an
    approximately 140-mA (inrush) current limit as seen in Figure 24.

    ....Once the inrush current falls about 10% less than the
    inrush current limit, the PD current limit switches to the operational level (about 970 mA).

    7.4.1.8 PD Hotswap Operation

    .....The internal hotswap MOSFET is protected against output faults and input voltage steps with a current limit and
    deglitched (time-delay filtered) foldback.

    However... :

    7.4.1.5 Inrush and Start-Up


    802.3at has a start-up current and time limitation, providing type 2 PSE compatibility for type 1 PDs. A type 2
    PSE limits output current to from 400 mA to 450 mA for up to 75 ms after power up (applying 48 V to the PI) to
    mirror type 1 PSE functionality. The type 2 PSE supports higher output current after 75 ms. The TPS23754
    implements a 140-mA inrush current, which is compatible with all PSE types. A high-power PD must control its
    converter start-up peak and operational currents drawn to less than 400 mA for 80 ms. The TPS23754 device's
    internal soft-start permits control of the converter start-up; however, the application circuits must assure that their
    power draw does not cause the PD to exceed the current/time limitation. This requirement implicitly requires
    some form of powering down sections of the application circuits.

    Now, I think that It was the reason for internal hot swap mosfet can be damaged if any application demand more current/time at start-up. In my case, I demanded it through too much output capacitance.

  • Hi Juan,

    There are a couple comments I have regarding this application:

    If you're looking to start fresh and replace components, the components that damage the most will be the IC, the FETs and BJTs.

    Increasing the VC cap will increase the time to charge the bulk cap (internal regulator can get damaged).

    Increasing the output at 12V, you should be careful about shutdown. You should look at the VDS of the synch FET during shutdown and make sure it's not exceeding the abs max rating of the FET.

    Note the TI design you reference ORs after the PD controller (between VDD-RTN). Any particular reason, if PoE is not being used, that it was OR'd at RTN? The reason I say this is because the PD controller front end (VDD-VSS) was designed per the ramp rate of a IEEE compliant PSE. If hotplugging an adapter between VDD-VSS, you may need to increase the input cap (larger than your VDD-RTN) cap. Or you can try modding the board so that both your input OR in reference to RTN.

    For the inrush spec you refer to, the PD must finish charging the VDD-RTN capacitance before it can do normal current limit operation; otherwise, the PD will go into thermal cycling. The converter or the load should not be ON at initial startup so the PD can get out of the inrush current limit state. However, referencing the adapters to RTN will bypass this (as another consideration).

    Regards,
    Darwin