Synchronous flyback circuit provides high-efficiency conversion
Edited by Bill Travis
Wayne Rewinkel, National Semiconductor, Phoenix, AZ -- EDN, July 22, 2004
Buck regulators are usually the first choice when you design nonisolated step-down regulators unless the ratio of VIN to VOUT is greater than 10, the input voltage is high, or both. Low duty cycle can be problematic for FET drivers and cause current-mode control loops to lose control. Efficiency can fall dramatically to 60 to 70% at low VOUT and current of only a few amps. The efficiency loss arises from switching losses, because the upper switch always sees full load current. Figure 1 presents a circuit that looks a little like a buck regulator and uses a buck controller but is actually a voltage-mode, synchronous flyback circuit. The application it targets needs 3.3V at 2A with an efficiency requirement of greater than 85% and an input-voltage range of 36 to 60V. This one appeared the most promising of several evaluated technologies because of efficiency and cost advantages over buck and asynchronous-flyback approaches.
The LM2743 controller derives its power after start-up from the MMBTA06 transistor and 6.2V zener diode and from a bootstrap winding. Its EN (enable) input is a comparator for UVL (undervoltage lockout) to prevent start-up below 28V. The controller drives a synchronous switch that provides lower loss than a Schottky diode and uses the lower FET's on-resistance as the sense resistor for current limiting. The 150-kΩ resistor at Pin 11 produces a switching frequency of 250 kHz. The flyback transformer, designed by Pulse Engineering (www.pulseeng.com), is a low-cost unit that provides 50 µH of primary inductance and a 3-to-1 turns ratio in a 13L×15W×11H-mm footprint. Its 3-to-1 turns ratio prevents the primary switch from seeing full output current, resulting in less switching loss than that of a buck regulator. The small LC filter at the output allows a single 10-µF ceramic capacitor to handle the high rms ripple current, and a low-cost aluminum capacitor also removes ripple and buffers load transients.
Figure 2 plots measured data at three input voltages and several output currents for the circuit in Figure 1. Efficiency is displayed on the left for the three uppermost curves, and the three lower curves show total loss in watts measured by the scale on the right. VOUT ripple measured 6 mV p-p at no load, rising to 20 mV p-p at 4A. The rapid fall in efficiency at 3.5A comes from current limiting. As with any switcher and especially for flyback designs, pc-board layout is important. You obtain best performance with four or more layers, separate power and ground planes, and short and wide gate-drive connections. Although the circuit of Figure 1 targets use in a 7W, single-output requirement, this synchronous flyback circuit applies to a wider power range; you can easily extend it to multiple outputs by adding secondary windings. The additional outputs can use either diode rectifiers or additional FETs driven from the low-gate driver.
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