Off-line applications that don't require isolation can take advantage of the circuit shown in Figure 1. It offers a very simple and inexpensive solution for supplying a regulated low-power dc output voltage. This circuit requires only a TL431 adjustable precision shunt regulator, two diodes, and six passives to generate a +5-V output from a 240-V ac input.

Although this circuit is designed for a load current of 14 mA, the passive components can be adjusted to provide up to 100 mA. In addition, the output voltage can be easily adjusted to any value in the 2.5- to 35-V range.

Figure 2 shows typical voltage waveforms. Due to the low dc voltage across the bulk capacitor (C2) and the configuration of the rectifiers, the input capacitor (C1) essentially sees the full sinusoidal input voltage. As a result, the input impedance is approximately equal to the impedance of the input capacitor. Consequently, the input capacitor must be rated for the full line voltage. Type X2 capacitors work well as input capacitors in this application.

The rectifiers conduct on alternating half-cycles of the input voltage, and the current through each diode appears as a half-wave rectified sine wave. All of the dc current flowing through diode D1 is forced by the bulk capacitor to flow to the TL431 and the load. As a result, the dc current through D1 determines the maximum load current and can be calculated by Equation 1:

I_{D1,DC} = 2√2 × V_{IN(RMS) }× C1 × f (1)

Note that the maximum load current is slightly less than the dc diode current because a small amount of current is necessary to bias the TL431.

The current through R1 is independent of the load current and equal to the dc current flowing through D1. Thus, the value of R1 determines the average voltage on the bulk capacitor. This resistor must only be large enough to ensure that the voltage across the bulk capacitor never dips below the output voltage. Keeping this resistance relatively low also minimizes the power losses in the resistor. For example, a 0603 surface-mount resistor can be used by picking a value of 100 Ω as shown in Figure 1.

The voltage rating of the bulk capacitor must be greater than the output voltage plus the voltage drop across R1. The average current through D1 and the value of the bulk capacitor determine the magnitude of the ripple voltage across the bulk capacitor. This ripple voltage can be calculated via Equation 2:

ΔV_{C2} = I_{D1,DC}/(2 × C2 × f) (2)

Typically, a capacitance in the range of 220 to 1000 µF is required to keep the ripple voltage at an acceptable level. Alternatively, the value of R1 can be increased to allow for more ripple voltage. However, this increases the amount of power losses in the power supply. Aluminum electrolytic, tantalum, and Oscon capacitors all perform well as bulk capacitors in this application.

The TL431 shunt regulator keeps the output voltage at a fixed level, determined by the resistor divider of R2 and R3. To guarantee stability of TL431, the output capacitor (C3) needs to be a 10-µF ceramic capacitor. TL431 keeps the output ripple voltage under 10 mV p-p.

This circuit draws a constant amount of apparent power and dissipates a constant amount of average power. Because the power losses in the supply are much smaller than the reactive power in the input capacitor, the supply appears as a capacitive load to the input line. As the load resistance changes, power losses shift between TL431 and the load. The maximum current limit of TL431 restricts any modifications of this circuit from providing more than 100 mA of load current.

The circuit also has an inherent short-circuit protection. The short-circuit current is limited by the value of the input capacitor and is equal to the maximum load current. As the load current rises above 14 mA, the output voltage drops sharply.