Differential-Mode Emissions In Fig. 13-5, the primary purpose of the capacitor C F, however, is to filter the full-wave rectified ac line voltage. The filter capacitor is therefore a large-value, high-voltage capacitor (typically 250 to 1000 mf with a voltage rating of 250 V or more), and it is far from an ideal capacitor. 18
Differential-Mode Emissions The simplified differential-mode equivalent circuit The differential-mode current, and therefore the LISN voltage, are both determined primarily by the parasitics ( L F and R F ) and the mounting of the filter capacitance. C F 19
Effect of the Filter Capacitor s ESL (Neglecting and assuming Z 0. ) C F R F 20
Once a fundamental frequency for the power supply is chosen, the only parameter under the designer s control, to reduce the maximum value of the differential-mode conducted emission, is the parasitic inductance L F of the input ripple filter capacitor. Example 13-2 Comparisons of Example 13-1 and 13-2. In high-voltage low-current supplies, the common mode emission will predominate, but in low-voltage high-current supplies the differential mode emission will predominate. JHLin, AppEMC; Conducted Emission 21
The criteria for the common-mode emission to be predominate is Effect of the Filter Capacitor s ESR At power line frequency (50 or 60 Hz), the capacitance reactance is the dominant impedance of this capacitor. Above about 1 MHz, the inductive reactance becomes the dominant impedance of the capacitor. At frequencies somewhere in between, the resistance will be the dominant impedance. JHLin, AppEMC; Conducted Emission 22
Differential-Mode Emissions The criteria for this break point to be equal to or less than 500 khz is that 23
Differential-Mode Emissions Some power converter designs use two filter capacitors in series for the input ripple filter capacitor. This is done to increase the voltage rating of the capacitor or to create a voltage doubler configuration so that the supply can work off of either a 115 V or 230 V ac power line. 24
Differential-Mode Emissions Notice also that, in this case, the common-mode and differential-mode currents add together when flowing through the LISN impedance that is connected to the phase (hot) conductor and subtract from each other when flowing through the LISN impedance connected to the neutral conductor. JHLin, AppEMC; Conducted Emission 25
Rectifier Diode Noise When a diode is forward biased, charge is stored in its junction capacitance. When the diode is turned off (reverse biased), this charge must be removed. This is referred to as diode reverse recovery; it produces a sharp negative spike on the voltage waveform when the diode turns off, which can produce substantial ringing and be a source of high frequency, differential-mode noise. Fast-recover diodes are usually preferred by power supply designers because they dissipate less power and are therefore more efficient. Fastrecovery diodes, however, produce higher frequency noise spectra than soft-recovery diodes. The primary offenders in this respect are the secondary side rectifiers, because these diodes operate at a much higher current level than the primary rectifiers. JHLin, AppEMC; Conducted Emission 26
Rectifier Diode Noise These noise pulses can be conducted out of the power supply secondary and/or can be coupled back through the switching transformer to the primary side of the supply. In both cases, the diode noise manifests itself as a differential-mode conducted emission. The snubber network consists of a series R C circuit. Typical values might be 470 pf and 10. 27
Rectifier Diode Noise Another approach is to add a small ferrite bead in series with each rectifier diode. JHLin, AppEMC; Conducted Emission 28
Power-Line Filters The power-line filter is a low-pass L C topology. The source (the power supply) and the load (the LISN) impedances determine the exact configuration of the filter. Because filter attenuation is a function of impedance mismatch, the role of a power-line filter is to maximize the mismatch between the source and load impedances. For common-mode noise, JHLin, AppEMC; Conducted Emission 29
Power-Line Filters The maximum value of the line-to-ground capacitors is limited because of leakage requirements imposed by various safety agencies. To obtain the large inductance required to suppress the lower order harmonics of the switching frequency, L 1 is wound on a high permeability core. Common-Mode Filtering In actual practice, the line-to-ground capacitors usually have a value of one half the maximum allowable by the leakage requirements. Typical values for the choke are from 2 to 10 mh. Differential-Mode Filtering To differential-mode noise, the two Y-capacitors are connected in series. These capacitors only contribute to the differential-mode attenuation above about 10 MHz where it is usually not required. Therefore, they are usually ignored with respect to differential-mode filtering. JHLin, AppEMC; Conducted Emission 30
Power-Line Filters To provide a significant amount of differential-mode capacitance, a line-to-line capacitor C 3 (X-capacitor) is added to the power line filter. Typical values for this capacitor range from 0.1 to 2. For safety reasons, a resistor, typically 1, is sometimes added in parallel with this capacitor. A second X-capacitor located across the power line and located on the power supply side of the common-mode choke can be helpful. Leakage Inductance F M Leakage inductance of the common-mode choke is important in power line filters because it determines the degree of differential-mode inductance present. As a result of leakage inductance, each winding of the choke will have in series with it a small differential-mode inductance. This differential-mode inductance along with the X-capacitor forms an L-C filter, which provides differential-mode filtering. Too much leakage inductance, however, can cause the common-mode choke to saturate at a low value of ac power current. Typical power line chokes will have leakage inductances somewhere between 0.5 and 5% of their common-mode inductance. 31
