Power tip: Boost discontinuous flyback efficiency
Figure 1: Self-driven synchronous rectifiers do not naturally commutate in a discontinuous flyback.
The switch is then turned off and the voltages at the dotted ends of the transformer windings rise until the body diode of Q2 clamps the voltage on the transformer secondary to the output voltage. Note the gate of Q2 is more positive than its source. Consequently, the current commutes from the body diode to the MOSFET channel and improves the rectification efficiency. The circuit is effectively latched in this state with a positive gate-to-source voltage connected through the transformer.
During this time the magnetizing inductance discharges and reverses direction. To exit this state, Q1 must be turned on to reverse the Q2 gate voltage and turn it off. This is a pretty stressful event as the two transistors are simultaneously on and current and voltage spikes are quite high. This simple circuit always operates in continuous conduction as at least one of the switches is on at all times.
The key to have synchronous rectifiers work in a discontinuous flyback is to make them work like the diodes they replace—that is, they must be turned off when the current in them reverses. The traditional approach is based on buffered current transformers while providing a positive drive voltage when current flow is in the proper direction, then reverse the drive when the current reverses. The downside is the cost and size of the current transformer, as well as the additional handful of discrete components for the buffer.
Several companies (TI included) have developed ICs as an alternative to current-sensing drive circuits as shown in figure 2. The synchronous rectifiers have been moved to low side of the transformer and a control IC provides the timing and gate drive. The benefit is that the source is connected directly to ground and the gate can be directly driven.
Figure 2: ICs appropriately drive a synchronous rectifier gate by sensing voltage reversal across drain voltage.
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