Electronic Design

Universal Off-Line Power Supply Uses Few Components

Certain off-line universal powersupply applications will have the ac input voltage vary from a minimum of 90 V ac to a maximum of 240 V ac, and the dc current requirements will drop to less than 20 mA. For these applications, the simple and cost-effective (less than $2 in production quantities) circuit shown represents a possible solution (Fig. 1). For simplicity and cost effectiveness, the input is half-wave-rectified by D1. C1 serves as the input ripple filter, and IC1, the IXCP20M35 current regulator, sets the sum of the currents into the load and Z1 (the output zener diode) to 20 mA.

The value of C1, the input filter capacitor, is chosen so that its voltage never goes below the sum of the voltages needed by the current regulator (IC1) and the desired output across Z1. With C1 = 3.3 µF, approximately 100 V p-p of ripple is present on the anode of IC1. Based on the published minimum dynamic resistance specification of 10k for the IXCP20M35, and using 14 Ω as the dynamic resistance of Z1, IC1 will keep the resulting output ripple to a maximum of 140 mV across Z1. (Actual measurements indicate ripple voltages under 10 mV.)

The output-voltage regulation also is determined by the load current variation. By using a maximum load current step of 20 mA, another 280 mV of variation appears in the output voltage. As a result, the total expected peak-to-peak output noise is 420 mV p-p. This number is still less than ±1.4% for a 15-V power supply and should be more than acceptable for most applications. Similar performance results can be achieved for output voltages from 3.3 V (Z1 replaced with a 1N4728) to 20 V (Z1 replaced with a 1N4747). Above 20 V, the value of C1 must be increased to assure minimum voltage across C1 is greater than 6 V above the desired output voltage. This ensures that IC1 remains in regulation.

Negative-output power supplies can be created by simply reversing all of the polarities in the circuit shown in Figure 1. In the negative power-supply configuration, D1 is reversed to provide the half-wave rectified voltage across C1 (Fig. 2). C1 is placed upside down so that the negative terminal goes to the output of D1. IC1 is also is reversed along with Z1.

Multiple-output, low-current, dc power supplies can be created by stacking zener diodes of different breakdowns (Fig. 3). For example, +15-V and +5-V power-supply sources are created by replacing Z1 in Figure 1 with two zener diodes stacked on top of each other. The lower 5.1-V zener diode (Z1a) creates +5 V; the upper 10-V zener diode (Z1b) creates the +15-V source. Care must be taken to manage the output current of VOUTA (+15 V), because any current taken from VOUTA will subtract from the VOUTB (+5 V) terminal. The current consumed from VOUTB can be the full-rated current available from IC1 without affecting the VOUTA output current.

Note: Since one of the terminals of this universal off-line power supply is directly connected to ac power, a potential shock hazard exists. The halfwave rectifier configuration allows the power supply’s common terminal to act as the ac “neutral” terminal to minimize this hazard.

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