The switching power supply does not require a heavy power transformer, and has the advantages of small size, light weight, high efficiency, and the integrated switching power supply integrated control module on the market, which simplifies the power supply design and debugging, so in most electronic devices ( Widely used in computers, televisions and various control systems. However, the various noises generated by the switching power supply itself form a strong source of electromagnetic interference. These interferences are significantly enhanced as the switching frequency increases and the output power increases, posing a potential threat to the normal operation of electronic equipment. Therefore, only by improving the electromagnetic compatibility of the switching power supply, the switching power supply can be adopted in those places where the power supply noise index is strictly required.
Causes of noise generated by switching power supplies
There are many types of switching power supplies. According to the circuit structure of the converter, it can be divided into two types: series-parallel type and DC-transform type. According to the excitation mode, it can be divided into self-excitation and its excitation. According to the combination of switch tubes, it can be divided into bridge type. Half bridge, push-pull, etc. However, regardless of the type of switching power supply, the semiconductor device is turned on and off, and the output voltage is controlled by the ratio of the on and off times. Since it usually operates at a switching frequency of 20 kHz or more, the dv/dt and di/dt in the power supply line are large, generating a large surge voltage, surge current, and various Other noises. They conduct outward through the power line in either common mode or differential mode, while also radiating noise to the surrounding space. Figure 1 shows a simplified diagram of a typical switching power supply circuit. The following is an example of the main cause of noise generation.
Primary rectification loop noise
In the primary rectification circuit, the rectifier diodes D1 to D4 flow only from the power supply input side at the moment when the ripple voltage exceeds the charging voltage of C1. Therefore, a rectifying loop produces a high-order distortion wave to form noise.
Switching loop noise
One is electromagnetic radiation. When the power supply is in operation, the switching tube T is in a high-frequency on-off state, and in the high-frequency current loop composed of the pulse transformer primary coil L, the switching tube T and the filter C, large spatial radiation noise may be generated. If the filtering of C is insufficient, the high-frequency current is also conducted to the AC power source in a differential mode. The second is the surge voltage caused by the inductive load. In the switching circuit, the load of the switching tube T is the primary coil L of the pulse transformer, which is an inductive load. Therefore, when the switching tube is turned on and off, a high surge voltage appears at both ends of the primary coil of the pulse transformer, which may cause Damage to the electronics of the same circuit (especially switch tube T).
Secondary rectification loop noise
One is electromagnetic radiation. When the power supply is working, the rectifier diode D is also in the high-frequency on-off state. The secondary winding L of the pulse transformer, the rectifier diode D and the filter capacitor C constitute a high-frequency switching current loop, which may radiate noise to the space. If the capacitance C is insufficiently filtered, the high-frequency current will be mixed in the differential mode to the output DC voltage, which will affect the normal operation of the load circuit.
Secondary rectification loop noise
The second is the surge current. When the silicon diode is forward-conducting, the charge in the PN junction is accumulated, and the accumulated charge when the diode is applied with a reverse voltage will disappear and generate a reverse current. Since the frequency of D in the secondary rectification loop is very high at the time of switching, that is, the transition from on to off is very short, and in the short time, the stored charge disappears to generate a counter current surge. Due to the presence of distributed capacitance and distributed inductance in the DC output line, the interference caused by the surge becomes a high-frequency fading oscillation.
Control loop noise
The pulse control signal in the control loop is the primary source of noise.
Noise caused by distributed capacitance
One is the role of Ci. Although there are insulating spacers between the heat sink K and the collector of the switching tube T, since the contact surface is large and the insulating mat is thin, the distributed capacitance Ci between the two cannot be ignored at high frequencies. Therefore, the high-frequency current flows through the Ci to the heat sink, flows to the chassis ground, and finally flows to the protective ground PE of the AC power source connected to the chassis ground to generate common mode radiation. The second is the role of Cd. The distributed capacitance Cd existing between the primary and secondary of the pulse transformer may directly couple the primary high frequency voltage to the secondary side, and generate the same phase common mode noise on the two power lines used as the DC output on the secondary side. .
Electromagnetic compatibility design of switching power supply
Suppressing the noise of a switching power supply can be based on three techniques.
