Figure 1 shows the commonly used EMC front-level schematic diagram in low-power power modules. FUSE is a fuse, MOV is a varistor, Cx is an X capacitor, LDM is a differential mode inductance, Lcm is a common mode inductance, Cy1 and Cy2 are Y capacitor, NTC is the thermistor. Although the main functions of Y capacitors and common mode inductances are not to improve the surge immunity of the circuit, they indirectly affect the design of the anti-surge circuit.
The surge voltage applied between ACL and ACN is called differential mode surge voltage, and the differential mode path is shown by the red line in the figure; the voltage applied between ACL (or ACN) and PE is called common mode surge voltage, The common mode path is shown by the blue line in the figure.
Before designing an anti-surge circuit, the corresponding "electromagnetic compatibility standard" must be determined, such as IEC/EN 61000-4-5 (corresponding to GB/T17626 ($2.1792).5), which stipulates surge immunity requirements and tests Method, test level, etc. Below we will discuss the design of anti-surge circuits on the basis of the provisions of this standard.
The surge generating circuit produces a surge voltage of 1.2/50μs when the output is open, and a surge current of 8/20μs when it is short-circuited.
The effective output impedance of the generator is 2Ω, so when the open circuit voltage peak value is XKV, the short circuit peak current is (X/2) KA.
When the anti-surge test is performed between ACL (or ACN) and PE, a 10Ω resistance is connected in series to the coupling circuit, ignoring the influence of the series coupling capacitance, the short-circuit peak current becomes approximately (X/12) KA.
Figure 1 Commonly used EMC pre-stage circuit
2. Introduction of related devices1. Varistor
The most important parameters for the selection of a varistor are: the maximum allowable voltage, the maximum clamping voltage, and the surge current that can be withstood.
First, ensure that the maximum allowable voltage of the varistor is greater than the maximum value of the output voltage of the power supply; secondly, ensure that the maximum clamping voltage does not exceed the maximum surge voltage allowed by the subsequent circuit; finally, ensure that the surge current flowing through the varistor Will not exceed the surge current it can withstand.
Other parameters such as rated power, the maximum energy pulse that can be withstood, etc., can be determined through simple calculations or experiments.
2.Y capacitor
When performing common-mode surge testing, if cost and other factors are considered, when varistors or other devices used for clamping voltage are not added to the common-mode path, ensure that the Y capacitor withstand voltage is higher than the test voltage.
3. Input rectifier diode
Assuming that after the surge voltage is clamped by the varistor, the maximum clamping voltage is greater than the maximum reverse voltage that the input rectifier diode can withstand, the diode may be damaged. Therefore, a diode whose reverse withstand voltage is greater than the maximum clamping voltage of the varistor should be selected as the input rectifier diode.
4. Common mode inductance
Theoretically, the common mode inductance only works in the common mode path, but because the two windings of the common mode inductance are not completely coupled, the uncoupled part will act as a differential mode inductance in the differential mode path, affecting EMC characteristics.
Three, case analysisBackground: Take a certain type of power module as an example. This module is a power module customized by Zhou Ligong Zhiyuan Electronics for a customer. The input is 85VAC~350VAC, and the EMC pre-circuit circuit is embedded in the module. Anti-surge requires a differential mode voltage of 3KV and a common mode voltage of 6KV. After replacing a larger fuse, it can withstand 6KV differential mode voltage. The front-level schematic diagram and corresponding physical diagram are shown in Figure 2
Figure 2 Schematic diagram and physical diagram of the example
1. Differential mode surge test
When selecting the varistor, firstly make the maximum allowable voltage slightly greater than 350V. The maximum clamping voltage of the varistor of this voltage level is about 1000V (under 50A test current). Secondly, on the differential mode path, it is equivalent to a voltage source with an internal resistance of 2Ω and a pulse voltage of 6KV in series with the varistor, the peak current is about (6KV-1KV)/2Ω=2500A ($0.3060). Finally selected 681KD14 as the varistor. Its peak current is 4500A, the maximum allowable working voltage is 385VAC, and the maximum clamping voltage is 1120V.
Don’t worry, because the uncoupled part of the common mode inductance will be part of the voltage as a differential mode inductance in the differential mode path. In fact, the circuit has been protected after the common mode inductance. The rectifier diode is verified by experiments. Choose the commonly used 1N4007 ($0.0093).
2. Common mode surge test
When testing a 6KV surge on ACL-PE or ACN-PE, that is, a common mode surge test, the common mode path is equivalent to a voltage source with an internal resistance of about 12Ω and a pulse voltage of 6KV in series with the common mode inductor and Y capacitor. Because the Y capacitor is a Y1 grade capacitor, its withstand voltage is higher, and the energy of the 6KV common-mode surge is not enough to damage it. Therefore, it is only necessary to ensure that the PE wiring and other wiring are indirectly connected to easily pass the common-mode surge. test.
However, because of the high voltage generated at both ends of the common mode inductor during the surge test, flashover occurs. If it is close to the surrounding devices, the surrounding devices may be damaged. Therefore, a discharge tube or varistor can be connected in parallel to limit its voltage, thereby playing the role of arc extinguishing. As shown in MOV2 in the picture.
Another method is to add discharge teeth at both ends of the common-mode inductor during PCB design, so that the inductor discharges through the two discharge tips, avoiding discharge through other paths, thereby minimizing the impact on surrounding and subsequent devices. Figure 3 is the physical picture of the discharging teeth added at the common mode inductance of the power supply module PCB of Guangzhou Zhiyuan Electronics Co., Ltd. with the model of PA1HBxOD-10W.
Figure 3 Physical picture of discharge gear
Four, summaryEMC tests are usually very practical, but if we master some basic principles, we will have more direction to conduct tests when designing EMC pre-circuits, thus shortening the time of project development. This article combines a simple example to introduce the selection of front-level circuit devices and typical circuits from the perspective of surge testing. In future articles, we will continue to explore the relevant content of anti-surge circuits in more depth, and learn from other EMC Design the EMC pre-stage circuit from the perspective of performance indicators.
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