Soft switching flyback converter

Abstract

In described examples of synchronous rectifiers and flyback converters (100), integrated circuits (101) and operating methods, a first switch (S1) is turned on to allow current to flow for a first time period (Tl) in a first direction in a transformer primary winding (108) responsive to a first switch voltage (VDSl) transitioning below a first threshold (VTHl), and a second switch (S2) is turned on for a second time period (T2) after the first time period (Tl) to transfer energy from a secondary transformer winding (122) to drive a load (125). In the same converter cycle, the second switch (S2) is again turned on for a third time period (T3) in response to a second switch voltage (VDS2) transitioning below a second threshold (VTH2) at one of a series of troughs of a resonant ringing voltage waveform across the second switch (S2), to cause current flow in a second direction in the primary winding (108) to discharge a capacitance (CSl) of the first switch (SI) to cause the first switch voltage (VDSl) to transition below the first threshold (VTHl) to initiate 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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