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LM25116: How to choose a MOSFET

nnn sato
Prodigy 170 points
Part Number: LM25116

I was designing and evaluating it, but the following symptoms occurred.

Design specification:
VIN=14V,Vout=10V
f=100kHz, Iout=3.5A

1. Voltage does not come out correctly when there is no load.
2. When the motor was connected and a load was applied to the motor, and the load current went from about 1A to 2A, something like a ripple was seen in the output waveform, which was higher than the normal voltage.

I had put a 9.1Ω resistor in the gate of the MOSFET (IRFH5300), but when I shorted it, "1" improved.
It seemed to be related to MOSFET.

So my question is,
1.Has the maximum operating capacity of the external MOSFET been determined?
If it is determined, how can I find it from the data sheet?

2.Isn't it usually used to put a resistor in the gate?
However, this seems to exceed the output current capacity of the gate driver on the power supply IC side.

3.Are Forced HO off-time and Minimum HO on-time of PWM COMPARATORS the same even if the Frequency of Operation changes?
Also, does this mean that I have to use a MOSFET that can be turned on with Minimum HO on-time (100ns) and a MOSFET that can be turned off with Forced HO off-time?

  • 0
    TI__Mastermind 49671 points

    Hello nnn 

    1. The device cannot drive MOSFETs if Qgate x Fsw x N is greater than VCC_current_limit (where N is the number of MOSFETs) 

    2. Gate resistor is commonly used in order to minimize EMI. Please double-check that the dead-time is enough after adding the gate resistor. 

    3. Forced HO off-time and Minimum HO on-time are not affected by switching frequency. Yes, the device should be able to switch on and off MOSFETs within the forced HO off-time and the minimum HO on-time limit. 

    -EL

  • 0 in reply to EricLee
    Prodigy 170 points

    Even if I don't put a resistor in the gate, won't the IC be damaged if it is operated for a long time?

  • 0 in reply to nnn sato
    TI__Mastermind 49671 points

    Hello nnn 

    If you want more investigation, please share the schematic and send me the completed quick start for my review,  

    Also, please send some waveforms including SW and inductor current.

    -EL

  • 0 in reply to EricLee
    Prodigy 170 points

    Regarding the circuit, it seems that the Qg of the MOSFET is too large, so I will review it.
    What I wanted to ask you about in the second question was not about the circuit I designed, but about the theoretical way of thinking when designing it.

    I would like to change the way I ask my question.
    Looking at the circuit diagram of the evaluation board, there is no resistor in the gate of the MOSFET, but will the power supply IC continue to operate for a long time without being damaged?
    I think that an excessive current flows instantaneously to charge the Qg of the gate, but if it does not break, what is the theory?

  • 0 in reply to nnn sato
    TI__Mastermind 49671 points

    Hello nnn

    Looking at the circuit diagram of the evaluation board, there is no resistor in the gate of the MOSFET, but will the power supply IC continue to operate for a long time without being damaged?==> Yes, there is no problem running for a long time without the gate resistors. 

    I think that an excessive current flows instantaneously to charge the Qg of the gate, but if it does not break, what is the theory?  ==> We designed the device to handle the peak current when charging the MOSFET gate capacitance. 

    -EL