X电容与Y电容容量的计算

June 1996 ® TOPSwitch®Power Supply Design Techniques for EMI and Safety Application Note AN-15 Offline switching power supplies have high voltage and high current switching waveforms that generate Electromagnetic Interference (EMI) in the form of both conducted and radiated emissions. Consequently, all off-line power supplies must be designed to attenuate or suppress EMI emissions below commonly acceptable limits. This application note presents design techniques that reduce conducted EMI emissions in TOPSwitch power supplies below normally specified limits. Properly designed transformers, PC boards, and EMI filters not only reduce conducted EMI emissions but also suppress radiated EMI emissions and improve EMI susceptibility. These techniques can also be used in applications with DC input voltages such as Telecom and Television Cable Communication (or Cablecom). Refer to AN-14 and AN-20 for additional information. The following topics will be presented: • EMI Specifications for North America, European Community, and Germany • Measuring Conducted Emissions with a LISN • Peak, Quasi-Peak, and Average Detection Methods • Safety Principles • EMI Filter Components • Flyback Power Supply EMI Signature Waveforms • Filter Analysis • Power Cord Resonances • Transformer Construction Techniques • Suppression Techniques • General Purpose TOPSwitch EMI Filters • EMI Filter PC Layout Issues • Practical Considerations Figure 1. FCC Class A and B Limits (Quasi Peak). 120 A m pl itu de (d Bµ V) 100 80 60 40 20 0 0.01 0.1 1 Frequency (MHz) 10 100 PI -1 62 3- 11 16 95 EN55022A QP EN55022A AVG EN55022B AVG EN55022B QP Figure 2. EN55022 Class A and B Limits (Average and Quasi Peak). 120 A m pl itu de (d Bµ V) 100 80 60 40 20 0 0.01 0.1 1 Frequency (MHz) 10 100 PI -1 62 2- 11 16 95 FCCA QP FCCB QP 120 A m pl itu de (d Bµ V) 100 80 60 40 20 0 0.01 0.1 1 Frequency (MHz) 10 100 PI -1 83 4- 04 22 96 Vfg243 QP Vfg46 AVG 120 A m pl itu de (d Bµ V) 100 80 60 40 20 0 0.01 0.1 1 Frequency (MHz) 10 100 Vfg243 QP Vfg1046 QP (VDE0871B QP) PI -1 62 4- 11 16 95 Figure 4. Vfg243 (Quasi Peak) and Vfg46 (Average) Class B Limits.Figure 3. Vfg1046 and Vfg243 Class B Limits (Quasi Peak). AN-15 A 6/962 Safety is a vital issue which determines EMI filter component selection, the transformer reinforced insulation system, and PC board primary to secondary spacing. In fact, safety is an integral part of the power supply/EMI filter design and is difficult to discuss as a separate issue. Throughout this application note, design guidance will also be presented for meeting safety requirements in TOPSwitch power supplies. EMI Specifications The applicable EMI specification must be identified for the intended product family and target market. In the United States, the Federal Communications Commission (FCC) regulates EMI specifications. Canadian specifications are similar to FCC specifications. Figure 1 shows the conducted emissions limits governed by FCC rules, Part 15, subpart J. Note that specification limits are given only for quasi-peak detection methods. A recent part 15 amendment allows manufacturers to use the limits contained in C.I.S.P.R. Publication 22 as an alternative when testing devices for compliance(1). The member countries of the European Community (EC) have established a harmonized program for electromagnetic compatibility. EN55022 for Information Technology Equipment is one of the first harmonized documents. EN55022 together with companion measurement document C.I.S.P.R Publication 22 set the conducted emission limits shown in Figure 2 for information technology products marketed to the European Community. In fact, EN55022 limits are the same as C.I.S.P.R Publication 22 limits. Note that class A and class B specification limits are given for both average and quasi-peak detection methods(2) (3). Figure 3 shows the well-known and most stringent VDE 0871 specification (narrow band limits) for German markets which has traditionally been the design target. German regulation Vfg 1046/1984 requires Information technology or Electronic Data Processing Equipment to meet the VDE 0871 class B narrow band limits from 10 kHz to 30 MHz. Note that specification limits are given only for quasi-peak detection methods. When marketing products only in Germany, there is a choice between meeting the regulation requirements of Vfg 1046/1984 or the new German regulation Vfg 243/1991 (as updated by Vfg 46/ 1992) which has relaxed limits from 10 kHz to 150 kHz and is harmonized with EN55022 from 150 kHz to 30 MHz. Vfg243/ 1991 sets quasi-peak limits and Vfg 46/1992 adds mean or average limits as shown in Figure 4. Figure 3 also shows Vfg243/1991 class B quasi-peak limits to compare with VDE0871(4) (5) (6). The EMI filter designed to meet VDE 0871 (per Vfg 1046/1984) will generally be higher cost than the EMI PI-1625-111695 VSL VSN LF LF CF CF LINE + _ + _ INPUT OUTPUT