any products including power supplies and household products are connected to the ac line. Within these products we find the so-called X capacitors used to provide line filtering. If no measures are taken to discharge these capacitors, they can retain a high-voltage charge even after ac power is removed from the product.

So, when the consumer removes the ac power cord plug of one of these products from the ac socket, there may be significant voltage remaining on the X capacitors. This high voltage may cause a shock to the consumer if their fingers touch the metal prongs of the ac plug during or shortly after its removal from the wall socket.

There are several regulations from the IEC/EN to prevent the consumer from being shocked if their fingers should touch the metal prongs of the ac power plug. These regulations are based on the IEC60335^[1], IEC62368^[2], and IEC/EN/ UL 60950^[3] standards.

IEC62368 applies to IT equipment and video and audio equipment, while the IEC60335 concerns appliances. Meanwhile, UL/EN/IEC 60950 is the harmonized standard for power supplies. UL has regulated the discharge of X capacitors for many years for TVs, where the X capacitor typically had a resistor value between a 1.0 MΩ and 2.2 MΩ to discharge them.

Satisfying the safety requirements for the discharge of X capacitors would be straightforward were it not for the energy efficiency standards, which are pervasive and over time are becoming more demanding. The simple act of using discharge resistors in a product's power supply can raise its power consumption under no-load or standby conditions to the point where it may exceed the mandated levels. This article discusses both the safety and energy efficiency requirements, looks at the different circuit approaches to discharging X capacitors and how well they do in terms of satisfying the energy efficiency regulations.

While the focus here is mainly on the X capacitors, note that there also Y-capacitors. Nick Davis ^[4] describes these X and Y capacitors along with the IEC ratings in his article, "Safety Capacitors First: Class-X and Class-Y Capacitors". 

The Y capacitor is connected from the mains to ground while the X capacitor is connected across the ac power mains. Fig. 1 shows both capacitor types. The Y capacitors are often ceramic and less than 0.0033 µF or 3.3 nF. The X capacitors cited in the standards are greater than 0.1 µF. These X capacitors are to be discharged to less than 34 V peak in 1 second after the power plug is removed from the socket. While it's only the X capacitors that pose the safety hazard, the Y capacitors are important to the discussion because they contribute to the ac leakage current.

Therefore, they also impact a product's ability to meet the energy efficiency rules.

Fig. 1. Generic schematic showing the front end of a power supply including discharge resistors, R, common-mode choke, differential-mode choke, X film capacitors, and Y ceramic capacitors for a product. The X capacitors are for EMI control, and the Y capacitors are limited in value due to leakage current to ground.

Discharge Resistor Power Consumption

While a product is plugged in, any discharge resistors associated with X capacitors will be continuously dissipating power, contributing to the product's continuous power loss.

The power loss produced by an X capacitor discharge resistor is a function of the ac line voltage and the resistor value:

Power Loss = (Vac Line)2/R

In the U.S., Canada, and Mexico, Vac is 120 Vrms nominal and R is typically 1 MΩ, so:

Power Loss = 0.0144 W or 14.4 mW

In Europe, Vac is 230 Vrms nominal and R is typically 1 MΩ, so:

Power Loss = 0.0529 W or 52.9 mW

This wattage seems small until you take into account the standby power requirements for some products. The U.S. Department of Energy ^[5] and the California Energy Commission ^[6] have new requirements for standby energy consumption that are, in the most stringent case, less than 2X the loss imposed by the discharge resistor. The no-load mode power is given below in Table 1.^[7]

Table 1. Efficiency requirements for single-voltage power supplies.

Discharge Time for X Capacitors

According to IEC 60335^[1] and IEC 62368^[2], the voltage across the X capacitors must be below 34 V, in less than 1 second. The X capacitors are part of the line-conducted EMI filter, and the values of these capacitors are determined at the timeline-conducted EMI testing is performed. The design engineer should use place holders on the pc board so the different values can be tried.   

