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Issues with TPS40060

vsc
Prodigy 45 points
Other Parts Discussed in Thread: TPS40060, TPS40061

Good Day to all gurus ,  We are in the process of developing a dc to dc convertor for an automobile application .

We are facing some issues like the following ;

The chip we use is TPS40060;

Basic Spec: Vin 24 V

Vout = 14 V; I out = 10 A;

1.      In case of No load, The output voltage fluctuates ie varies up to 14.7 ;?

2.      In case of light load ( up to 500 mA) the PWM wave shape is not normal ?

Can you please give me some first hand information’s regarding this problem

  • 0
    TI__Guru 56825 points

    VSC,

     

    The TPS40060 controller is a source only controller.  The Source Only function (decribed in the datasheet) results in diode emulation of the low-side MOSFET.  That is, when the TPS40060 controller senses zero inductor current during the LDRV (synchronous MOSFET driver) "ON" time, the low-side MOSFET is turned OFF to prevent negative inductor current.

    When this occurs the switch node rises to the output voltage and often "rings" as the parasitic inductances of the switch-node source traces react with the parasitic capacitance of the switching node.

    This causes the power stage to operate in discontinuous conduction at load currents below the critical conduction current (1/2 the nominal peak-peak inductor ripple).  This requires a lower than normal duty cycle, based on delivering an average current to the load.  This required duty cycle gets lower and lower as the load current falls.  This causes 1 possibilities:

    1) The control loop is unstable in discontinuous conduction.  This would likely cause a problem whenever the load current was less than 1/2 the peak to peak inductor current

    2) The required pulse width to maintain operation is less than the minimum controllable pulsewidth of 150ns, as such the TPS40060 turns on the HDRV driver (High-side MOSFET driver) for the minimum pulse width, delivers too much output current and then waits for the output voltage to fall and the control loop demands another HDRV pulse.  This causes very high ripple, a low switching frquency and an increase in average measured voltage. 

     

    One way to check which is the most likely canidate is to compare the problem current to the peak to peak ripple current of the inductor.

    Ipk-pk =1/L x (Vin - Vout) x Vout / Vin x 1/Fsw  - Since you havent provided the switching frequency or inductor value, I can't calculate this for you.  But if this comes to 1-2A, most likely you have a control loop that is unstable in discontinuous conduction mode and you need to modify your compensation.

    If it comes to a much higher current or you're running at very high frequency (500kHz or higher) it could be the latter and you'll want to look at the pulse width of the switch node (junction of the High-side Source, Low-side Drain and Inductor) at a load current just high enough to maintain stead operation and compare it to 200ns.

     

    You could also consider use the TPS40061 controller, which is a direct replacement for the TPS40060 other than it operates fully synchronous, allowing the power stage to both source current to the load and sink current from the load.  This Quadant 1 & 3 operation allows the control loop and pulse-width to be nearly the same at no load and full load.