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Switching power supply noise problem

Author:Roadsunny Date:2012-12-19 23:28:08
Switching power supply noise problem
The noise from the PCB design / circuit oscillation / magnetic components in three areas:
1) circuit oscillation, power output very low frequency stability wave. The circuit stability redundancy due to insufficient. Theoretically can use the system control theory in the frequency domain method / time domain method or Routh criterion do theoretical analysis. Now; convenient means of computer simulation verify circuit stability, in order to avoid self-oscillation occurs, there are a variety of software can be used. Ready for the circuit, you can increase the output filter capacitor or inductor / change the position of signal feedback / increase the integration capacitor PI regulator / reduce the open-loop amplification method to improve.
2) PCB design
A) EMI noise caused the RF Noise adjust PI regulator, the output error signal contains disturbance. Main view the high-frequency capacitance is too far left off components, whether large C-shaped surround wiring and so on ...
B) at least more than two o'clock PCB line control circuit and power circuit shared. PCB overlying copper is not a perfect conductor, it can always be equivalent to the inductor or resistor element, when the power current flowing through the common PCB line and the control loop to generate a voltage drop on the PCB, the control circuit of each node is dispersed at different positions voltage drop caused by the power current into the control network hotels disturbance, the circuit noise. This appeared to occur in the power ground line, pay attention to the single-point ground can be improved.
3) magnetic components
Magnets have magnetic characteristics to the strain, the enameled wire will leak magnetic field by the left and right of the electric power, under the combined effect of these factors, the local overtones or 1 / N frequency resonance occurs. Changing the switching frequency and the magnetic element Impregnation can be improved.
This is usually a little experience, try.
Do not know you say the noise is mechanical vibration noise or high-frequency AC component in the output voltage?
Both noise often encountered in switching power supply
Mechanical noise is mostly because of the circuit, the presence of abnormal electrical shock, and the frequency is less than 20K, transformers, inductors, magnetic core, the sound, the human ear can hear. The solution is to adjust the compensation, to reduce the input impedance of the amplifier, in a sensitive place interference, plus absorption circuit.
Output ripple noise is mainly due to the switching of the tube as of the moment, due to voltage spikes caused by the leakage inductance of the transformer and line inductance, it is caused by the output ripple noise, high frequency switching power supply in general we do, much larger than 20K, so, if no abnormal circuit shock, we can not hear the sound




V1 ~ V4 finishing by a bridge rectifier, the AC input switching power supply into a DC voltage Vi is applied to the high-frequency transformer primary L1 and the switching valve V5. The base input of the switch tube V5 a tens to hundreds of kHz, high frequency rectangular wave, the repetition frequency and the duty cycle is determined by the requirements of the output DC voltage VO. Pulse current switch amplified by the high-frequency transformer coupled to the secondary loop. High frequency transformer primary to secondary turns ratio is determined by the requirements of the output DC voltage VO. High-frequency pulse current through the diode V6 rectifier and C2 filtered into a DC output voltage VO. Switching power supply in the following areas will produce noise formation of electromagnetic interference.

(1) high-frequency transformer primary L1, switch V5 and filter capacitor C1 form of high-frequency switching current loop, may
Larger space radiation. If the capacitor filter, the high-frequency current but also the differential mode conducted to the input AC power. Figure 1 in the I1.

(2) high-frequency transformer secondary L2, the rectifier diodes V6, filter capacitor C2 also constitute a high-frequency switching current loop
Generation of space radiation. If the capacitor filter, the high-frequency current differential mode will be mixed in the form of conduction out of the output DC voltage. Figure 1 in the I2.

(3) high-frequency transformer primary and secondary distributed capacitance Cd, junior high frequency voltage electricity through these distribution
Content will be coupled directly to the secondary up phase common mode noise generated in the the secondary two output DC power supply line. If the two line-to-ground impedance imbalance, but also into the differential mode noise.

(4) The output rectifier diode V6 will produce reverse surge current. Diode in the forward conduction PN junction charge
Accumulation, will disappear and a reverse current diode reverse voltage accumulated charge. Because the switching current required to be provided by the diode rectifier, the diodes by conduction into the off time is very short, to disappear in a short period of time to let stored charge to produce a reverse current surge. Surges caused by the oscillation frequency attenuation due to the distributed inductance in the DC output line, the distributed capacitance, which is a differential mode noise.

(5) the load of the switching valve V5 is the primary coil L1 of the high-frequency transformer, is an inductive load, the switch-off will appear at both ends of the tubes when a high surge voltage spikes, this noise will be transmitted to the input and output ends to.

