EMC (Electromagnetic Compatibility) has become a household name over the past decade. In the mid-1990s, Europe mandated reductions in radiated and conducted emissions from products sold within the region. Since then, many products have incorporated EMC testing into their design phase, a trend that continues to this day. A frequently asked question is: What is EMC? EMC is the ability of a component, product, or system to function properly in a predetermined electromagnetic environment (where electromagnetic interference (EMI) exists) without degrading or becoming a source of interference. To achieve this functionality, EMC standards must be followed, established by organizations such as the IEC and CISPR. This article will discuss EMC regulations concerning radiated and conducted emissions, including common and differential mode emissions, explore how to design power line filters to reduce input and output noise, and finally provide some printed circuit board design techniques to reduce noise. 1 EMC Regulations To achieve reliable EMC designs, it is essential to understand EMC requirements. These requirements apply not only to modular power supplies but also to system-level standards shared by Europe and North America. The IEC (International Electrotechnical Commission) is responsible for developing European specifications, while CISPR (International Special Committee on Radio Frequency Interference) is responsible for EMC testing using CISPR 22, which defines the most stringent limits for conducted emissions. These limits (conducted emissions) are now described by product standards EN55022 and EN55011. Class A and Class B requirements refer to industry standards and domestic standards, respectively. European standards impose two limits depending on the antenna used for testing noise. The higher limit applies to quasi-peak antennas, while the lower limit applies to general antennas; both limits must be met for the device to pass the requirements. The FCC standards used in North America are similar to the European EN requirements. Two European standards, EN55011 and EN55022, are used when testing power supplies. In North America, radiated EMI is typically measured in the 30MHz to 10GHz frequency range (according to FCC regulations), while conducted EMI is typically measured in the range of several to 30MHz (according to FCC regulations). The goal here is to develop systems that meet all or some of the aforementioned emission-related requirements, either as standalone devices or integrated into larger systems. 2. Common-Mode and Differential-Mode Noise Common-mode and differential-mode noise are two main sources of noise. Common-mode noise originates from common-mode current. Common-mode energy coexists on both power lines of a single-phase system and travels in the same direction between all conductors and ground, and along all power lines or conductors. Because both conductors have the same voltage level simultaneously, there is no attenuation between the conductors. Common-mode noise from common-mode current is always present on the cables entering the equipment. One way to reduce this current is to test the cables on the original model as early as possible (so that designers can make any necessary changes before the final design is delivered to production), and before performing EMC compliance testing. In many cases, if a device fails the common-mode current test, it will also fail the radiated emissions test. Common-mode current can be easily tested using a current probe with high-frequency clamping and a spectrum analyzer. A current probe with a response range up to 250MHz is sufficient. Differential mode noise is the opposite of shared mode noise. Differential mode noise is generated by the refraction of current through a charged or neutral conductor from another conductor. This creates a noise voltage between the charged and neutral conductors. 3 AC Power Line Main Filter This is an example illustrating a single-phase AC power supply filter. This type of filter is often used to reduce differential mode and shared mode noise in the input and output power supplies. 4.1 Part A: Inductors L1/L2 and capacitor C1 form a differential filter to handle all noise attempting to enter the power supply. Differential mode noise is generated by the refraction of current through a charged or neutral conductor from another conductor. The combination of L1 and C1 or L2 and C1 forms a voltage divider. Depending on the frequency of the noise, capacitor C1 presents a smaller impedance (larger load) to the signal, thus reducing noise on the power line. For example, at a specific frequency, if the equivalent impedance of L1 is 10KΩ and the equivalent impedance of C1 is 1KΩ, then the noise passing through the filter is one-tenth of its original strength, or a reduction of 20dB in noise. 4.2 Section B capacitors C2 and C3 constitute a shared-mode filter with a ground reference. Common-mode noise becomes significant when the current is in phase with the current in the charged and neutral conductors and returns via a safe ground. This generates a noise voltage between the charged/neutral conductors and ground. C2, C3, C4, and C5 are all equal, and all shared-mode noise on these lines will be shunted to ground. Note that Section B cannot be used in medical devices due to leakage current. 4.3 Section C unreferenced Zorro inductors (shared-mode chokes). Choosing the direction of each winding to produce opposite currents eliminates all noise. The magnetic flux caused by the shared-mode current accumulates and generates impedance, thus reducing noise on the power lines. Since the currents in differential mode flow in opposite directions, the magnetic flux generated by the differential mode currents cancels each other out, so no impedance is generated and differential mode noise cannot be reduced. Capacitors C1 and C16 are Class X capacitors used to reduce differential noise and need to withstand the supply voltage. Class X capacitors are typically in the range of 0.01uF to 2uF. Capacitors C2 to C5 are Class Y capacitors for common mode noise and need to be able to withstand short circuits (more expensive than Class X capacitors). Class Y capacitors have smaller capacitance values, typically between 0.002uF and 0.1uF. 5 Design Guidelines for Reducing Internal and External Noise in Power Converters AC to DC power supplies have three areas of noise generation: (1) Noise already present in the AC power supply enters the power supply unit (common mode/differential mode); (2) Noise caused by the switching frequency of the power supply (common mode); (3) Fast switching edges generated when the MOSFET is turned off and the resulting ringing (common mode). 