Here's a simple way to create a power supply, operating on ac-line voltage, that generates an isolated, regulated 5-V output capable of producing 40 mA (Fig. 1). Because the isolation transformer used operates at 25 kHz, it's much smaller and lighter than similar input transformers running at 60 Hz.
In this setup, the voltage drop across R1/C1 allows a direct connection between IC1 and the ac line. R2 limits the current into C1. IC1, which is an ac-to-dc regulator containing a full-wave rectifier, 12.4-V Zener diode, and series output regulator, provides a regulated supply voltage to IC2. Typically 5 V ±4%, this output (VREG) is adjusted to approximately 5.3 V by introducing an external divider (R5/R6) in the regulator's feedback path. This feedback regulates the 5-V output by sensing changes in IC2's output voltage (pin 5) and adjusting VREG accordingly.
IC2 is a step-down switching regulator that usually operates with an external inductor in the flyback mode. In this circuit, it operates with a transformer in the forward mode, applying a chopped -5.3 V to the transformer primary. The secondary voltage is then half-wave rectified and filtered to deliver the final 5-V output.
Within IC2 exists a large, 1-Ω PMOS switch that connects Lx and V+ (pins 5 and 6). The small voltage drop across this switch is compensated by feedback, as is the drop across diode D4.
For further compensation for the effects of voltage and temperature variations in the diode, a similar diode (D1) is included, connected in series with the feedback network. Because load current flows in D4 but does not flow in D1, the compensation is partial, resulting in finite load regulation.
D3 and D2 (a 4-V Zener diode) clamp the Lx output to approximately -5 V, which makes it possible for flux in the transformer core to reset on every cycle. The resistor-divider R3/R4 applies 2 V to IC2's pin 1, thus setting the internal oscillator to 25 kHz with a 40% duty cycle. In addition, R7 provides a continuous load that bleeds off the energy contributed by leakage inductance in the transformer. Without R7, the output will rise to a higher voltage than the desired 5 V.
The circuit delivers 0 to 40 mA with a drop of less than 250 mV (Fig. 2). To improve the steep drop in voltage near zero load, the output can be loaded more heavily by lowering the value of R7.
For better load regulation, VREG can be raised either by adjusting the R5/R6 ratio or by altering the transformer turns ratio. The desired output voltage level then can be obtained by adding a linear regulator.