Power Tip 67: Improve discontinuous flyback efficiency with synchronous rectifiers
It is well established that synchronous rectifiers can significantly improve the efficiency of a power supply by replacing the rectifier junction voltage with a lower resistive switch voltage drop. The challenge has been to develop robust control strategies and drive components to maximize the impact. Implementation in a discontinuous flyback is a significant challenge compared with a continuous flyback. Figure 1 shows a simplified schematic of a flyback with a synchronous rectifier along with associated waveforms. At t=0, the primary switch Q1 is on and its drain current is ramping positively.
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.
Since the device works by monitoring the drain-to-source voltage, the circuit is also less noise prone with the source connected to the system ground. This circuit now functions as a discontinuous flyback and several of the ideal waveforms are shown to the right. Of particular note is the voltage stress on the output rectifier, which is also the drain voltage (VD) connection on the IC. In actuality the voltage will be higher due to ringing, but the voltage in the ideal case is equal to the reflected input voltage plus the output voltage. With output voltages greater than 5 volts, or with widely ranging input voltages, the voltage on this node easily can exceed the IC’s 50-V rating.
Figure 3 presents a simple, inexpensive circuit to work around the voltage rating of the VD pin comprising only two components. As shown on the right, the voltage on the VD pin is limited to the output voltage. When the primary FET Q1 is on, the voltage on Q2 and Q3 drains is equal to the reflected input voltage plus the output. Since the gate of Q3 is connected to the output voltage, the source voltage will be about a threshold voltage less. When Q2 is on, Q3 body diode turns on and its source is pulled below the output voltage, enhancing Q3 so the VD pin is connected to the drain of Q3.
Figure 3. A simple clamp extends useful operating voltage.
To summarize, you cannot make a self-driven synchronous rectifier for the discontinuous flyback converter. It takes some circuitry to determine when to drive them. There are both current transformer-driven and semiconductor circuits to accomplish this. The semiconductor circuits have a size and cost advantage. There are a number of vendors that have developed these circuits, but they may need a buffer to properly interface with the high voltage and currents found in power supplies.
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