EMI test issues have become a critical element of switching power supply reliability

1、 Preface

With the increasing volume and power of switching power supply, EMI test operation has become a key factor in the reliability of switching power supply. EMI filtering technology, shielding technology, sealing technology and grounding technology can effectively suppress and eliminate the coupling and radiation between the interference source and the disturbed equipment, disconnect the transmission path of the interference signal, and improve the electromagnetic compatibility of the switching power supply.

Switching power supply is a kind of power semiconductor device and power conversion technology, electronic electromagnetic technology, power and electronic equipment, such as automatic control technology. Switching power supply has weak transient response and is prone to generate interference signals (EMI), and EMI test signals have a wide frequency range and certain strength. EMI signals pollute the electromagnetic environment through conduction and radiation, and affect communication equipment and instruments. Therefore, the application of switching power supply is limited to a certain extent.

2、 Cause of interference signal

Electromagnetic interference (EMI) is a kind of electronic system or subsystem suffering from unexpected electromagnetic oscillation. It is composed of three basic premises: interference source, i.e. equipment that interferes with signal energy; Lotus mode, that is, the channel or medium for transmitting interference signals; Sensitive equipment, equipment, subsystem or system damaged by interference signal. In view of this, the basic countermeasures to control the interference signal are to suppress the interference source, disconnect the coupling mode, and reduce the impact of sensitive equipment on the electromagnetic sensitivity level.

According to the working principle of switching power supply: switching power supply firstly rectifies the working frequency AC to DC, then inverts to high-frequency AC, and finally obtains stable AC voltage through rectification and filtering output. In the circuit, the power supply triode. The main work of the diode is the switch tube, which works on the microsecond scale. The triode rises in the opening and closing rotating link. The current changes greatly in the falling time. It is easy to generate radio frequency energy and interference sources. At the same time, the peak value caused by transformer leakage and diode reverse recovery current will also form a hidden interference signal.

Switching power supply usually works at high frequency, so its distributed capacitance cannot be ignored. On the one hand, due to its large contact area and thin insulation, the insulation sheet between the radiator and the insulating sheet of the switch tube cannot be ignored at high frequency. High-frequency current flows into the radiator through the distributed capacitance, and then flows into the shell, causing common-mode interference; On the other hand, there is distributed capacitance in the middle of the primary of the pulse transformer, which can directly couple the resistance voltage of the primary winding to the secondary winding, resulting in common mode interference on the secondary winding.

Therefore, the interference source in the switching power supply mainly lies in the voltage and current changes greatly, such as the switch tube, diode, high-frequency transformer and other components and the AC input and rectifier output circuits.

3、 Measures to suppress interference signals of switching power supply

EMI filtering technology of general switching power supply is usually used for operation, shielding technology, sealing technology, grounding technology, etc. EMI is divided into conducted interference and radiated interference according to the transmission path. Switching power supply is usually conducted interference, with the frequency range of 10kHz to 30MHz. Most of the countermeasures to suppress conducted interference are in three frequency bands above 10kHz - 150kHz. 150kHz - 10MHz. 10MHz.

① The key factor in the range of 10kHz to 150kHz is the normal impact, and the general LC filter is generally used to solve the problem.

② 150kHz - 10MHz common mode suppression filter is usually used to solve common mode interference within the range.

③ The countermeasures for the above frequency band of 10MHz are to improve the appearance of the filter and adopt electromagnetic shielding measures.

1. Select AC input EMI filter

Generally speaking, there are two ways to transmit the influence current on the wire: common mode method and differential mode method. Common mode interference refers to the influence between the load liquid and the ground. The magnitude and direction of the influence have any power supply relative to the ground, or between the neutral line and the ground, which is generally caused by du/dt, and di/dt will also produce certain common mode interference.

Differential mode interference is the influence between the load liquid: the influence of the same size. On the contrary, there is an intermediate position between the power supply phase line and the neutral line, between the phase line and the phase line. The influence current can be transmitted not only in common mode, but also in differential mode. However, only when the common-mode interference current becomes the differential-mode interference current can the useful signal be affected.

The AC input network has the above two effects, which are generally low frequency band differential mode interference and high frequency band common mode interference. Generally speaking, the intensity of differential mode interference is small, the frequency is low, and the impact is small; Wide range of common mode interference At high frequency, radiation can also be generated through the wire, causing greater impact. If proper EMI filter at AC input is selected, interference signal can be effectively suppressed. The basic concept of the power line EMI filter is shown in Figure 1, where the differential mode capacitor C1.C2 is used for the short circuit differential mode interference current, and the intermediate grounding capacitor C3.C4 is used for the short circuit common mode interference current. The common mode choke is composed of two equally thick coils, which are wound in the same direction in a magnetic core. If the magnetic coupling between the two coils is very close, the leakage inductance will not be large, and the differential mode reactance within the frequency range of the power line will become very small. When the load current passes through the common-mode choke, the magnetic induction line formed by the coil in series on the phase line and the magnetic force line formed by the coil in series on the neutral line are in the opposite direction, and cancel each other in the magnetic core. Therefore, even under the condition of large load current, the magnetic core will not be saturated. However, for the common-mode interference current, the magnetic field generated by the two coils is in the same direction, and there will be a large inductance, resulting in the loss of the common-mode interference signal. Here, the common-mode choke adopts high permeability. The ferrite permanent magnet material has good frequency characteristics.

