The Research And Realization Of ZCT Push-Pull Forward DC/DC Converter

A set of DC stabilized voltage supplies with electric isolation at different levels is required in the second-stage 985 engineering project"The study of key technologies for lunar rover and lander"in Harbin Institute of Technology. After comparing of several different DC/DC converters, push-pull forward converter (PPF) is selected. PPF can suppress switch's drop-out peak-voltage and magnetic biasing by adding a clamping capacitor on primary side of the traditional push-pull converter's transformer.This paper studies the clamping capacitor's influence over the circuit and the selection principle through freewheeling current analysis with PSPICE simulation and based on the work principle research: The clamping capacitor supplies a release circuit of the leakage energy so that the switch's drop-out peak-voltage is suppressed. But too big clamping capacitance will make the magnetic biasing effect lost. So the clamping capacitor's value should be compromised. The transformer's leakage inductance increases the duty cycle loss and cut down the utilization ratio of the transformer. So the leakage inductance should be decreased as far as possible.This paper adopts peak current control. Because most common converters have only one primary winding, so we only need sample its current when designing the inner loop sample circuit. Theoretical analysis show that sampling one winding's current can not acquire ideal slope signal. So the proposal that samples the D-value of the two windings circuit is designed. It can avoid the subharmonic oscillation that the former causes. Through adding slope compensation link the power can be stable when D >50% and the problem that the power is out of control when the load is light or zero can be also solved.A good power supply should meet the challenge of stability, rapidity and accuracy. This paper builds the stable state mathematic model of the control link and gives the simulation analysis through which it can be known that the low-frequency gain is little and the steady state error is big. So the lag-lead correction is added to increase the low-frequency gain and to make the high frequency interfering signal decrease rapidly so that the power supply's stable and instantaneous property can both meet the design demands.This paper also investigates the soft switch realization of PPF and emphasizes the zero current transition's work principle. By adding resonance networks to secondary side of the transformer, the switch can be on and off with zero current so that the switch loss is decreased and the efficiency as well as reliability of the power are increased.Based on the principle analysis the submodule of 24V/15A is manufactured. The experimental analysis and tests verify the theory that the paper proposes and the power's various property indexes meet the design demands.