Using a three-phase rectifier and a step-down converter to make an offline single-phase power supply

Some industrial applications require a three- phase supply to be fed into a low-power dc/dc converter that may have to handle 200V or 400V effective line-to-line voltages. In addition, there may be no AC neutral available, so a line-to-line voltage link has to be used, which means a higher input voltage. Voltage faults may occur between single and double phases, and some applications require the power supply to continue to operate under such problems. To cope with these problems, an uncontrolled rectifier and a capacitor can be used to convert the three-phase AC power to a DC voltage, but this voltage may be higher than the maximum input voltage that a standard converter can support. It can be difficult to find a dc/dc converter that can be used for these voltages, which are generally 564V DC or higher.

The circuit of Figure 1 can be used to obtain a DC voltage less than a given value, which is determined by the ratio of R1 to R3. With the resistor values ​​shown, this voltage is about 340 V when the three-phase voltage between the lines is 400 V effective or higher. This value can be obtained from two-phase or three-phase power, with or without a neutral line, or from a single-phase power line with a neutral line. This circuit omits the two diodes of the traditional three-phase rectifier bridge and includes a neutral line diode pair, so that a voltage less than 340 V is obtained on the energy storage capacitor C1, and it reaches 0 V at the initial start (Figure 2). If the neutral line is connected, it must be connected to a double diode arm of the rectifier to obtain a starting voltage of 0 V; however, the phase lines can be connected randomly.

Figure 1: A three-phase rectifier uses a switching IGBT and a capacitor to step down the voltage to within the range of a standard off-line dc/dc converter.

Figure 2, the circuit omits the two diodes of the typical three-phase rectifier bridge and adds a neutral line diode pair, so that the voltage on the energy storage capacitor C1 is less than 340V, reaching 0V at the initial start.

Shunt regulator IC1 acts as a comparator. After initial startup, once the instantaneous rectified voltage VI is greater than 340V, IC1's reference-anode voltage is higher than its internal reference of 2.495V, reducing the anode-cathode voltage to about 2V, turning off Q2. When the rectified voltage is below 340V, IC1 does not pull current. Q2 is then turned on by R2 bias, connecting the energy storage capacitor C1 and the dc/dc converter load to the rectifier.

At power-up, if C1 is fully discharged and the instantaneous rectified AC line voltage is greater than approximately 50V, MOSFET Q1 turns on, keeping IGBT Q2 off; no charging current flows through the capacitor. If the instantaneous rectified voltage is lower than the sum of the storage capacitor and 50V, Q1 turns off and Q2 turns on, connecting the capacitor and the load to the rectifier.

Note (especially at power-up) that when Q2 turns off, Q2's VCE = VI - VLOAD rises to a large value, so R5 must be as large as possible to withstand about 0.5W of power. Increasing the value of R5 means that the value of R4 must be increased, which makes Q1 turn off more slowly and may cause glitches at start-up. For the values ​​of R4, R5 and D8, a balance must be made when practical. Considering that D8 limits the maximum gate voltage on Q1, its Zener voltage must be as close to the threshold voltage as possible, so that it is turned off faster through R4. A good choice for Q1 is BS170. A snubber network consisting of R6 and CS can be connected across the collector-emitter junction of Q2 to limit the noise generated.

When the actual load voltage is 340V, the reference voltage of IC1 is about 0.5V higher than its cathode, and the input voltage begins to conduct through the collector junction. This cathode voltage must be measured at 0.5V, 45μA, and must be taken into account if new values ​​for R1 and R3 are calculated. The simulation in Figure 2 does not take into account this input leakage, the switch is 310V.