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AN3276 Datasheet(PDF) 5 Page - STMicroelectronics |
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AN3276 Datasheet(HTML) 5 Page - STMicroelectronics |
5 / 22 page AN3276 Existing solutions Doc ID 17975 Rev 1 5/22 This soft switching solution is well known, but it requires that several technical criteria be met: ● Reset the current in the inductor L at each switching period, whatever the variations of the current, and input and output voltages. ● Recover the saved inductive energy without losses. ● Limit any overvoltage and overcurrent stress in the semiconductor devices. ● Keep cost down when adding any device. ● Maintain a similar power supply density. There are many circuits that are classified in two families of recovery circuits: ● active ● passive 1.3 Active recovery circuit In the active recovery circuit family, the zero voltage transition (ZVT) [see Section 5: References, 3.] shown in Figure 4 is well known by designers. This circuit allows both switch-on and switch-off power losses to be removed. Figure 4. Zero voltage transition (ZVT) active recovery circuit A theoretical study indicates that ZVT is an excellent topology for the PFC application, since all the switch losses are removed. In addition, this circuit can work whatever the input and output power variations. Nevertheless, in practice, the recovery current from the boost diode DB significantly affects the ZVT behavior leading to some constraints on both inductance and minimum duty cycle. During the reset current in the small inductor L, the recovery current from D2 involves a high-stress voltage and damping parasitic oscillation. Finally, the dynamic behavior of the PN diode affects the global ZVT efficiency because conduction times in the transistor should increase and a dissipative snubber is mandatory to reduce the electrical stress across the semiconductors. In terms of cost the ZVT circuit requires an additional power MOSFET and a specific PWM controller. Several derivative circuits of the ZVT circuit have the same technical issue and their higher price makes these circuits less than ideal for mass market applications. Therefore, the passive recovery circuit can be more attractive. VOUT Vmains DB D2 D1 L TZVT TR LB Rdamping Ddamping |
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