tailieunhanh - Theory Power Electronics phần 3

Cô lập một kết thúc Topology 1. Việc chuyển đổi flyback (Hình 30,21) là một phiên bản bị cô lập của bộ chuyển đổi buck-tăng. Trong chuyển đổi này (Hình 30,21), khi bóng bán dẫn trên, năng lượng được lưu trữ trong điện dẫn cùng (không phải là một biến áp), và năng lượng này được chuyển tải khi chuyển đổi được. | ---------- _ _ _ . . . FIGURE Nonisolated Cuk converter. FIGURE a Flyback converter. The clamp winding shown is optional and is used to clamp the transistor voltage stress to Vin nVo. b Flyback converter waveforms without the clamp winding. The leakage inductance spikes vanish with the clamp winding. Isolated Single-Ended Topologies 1. The flyback converter Fig. is an isolated version of the buck-boost converter. In this converter Fig. when the transistor is on energy is stored in the coupled inductor not a transformer and this energy is transferred to the load when the switch is off. Some of the advantages of this converter are that the leakage inductance is in series with the output diode when current is delivered to the output and therefore no filter inductor is required cross regulation for multiple output converters is good it is ideally suited for high-voltage output applications and it has the lowest cost. Some of the disadvantages are that large output filter capacitors are required to smooth the pulsating output current inductor size is large since air gaps are to be provided and due to stability reasons flyback converters are usually operated in the DCM which results in increased losses. To avoid the stability problem flyback converters are operated with current mode control explained earlier. Flyback converters are used in the power range of 20 to 200 W. 2. The forward converter Fig. is based on the buck converter. It is usually operated in the CCM to reduce the peak currents and does not have the stability problem of the flyback converter. The HF transformer transfers energy directly to the output with very small stored energy. The output capacitor size and peak current rating are smaller than they are for the flyback. Reset winding is required to remove the stored energy in the transformer. Maximum duty cycle is about and limits the control range. This topology is used for power levels up to about 1 kW. The flyback .