Electromagnetic compatibility of power converters

　　EMC (Electromagnetic Compatibility ) has become a household word in the past decade. In the mid-1990s, Europe required lower radiated and conducted emission levels for products sold into the region. Since then, many products have begun to incorporate EMC testing into their design phase. This trend continues in product development today.

　　A question that is often asked is: What is EMC? In fact, EMC is the ability of a component, product or system to work properly in a predetermined electromagnetic environment (existing electromagnetic interference EMI) without being degraded and becoming a source of interference. To design such functions, it is necessary to follow EMC standards, which are formulated by groups such as IEC and CISPR. This article will discuss EMC regulations on radiation and conduction, including common mode and differential mode emissions, and explore how to design power line filters to reduce input and output noise. Finally, some printed circuit board design tips that can reduce noise are provided.

　　1 EMC regulations

　　In order to obtain a reliable EMC design, it is necessary to understand the EMC requirements. These requirements are not only for module power supplies , but also for system-level standards common to Europe and North America.

　　The IEC (International Electrotechnical Commission) is responsible for the European specifications, while CISPR (International Special Committee on Radio Frequency Interference) is responsible for EMC testing with CISPR 22, which defines the most stringent limits for conducted emissions. These limits (conducted emissions) are now described by product standards EN55022 (Figure 1) and EN55011 (Figure 2). The Class A and Class B requirements in Figures 1 and 2 refer to industrial and domestic standards, respectively. The European standard has two limits depending on the antenna used to test the noise. The higher limit is for quasi-peak antennas and the lower limit is for general antennas, but both limits must be met in order for the device to pass the requirements. The FCC standard specifications used in North America are similar to the European EN requirements, please refer to Figure 2. Two European standards are used when testing power supplies: EN55011 and EN55022. In North America, radiated EMI is usually measured in the frequency range of 30MHz to 10GHz (according to FCC regulations), while conducted EMI is generally measured in the frequency range of a few to 30MHz (according to FCC regulations).

　　The aim here is to develop systems that meet all or part of the above emission-related requirements, either as stand-alone equipment or integrated into a larger system.

　　2 Common Mode and Differential Mode Noise

　　Common mode and differential mode are the two main sources of noise. Common mode noise comes from common mode current. Common mode energy coexists on both power lines of a single-phase system and is transmitted in the same direction between all wires and ground and on all power lines or wires. Since both wires have the same level at the same time, there is no attenuation to the equipment between the wires.

　　Shared mode noise from shared mode currents is always present on the cables that enter the device. One way to reduce this current is to test the cables early on, on the original model (so that the designer can make any changes needed before the design is finally delivered to production), and before EMC compliance testing is performed. In many cases, if the device does not pass the shared mode current test, it will not pass the radiated emissions test either. Shared mode currents can be tested simply by using a current probe with a high frequency clamp 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 caused by current flowing through either the live or neutral conductor and then refracting through the other conductor. This creates a noise voltage between the live and neutral conductors.

　　3AC power line main filter

　　This is an example of a single-phase AC power filter. This type of filter is often used to reduce differential mode and common mode noise on the input and output supplies.

　　4.1 Part A

　　Inductor L1/L2 and capacitor C1 form a differential filter to handle all noise that tries to enter the power supply. Differential mode noise is caused by current flowing through the live or neutral conductor and then refracting from the other 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 less impedance (greater load) to the signal, thus reducing the noise on the power line. For example, at a specific frequency, the equivalent impedance of L1 is 10K and the equivalent impedance of C1 is 1K, then the noise that passes through the filter is one-tenth of its original strength, or 20dB of noise reduction.

　　4.2 Part B

　　Capacitors C2 and C3 form a shared mode filter with a ground reference. Common mode noise becomes apparent when the current is in phase with the current in the live and neutral conductors and returns via a safe ground. This creates a noise voltage between the live/neutral conductors and ground. C2, C3, C4, and C5 are all equal, and any shared mode noise on these lines will be shunted to ground. Note that Part B cannot be used in medical equipment due to leakage current.

　　4.3 Part C

　　Zorro inductor (shared-mode choke) without reference. The direction of each winding is chosen to produce opposite currents, which cancels out all noise. The magnetic flux caused by the shared-mode currents converges and creates impedance, thus reducing noise on the power line. Since the differential-mode currents flow in different directions, the magnetic fluxes created by the differential-mode currents cancel each other, so there is no impedance and no reduction in differential-mode noise.

　　Capacitors C1 and C16 are class X capacitors for differential noise reduction and need to be able to withstand the power supply voltage. Class X capacitors are typically in the 0.01uF to 2uF range. Capacitors C2 to C5 are class Y capacitors for shared mode noise and need to be able to guarantee that they will not fail in a short circuit (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

　　There are three areas where noise is generated in an AC to DC power supply:

　　(1) Noise already present in the AC power supply enters the power supply device (common mode/differential mode);

　　(2) Caused by the switching frequency of the power supply (common mode);

　　(3) The fast switching edge and resulting ringing (common mode) generated when the MOSFET turns off.