NEUTRAL CC CC RSL RSN Figure 5. Line Impedance Stabilization Network (LISN). filter designed to meet Vfg/243 regulation requirements. Measuring Conducted Emissions Details of testing apparatus and methodology are governed by the various EMI regulations, but share the same general concept. Conducted emissions measurements are made with a Line Impedance Stabilization Network (LISN). Figure 5 shows the effective filter, represented by LF and CF, inside the LISN which passes line frequency currents but forces higher frequency power supply conducted emission currents to flow through coupling capacitor CC and sense resistor RS. A spectrum analyzer or EMI receiver reads the current emission signal magnitude as sensed voltages VSL and VSN across RSL and RSN in dBµV. A 6/96 AN-15 3 LISN Bonded to Reference Plane Non-conducting Table 40 cm 80 cm 80 cm 80 cm minimum height PI-1626-111695 Unit Under Test Load This Edge Flush Up Against Vertical Reference Plane PI-1627-111695 0ACCURRENT AC IN CIN L L V+ V- First Pulse Steady State Peak Current Conduction Time ≅ 3 mS ID Figure 7. Differential Mode Currents Charging Input Capacitor CIN. Figure 6. Typical Conducted Emissions Precompliance Test Set Up. Figure 6 shows a typical conducted emissions pre-compliance test setup on a wooden table at least 80 cm high constructed with non-metallic fasteners(7). The unit under test, LISNs, and load are all placed 40 cm from the edge of the table as shown. Six foot cables are used between the unit under test and both the LISN on the AC input and the load on the DC output. The LISN and load are each located 80 cm from the unit under test with excess cable bundled non-inductively. The edge of the table is placed flush against a vertical reference plane at least two meters square. The LISN is bonded to the reference plane with a low impedance, high frequency grounding strap or braided cable. In applications where the power supply and load are located in the same physical package, the cable can be omitted between the unit under test and the load. For design, investigation and precompliance testing, a spectrum analyzer is highly recommended compared to EMI receivers which are more expensive and more difficult to use. For conducted and radiated emissions testing, the spectrum analyzer should have a frequency range of 10 kHz to 1 Ghz, wide range of resolution bandwidths (including C.I.S.P.R. specified bandwidths of 200 Hz, 9 kHz, 120 kHz), built in quasi-peak detector, video filter bandwidth adjustment capability down to 3 Hz or below for average measurements, maximum hold for peak measurements, and an accurate and temperature compensated local oscillator capable of centering a 100 kHz signal in the display with insignificant frequency drift. The HP 8591EM and Tektronix 2712 (option 12)(8)are two examples of lower cost spectrum analyzers sufficient for conducted emissions precompliance testing. Peak, Quasi-Peak, and Average Detection Power supplies operating from the 50 or 60 Hz AC mains use a bridge rectifier and large filter capacitor to create a high voltage DC bus from the AC input voltage as shown in Figure 7. The bridge rectifier conducts input current for only a short time near the peak of AC mains voltage. Actual conduction time is typically 3 mS out of effective line frequency periods of 8.3 to 10 mS which defines an effective “line frequency duty cycle” of 30% to 36%. Conducted emission currents can flow in the AC mains leads (and are sensed by the LISN) only during the bridge rectifier conduction time. The conducted emissions signal is actually applied to the spectrum analyzer or receiver detector input only during bridge diode conduction time which defines a “gating pulse” with pulse repetitive frequency (PRF)(8)(9) equal to the AC mains frequency (50 or 60 Hz) and “line frequency duty cycle” just defined. The “gating pulse” effect due to bridge rectifier conduction time causes the measured signal magnitude to change depending on whether peak, quasi-peak, or average detection methods are used. A spectrum analyzer or EMI receiver displays the RMS value of the signal(9). For example, a 100 kHz continuous sinusoidal AN-15 A 6/964 voltage when viewed on an oscilloscope may have a peak voltage of 1 Volt and hence an RMS voltage of 0.707 Volts. The spectrum analyzer (with 50 Ω input) will display a value for this 100 kHz signal of 0.707 volts (or 117 dBµV or 10 dBmW) regardless of which detection method is used (peak, quasi-peak, or average) because the signal is continuous, narrow band, and not modulated or gated. If the signal was broadband, modulated, gated at a duty cycle, or in some other way not continuous, the displayed RMS value will change with the detection method. The measured display will then be the magnitude of an equivalent continuous sinusoidal signal with an RMS value equal to the RMS content of the LISN signal measured at the output