Circuit provides universal AC-input-voltage adapter

JB Castro-Miguens, Cesinel, Madrid, Spain, and C Castro-Miguens, University of Vigo, Vigo, Spain

Use a TRIAC and optocoupler to automatically change between doubler and bridge configurations

The input-rectifier stage of an offline power supply converts an AC-line voltage to a DC voltage, which powers a DC/DC converter. If you configure the rectifier section as a full-wave bridge for a universal mains input of 92 to 276V rms and 47 to 63 Hz, you must design the DC/DC converter to operate over a wide range of input voltages. This approach increases the cost of components and reduces the efficiency of the dc/dc converter.

For 115V-rms operation, if you configure the rectifier stage as a simple bridge, the output-voltage ripple ranges from 113 to 192V, given a ±20% tolerance on the AC input voltage (Table 1). Note that the voltage ripple corresponds to an effective output capacitance of 165 μF, the series equivalent of two 330-μF capacitors, and to a load of 250W. However, if you configure the rectifier stage as a voltage doubler for 115V-rms operation, the output voltage ranges from 200 to 372V—much closer to the 215 to 387V voltage ripple for 230V-rms operation.

Table 1. 200W input-rectifier/filter section

This Design Idea proposes a rectifier stage that automatically configures itself as a voltage doubler for 115V-rms±20% line operation. Figures 1 and 2 show the proposed rectifier stage. The STMicroelectronics snubberless BTB16-600BW TRIAC (triode alternating current) is suitable for a 250W load (Figure 1).

Figure 1.	This AC-rectification circuit uses a TRIAC and an optocoupler to automatically switch between doubler and bridge configurations.

For 115V-rms±20% line operation, the Reference terminal of the TL431 programmable shunt regulator has a voltage lower than its 2.5V internal reference (Figure 2). In this situation, the MOSFET is on, and the IL4216 optocoupler continuously fires the TRIAC.

Figure 2.	This control circuit uses a programmable shunt-regulator IC to sense the AC-input-voltage range.

For 230V-rms±20% line operation, the Reference terminal has a voltage higher than 2.5V, and the MOSFET and the TRIAC are off. C₁ is necessary to prevent the rectifier from starting in doubler mode when you turn on the supply with a low rectified line voltage.

This approach prevents the rectifier stage from changing from doubler mode to bridge mode during start-up, which would create a voltage drop in the bulk output capacitors that would differ from the voltage in steady-state operation. This difference would cause one of the capacitors to have an abnormally high voltage and the other to have an abnormally low voltage.

A practical value of C₁ is 1 μF, which introduces an approximately 8-msec time delay for starting in doubler mode. You can use a 15V zener diode to clamp the gate voltage of the MOSFET.