EMC performance is an important index for power module design

In the field of electromagnetic compatibility, everyone must be familiar with power modules, and EMC performance is an important indicator of power modules. Do you know how to deeply understand the EMC performance of various power modules? How can I improve the EMC protection capability of a module in an application? This article will answer for you.

As we all know, EMC refers to electromagnetic compatibility testing, which means that the electromagnetic energy generated by equipment neither interferes with other equipment nor interferes with the electromagnetic energy of other equipment. EMC testing of isolated power modules includes EMI testing and EMS testing. How can I ensure the EMC performance of power modules? This article will be revealed to everyone.

Everyone must be familiar with power modules, and EMC performance is an important indicator of power modules. Do you know how to deeply understand the EMC performance of various power modules? How can I improve the EMC protection capability of a module in an application? This article will answer for you. As we all know, EMC refers to electromagnetic compatibility testing, which means that the electromagnetic energy generated by equipment neither interferes with other equipment nor interferes with the electromagnetic energy of other equipment. EMC testing of isolated power modules includes EMI testing and EMS testing. How can I ensure the EMC performance of power modules? This article will be revealed to everyone.

1、 Introduction to EMC

EMI refers to the ability of the equipment under test to interfere with surrounding equipment, mainly including conducted disturbance CE and radiated disturbance RE. Power module EMS electromagnetic interference refers to the power module tests specified in the national standard GB/T16821-2007 "General Test Methods for Communication Power Supply Equipment", which mainly include group pulse immunity (EFT), surge immunity (SURGE), electrostatic discharge reactance interference, radiation immunity, etc.

EMC's production must have three elements: interference sources, transmission media, and sensitive equipment, as shown in Figure 1 below. If none of the three cannot constitute an EMC problem, the design of the power module can only achieve EMC protection by correcting one of these aspects, such as eradicating interference sources, improving transmission media, avoiding interference with transmission, or keeping sensitive devices away from interference sources.

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图1.EMC三要素

2、 EMC Interference Protection - Devices

The selection of power module components will directly affect the overall performance of the module. Next, we will introduce power chips, high-frequency transformers, field effect transistors, and common mode inductors, as shown below.

Field effect transistor: Pay attention to two parameters: conductive resistance and low gate charge, which affect the EMC performance and overall efficiency of the module, so it is necessary to balance the two;

High frequency transformer: It should ensure low DC loss, low AC loss, and small leakage inductance. A good winding layout is required between windings to minimize the leakage peak value during switching power supply operation;

Filter capacitor: applied to input filter; Apply to output terminal to absorb switching frequency and high harmonic current components. The demand trend is small, large capacity, high frequency, low impedance, and high voltage resistance;

Common mode inductance: Like other passive components, pay attention to their electrical parameters, such as rated voltage, rated current, inductance, leakage inductance, etc.

Varistor: The maximum DC working voltage is required to be greater than the DC working voltage of the power and signal lines.

3、 EMC interference protection - circuit design

Power modules with high power density and high conversion efficiency are typically switching power supplies. When the switch is opened and closed, the voltage and current will be cut off, resulting in large transient changes (di/dt, dv/dt). Therefore, regardless of the topology used by the power module, as long as the power supply is turned on and off, there will be a certain degree of EMC interference, as shown in Figure 2.

Figure 2. Common topology and chopping of switching power supplies

By optimizing its topology and standardizing PCB design, the EMC performance of power modules can be improved:

In circuit design, the protective device should be placed closest to the electrostatic inlet of the product. The principle is to protect before filtering;

In the topology design, select a continuous conduction mode (CCM) topology, such as Boost, full bridge, push pull, and other topologies;

In terms of circuit protection, it is recommended to add an RC absorption circuit and an RCD absorption circuit to the switch tube, and place them near the switch tube to reduce the peak voltage. Filter circuits such as π type filters and full wave rectification circuits are used in EMC transmission circuits. See Figure 3 for details;

In PCB design, try to lay a large area of ground, minimize the division of the ground, reduce the circuit area, and thereby reduce interference. Avoid large areas of isolated copper areas, which may affect the reliability of the module due to electromagnetic and other reasons; Reduce wiring length, thereby reducing inductance at dynamic nodes, and avoiding strong electromagnetic fields.

Figure 3. EMC Optimized Topology Power Module

4、 EMC Interference Protection - Peripheral Protection

As a modular product, the power module requires a high volume. If only the internal design of the power module is relied on to meet the requirements, the product will be very large in size and expensive in cost. Due to the large volume of electronic components that absorb EMS, advanced EMC interference protection can only meet the EMS requirements of the system through peripheral circuit design.

According to the national standard "General Test Methods for Communication Power Supply Equipment" (GB/T16821-2007), the waveform of conducted interference (CE) is generally divided into three components: low frequency (150KHz~0.5MHz), intermediate frequency (0.5MHz~5MHz), and high frequency (5MHz~30MHz). Different situations require different peripheral circuits to solve.

Low frequency: It belongs to differential mode disturbance and is solved by differential mode filter circuit;

Intermediate frequency: Differential mode and common mode disturbances solved jointly by common mode filter circuits and differential mode filter circuits;

High frequency: It belongs to common mode disturbance and is solved by differential mode filter circuit.

Common mode and differential mode interference often coexist in power lines, so the power EMI filter consists of a common mode filter circuit and a differential mode filter circuit, as shown in Figure 5.

Figure 4. EMC Peripheral Recommended Circuit Power Module

5、 EMC Interference Protection - Premium Power Module

The design of electromagnetic compatibility (EMC) is becoming increasingly important. For independently built power modules, it is not only a long research and development cycle, but also a high production cost, and it is difficult to ensure product consistency and reliability. At this point, high-quality power modules can be selected for product design.