The leakage inductance of a common-mode choke can easily be measured by shorting one of the windings and measuring the inductance across the other winding. 32
Power-Line Filters The common-mode filter is usually designed first and then the differential mode filter is designed by starting with the leakage inductance of the common-mode choke, and choosing a value for the line-to-line capacitor C 3, to provide the required attenuation. If additional differential-mode attenuation is required, Values for these differential-mode inductors are typically a few hundred microhenries. 33
JHLin, AppEMC; Conducted Emission 34
Filter Mounting Power-Line Filters The performance of this filter is as much, if not more, a function of how and where it is mounted, and how the leads are routed, as it is of the electrical design of the filter. Problem 1: the filter is not mounted close to the point where the power line enters the enclosure. Problem 2: the wire grounding the filter to the enclosure has a large inductance, which decreases the effectiveness of the Y-capacitors in the filter. JHLin, AppEMC; Conducted Emission 35
Power-Line Filters Problem 3: capacitive coupling occurs between the noisy power-supply-to-filter wiring and the ac power line. The cable between the filter and the power supply should be routed close to the enclosure to minimize any pickup. The filter s input leads should also be kept away from any signal cables (especially digital cables) and should not be routed over, or near, a digital logic PCB. JHLin, AppEMC; Conducted Emission 36
Power-Line Filters An additional improvement over the arrangement shown in Fig. 13-23 is to mount the power supply directly adjacent to the power line filter. A power-line filter having an integral ac power cord connector as shown in Fig 13-24. JHLin, AppEMC; Conducted Emission 37
Power-Line Filters Power Supplies with Integral Power-Line Filters Some switched-mode power supplies have the power-line filter built into the supply on the same PCB as the power converter. This is usually done to reduce size and costs. However, this arrangement often violates some, if not all, rules for proper filter mounting and wiring discussed above. Problem 1: long traces (too much inductance) connecting the Y-capacitors to the enclosure. Problem 2: Magnetic coupling to the unshielded common-mode choke. This problem can be overcome by proper layout and orientation of the common-mode choke on the board, or by placing a shield over the choke or power line filter portion of the board. Problem 3: Input and output traces to the filter, routed in such a way as to maximize the parasitic capacitance between the two, thus coupling noise around the filter to the power line. Filters integral with the power supply can be effective, but only if all the issues previously discussed relating to proper filter mounting and layout are considered during the design process. 38
Power-Line Filters High-Frequency Noise (> 10 MHz) The high- frequency attenuation of the power supply noise is limited primarily by the interwinding capacitance of the common-mode choke and the inductance in series with the Y-capacitors. The best way to deal with this problem is at the source, the digital logic PCB. JHLin, AppEMC; Conducted Emission 39
Power-Line Filters The capacitor value should be chosen to have an impedance less than a few ohms at the lowest frequency of interest. A 1000-pF capacitor is usually satisfactory, if filtering is only required above 30 MHz. Below 30MHz, a 0.01- F capacitor would be preferred. The ferrite bead material should be chosen to provide about 50 of impedance at the lowest frequency of interest and the ferrite must be capable of carrying the output current without saturation. JHLin, AppEMC; Conducted Emission 40
Primary-to-Secondary Common-Mode Coupling JHLin, AppEMC; Conducted Emission 41
Primary-to-Secondary Common-Mode Coupling JHLin, AppEMC; Conducted Emission 42
Primary-to-Secondary Common-Mode Coupling Usually, a Y-capacitor is used with a value of 1000 to 4700 pf. To be effective, the bridge capacitor must be placed on the PCB in a location that minimizes the trace inductance (use short, wide traces) in series with it, and the traces must maintain a small loop for the common-mode current. Other ways to eliminate or minimize this problem are by using a transformer that contains a Faraday shield, or by adding a commonmode choke in the dc output-leads to reduce the common-mode current. JHLin, AppEMC; Conducted Emission 43
Frequency Dithering Vary the switching frequency over a narrow range to spread the energy out over a larger frequency band, which decreases the peak amplitude at any one frequency. Several integrated circuit (IC) pulse-width-modulation power supply controllers have this feature built in. The modulation waveform is usually a triangular wave with a frequency of a few hundred Hz and a maximum deviation of a few khz. In products with a low leakage requirement where Y-capacitors cannot be used in the power-line filter, the use of frequency dithering and/or a bridge capacitor are extremely useful techniques to reduce the commonmode emissions. 44