For switched-mode power supplies operating at low power levels, a flyback topology typically offers the best efficiency, but there is a risk of potentially high voltage stress on the switch. Using a quasi-resonant switching method can help avoid this situation, and can also improve EMI behavior. A PWM converter with support for valley switching provides an optimal solution, and can be the basis for a variety of power supplies with different power ranges.
Figure 1 gives the schematic for a generic quasi-resonant flyback converter.
The left side of the schematic shows the input connection, with a bridge rectifier. The control IC is supplied through a diode and resistor for the startup. During operation, the control IC draws its own power through the auxiliary winding (shown in the middle of the schematic and in part of the transformer). The auxiliary winding includes a resistor divider that enables the controller to measure voltage waveforms across the winding and determine the turn-on moment. An external MOSFET is used as the main power switch, in series with a shunt resistor to measure the current on the primary-side. The secondary-side includes a rectification and regulation loop, with a reference and an optocoupler.
Using the generic schematic shown in Figure 1 as the starting point for development, we implemented a sample design using the highly integrated Fairchild FAN6300A/H PWM controller. We chose the FAN6300A because it includes several features that enhance performance in quasi-resonant topologies, like valley-switching support. The device selects one of the voltage minima to turn on for the next cycle, as shown in Figure 2.
The yellow trace is the drain-source voltage of the main switch, and the blue trace is the current through the main switch. Roughly two switching cycles are visible. The controller operates in discontinuous mode, as shown by the low-frequency ringing after the energy is transferred. One of these minima is then chosen to turn on the main switch — in this case, the fourth minimum. This ensures low-voltage turn-on, and that minimizes the switching losses. The controller has a minimum off time to ensure that the switching frequency doesn’t get too high at high-load. At low-load, the off time is modulated to become longer, effectively reducing the switching frequency and any associated losses.
Figure 3 shows the FAN6300A with a 30W power supply capable of handling up to 1000V DC at its input, and operating with very good efficiency across the entire load range.
To cope with the high DC voltage several capacitors are connected in series at the input. A TVS diode is used to perform the snubber function and reduce power dissipation. The main switch is realized with a cascode, consisting of a low-voltage MOSFET, a bipolar transistor and an optocoupler and is used to create the feedback loop.
The cascode delivers several advantages. The switching speed can be very high, as the Miller capacitance is ineffective when the bipolar transistor switches. The output capacitance is low, and the design is very robust. Furthermore, a high breakdown voltage is relatively easy to achieve with this setup, and the MOSFET represents an easy-to-drive load for the controller.
Figure 4 shows the efficiency levels for the schematic given in Figure 3.
At each input voltage level, the efficiency is very good across the output power range. In most cases efficiency is above 80 percent, and at higher power it’s in excess of 85 percent. Efficiency is essentially independent of the input voltage, demonstrating the very low switching losses associated with the cascode circuit when it operates in a quasi-resonant switching scheme. Although not shown in Figure 4, the power supply can easily operate down to 80V AC, and up to 1200V DC, exhibiting outstanding robustness.
Quasi-resonant operation, as implemented with a modern PWM controller that includes various protection functions, meets the requirements for many industrial applications by enabling good performance and superior robustness, across a wide input voltage range, with excellent efficiency. The 30W design described here is available as a fully tried and tested reference design, complete with characterization report and build data. Other designs based on similar control schemes and intended for 8W and 80W systems are also available. Please contact Fairchild Semiconductor for more information.
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