Research On Stability Issues Of High Performance DC Converter System

With different demands of source and load, high order topologies and cascaded connection are widely used in DC-DC converter system to realize high performance. The stability issues of DC-DC converter system are always focused on. The small signal model parameters are the important factors which affect the stability of DC-DC converter system. In this paper the key parameters, which are summarized as controlled object, audio susceptibility, input impedance and output impedance, are analyzed and the corresponding improvements are proposed.Many literatures have focused on the basic second-order converters and the stability problems have been solved. But when these converters are used as the interface connected with the solar array and battery, the extra filter should be added into the topologies to meet the continuous current demand. The high-order converter topology with continuous input or output current is another solution. By these means, there are many poles and zeros in the model. Especially when there are right-half-plane (RHP) zeros, the phase of bode plots may pass through-180°in low frequency, therefore the bandwidth of closed-loop system may be limited. In this paper, the damping optimizations for controlled object in high-order converter topologies are summarized. A simple parameter design method for damping networks of Superbuck topology to avoid right-half-plane (RHP) zeros is presented. By this method the controlled object can be considered as a minimum phase system.The feedforward control can realize zero audio-susceptibility in converters. But when the order of the model is high, the desired feedforward controller is complicated and the derivation is redundancy. A method for simplification of the converter when computing the feedforward controller is presented so that the related components of the circuit can be easily established. In this paper, the proportional feedforward controller instead of the accurate form is used to realize zero audio susceptibility in certain frequency range. It shows that the proportional feedforward controller only depends on the DC gain of the converter.Because of the impedance interaction of source converter and load converter, the cascaded system may be not stable when the converters are stable independently. To improve the stability of cascaded system, the intersection range of output impedance and input impedance should be decreased. One side, the output impedance of source converter should be decreased, the other, the input impedance of load converter should be increased.Based on the unified small signal model of DC-DC converter, the double loop control method with input current inner loop and output voltage outer loop is proposed. The input impedance is analyzed under this situation and the paper gives the quantization of the input impedance when the current sampling factor is changed. It is shown that when the cross over frequency of current loop is larger than that of the voltage loop, the input impedance can be significant increased when the current sampling factor is increased. Therefore the impedance intersection of source and load converters can be decreased so to improve the stability.Similarly, to improve the output impedance of the converter, an active damping method to emulate a virtual resistor, which is connected in parallel with the output of the converter’s small signal model, is presented. This method can be applied into the converter whose model has no RHP zeros. The small signal model is improved but the steady-state operating point is not affected. By this means, the resonant peaks of output impedance can be decreased and the output impedance can be improved.