Careful Rectifier Diode Choice Simplifies and Reduces the Cost of EMI Filter Circuitry

This circuit simplifies and reduces the cost of the EMI filter circuitry in AC/DC converters.

Meeting EMI standards with AC/DC power supplies requires a number of EMI filter components, such as X and Y capacitors. The standard input circuitry of AC/DC power supplies includes a bridge rectifier, which rectifies the line voltage (typically 50-60 Hz). Since this is low frequency AC input voltage, standard diodes, such as the 1N400X series of diodes, are used since they are the least expensive.

These filter components are used to reduce the EMI generated by the power supply in order to meet published EMI limits. However, the standard diodes used in the bridge rectifier (see Figure 1) have slow reverse recovery times and are typically not directly associated with EMI generation since measurements used to record EMI start only at 150 kHz while the AC line frequency is only 50 or 60 Hz.

However the input filter circuitry in the past has on occasion included capacitors in parallel to the bridge rectifier diodes to damp out any high frequency waveforms caused by the rectification of the low frequency line voltage.

Use of fast recovery diodes in the bridge eliminates the need for these capacitors. These diodes recover very quickly when the voltage across them reverses (see Figure 2). This reduces excitation of stray line inductance in the AC input line by reducing the subsequent high frequency turnoff snap and hence EMI. Only 2 of the 4 diodes need to be fast recovery type, since 2 diodes conduct in each half cycle. As such, only one of the two diodes conducting during each half cycle is required to have a fast recovery characteristic.

Figure 1. Typical input stage of an SMPS using bridge rectifier at AC input

Figure 2. Input Voltage and current waveforms showing diode snap at end of reverse recovery