The majority of notebook power adapters use a flyback converter as shown in FIG. 1. To improve efficiency over diode rectification, most notebook power adapter manufacturers use synchronous rectification (SR). In other words, a synchronous rectifier FET is used to replace diode D on the secondary winding T1 of the transformer T in the flyback converter shown in FIG. 1. A major disadvantage of SR over diode rectification is the higher cost, which is associated with the control signal that needs to be sent across the isolation barrier (the transformer).
Instead of sending the control signal across the isolation barrier, in certain prior art systems, the signal can also be derived from the voltage across the synchronous rectifier FET. One of the issues in doing this is that the voltage signal across the FET is very small compared to the dynamic range (millivolts versus tens of volts). To avoid false triggering, bandwidth is sacrificed. This leads to increased turn-off times, leading to efficiency losses.
Some prior art systems use a transconductance amplifier to slow the turn on of the synchronous rectifier FET, which solves the false triggering problem. However, slow turn-on makes it not useful in continuous conduction mode (CCM) applications. Furthermore, the transconductance amplifier brings a slow turn-off, which causes even more switching losses.
Therefore, there is an unmet need to provide a solution having a much faster turn-off and hence reduced switching losses. Moreover, the solution should be used in any type of flyback converter (CCM, discontinuous conductance mode (DCM), and quasi-resonant).