Safety standards such as EN60950 require that any X-capacitors in EMC filters on the AC side of the bridge rectifier quickly discharge to a safe level when AC is disconnected. This discharge requirement ensures that any high-voltage level present at the pins of the AC plug does not present an electric shock hazard. The standards require that the voltage across the X-capacitor decay with a maximum time constant of 1 second. Typically, this requirement is achieved by including a resistive discharge element in parallel with the X-capacitor. However, this resistance causes a continuous power dissipation that impacts the standby power performance. The power dissipation in the discharge resistors depends on the X-capacitor value. Assuming that the discharge resistor meets the 1-second time-constant requirement, (in other words, the R-C product is 1 second) the dissipation is described in Equation 2.
Thus at 230 VAC, the discharge resistor causes 5.3-mW dissipation for every 100 nF of X-capacitance – for a typical 470-nF X-capacitor value, that causes 25 mW to be lost in the discharge resistors.
The safety standard does not mandate that the X-capacitor is fully discharged to zero within one second. It simply requires the discharge rate to exhibit a 1-s time constant. Figure 18shows the discharge characteristic (for a 1-s discharge time constant) versus time, for disconnection at the peak of 90 VAC, 115 VAC, 230 VAC and 264 VAC. For AC inputs above 115 VAC, with 1-s discharge time constant, the voltage does not drop below the Safety-Extra-Low-Voltage (SELV) 60-V level until 1 s or longer. In fact, at 264 VAC, 1.83 seconds elapse before reaching 60 V.