of the detector stage. Peak detection is the simplest and fastest method when measuring conducted emissions. Resolution bandwidth is set to 200 Hz for measurements from 10 kHz to 150 kHz and set to 9 kHz for measurements from 150 kHz to 30 MHz. Sweep times are relatively low. When displaying emissions in real time with no averaging, the peaks are not constant but change in magnitude with each measurement sweep due to the bridge conduction gating pulse effect described above. Most spectrum analyzers have a “maximum hold” feature which displays the highest peak occurring over many measurement sweeps. The peak detector measures the magnitude of the largest signal occurring during the bridge conduction gating pulse. The average detector is simply a low pass filter with corner frequency sufficiently below the gating pulse repetitive frequency or PRF. In typical spectrum analyzers, the video filter bandwidth can be reduced to 30 Hz or below to average the signal but the sweep time must be increased for a calibrated measurement. For test purposes, the full conducted emissions range starting at 10 kHz (or 150 kHz or 450 kHz, depending on the regulation) up to 30 MHz should first be examined with a peak detection measurement. Peak detected emissions with insufficient margin compared to the regulation average limit should be centered on the spectrum analyzer display with the lowest possible frequency span per division setting before reducing video bandwidth and performing the average measurement sweep(10). Figure 8 shows typical conducted emissions from 10 kHz to 500 kHz with both peak detection and average detection. Note that peak detection picked up an envelope of high order harmonics from line frequency rectification in addition to the fundamental and first three harmonics of the 100 kHz switching frequency. The quasi-peak detector is designed to indicate the subjective annoyance level of interference. As an analogy, a soft noise that happens every second is much more annoying than a loud noise 110 A m pl itu de (d Bµ V) 100 90 80 70 60 50 40 30 20 100 200 Frequency (KHz) Peak Data Average Data 300 500400 PI -1 62 8- 11 16 95 Figure 8. Peak Data vs Average Data. that happens every hour. A quasi peak-detector (actually a calibrated, intermediate bandwidth video filter) behaves as a leaky peak detector that partially discharges between input signal pulses. The lower the pulse repetitive frequency (PRF), the greater the dB differential between the peak and quasi-peak measured response (8) (9). Quasi-peak and average detection methods will always give a lower measured value compared to peak detection. If a peak detector measurement meets the average or mean specification limit with sufficient margin, additional measurements using average detection are not necessary. When no average limit is specified, if the peak measurement meets the quasi-peak limit with sufficient margin, additional measurements using quasi- peak detection are not necessary. In general, when testing TOPSwitch power supplies to the C.I.S.P.R. Publication 22, EN55022, or Vfg 243/91(and Vfg 46/92) limits, peak measured data usually meets the quasi-peak limit but, in some areas, may have insufficient margin when compared with the average limit. In this case, further measurement is necessary using average detection. Safety Principles Safety principles must be examined before proceeding further with EMI filter concepts because safety requirements place several constraints on EMI filter design. Virtually all equipment including computers, printers, televisions, television decoders, video games, battery chargers, etc., must be safety recognized by meeting the safety standard for the intended market and carrying the appropriate safety mark. Safety principles are very similar among the various standards. This application note will focus on the electric shock hazard requirements of one popular standard, IEC950(11). A 6/96 AN-15 5 The European International Electrotechnical Commission Standard IEC950 entitled “Safety of Information Technology Equipment Including Electrical Business Equipment” provides detailed requirements for safe equipment design. Application of IEC950 is intended to prevent injury or damage due to hazards including electric shock, energy hazards, fire hazards, fire, mechanical and heat hazards, radiation hazards, and chemical hazards. IEC950 specifies the following definitions and requirements applicable to TOPSwitch power supplies. (This is only a partial list of the key requirements targeted specifically at typical TOPSwitch power supply implementations. The appropriate IEC950 section is identified by parentheses.) IEC950 Definitions (Applicable to TOPSwitch Power Supplies): (Introduction): Electric shock is due to current passing through the human body. Currents of approximately 1 mA can cause a reaction in persons of good health and may cause indirect