First, it is to reduce the interference energy of the interference source;
Second, it is to destroy the interference path;
Third, it is to use shielding.
Reduce interference source energy
Since the source of interference from the switching power supply is impossible to eliminate, it is necessary to reduce the energy of the interference source. The general measures taken are:
(1) Connect the RC circuit in parallel. An RC absorbing circuit is added at both ends of the switching tube T; an RC absorbing circuit is added across the rectifying diode D in the secondary rectifying circuit to suppress the surge voltage.
(2) Serially connect the saturable core coil. In the secondary rectification circuit, the rectifying diode D is connected in series with the coil with the saturable magnetic core, and the saturation magnetic core coil is saturated when the normal current is passed, and the inductance is small, which does not affect the normal operation of the circuit; When flowing in the opposite direction, the core coil will generate a large back EMF, preventing the reverse current from rising, so connecting it in series with the diode D can effectively suppress the reverse surge current of the diode D. At present, there is an ultra-small amorphous magnetic ring finished product, which can be directly placed on the positive lead of the diode, and is convenient to use.
Destruction interference path
One is to take measures against the electric field noise caused by the distributed capacitance in the switching power supply. The main anti-jamming measures are:
(1) Reduce the coupling capacitance Ci between the collector of the switch tube and the heat sink. Use a low dielectric constant material as the insulating mat, thicken the thickness of the gasket, and use electrostatic shielding.
Generally, the outer casing of the switch tube is a collector, and a layer of sandwich insulation is placed between the collector and the heat sink, that is, a layer of copper foil is sandwiched between the insulators as an electrostatic shielding layer, which is connected to the input DC 0V ground, and the heat sink is still connected. On the chassis floor, this will greatly reduce the coupling capacitance Ci between the collector and the heat sink, which reduces the electric field coupling between them.
(2) Reduce the distributed capacitance Cd of the pulse transformer. An electrostatic shielding layer is applied between the primary side and the secondary side. The shielding layer should be as close as possible to the emitter and grounded, so that the coupling capacitor Cd is also divided into a series of Cd1 and Cd2.
Reduce the coupling interference of the electric field on the primary and secondary sides. The second is to take measures against the characteristic that the switching power supply transmits noise through the power line, that is, the filtering technology is used to destroy the interference. The filtering techniques used are:
(1) AC side filtering. The input terminal of the AC power line of the switching power supply is inserted into the common mode and differential mode filter to prevent the common mode and differential mode noise of the switching power supply from being transmitted to the power line, affecting other power devices in the power grid, and also suppressing noise from the power grid. AC side filter, in which LD and CD are used to suppress differential mode noise. Generally, LD takes 100 to 700 μH, and CD takes 1 to 10 μF, which is effective for suppressing noise of 10 to 150 kHz. LC and CC suppress common mode noise. Generally, LC takes 1~3mH and CC takes 2000~6800pF, which is effective for suppressing common mode noise above 150kHz. For the specific switching circuit, the parameters of the above components should be debugged and determined.
(2) DC side filtering. A power supply filter is inserted in the DC output side of the switching power supply, which is composed of common mode chokes L1, L2, choke L3 and capacitors C1, C2. In order to prevent the magnetic core from being saturated under a large magnetic field strength and causing the choke to lose its function, the core of the choke must use a constant μ core with high frequency characteristics and high saturation magnetic field strength.
shield
An effective way to suppress radiated noise is to shield. The electric field is shielded by a conductive material, and the magnetic field is shielded by a material having a high magnetic permeability. In order to prevent the magnetic field leakage of the pulse transformer, a closed magnetic ring can be used to form a magnetic shield to shield the entire switching power supply. The heat dissipation and ventilation problems should be considered when shielding. The ventilation holes on the shielding box are preferably round, and the joints are preferably welded to ensure electromagnetic continuity.
There are many factors to consider in the electromagnetic compatibility design of switching power supplies, such as the production of printed boards, the layout of components, and the bundling and configuration of various power lines and signal lines. There is much work to be done. The comprehensive suppression of various noises of the switching power supply will greatly improve the electromagnetic compatibility of the switching power supply, and the switching power supply will be more widely used.
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