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TPS40200-HT: Signal Before Inductor

Marius Raducanu
Intellectual 680 points
Part Number: TPS40200-HT
Other Parts Discussed in Thread: LM5145, , TPS40200, LM25085, LM5085

Hello,

I am using TPS40200HD in my design, 32-42V in, 5V out. It works good but its temperature is higher than I expected, 95F at room temperature. The signal before the inductor is not perfect rectangular - see the picture below (probe 1X10).

Is this the normal behavior or I can improve it, for instance adding a resistor to the pin 6 to decrease the MOSFET current?

  .  

Thanks,

Marius Raducanu

  • 0
    TI__Guru 60727 points

    Hi Marius,

    This looks like DCM. Try increasing the load current to see if it goes into CCM with a rectangular waveform on SW.

    In terms of temperature rise, this is a non-sync controller, so the efficiency is lower than a synchronous device (such as the LM5145). You may also be able to improve the inductor - take a look at the Coilcraft XGL6060 family for example.

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    Intellectual 680 points

    Hi Timothy,

    Thanks for your answer.

    The current is ~35mA at 32V. Even if the shape of the signal improves, increasing the current, it doesn't help me - I don't want to increase the power consumption. Maybe decreasing the frequency, increasing the duty-cycle, will improve the signal before inductor shape.  

    LM5145 is rated 150C and TPS40200-HT is rated 210C - I need at least 175C.

    I expect the most part of the heat to be on the inductor and external MOSFET. I suppose, the heat on the TPS40200 is created by the internal MOSFET driver (200mA). Maybe, reducing this current (adding a resistor) would decrease the heat. I am aware that decreasing the MOSFET driver current decreases the rising time that can increase the heat in the external MOSFET - I am looking for a trade-off. 

    Regards,

    Marius Raducanu 

  • 0 in reply to Marius Raducanu
    TI__Guru 60727 points

    Send on the schematic and I'll take a look. The FET and inductor are possible optimizations - what are you currently using? If Vin-max is 42V, you'll need a 60V p-channel FET (not higher).

  • 0 in reply to Timothy Hegarty
    Intellectual 680 points

    Hi Timothy,

    This is the schematic used in a prototype board. I was aware that the inductor is critical and I ordered a few types for testing. The initial inductor was 180uH, big enough to be able to decrease its inductance (number of turnings), now is 90uH.

    Thanks,

    Marius

     

  • 0 in reply to Marius Raducanu
    TI__Guru 60727 points

    okay, it will be interesting to see if a better inductor helps. Also, the FET could be improved. D2PAK devices are typically old, lossy devices -- try a 5 x 6mm package device with low Qg.

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    Intellectual 680 points

    I've chosen the bigger MOSFET package thinking that it will help with heat dissipation - system rated 175C. On the actual board, the MOSFET temperature is 87F, Inductor 85F, TPS40200-HT 89F in a 77F room temperature - maybe this is not so bad. What worried me most is that keeping my finger on the TPS40200-HT, the temperature increased in ~10s enough to be considered hot - maybe this is because the heat is not released when keeping the finger on the converter. 

    I will try other inductors and let you know.

    Regards,

    Marius

  • 0 in reply to Marius Raducanu
    TI__Guru 60727 points

    The MOSFET  and inductor are the main heat dissipating sources and are likely heating up the adjacent controller. The controller power dissipation is mainly the gate drive losses (=Qg*Fsw*Vcc), which is independent of the series gate resistor.

    Here is the FET datasheet: NP50P06KDG DS (renesas.com)

    The gate charge at 95nC at Vgs = -10V is huge -- I recommend finding a FET with lower Qg. Also, the Qgd at 26nC is way too high. Take a look at the p-channel FETs use in the LM5085 / LM25085 datasheets and EVMs as potential candidates.

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    Intellectual 680 points

    The Si7465 recommended in LM5085 has the Qg 26nC. The biggest constrain is the Maximum Operating Temperature 150C - I need 175C.

    I will order and test a few MOSFETs.

    Regards,

    Marius

  • 0 in reply to Marius Raducanu
    TI__Guru 60727 points

    Look for automotive MOSFETs as they are typically rated to 175.

  • 0 in reply to Timothy Hegarty
    Intellectual 680 points

    Found: IRF9Z14PBF, AOD407, NTBV5605T4G, SPD18P06PGBTMA1 (the last 3 are easy to test, it has the same original footprint/package).

    Regards,

    Marius

  • +1 in reply to Marius Raducanu
    TI__Guru 60727 points

    Sounds good, Marius. You don't need low Rdson given the duty cycle.

    Regards,

    Tim