Figure 1. Basic circuit diagram of power line filter

2. Use absorption circuit to improve switching wave mode

The switch tube or diode is easy to switch the peak voltage of the collector, emitter and both sides of the diode due to the leakage of the transformer and the line inductance, the storage capacitance and the distributed capacitance of the diode. Generally speaking, RC/RCD absorption circuit is selected, and the surge voltage absorption circuit shown in RCD Figure 2 is selected.

Figure 2. RCD surge voltage absorption circuit

When the voltage on the absorption circuit exceeds a certain strength, each device will be disconnected quickly to release the surge energy and limit the surge voltage to a certain strength. Saturated magnetic core coil or microcrystalline magnetic beads are connected in series on the positive wire of the switch tube collector and the output diode. The material is generally cobalt (Co). When the magnetic core is saturated according to the normal current, the inductance is small. Once the current passes in the reverse direction, a large reverse potential will be generated, which can effectively suppress the diode VD reverse surge current.

3. Use switching frequency modulation technology

The frequency control system is based on the switch, which mainly affects the energy of a specific number of times and has a large frequency band peak. If these kinetic energy can be dispersed in a wider frequency band, the effect of reducing the peak value of the disturbing frequency band can be achieved. Generally speaking, there are two processing methods: arbitrary frequency method and modulated frequency method.

① Arbitrary frequency method

Add an arbitrary oscillating component in the circuit switch spacing to disperse the influence kinetic energy of the switch within a certain range. It is found that the switching influence frequency band has changed from discrete peak pulse interference to continuous distribution influence, and the peak value has been greatly reduced.

② Modulation frequency method

Add modulation wave (white noise) to the sawtooth wave, generate edge frequency band around the affected discrete frequency band, and allocate the affected discrete frequency band into a ubiquitous frequency band. In this way, the influence kinetic energy will be dispersed into these ubiquitous frequency bands. This control measure can well inhibit the opening without affecting the operating characteristics of the converter.

4. Select soft switch technology

One of the effects of switching power supply is du/dt when the power switch is on/off. Therefore, reducing du/dt when the power switch is on/off is an important countermeasure to suppress the effect of switching power supply. Soft switching technology can reduce the number of switch tubes/disconnects du/dt.

If a small inductor, capacitor and other resonant elements are added under the premise of switching circuit to form an auxiliary network. The resonance process is introduced before and after the switching process to reduce the voltage before the switch is opened to zero, thus clearing the open voltage, reducing the current coincidence, and even clearing the loss and influence of the switch. This circuit is called soft switching circuit.

According to the above principles, two methods can be used to make the current zero before the switch is closed, and there will be no loss and impact when the switch is closed. This method of closing is called zero current closing; Or make the voltage zero before the switch is opened, and there will be no loss and impact when the switch is opened. This opening method is called zero voltage opening. In many cases, opening or closing is no longer emphasized, but referred to as zero-current switch and zero-voltage switch. The basic circuit is shown in Figure 3 and Figure 4.

Figure 4. Zero-current switching resonant circuit

Figure 5. Zero-current switching resonant circuit

Generally, the soft switch circuit control system, reasonable component layout and printed circuit board layout, and grounding technology are used to improve EMI of switching power supply.

5. Select electromagnetic shielding countermeasures

Electromagnetic shielding measures can generally effectively suppress the electromagnetic radiation effect of switching power supply. Shielding countermeasures for switching power supply are mainly used for switch tubes and high-frequency transformers.

① For switch tube

The switch tube generates a large amount of heat when it works, so it is necessary to install a radiator to generate a large distributed capacitance between the collector and the radiator of the switch tube. Therefore, an insulating shielding metal layer is placed between the collector and the radiator of the switch tube, and the radiator is connected with the shell. The metal layer receives the zero potential of the hot end, reducing the connection capacitance between the collector and the radiator, and reducing the radiation interference caused by the radiator.

② High frequency transformer

For high frequency transformer, firstly select the magnetic structure according to the magnetic shielding characteristics, such as tank iron core and El iron core. The magnetic shielding effect is good. When the transformer is shielded, the shielding box should not be close to the outside of the transformer and should leave a certain magnetic density. If you choose double-layer shielding with magnetic density, the shielding effect is better. In addition, in high-frequency transformers, the distribution capacitor between the primary and secondary coils must be removed frequently. Copper strips can be placed between the coils along the length of the coil to reduce the disaster between them. The lead belt ring is not only connected with the iron core of the transformer, but also connected with the ground of the power supply, playing the role of electrostatic shielding. If conditions permit, installing a shield on the entire switching power supply will better suppress radiation interference.

4、 Conclusion

With the volume and power of switching power supply becoming smaller and smaller, EMI testing has become a key factor in the reliability of switching power supply. According to the above analysis, EMI filtering technology, shielding technology, sealing technology and grounding technology are selected to effectively suppress and eliminate the confluence and radiation between the interference source and the interference equipment, cut off the transmission path of the interference signal, and improve the electromagnetic compatibility of the switching power supply.