5.1 For AC power supplies with noisy power lines, an AC power line filter can be used. When using an AC power line filter, ensure it is installed as close as possible to the AC power line entry point on the PCB. The filter's ground connection should also be as short as possible to connect to the power supply's primary ground plane. To reduce shared-mode and differential-mode noise from incoming and outgoing devices, an AC power line filter should be used. See the AC power line filter section. 5.2 The switching frequency of the power supply is similar to that of systems using a system clock. Many power supplies employ pulse-width modulation (PWM) components that operate at a specific frequency to control the output voltage. Therefore, the system clock and PWM controller require careful placement on the PCB. For transformer designs using flyback, forward, or other topologies, it is crucial to design the lead between the primary winding and the drain of the switching MOSFET to be as wide and short as possible. This shortens the inductor path and keeps ringing to a minimum. Ideally, both the MOSFET and PWM controller should be connected to the ground plane simultaneously, minimizing the number of vias on the ground plane (avoiding an appearance resembling Swiss cheese). A ground line should run parallel to the current return lead (if there are no stray capacitance issues). If noise problems persist, remove the ground plane below the lead to minimize the capacitance from the drain lead to the transformer. MOSFET switching structures already have parasitic capacitance that spools current between the component and ground. If the ground plane below the trace shown in the green line is not removed, additional current will flow into the ground plane, causing greater shared-mode conducted noise. The source of the switching MOSFET must be reliably connected to the primary ground plane of the power supply. Therefore, a large pad should be made for the ground terminal to allow for a reliable connection to the ground plane using an appropriate number of bridges (depending on the sink current). 5.3 The PWM switching edge and concurrent ringing are handled by a resistor-capacitor-diode (RCD) circuit (R1, C1, and D1), which serves two purposes: first, C1 slows down the rise time of the collector voltage of Q1 when it is turned off (smoothing and reducing radiated EMI); second, it maintains the input voltage at 2VCC, which does not exceed the breakdown voltage of the switching MOSFET. With a sufficiently large C1, the rising collector voltage and falling collector current intersect at a very low point, thus significantly reducing transistor power consumption. The ringing circuitry of C2 and R2 is also important, used to reduce ringing on the transformer primary side, which is caused when the MOSFET discharges the input voltage. As a first pilot test, here is a method to determine the values ​​of C2 and R2: (1) Determine the frequency of the ringing waveform and calculate the period; (2) Multiply the period determined in step one by 5; (3) Set the value of the resistor (usually less than 100R); (4) Divide the value obtained in step two by the resistor determined in step three. The advantage of using a network of resistor R2 and capacitor C2 is that it reduces ringing, but the disadvantage is that the high-frequency ripple through capacitor C2 will be dissipated as heat on resistor R2. If reducing noise is more important than efficiency, it can be used; otherwise, it will reduce efficiency. 6 Printed Circuit Board Design Guidelines (1) Place and determine the orientation of components properly; (2) If using a heatsink, be sure to ground it; (3) Component shielding may be required; (4) Shared mode capacitors should have a low ESR value and the ground lead length should be shortened; (5) If a buffer circuit is connected across the transformer to slow down the rise time of the MOSFET switch turn-off, remember to shorten the trace length of the drain and both source transformer leads. If possible, place the buffer circuit between the two primary leads; (6) Avoid using slots in the ground plane and power board (if used); (7) Traditional decoupling methods are effective below 50MHz (considering harmonics of the PWM controller). One or two decoupling capacitors (typically 0.1 or 0.01uF) can be used near the IC power and ground leads. Consider the loop area formed between the IC and the decoupling capacitor and place the capacitor to minimize the loop area; (8) Make the ground wire as short and thick as possible; (9) Avoid sharp corners on the trace; (10) Where shielding is required, concentrate all noisy components in the same area as much as possible; (11) Use multilayer printed circuit boards if possible. 7 Safety of Medical Devices For applications sensitive such as medical devices, shared-mode noise is indeed a problem. If the device comes into contact with a patient, the overall system leakage current will be limited to below 100uA, which means that most power supply designers need to limit the leakage current to 20 to 40uA. To meet these stringent requirements, medical devices do not use shared-mode filters with capacitor grounding. These shared-mode conducted emission pulses can be reduced by using shared-mode chokes, feeding high-frequency noise to ground through capacitors (where it is shunted to chassis ground instead of signal ground), and by adding transformers or isolating power lines in the power supply. Medical devices comply with IEC 950/UL 1950 Class II safety standards. 8. Conclusion EMC is a critical consideration in modern system design, and its regulations are becoming increasingly stringent over time. Remember that noise, both conducted and radiated, also occurs during switching. This article introduced board-level techniques to reduce noise. For further noise reduction, especially in radiated areas, using conductive housings is a good option. Of course, these methods will increase costs. Design engineers must evaluate standards compliance, safety compliance, and the cost of the final product.