| Abstract |
Dc-dc switch mode power converters (SMPCs) appear as negative input impedance for the dc power line.
This can cause instability and therefore it is worth studying how converter’s input impedance depends on
various factors. This paper studies in detail the input impedance of a buck converter operating in
continuous conduction mode (CCM) by using converter’s state-space average (SSA) model. Three
different PWM control schemes are considered – the voltage-mode control (VMC), input voltage feedforward
(IVFF) and peak current mode control (PCMC).
Results show, that for a given operating point, the input impedance depends on converter’s control.
According to the theory a step down converter with IVFF has input impedance, which is a pure negative
resistance regardless of converter’s regulation bandwidth, i.e. within the entire frequency range, where the
SSA model is valid. The same applies for a PCMC buck converter, if optimal slope compensation is used.
The input impedance of a VMC step-down converter is a second order transfer function, which is
approximately negative real number only in the lowest frequency range of the control bandwidth. |
