EMC testing for isolated power modules includes two aspects: EMI (electromagnetic interference) testing and EMS (electromagnetic immunity) testing. So how can we ensure the EMC performance of power modules? This article will reveal the answer.
I. Introduction to EMC
EMI (Electromagnetic Interference) refers to the ability of a device under test to interfere with surrounding equipment, mainly including conducted interference (CE) and radiated interference (RE). The EMS (Electromagnetic Immunity) of a power module refers to the ability of the equipment or system to withstand electromagnetic interference within the range specified by relevant standards during normal operation. According to the national standard GB/T 16821-2007 "General Test Methods for Power Supply Equipment for Communication," power module testing mainly includes items such as surge immunity (EFT), surge immunity (SURGE), electrostatic discharge immunity, and radiated immunity.
EMC (Electronic Computation) requires three essential elements: an interference source, a transmission medium, and sensitive equipment, as shown in Figure 1. The absence of any one of these elements will not constitute an EMC problem. Therefore, in the design of power modules, only one aspect needs to be addressed to achieve EMC protection. This can be achieved by eliminating the interference source, improving the transmission medium to prevent interference propagation, or moving sensitive equipment away from the interference source.
Figure 1. Three elements of EMC
**II. EMC Interference Protection: Step One – Circuit Design**
High power density and high conversion efficiency power modules are generally switching power supplies. When the switching transistor is turned on and off, the voltage and current are chopped, causing large transient changes (di/dt, dv/dt). Therefore, regardless of the topology used, as long as it is a switching power supply, it will generate a certain degree of EMC interference, as shown in Figure 2.
Figure 2 Common topologies and choppers for switching power supplies
The EMC performance of a power module can be improved by optimizing its topology and standardizing PCB design. For example:
In circuit design, the principle of protection before filtering should be followed, and protection devices should be placed as close as possible to the electrostatic discharge port of the product.
In topology design, choose a continuous conduction mode (CCM) topology, such as Boost, full-bridge, push-pull, etc.
In terms of circuit protection, it is recommended to add RC snubber circuits and RCD snubber circuits to the switching transistors and place them close to the switching transistors to reduce peak voltage. π-type filters and full-wave rectifier circuits should be used on the EMC transmission path, as shown in Figure 3.
In PCB design, maximize the area covered by ground plane and minimize the segmentation of the ground plane to reduce loop area and thus reduce interference. Avoid large isolated copper areas, as these can affect module reliability due to electromagnetic interference. Reduce the length of traces to decrease inductance at dynamic nodes and avoid generating strong electromagnetic fields.
Figure 3 Power module EMC optimized topology
III. EMC Interference Protection, Part Two – Device Selection
The selection of components in a power module directly affects the overall performance of the module. The following section will introduce the components, including power chips, high-frequency transformers, MOSFETs, and common-mode inductors.
High-frequency transformers should ensure low DC loss, low AC loss, low leakage inductance, and a good winding layout to provide good shielding between windings, thereby minimizing the spikes generated at the drain when the switching power supply is operating.
Field-effect transistors (FETs): Pay attention to their on-resistance and low gate charge, as these two parameters affect both the module's EMC performance and overall efficiency, so a balance must be struck between them;
Common-mode inductors: Similar to other passive components, focus on their electrical parameters, such as rated voltage, rated current, inductance, and leakage inductance.
Filter capacitors: used for filtering at the input end; used at the output end to absorb switching frequency and high-order harmonic current components. The demand trend is towards smaller size, larger capacitance, higher frequency and lower impedance, and higher voltage withstand.
Varistors: The maximum DC operating voltage must be greater than the DC operating voltage of the power supply and signal lines.
IV. EMC Interference Protection, Third Step – Peripheral Protection
As a modular product, power modules have high requirements for size. If the internal design of the power module is used to meet the requirements, the product will be very large and the cost will be very high. This is because the electronic components that absorb EMS are all very large. Therefore, high-level EMS interference protection can only meet the EMS requirements of the system through the design of the external circuit.
According to the national standard GB/T 16821-2007 "General Test Methods for Power Supply Equipment for Communication", conducted interference (CE) waveforms are generally composed of three components: low frequency (150KHz to 0.5MHz), medium frequency (0.5MHz to 5MHz), and high frequency (5MHz to 30MHz). Different peripheral circuits are required to address different situations.
Low frequency: This is differential-mode interference, which can be resolved using a differential-mode filter circuit;
Intermediate frequency (IF): Simultaneous differential-mode and common-mode interference is present, which is addressed by both common-mode and differential-mode filtering circuits.
High frequency: This is common-mode interference, which can be resolved using a differential-mode filter circuit.
Common-mode and differential-mode interference often coexist in power lines. Therefore, power EMI filters are composed of common-mode and differential-mode filter circuits, as shown in Figure 5.
Figure 5 Recommended peripheral circuit for power module EMC
V. The Fourth Form of EMC Interference Protection – High-Quality Power Supply Modules
For self-built power modules, not only is the R&D cycle long and the production cost high, but the consistency and reliability of the products are also difficult to guarantee. In this case, a high-quality power module can be selected for product design.
ZLG Zhiyuan Electronics independently develops and manufactures isolated power modules with a wide input voltage range, offering multiple series including 1000VDC, 1500VDC, 3000VDC, and 6000VDC isolation. These modules are available in various packaging forms, compatible with international standard SIP and DIP packages. Furthermore, Zhiyuan Electronics has built a first-class testing laboratory in the industry, equipped with state-of-the-art testing equipment. All series of isolated DC-DC power supplies have passed complete EMC testing, achieving electrostatic discharge immunity up to 4KV and surge immunity up to 2KV. These modules can be applied to most complex and harsh industrial environments, providing users with stable and reliable power isolation solutions.
