Use a three-phase rectifier and voltage reducer for offline single-phase supplies

JB Castro-Miguens, Cesinel, Madrid, Spain; C Castro-Miguens, University of Vigo, Spain; M Pérez Suárez, University of Vigo, Spain; and A Abal Pena, Tekplus, Vigo, Spain - April 6, 2012

Some industrial applications require you to feed a low-power dc/dc converter from a three-phase source and possibly to deal with line-toline voltages of 200 or 400V rms. Further, the neutral terminal of the ac source may be unavailable for connection, forcing you to use a line-to-line voltage link, which means a higher input voltage. Voltage faults can happen in one or two phases, and some applications require the power supply to keep working despite this problem. To handle these problems, you can use an uncontrolled rectifier and a capacitor to convert three-phase ac into a dc voltage, but this voltage may be higher than the maximum input voltage that a standard converter supports. It is difficult to find a dc/dc converter for these voltages, which are typically 564V dc or higher.

Use a three-phase rectifier and voltage reducer for offline single-phase power supplies figure 1

You can use the circuit in Figure 1 to obtain a dc voltage lower than a given value, which the ratio of R₁ to R₃ sets. This value is approximately 340V peak with the values in the figure, from a 400V-rms or higher line-to-line three-phase voltage. You can feed this value from two or three phases with or without the neutral wire or from just one phase with the neutral wire. This circuit omits two diodes from the typical three-phase rectifier bridge and includes a neutral-wire diode pair to obtain voltages below 340V across bulk capacitor C₁ and to reach 0V for initial start-up (Figure 2). If you connect the neutral wire, you must link it to a two-diode arm of the rectifier to achieve a 0V start-up; however, you can randomly connect the phase wires.

Use a three-phase rectifier and voltage reducer for offline single-phase power supplies figure 2

Shunt regulator IC₁ acts as a comparator. After the initial start-up, whenever the instantaneous rectified voltage, V_I, is higher than 340V, the IC₁ reference-to-anode voltage is higher than its internal reference of 2.495V, decreasing the anode-to-cathode voltage to approximately 2V and turning off Q₂. When rectified voltage is lower than 340V, IC₁ does not sink current. Thus, Q₂ turns on through the R₂ bias, connecting bulk capacitor C₁ and the dc/dc-converter load to the rectifier.

At turn-on, if C₁ is completely discharged and the instantaneous rectified ac-line voltage is greater than approximately 50V, MOSFET Q₁ is on, keeping insulated-gate bipolar transistor Q₂ off; no charging current flows into the capacitor. If the instantaneous rectified voltage is lower than the voltage across the bulk capacitor plus 50V, Q₁ is off and Q₂ turns on, connecting the capacitor and the load to the rectifier.

Notice that, especially at turn-on, the value of Q₂’s V_CE=V_I−V_LOAD rises to large values when Q₂ turns off, so R₅ must be as large as possible and must handle approximately 0.5W of power. Increasing the value of R₅ means that you must increase the value of R₄, more slowly turning off Q₁ and possibly resulting in a start-up malfunction. You must reach a practical compromise for the values of R₄, R₅, and D₈. Considering that D₈ limits the maximum gate voltage at Q₁, it must have a zener voltage as close as possible to the threshold voltage for a faster turn-off through R₄. A BS170 is a good choice for Q₁. You can add a snubber network comprising R₆ and C_S across the collector-emitter of Q₂ to limit the generated noise.

With the actual load voltage at 340V, IC₁’s reference voltage is approximately 0.5V higher than its cathode, and the input transistor begins to conduct through the base-collector junction. You must measure this cathode voltage at 45 μA at 0.5V, and you must account for this value if calculating new values for R₁ and R₃. The simulation in Figure 2 does not account for this input leakage and appears to switch at 310V.