Case Study: Building a Bridge Between Product Safety and EMC

The product safety analysis of an ac filter includes testing for capacitor discharge, current leakage, and dielectric strength, as well as inspection of the filter's chokes. The EMC analysis of an ac filter involves determining whether the filter provides adequate attenuation. The filter lies between the line impedance stabilization network and the intended load. By combining the product safety and conducted emissions tests, compliance engineers can determine optimum filter component values and subsequently the minimum and maximum component values for different loads.

To follow the methods used for this article, the influence of the shunt resistor, X and Y capacitors (specifically regarding capacitor discharge), current leakage, and hipot tests must first be understood. The values of the X capacitors and shunt resistor determine capacitor discharge rate. (The load, if turned on, also influences discharge rate, but for these experiments no loads were used.) The values of the Y capacitors determine the amount of current leakage from the mains power to ground. UL 1950 limits current leakage for ITE to 3.5 mA.² The Y capacitors also determine the hipot test current. There is no limit to the hipot current, but the current must be determined to properly set the hipot equipment. Setting the equipment's upper-current detection too low will result in defective operation.

The capacitor discharge test ensures that if an ac plug is abruptly removed from its receptacle, the voltage across the line and neutral terminals will not exceed a safe level. Per UL 1950, voltage across a capacitance greater than 0.1 µF must decay to 37% of the ac-input peak voltage in 1 second for type A equipment and 10 seconds for type B equipment. IEC 61010-1 requires that the pins not be hazardous (live) at 5 seconds after disconnection from the supply.³

Before each measurement, the X capacitance of the filter was measured using a network impedance analyzer. The X-capacitor value is the capacitance of the filter construction plus the parallel total of the actual values of the physical X capacitors (C3||C4). The resistance of the shunt resistor, R1, was measured with a multimeter to obtain R1_meas. C = capacitance of the X-capacitor system, as listed in Table I.

(1) C = (capacitance of the ac filter construction) + C3||C4

The capacitor voltage, V_cap, is to be 37% of V_pk after one time constant. Equation 4 derives the calculated capacitor voltage at any given time.

R = R1

The measured V_cap is within a few volts of the calculated value. Even though R1 was not physically placed in the ac filter, 10 MW was measured. The resistance was the resistance of the scope probe. Product safety engineers still must take into account the resistance value of the probe when calculating t, be-cause the probe is parallel with R1. Use a probe with a resistance at least 10 times greater than that of the shunt resistor. Figure 2 shows the discharge voltage of filter no. 1 after one time constant.

Figure 1. Schematic of an ac filter.

Figure 2. Discharge voltage of filter no.1 after one time constant.

Current Leakage

To minimize the calculated error for I_leakage, each Y capacitor was measured with a network impedance analyzer. To maintain a safety margin of 10% for the ac mains input frequency, the analyzer was set to 66 Hz. I_leakage included the contribution of current from the power cord and the ac filter construction, 18 µA and 3 µA, respectively. For I_leakage (equation 5) the variables are as follows: E_leakage, the input voltage to the circuit when measuring for leakage current, equals 288 V rms; f_leakage, the frequency of the ac voltage input when using a variable ac power source, equals 66 Hz; and C represents the capacitance of C1 or C2 in nF. I_leakage, the leakage current through one of the Y capacitors, was calculated and measured for each Y capacitor, C1 and C2. This was done by reversing the mains polarity.

Typical Y-capacitor values of 3.3–;4.7 nF result in leakage less than the 3.5 mA limit (see Table II). Because filter no. 4, however, has larger Y capacitors, its leakage current exceeds most safety standard requirements. Product safety engineers must account for more than the stated value of capacitance. The positive capacitance tolerance value is equally important. The Y capacitors set the current leakage, but the added tolerance could cause excess leakage.

Ac filter		Ileakage				Y-capacitor system measured at 66 Hz
No.	Calculated		Measured			Y capacitor, C1	Y capacitor, C2
1	570 µA		568 µA			4.6 nF	4.6 nF
2	558 µA		556 µA			4.5 nF	—
	606 µA		603 µA			—	4.9 nF
3	439 µA		435 µA			3.5 nF	—
	427 µA		432 µA			—	3.4 nF
4	4.07 mA		4.05 mA			33.9 nF	33.9 nF
Table II. Current leakage.

