AN-9752

APPLICATION NOTE

© 2011 Fairchild Semiconductor Corporation

www.fairchildsemi.com

Rev. 1.0.1 • 1/23/13

2

Semiconductor’s mWSaver™ technologies offering best-in-

class minimum power consumption in no-load and light-

load conditions to meet the latest ENERGY STAR

specifications and 2013 ErP Standby Power Regulation

(less than 0.5 W consumption with 0.25 W load for ATX

power and LCD TV power).

The EMI filter in the front end of the Switched-Mode

Power Supply (SMPS) typically includes a capacitor across

the AC line connector, as shown in Figure 2. Most of the

safety regulations, such as UL 1950 and IEC61010-1,

require the capacitor to be discharged to a safe level within

a given time after the power supply is unplugged from the

power outlet.

Figure 2.

Typical Circuit of Line EMI Filter

 UL1950: voltage across a capacitance greater than 0.1 µF

must decay to 37% of the AC input peak voltage in one

second for type-A equipment and 10 seconds for type-B

equipment.

 IEC61010-1: The pins must not be hazardous (live) at

five seconds after disconnection from the supply.

The discharge resistor must comply with Equation (1) to

meet the discharge time of less than one second. The power

loss of discharge resistor paralleled with X-cap is shown in

Equation (2):

s

R

C

DIS

X

DIS

1

≤

×

=

τ

(1)

DIS

AC

Loss

R

V

P

2

(RMS)

=

(2)

Table 1 shows the relationship between rated output power,

typical effective X capacitor value, and power dissipation of

the discharge resistor. As the power level increases, the

EMI filter capacitor tends to increase and, therefore,

requires a smaller discharge resistor to maintain the same

discharge time. This typically results in more power

dissipation in high-power applications. Power dissipation in

the discharge resistor is a major cause of standby power

consumption in high-power applications.

Table 1. Power Dissipation in Discharge Resistor

for Different Rated Power System

Effective

X-CAP

Typical Rated

Output Power

Discharge

Resistor

Power

Dissipation in

Discharge

Resistor at

250 nF

20~50 W

4 MΩ

14.4 mW

500 nF

50~100 W

2 MΩ

28.8 mW

1 µF

100~200 W

1 MΩ

57.6 mW

2 µF

200~400 W

500 kΩ

115.2 mW

4 µF

400~800 W

250 kΩ

230.4 mW

8 µF

800~1,600 W

125 kΩ

460.8 mW

developed to eliminate X-cap discharge resistors while

meeting safety requirements.

2.1 Proposed Solution

Figure 3 shows the typical application circuit and internal

the filter capacitor only when the power supply is

unplugged from the power outlet. Since the AX-CAP

discharge circuit is disabled in normal operation, the power

loss in the EMI filter can be virtually removed. In normal

operation, the line voltage is detected by sensing X

capacitor voltage with a switched voltage divider composed

narrow pulse such that the power consumption of the

voltage divider can be minimized.

Figure 3.

AX-CAP

zero crossing of X capacitor voltage. In normal operation, X

capacitor voltage drops down to zero repetitively as long as

it is connected to the line. Once the power supply is

disconnected from power outlet, the bridge rectifier is

reverse biased and the only discharge path for the X

capacitor is through the switched voltage divider. Then, the