Choosing primary inductor and output capacitor values for a buck converter
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Posted by Michael on October 21, 2007, 12:32 pm
Please log in for more thread options able to put my finger on it. Both your inductor and your output capacitor affect Vout ripple on a buck converter. But are there reasons to increase one and not the other, or anything like that? For example, if you were to design the DC/DC converter to be able to handle a large step in the output current without a large change in output voltage, would there be one of these that you would want to change more than the other? Thanks! -Michael | |||||||||||||
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Posted by John Popelish on October 21, 2007, 12:46 pm
Please log in for more thread options I think the generalities are: For a given frequency, ripple voltage and load current, higher inductance improves the losses in the switch, since the peak current gets closer to the load current as the inductance rises. For a given frequency, ripple voltage and load current, the transient response (settling time after a step load or input voltage change) gets shorter as the inductance gets lower, since the inductor current changes more in a single cycle. Those generalities do not imply that changing the frequency is not something you should also consider. | |||||||||||||
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Posted by Nobody on October 21, 2007, 1:25 pm
Please log in for more thread options On Sun, 21 Oct 2007 16:32:34 +0000, Michael wrote:
> Hi there - I've been working this through my head, and I haven't been
> able to put my finger on it. Both your inductor and your output > capacitor affect Vout ripple on a buck converter. But are there > reasons to increase one and not the other, or anything like that? For > example, if you were to design the DC/DC converter to be able to > handle a large step in the output current without a large change in > output voltage, would there be one of these that you would want to > change more than the other? The larger the inductor, the greater the voltage spike if you suddenly disconnect the current (or the voltage drop if you suddenly increase the current drain). The larger the capacitor, the lower the voltage change. To allow for sudden drops in current draw, it helps if 0.5*LI^2 is substantially less than 0.5*CV^2 (i.e. if the current drops to zero, you can dump all of the inductor's energy into the capacitor without a significant voltage increase). For the case of the output current suddenly decreasing, you could just dump the energy into an overvoltage protection circuit if it's likely to be an infrequent occurrence (i.e. not a PWM load). For the case where the current suddenly increases, you can't increase the inductor current any faster than dI/dt = Vin/L, so any excess has to come from the capacitor. tl;dr version: a lower L/C ratio = less voltage fluctuation for a given current fluctuation. | |||||||||||||
> able to put my finger on it. Both your inductor and your output
> capacitor affect Vout ripple on a buck converter. But are there
> reasons to increase one and not the other, or anything like that? For
> example, if you were to design the DC/DC converter to be able to
> handle a large step in the output current without a large change in
> output voltage, would there be one of these that you would want to
> change more than the other?