Dielectric Strength (Hipot)

From the previous current leakage experiment, we obtained the I_leakage for each of the ac filter configurations: E_hipot, the voltage to which the hipot equipment is set, equals 1500 V rms; E_leakage = 288 V rms; f_hipot, the frequency of the hipot equipment, equals 60 Hz; and f_leakage= 66 Hz. If the Y capacitors were not equal, the I_leakage was calculated to be the average measured current flowing through each Y capacitor.

Table III indicates that the typical Y-capacitor values of 3.3–;4.7nF result in hipot currents of 4.3–;5.9 mA. The high-value Y capacitor of filter no. 4 results in a high-level hipot current. Product safety engineers should make allowances for a variety of ac filters that could be tested on the production line. When setting hipot equipment for the current, a minimum current should also be included to alert an operator that the on-off switch is in the on position.

Ac filter	Ihipot		Total capacitance of Y capacitors	Y capacitor, C1	Y capacitor, C2
No.	Calculated	Measured
1	5.3 mA	5.9 mA	9.2 nF	4.6 nF	4.6 nF
2	5.5 mA	5.6 mA	9.4 nF	4.5 nF	4.9 nF
3	4.1 mA	4.3 mA	6.9 nF	3.5 nF	3.4 nF
4	38.0 mA	39.1 mA	67.8 nF	33.9 nF	33.9 nF
Table III. Current hipot.

Ac filter	R1	X capacitors Y capacitors					Choke
No.		C3	C4	C1	C2
1	330 kW	0.33 µF	0.33 µF	4.7 nF	4.7 nF		1 mH
2	417 kW	0.10 µF	0.10 µF	4.9 nF	4.9 nF		2.5 mH
3	1 MW	0.47 µF	0.47 µF	3.0 nF	3.0 nF		2 ¥ 50 µH
4	1 MW	0.47 µF	0.47 µF	33 nF	33 nF		2 ¥ 50 µH
Table IV. Ac filter components.

Chokes

The chokes of an ac filter are to be constructed to meet the intended safety standard. Per UL 1950, operational insulation is needed from line to line. UL 1950 requires 1.5 mm for clearance and, depending on the material, 1.6–;3.2 mm for creepage. There is no requirement for distance through the insulation. The dielectric strength from line to ground is 1500 V_pk. Per IEC 61010-1, basic insulation is needed from line to line. IEC 61010-1 requires 1.5 mm for clearance and, depending on the material, 1.6–;3.0 mm for creepage. The distance through the insulation is not addressed in IEC 61010-1. The dielectric strength from line to ground is 1900 V_pk.

If a product safety engineer intends to evaluate open-framed ac filters, then accessibility to the components must be considered to ensure operator and service personnel safety. Off-the-shelf filter components are most likely potted and metal encased, so the accessibility of the filter's ac input and output terminals is the primary concern.

The EMC analysis of an ac filter consists mainly of performing conducted emissions testing. Performing the emissions tests after the components have been selected based on safety testing minimizes testing time. Using this order, if a filter does not meet relevant conducted emissions requirements, its components can be easily changed and retested or EMC compliance. Once new values have been selected, a filter can be retested to ensure that it still meets safety requirements. Because ac filter components can be easily swapped and tested, a variety of components should be tried prior to conducted emissions testing. Because EMC equipment tends to be more expensive than safety test equipment, access to EMC equipment may not be as readily available as safety equipment.

Figure 3 graphs all four ac filter configurations. In addition to the four configurations, a test without the filter was performed to provide a base-line for a product started without filtering. The greatest amount of filtering can be seen after 1 MHz. Figures 4 and 5 expand the regions in which the product's frequencies can still be seen. Note that with different value combinations, engineers can identify values that maximize attenuation. Once those values are known, it is much easier to identify off-the-shelf ac filters that closely match engineered values. When selecting off-the-shelf filters, it is also important to consider the filter's mechanical size and ensure that the chokes can handle the current.

Figure 3. Conducted emmissions on the four ac filters.

Figure 4. Conducted emissions on ac filters no. 1 and 2, from 10 kHz to 1 MHz.

Figure 5. Conducted emissions on ac filters no. 3 and 4, from 10 kHz to 1 MHz.

The components used in a line filter can create problems for safety engineers, but with capacitance reduction and proper choke selection, most product safety problems can be avoided. Selecting parts based on safety requirements provides the parameters for identifying filter options for conducted emissions testing. By fabricating an ac filter with components that can be easily substituted, engineers can experiment with a variety of configurations and quickly determine whether specific component values will meet both safety and EMC requirements.