Soft Switching Flyback Converter

Abstract

In the illustrated example of a synchronous rectifier and flyback converter (100), integrated circuit (101) and method of operation, the first switch (S1) to allow current to flow in a first direction in the transformer primary winding (108) for a first time period (T1), and after said first time period (T1) to turn on a second switch (S2) for A second time period (T2) is used to transfer energy from the secondary transformer winding (122) to drive the load (125). In the same converter cycle, at one of a series of valleys of the resonant ringing voltage waveform across the second switch (S2) in response to the second switch voltage (VDS2) transitioning below the second threshold ( VTH2) and turn on the second switch (S2) again for a third time period (T3) to cause current to flow in the primary winding (108) in the second direction to turn the first switch (S1) The capacitor ( CS1 ) of V is discharged to cause the first switching voltage ( VDS1 ) to transition below the first threshold ( VTH1 ), thereby initiating a subsequent converter cycle.

Description

[0001] This relates generally to flyback converters, and more specifically to soft switching and synchronous rectification flyback converters. Background technique [0002] Synchronous rectifiers are used to perform DC-DC conversion to drive output loads, where transformers are often used to construct flyback converters with secondary side switches to provide efficiency advantages over passively rectified flyback converters. In many applications, efficiency is a primary design goal and it is desirable to reduce or mitigate switching and conduction losses in the primary and secondary side switches. Soft switching or zero voltage switching (ZVS) involves turning on the primary side switch and / or the secondary side switch when the voltage across the switches is low, preferably zero. Ideally, switching at zero volts minimizes switching losses, but this is difficult due to the drain-source capacitance of field effect transistor (FET) type switches. Conduction losses occur when the...

AI Technical Summary

Problems solved by technology

However, the resonant voltage oscillation is not close to zero volts even at the valley, especially for high input voltage conditions

Therefore, true zero-voltage switching cannot be achieved across the wide range of operating conditions of conventional synchronous rectification flyback converters, and switching losses can be substantial

Therefore, under certain operating conditions, discontinuous mode (DM) switching operation must be used, which increases conduction losses and reduces converter efficiency

Furthermore, hard switching (i.e., without the ability to reliably achieve true zero-voltage switching) inhibits the ability to increase switch size to counter conduction losses and results in increased common-mode electromagnetic interference (EMI)

Hard turning on of the primary side switch can also cause a resonant multiplication of the voltage on the secondary side rectifier, resulting in increased synchronous rectifier blocking voltage and further increased conduction loss due to higher RDSON

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