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active clamped forward converter using LM5025C

Siddartha koganti
Prodigy 70 points
Other Parts Discussed in Thread: LM5025C

I have a query regarding LM5025C IC
Topology-Isolated, (Active clamp forward converter)
Input voltage- 18-75VDC
Output- 3.3V/12A
We used transistor based linear regulator for powering IC(15V). Our input voltage is 18-75VDC. When we supply more than 30VDC the linear regulator is getting heated up. What we observed is the LM5025IC is taking 55mA supply current

 

Let us know why VCC is going in to current limit mode(55mA)? or will it take 55mA supply current?

Siddartha

  • 0
    TI__Genius 9130 points
    Siddartha,

    The LM5025C abs max VDD is 16 V. Depending on the components and tolerances of your linear regulator cct, the nominal 15-V VDD level at the IC may be reaching or exceeding 16 V max. Especially the zener on the pass transistor base will have a positive TC, so the voltage will rise as it gets hotter.

    I would recommend changing the linear regulator design for more like 11-12 V typical, and analyse the tolerance of your cct to make sure that the worst case VDD level is < 16 V.

    Thanks,
    Bernard
  • 0 in reply to Bernard Keogh
    Prodigy 70 points
    HI bernard,

    Thanks for the reply. We observe degradation of the IC when operated at full load and minimum voltage very near to the UVLO voltage. It is drawing high current upto 500mA. and it is going to thermal shutdown. Can you please help with the possible cases of the IC failure
    Thank you
  • 0 in reply to Siddartha koganti
    TI__Genius 9130 points
    Siddartha

    Can you share more details, like your schematic? I really need to see more detail to be able to help. If you want to send to me separately, my email is bernardkeogh@ti.com.

    The 55 mA figure above is very familiar, this is the current limit of the internal VCC linear regulator.

    Are you using the internal linear regulator? You mentioned a transistor-based external linear reg, if you are doing this instead of the internal regulator, then VIN & VCC pins must be shorted together, and the external VCC level must be > 8 V, but also < 15 V.

    Do you supply the VCC from an aux winding after startup, or is the linear reg always in use?

    In your latest post, you say "minimum voltage very near to the UVLO voltage" - do you mean min voltage and UVLO level of the power converter, or the IC VCC level? If the linear reg cannot maintain VCC > 8 V at min input voltage to the power converter, this could cause the internal regulator to enable and pass current, especially if the VIN pin is not shorted to VCC.

    Thanks,
    Bernard
  • 0 in reply to Bernard Keogh
    TI__Genius 9130 points
    Siddartha,

    Based on an offline sch review, here are further comments to add to the above.

    If an external LDO is to be used instead of the internal one, the LM5025C datasheet recommends that VIN & VCC should be tied together, and then to the output of the external LDO (see page 10 “High Voltage Startup Regulator”). If you were to use both the internal LDO & an external LDO feeding VCC, maybe there could be issues where the ext LDO sinks current during startup and acts as a load on VCC, triggering it’s 55 mA current limit, maybe clamping VCC too low, and thereafter preventing either regulator from letting go?


    If you want to supply the bias power more efficiently from a transformer aux bias winding, the winding voltage must be rectified in the same way as the main output – 2 rectifying diodes (1 fwd and 1 free-wheel) plus an L-C filter. This is more complexity and components, however, it will be far more efficient, it will allow you use the LM5025C internal LDO just for startup, and the aux rail should have good enough regulation to not require any external LDO at all.


    Another possibly simpler alternative would be to add an aux winding to the output inductor, this can then be peak rectified with a single diode into a cap to get a rail that should be pretty well proportional to Vout, with the second diode or the filter L. This is shown in Fig. 1 and in the application sch on page 14 of the datasheet.

    I hope this helps to resolve your issues.

    Thanks,
    Bernard