　　5.1 AC Power Supply

　　If there is a noisy power mains, an AC power line filter can be used. When using an AC power line filter, make sure to install it as close as possible to where the AC power line enters the printed circuit board (PCB). The ground connection of the filter should also be as short as possible to connect to the ground plane of the power supply primary.

　　To reduce shared-mode and differential-mode noise from entering and leaving the device, an AC power line filter should be used. See the AC Power Line Main Filter section.

　　5.2 Switching frequency of power supply

　　Like systems that use a system clock, many power supplies use a pulse width modulation (PWM) component that operates at a certain frequency to control the output voltage. Therefore, the system clock requires careful layout on the circuit board, and the same goes for the PWM controller.

　　For transformer designs using flyback, forward or other topologies, it is very important to design the trace between the primary winding and the drain of the switching MOSFET so that it is as wide and as short as possible. This shortens the inductive path and keeps ringing to a minimum. It is best to connect both the MOSFET and the PWM controller to the ground plane to minimize the number of holes in the ground plane (and not look like Swiss cheese). The current return trace should have a ground trace running parallel to it (if there are no stray capacitance issues), and if noise is still a problem, remove the ground plane under the trace to minimize the capacitance of the drain lead to the transformer. The MOSFET switching structure already has parasitic capacitance that will sink current between the component and ground. If the ground plane under the "green line" trace is not removed, additional current will enter the ground plane, causing greater shared mode conducted noise.

　　The source of the switching MOSFET must be reliably connected to the ground plane of the power supply primary. Therefore, a large pad is made for the ground terminal so that it can be reliably connected to the ground plane using an appropriate number of jumpers (depending on the sink current).

　　5.3 PWM Switching Edges and Concurrent Ringing

　　The resistor-capacitor-diode (RCD) circuit (R1, C1, and D1) has two functions. First, C1 can slow 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, that is, not exceeding the breakdown voltage of the switching MOSFET. When C1 is large enough, the rising collector voltage and the falling collector current intersect at a very low position, thus greatly reducing the power consumption of the transistor.

　　The ringing circuitry of C2 and R2 is also important to reduce the ringing on the transformer primary that is caused when the MOSFET delivers power to the input voltage.

　　As a first trial, here is one way to determine the C2 and R2 values:

　　(1) Determine the frequency of the ringing waveform and calculate the period;

　　(2) Multiply the period determined in the first step by 5;

　　(3) Set the resistance value (usually less than 100R);

　　(4) Divide the value obtained in step 2 by the resistance determined in step 3

　　The advantage of using a resistor R2 and capacitor C2 network is that it reduces ringing, but the disadvantage is that the high-frequency ripple through capacitor C2 will be dissipated as heat in 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) Properly place and orient components;

　　(2) If a heat sink is used, be sure to ground it;

　　(3) Component shielding may be required;

　　(4) The ESR value of the shared mode capacitor should be small and the length of the ground lead should be shortened;

　　(5) If you connect a snubber circuit 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 two source transformer leads. If possible, place the snubber circuit between the two primary leads;

　　(6) Avoid using slots in ground planes and power planes (if used);

　　(7) Below 50MHz (considering the harmonics of the PWM controller), the traditional decoupling method is effective. One or two decoupling capacitors (usually 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 traces;

　　(10) When shielding is necessary, try to concentrate all noise components in the same area;

　　(11) If possible, use multi-layer printed circuit boards.

　　7. Safety of Medical Devices

　　Shared mode noise is indeed a problem for devices with sensitive applications such as the medical field. If the device is in contact with the patient, the overall system leakage current will be limited to less than 100uA, which means that most power designers need to limit the leakage current to 20 to 40uA. To meet this stringent requirement, medical equipment will not use shared mode filters with capacitor grounding. Using shared mode chokes, feeding to ground through capacitors (high frequency noise is shunted to chassis ground instead of signal ground), and adding transformers or isolating power lines in the power supply can reduce these shared mode conducted emission pulses. Medical equipment uses the IEC950/UL1950 Class II safety standard.

　　8 Conclusion

　　EMC is an important consideration in today’s system design, and its rules will become more stringent over time. Remember that when switching occurs, noise will also be present, whether it is conducted noise or radiated noise. This article describes board-level techniques that can reduce noise. If further noise reduction is required, especially in terms of radiation, the use of conductive enclosures is a good choice. Of course, these methods will add additional cost. Design engineers must evaluate standards compliance, safety compliance, and the cost of the final product.