Open-loop power supply delivers as much as 1W
Christophe Basso, On Semiconductor, Toulouse, Cedex, France -- EDN, January 18, 2001
For VCRs, TVs, and other equipment that requires a standby mode, you must supply power to a µP when other components are asleep to receive and interpret any wake-up signal from the remote control or from the broadcasting company. These types of systems have rather low power consumption, and classical switch-mode power-supply ICs represent a clear overkill for less-than-1W output levels. Any active power-supply circuit also needs to be more cost- effective than the standard structure using a metallic transformer. The circuit in Figure 1 reduces the cost by eliminating the use of the optocoupler.
IC1 directly drives an external 600V MOSFET. The lack of an auxiliary winding greatly simplifies the overall application circuitry; the controller's integrated dynamic self supply provides VCC. IC1 works as a peak-current PWM controller, combining fixed-frequency operation at 40, 60, or 100 kHz and the skip-cycle method for low standby-power consumption. IC1 regulates the peak current and allows operation over universal mains. Because the circuit operates at constant output power, the following formula determines the necessary peak current:
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With an internal error amplifier that clips at 1V maximum, RSENSE is equal to 1/IP(maximum). In this example, a 40-kHz circuit and a 6.8Ohm sense element deliver as much as 1W of continuous power with LP=2.8 mH. You can recompute RSENSE for lower or higher output-power requirements. The 12V zener diode prevents the circuit from generating overvoltages. R1 deactivates the internal short-circuit protection, which normally reacts upon feedback-path loss.
Thanks to its avalanche capability, the MTD1N60E requires no clipping network, which further eases the design. The efficiency measured 64% (low line, POUT=866 mW) and 61% (high line, POUT=1.08W). Figure 2 plots the input-voltage rejection, which stays within 1V from 130 to 260V-ac mains. This figure illustrates current mode's inherent audio susceptibility.