danger due to involuntary reaction. Higher currents can have more damaging effects. Voltages up to about 40 V peak, or 60 VDC are not generally regarded as dangerous under dry conditions, but parts which have to be touched or handled should be at earth ground potential or properly insulated. (1.2.4.1): Class I Equipment: equipment where protection against electric shock is achieved by: a) using basic insulation, and also b) providing a means of connecting to the protective earthing conductor in the building wiring those conductive parts that are otherwise capable of assuming hazardous voltages if the basic insulation fails. (1.2.4.2): Class II Equipment: equipment in which protection against electric shock does not rely on basic insulation only, but in which additional safety precautions, such as double insulation or reinforced insulation, are provided, there being no provision for protective earthing or reliance upon installation conditions. (1.2.8.1): Primary circuit: An internal circuit which is directly connected to the external supply mains or other equivalent source. In a TOPSwitch power supply, this includes the EMI filter, discrete or common mode chokes, bridge rectifier, transformer primary, TOPSwitch, and any components connected to TOPSwitch such as primary bias windings and optocoupler output transistors. (1.2.8.2): Secondary circuit: A circuit which has no direct connection to primary power (except through properly selected Y-capacitors) and derives its power from a transformer. (1.2.8.5): Safety extra-low voltage (SELV) circuit: A secondary circuit which is so designed and protected that under normal and single fault conditions, the voltage between any two accessible parts, or between one accessible part and the equipment protective earthing terminal for class I equipment, does not exceed a safe value. (1.2.9.2): Basic Insulation: insulation to provide basic protection against electric shock. (1.2.9.3): Supplementary Insulation: Independent insulation applied in addition to basic insulation in order to ensure protection against electric shock in the event of a failure of the basic insulation. (1.2.9.4): Double Insulation: Insulation comprising both basic insulation and supplementary insulation. (1.2.9.5): Reinforced Insulation: A single insulation system which provides a degree of protection against electric shock equivalent to double insulation. (1.2.9.6): Working voltage: The highest voltage to which the insulation under consideration is, or can be, subjected when the equipment is operating at its rated voltage under conditions of normal use. (1.2.9.7): Tracking: the progressive formation of conducting paths on the surface of a solid insulating material (such as PC board or transformer bobbin) due to the combined effects of electric stress and electrolytic contamination on this surface. (1.2.10.1): Creepage distance: the shortest path between two conductive parts, or between a conductive part and the bounding surface of the equipment, measured along the surface of the insulation. In a TOPSwitch power supply, the most important creepage distance is between all primary circuits and all secondary circuits (typically 5mm to 6 mm). (1.2.10.2): Clearance: the shortest distance between two conductive parts, or between a conductive part and the bounding surface of the equipment, measured through air. (1.2.11.1): Safety Isolating Transformer: the power transformer in which windings supplying SELV circuits are isolated from other windings (such as primary and primary bias windings) such that an insulation breakdown either is unlikely or does not cause a hazardous condition on SELV windings. AN-15 A 6/966 IEC950 Requirements (Applicable to TOPSwitch Power Supplies) (1.4.5): In determining the most unfavorable supply voltage for a test, the following variables shall be taken into account: • multiple rated voltages • extremes of rated voltage ranges • tolerance on rated voltage as specified by the manufacturer. If a tolerance is not specified, it shall be taken as +6% and - 10%. (1.6.5): Equipment intended to operate directly from the mains supply shall be designed for a minimum supply tolerance of +6%, -10%. (2.1.10): Equipment shall be so designed that at an external point of disconnection of the mains supply, there is no risk of electric shock from stored charge on capacitors connected to the mains circuit. Equipment shall be considered to comply if any capacitor having a rated capacitance exceeding 0.1 uF and connected to the external mains circuit, has a means of discharge resulting in a time constant not exceeding 1 second for pluggable equipment type A (non-industrial plugs and socket-outlets). This requirement specifically applies to any EMI filter capacitor connected directly across the AC mains which could cause a shock hazard on the exposed prongs of an unplugged power cord. (5.2.2): Earth Leakage Current: Maximum