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TPS40210 instability in light load

treez
Intellectual 970 points
Other Parts Discussed in Thread: TPS40210

Hello,

We are doing  TPS40210 boost converter.

Vin = 12V

Vout = 19V

Iout(max) = 1.62A

F(sw) = 500KHz (Sync'd with diode method)

C(in)  = 20uF ceramic

C(out) = 20uF ceramic, then 1.5uH series inductor, then 10uF ceramic capacitor.

Current sense resistor =  10mR

Upper feedback divider resistor = 61.9K

RC series network connected round error amplifier = 4.99K, 22nF

(a 47pF ceramic cap is across this RC series network)

Boost inductor = 10uH.

 

We are OK in max or high load, but in light load, we are seeing an instability involving a ripple on the Vout of frequency 5KHz.

-Do you  know why this is?

  • 0
    Expert 1920 points

    Try applying a electrolyte capacitor on the output. Try a 10-47uH 35V version.

    Some times it's the lack of ESR in the output capacitors that makes the loop instable.

  • 0 in reply to CWilson
    Intellectual 970 points

    thanks,

    but we are stable at max load ..no problem...its just light load.

    unfortunately  we are not able to re-do the pcb as its  too expensive, -we are stuck with the pcb as it is...so we cant add more capacitance

  • 0 in reply to treez
    Expert 1920 points

    Tried to add a electrolyte capacitor on the output? Just solder it on to of one of the ceramic.

  • 0 in reply to CWilson
    Intellectual 970 points

    sorry but we cannot do this, as it would mean a PCB change.

    There are two 10uF 1210 ceramic capacitors on the output...and we are stuck with those only.

    the tps40210 datasheet blames light load instability on a change from ccm to dcm, but i am certain that a boost converter correctly compensated for ccm can go into dcm and still be stable (its the other way round thats the problem) ?

  • 0 in reply to treez
    Expert 1920 points

    For the experimental, please try to solder an electolyte cap (10uF) onto one of the ceramic capacitors. Try using a SMD version of the electrolyte. You can get some at Ø4mm.

  • 0
    TI__Genius 17240 points

    Hi Treez,

    I am wondering whether it was caused by the minimum controllable on-time of 275ns (typical).

    Regards,

    Na

  • 0 in reply to treez
    TI__Mastermind 35165 points

    Can you capture a waveform of your output voltage ripple including the switching node too?

    Typically a converter stable in CCM will be stable in DCM. Can you point me to where your referring on the data sheet mentioning instability when changing from ccm to dcm? I can't seem to find it.

    Thanks,
    Anthony

  • 0 in reply to Anthony F
    Intellectual 970 points

    in the datasheet on page 30 they only do the feedback loop for the light load case, because they say thats where all the problems are...a TI emplyee tells us that they go unstable when they go into discontinuous mode.

  • 0 in reply to treez
    TI__Mastermind 35165 points

    Hi treez,

    Do you have any updates on this? Can you get us a screenshot clearly showing your output ripple and the switching node?

    I now see why it can go unstable in DCM because with peak CMC, the gain from the control to output will be higher pushing to a higher crossover frequency. This is why it  suggests to compensate at the lightest load you will see to avoid any stability issues.

    Thanks,
    Anthony

  • 0 in reply to Anthony F
    Intellectual 970 points

    sorry but i can garantee that if  you have a ccm current mode boost converter which  is stable at full load......then when it goes into light load and its current goes discontinuous............it is DEFINETELY stable in DCM

    there is no instability which suddenly appears just because its so lightly loaded that it goes into DCM.

     

    I garantee this to all

  • 0 in reply to Anthony F
    Intellectual 970 points

    no ccm boost in current mode goes unstable when it gets so lightly loaded that it goes dcm....i garantee it

  • 0 in reply to Anthony F
    Intellectual 970 points

    no ccm cm boost goes unstable when in dcm & light load

  • 0 in reply to treez
    TI__Mastermind 35165 points

    Hi treez,

    If it is a stability issue targetting a lower crossover frequency when designing your compensation typically fixes it. It will be hard for us to help you troubleshoot this without seeing some screenshots so we know exactly what you are seeing. Any measurements which show your issue will help. At what load do you start seeing the instability occur, is it right when it enters DCM?

    Also if you can share a schematic and bom (in particular part number or more specs for the output caps and inductors) I can build up the circuit on an EVM to help you troubleshoot this further. I will then be able to measure the loop to get some more insight as to what is happening with the stability.

    Regards,
    Anthony

  • 0 in reply to Anthony F
    Intellectual 970 points

    When a CM , CCM Boost goes into DCM, because of light loading, the crossover frequency gets LESS, and the phase margin improves.

     

    therefore, the instability cannot possibly be due to going into DCM.

     

    This instability is due to the unfortunate minimum on time of the TPS40210

  • 0 in reply to treez
    TI__Guru**** 191445 points

    For your design, assuming the maximum possible minimum off time of 400 nsec, you will start to have minimum off time constraints at loads below 147 mA.  To properly design the compensation network, you need to design based on maximum output impedance (or minimum load conditions).  What minimum current did you use for the calculations of equations 58 thru 67?  If we can see some waveforms, we can determine exactly what you situation is.