Rectifier Bridge Sizing Formulas

Bridge rectifier formulas: the DC voltage from the AC voltage, the peak reverse voltage, the diode current and the power lost in the diodes.

The formulas

Vrms is the RMS AC voltage in volts, Vf the forward drop of one diode, Idc the load current and VL the line voltage of a three-phase supply. The 0.9 is 2√2 / π, the average of the rectified wave, and 1.35 is 3√2 / π for a three-phase bridge with the line voltage. The figures are for a resistive load without a filter capacitor. With a capacitor the DC voltage is closer to the peak, and the peak current in the diodes is much higher than the average. Pick a diode with a reverse voltage rating well above the PIV (at least about 1.5 to 2 times) and a current rating that covers the surge. The values here are examples.

Worked example: 24 V RMS at 5 A, silicon diodes

Vf = 0.7 V.

1. Vpk = √2 × 24 = 33.94 V, so each diode needs a PIV above 33.94 V
2. Vdc = 0.9 × 24 − 2 × 0.7 = 20.2 V
3. Id = 5 / 2 = 2.5 A and the loss is 2 × 0.7 × 5 = 7 W

Worked example: a three-phase bridge on 400 V

1. Vdc = 1.35 × 400 = 540 V

Peak voltage and ideal DC voltage by AC voltage

Frequently asked questions

Why are two diode drops lost?

In a single-phase bridge the current flows through two diodes at a time, one on each side of the load.

What changes with a smoothing capacitor?

The DC voltage rises toward the peak and the diodes carry short, large pulses, so choose them for the peak current and not only the average.

What is the PIV?

The peak reverse voltage the diode must block. In a bridge it is the peak of the AC voltage, and a margin is added for surges.

Does the diode need a heat sink?

It may. The loss is the forward drop times the current, so check the heat against the data sheet and the thermal resistance in the heat sink article.

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