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DC-DC converter with TPS40055 high drop voltage

Nico Aprile
Prodigy 30 points
Other Parts Discussed in Thread: TPS40055

I made the attached diagram that uses the switching regulator TPS40055. The features of the converter are the following: Vin = 18V to 32V; Vout = 13V; Iout MAX = 20A. Without load connected, the output voltage at the pin "+12V1" is 13V but with 10A load current, (half of the maximum load), the Vout drops to 10.5V.

My question is the following: It is normal that the voltage drops so much?

Many thanks

  • 0
    Prodigy 30 points
    I resolved my problem removing the feedback network D4-C23-R7-R8-R9. In this condition, the pin named "+12V1" has the following voltage value:

    13.5 V @ Iout = 0 A
    13.0 V @ Iout = 11 A
  • 0 in reply to Nico Aprile
    Prodigy 30 points
    Now, I have another problem. There is an excessive heat production on the mosfet Q1-Q3 (high side) and Q2-Q4 (low side). Without connect any metal heat sink, with Vin = 32V and Iout = 11A, the pair Q1-Q3 and Q2-Q4, reach the following temperatures:

    190°C for Q1-Q3
    150°C for Q2-Q4

    The Q1-Q3 mosfet have the following features:

    Producer: FAIRCHILD
    Model: FDB070AN06A0
    RDS(on) = 6.1 mΩ ( Typ.) @ VGS = 10 V, ID = 80 A
    Qg(tot) = 51 nC ( Typ.) @ VGS = 10 V

    The Q2-Q4 mosfet have the following features:

    Producer: FAIRCHILD
    Model: FDB52N20
    RDS(on) = 49 mΩ (Max.) @ VGS = 10 V, ID = 26 A
    Qg(tot) = 49 nC ( Typ.) @ VGS = 10 V

    I try to decrease the switching frequency from 300kHz to 200kHz and the temperatures (without connect any metal heat sink) are the following

    160°C for Q1-Q3
    115°C for Q2-Q4

    Anyone has any suggestions to further improve the heat dissipation (apart from the fact of using a metal heat sink)? Maybe use the faster mosfet?

    Many tanks