ASSESSMENT OF INPUT FILTERS INFLUENCE ON THE PARAMETERS OF DC PULSE CONVERTERS

Abstract

The article analyzes the influence of input filters on the characteristics of DC-DC 
converters, including the frequency characteristics of the transfer function. Using the 
theorem on the additional element, the effect of the filter's output impedance on the 
parameters of the DC-DC converter is considered. The main focus is on the "control
output" transfer function, which is adjusted by a correction factor depending on the system 
impedances. 
It is shown that connecting an input filter changes the transfer function by the 
magnitude of the correction factor. If the condition: ∥
∥≪∥ ∥, ∥ ∥≪∥ ,is met, 
the correction factor is approximately equal to one, and the input filter has little effect on 
the "control-output" transfer function. These inequalities limit the maximum permissible 
output impedance of the input filter and represent useful design criteria for the filter. 
For example, in a buck DC-DC converter with an input L−C filter, Bode plots are 
constructed for 
( ) and ( ) and compared with the plot of the filter’s output 
impedance ( ( )). This provides insights necessary for developing an input filter that 
meets the condition ∥
∥≪∥ ∥, ∥ ∥≪∥ . 
It is important to note that selecting identical resonance frequencies for the input and 
output filters is undesirable, as it makes it more difficult to satisfy the above inequality. On 
the contrary, the resonance frequencies of the input and output filters should be chosen as 
far apart as possible. 
Thus, adding an input filter to the converter changes the "control-output" and other 
transfer functions of the converter, and the influence of the input filter must be considered 
when designing the converter’s control system.