why in DC-DC converter...

Hi All,

why in DC-DC converter(buck..boost...) inductor is used...why not capacitor?

would any one let me know the advantages?

Thanks

are you sure they do not use capacitor?

It absolutely exists, switched capacitors reducing converters are an example, charge pumps too !

The principle idea about DC-DC conversion with L and C is that they allow interchange of magnetic and electrostatic energy with a variable factor. Its like an ideal transformer operating at DC.

If you connect a capacitor and inductor and place an initial energy on one of both they start to oscillate and periodically interchange magnetic and electrostatic energy.

If you plot the inductor current and capacitor voltage in the plane you wil get a circle.

In the most simple converter, I think the buck-type, you have a voltage source in series with the inductor typically as a power source and a load current source parallel to the capacitor. The two switches places the hole converter in two operting modes. The first is where the inductor is charged by the voltage source and the capacitor is discharged by the load current source. the second mode is where in addition the inductor and the capacitor oscillator. But the the switching of the modes is faster than the oscillation frequency. So if you look at operation in the plane described before the cyclic figure is in the first mode a straight line and in the second mode are segment of a circle. But without the circle part there is no DC-transformer operation possible.

With pure inductors or capacitors you can built only rational conversion factors. For instance connect from 6 capacitor 3 in series and 2 groups of them to the power source and then in the second mode 2 in series with 3 groups. So you convert the voltage by 2/3.

Yeap, they always have both inductors and capacitors to exchange energy efficiently with an ideal loss of 0. Obviously, since in real life both inductors and caps have parasitic resistances, the circuit can't never provide an efficiency of 100%.

Now if your concern is... "why not JUST using caps and avoid the inductors" you can see (after deriving the expressions) that when you transfer energy between capacitors there is also an intrinsic energy loss due to spiking... now if you have caps and inductors they both serve as temporary voltage/current sources for one-another making the energy transfer smooth and loss-less.

Hope this helps,

diemilio

Thanks

for the info...

Added after 5 hours 39 minutes:

hi all,

can any one provide good material on PFM(pulse frequency modulation) for DC to DC Converters...

Thanku...