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Simple Switchers output capacitors are much larger than recommended

I inherited some simple switcher designs... the output capacitors are way too big....like 10 times larger than recommended.  Should I worry?

The output voltages look pretty quiet and well regulated.

I suspect the controllers might be running in a bang bang mode because the loop compensation is so screwed up.  

 So these switchers are probably more noisy on the input side than they could  be...

And of course the layout area is much larger than it needed to be with the 10 times capacitance.

However the folks around me say if it ain't broke don't fix it.   Any thoughts?

  • Hi Tom,

    There are two main effects from oversized output capacitors.

    1) During Startup as the output voltage is ramped up to Vout, a large input current = Cdvt/dt.  If this current is close to the current limit of the parts, then you can see start-up issues.  Many of our parts have adjustable soft-start, which slows the dv/dt, thus lowering the input current during start-up. This doesn't sound like an issue since this part has already been used in the past.

    2)  The second thing is that the loop crossover frequency will be much lower.  This will actually slow down the response time during transients and other events.  This is usually somewhat offset by the fact that the larger capacitor can provide a much larger discharge current then a smaller one. 

    If the large capacitance has a very high esr than this can move the esr zero in the design and potentially cause instability.  To check this, I would run an output voltage transient response.  Hit the output voltage with a step load current (fet, resistor and signal generator if you don't have an electronic load) and observe the shape of the output voltage recovery.  You should see the initital dip as the step current occurs, the part should then recover to the original Vout.  If it does so without any ringing than it is an overdamped response and stable.  If it rings just once than you have 45 degrees of phase margin.  If it rings more the once then your phase margin is below 45 degrees. If the phase margin is below 45 you might want to consider a re-design.

    3) Another question you might want to ask is whether the capacitor was put there for hold-up time or some other system purpose.  I have seen several designs that have a large output capacitance to keep the output voltage up for a certain amount of time after the system is turned off.  If this is the case then you wouldn't want to decrease the capacitance.

    Regards,

    Marc

     

  • Hi Mark,

    Thanks for the excellent reply... I need to roll all that around in my brain a bit but much of what you say rings a bell..