Design and Effects of Input Filter for DC-DC Power Converters

Abstract

The DC-DC Power Electronic Converters work on the principle of converting a particular level of DC voltage/current to another level of DC voltage/current. In order for the accurate and reliable working of the desirable converter, the input voltage should be made ripple-free. Also due to switching action, ripple values are reflected in the source current. In order to avoid these disturbances, a filter is designed for the input side which can pass the ripple component through it. It is designed on the basis of the maximum charge to be flown through it which is contributed by both voltage ripples as well as switching ripple (Current Ripple). The designed filter passes the ripple thereby providing a constant voltage and current on the input side.
A unique theorem of Middlebrook namely the Extra Element Theorem (EET) provides the relationship between the gain of the system and the gain of the same system when an extra element (Filter) is added into it. The same theorem is implemented for the filter addition where the gain parameters of the converter change when the filter is added in the circuit. The small signal transfer functions i.e. control-to-duty ratio, line-to-output etc. change by a common factor which depends on the single injection impedance ( ), null double injection impedance ( ) on the small signal model as well as on the impedance of the filter element. These impedances are calculated by applying special conditions on the small signal model to obtain their driving point impedances.
The modified system is made stable by attempting to make it similar to the original system. This is done by imposing the criteria that << Z & << Z. When the criterion is satisfied, the system tends to the original system. If the original system is known to be stable, by the above constraints the modified system also becomes stable. The Middlebrook criteria hence provides a unique method of designing the input filter which when added to single or multiple input/output systems ensures stability with respect to the driving point impedances obtained under special constraints applied to the system.