Table 2 was created to show the value of the two resistors in series in Fig. 1 to discharge the total of X2 capacitors. The value of the X capacitors ranges from 0.1 µF to 5.6 µF. The voltage used for the calculation was 230 Vac. In addition, the power dissipation was calculated for the resistors in series. The value of the capacitance is the sum of the EMI capacitors including X-1, X-2, X-3 and the MOV capacitance. 

Table 2. Discharge resistor values for power supply circuit in Fig. 1.

The power dissipation shown exceeds the 0.1 W in the U.S. Department of Energy's standby power no-load regulations for some low-wattage power supplies and battery chargers of cell phones. To reduce the power dissipation in the standby mode, the semiconductor industry created X2 discharge ICs such as the Power Integrations CAPZero ^[8] family of parts. In a nutshell, these ICs disconnect the discharge resistors from the circuit when a product is plugged in and reconnect these resistors at the time ac power is removed. In this way, the discharge resistors are not dissipating unnecessary power that contributes to the product's standby or no-load power consumption.

Other companies have similar parts such as the TEA1708^[9] and HF81.^[10] This approach is shown in Fig. 2, which is taken from the Power Integrations Application Note AN-48.^[15]

Fig. 2. CAPZero X capacitor discharge IC approach. While the product is plugged in, the CAPZero IC disconnects the discharge resistors so that they are not dissipating power.

The semiconductor industry has found other ways to discharge the X capacitors and these methods have been implemented in pulse width modulation control ICs for the power supplies. This approach eliminates a separate X capacitor discharge control IC.

Some of these ICs were found via web search using "X capacitor discharge" as the search topic: FAN6756,^[11] NCP12400,^[12] TEA19363,^[13] and BM1C102F. ^[14] This is not a complete list. The PWM control IC approach shown in Fig. 3 is taken from the NCP12400.

The application notes AN-48^[15] from Power Integrations was helpful in determining the values of the discharge resistors and the time to discharge the X capacitors. The X capacitors used in the EMI or RFI filter capacitors can be classified as either X1 or X2 depending upon the construction of the capacitors; reference 4 explains the difference and uses of these capacitors.

Fig. 3. The discharging of the X capacitors using the NCP12400.

Summary

This article explains why there are discharge resistors across the X capacitors of a product. The resistor is used to discharge the voltage left on the capacitor to a safe level so as not to cause an electrical shock to the consumer when the product in unplugged. The time allowed to discharge the capacitor is 1 sec.

However, whenever a product is plugged in, the discharge resistors are consuming power, unless there is a control IC used to eliminate the resistors' constant current draw. Without such a control IC, the power draw of the discharge resistors contributes to the product's power consumption in the standby or no-load mode and may exceed the power consumption mandated by regulatory agencies, including the U.S. Department of Energy and the California Energy Commission. Similar regulations apply in Europe. 

References:

1. IEC 60355.
2. IEC 62368.
3. UL 60950-1.
4. "Safety Capacitors First: Class-X and Class-Y Capacitors" by Nick Davis, All About Circuits, May 06, 2019.
5. U.S. Department of Energy.
6. California Energy Commission.
7. https://ac-dc.power.com/green-room/regulations-agency/us-doe-eisa2007-energy-independence-and-security-act/ 
8. CAPZero-3 and CAPZer0-2 product page.
9. TEA1708T data sheet.
10. HF81 data sheet.
11. FAN6756 product page.
12. NCP12400 product page.
13. TEA19363 data sheet.
14. BM1C102F data sheet.
15. "CAPZero Design Considerations," Power Integrations application note AN-48.

Acknowledgements

I would like to thank Edward Ong of Power Integrations and Carl Walding of ON Semiconductor for their input to this article. - Jim Spangler

Authors:

Kevin Parmenter Director of Applications Engineering Taiwan Semiconductor America

Jim Spangler President Spangler Prototype Inc. (SPI)

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