(6) the switch between the distributed capacitance CI V5 collector and heat sink K high frequency switching current to flow through the CI heatsink K, and then flow to the chassis ground, and eventually flow to the chassis ground phase AC power cord to connect the protective earth PE, resulting in the common-mode radiation. L and N of the power supply line of PE presence of a certain impedance, such as impedance imbalance of common mode noise will be converted into a differential mode noise. Figure 1 in I3.

By the analysis above, we can know that a lot of switching power supply noise source interference pathway is varied, affecting a large noise source can be summarized as the following three:

(1) diode reverse recovery time due to interference.

(2) harmonic interference switch work

Power switches in the conduction flow through large pulse current during the closing, the high-frequency transformer winding leakage inductance caused by the current mutation will produce spikes interference.

(3) AC input circuit interference

Switching power supply input rectifier reverse recovery period can also cause high frequency attenuation oscillation interference. Usually after the rectifier circuit is always connected to a relatively large filter capacitor, and thus the conduction angle of the rectifier is small, will cause a lot of charging current, so that the alternating current of the AC input side distortion, affecting the quality of the power grid. In addition, the filter capacitor equivalent series inductance interfere also have a greater impact.

All these disturbances route of transmission can be divided into two types of conduction and radiated interference. The interference peak interference and harmonic interference generated by the switching power supply energy by switching power supply input and output lines spread formation called conduction disturbances. Harmonic and spurious oscillations energy, through the propagation of the input and output lines, the generated electric and magnetic fields in the space, the interference generated by the electromagnetic radiation is called radiation interference.

Because of the switching power supply itself is a strong source of interference, so in addition to take measures to suppress electromagnetic interference generated circuit, should also switching power effective electromagnetic shielding, filtering and grounding.
3 switching power supply noise rejection

The three elements of the formation of electromagnetic interference is the source of interference, transmission routes and disturbed equipment, therefore, suppress electromagnetic interference should also proceed from the three aspects. The route of transmission; should first sources of interference suppression directly eliminate the cause of the disturbance; followed by the elimination of the coupling between the source of interference and disturbed equipment and radiation, cutting electromagnetic interference third is to improve the immunity disturbed equipment to reduce the noise sensitivity. The third point is not the scope of this article.

The power factor correction (PFC) technology and soft switching power conversion technology can greatly reduce the amplitude of the noise.

(1) circuit on the measures

Mainly due to the sharp change of the voltage and current switching power generating electromagnetic interference and therefore require as much as possible to reduce the rate of change (du / dt, di / dt) of the voltage and current in the circuit. Suppress electromagnetic interference absorption circuit is also a good way. The basic principle of the absorption circuit is for the switch when the switch is turned off to provide bypass, absorbing the energy accumulated in the parasitic distributed parameter, thereby suppressing the interference occurs. The commonly used snubber circuit RC, RCD, LC passive absorption of the network and the active absorption Network.

The filter is a good way to suppress conducted interference. For example, on the power input termination filter can inhibit the switching power supply to produce interference and feedback to the grid, from the grid against noise on the power supply itself can be suppressed. In the filter circuit also uses many dedicated filter element, such as the feedthrough capacitor, three-terminal capacitor, ferrite bead, and that they can improve the filter characteristic of the circuit. Appropriate design or choose a filter, and the right to install the filter, is an important part of the anti-jamming technology.

Specific measures are as follows:

a. Installation of power at the AC input of the filter, the filter circuit of the type shown in Figure 2. LD, CD is used to suppress differential mode noise, LD generally take 100 ~ 700μH the CD take ~~ 10μF, 10 ~ 150KHz. LC, CC is used to suppress common mode noise, and generally and LC take 1 ~ 3μH CC take 2000 ~ 6800pF 150KHz to suppress. The above effective common-mode noise. The device parameters to be adjusted in practice.
 


In addition, the power supply filter installation should pay attention to:

When the installation of the power supply filter must be grounded. In addition to the the manufacturers special instructions allow ungrounded filter can use ungrounded, all power filter must be grounded, because the common mode filter bypass capacitor must be grounded in order to work. In addition to the phase filter with metal chassis ground, but also with thicker wire connected to the grounding point of the filter housing and equipment. Grounding impedance is lower, the better the filtering effect.
Power as close as possible to the entrance. Installation, the input / output terminal of the filter as possible, and avoid the interference signal from the input terminal is coupled directly to the output terminal. When necessary, the use of shielding partition separated.

b retrofitting in the output terminal of the power supply to the common mode noise filter. Put a ferrite bead on the output line, made of conjugated choke, plus mounted high-frequency capacitance, which can inhibit some of the common mode noise. Increase the inductance of the output filter inductor and filter capacitor capacitance can suppress differential mode noise, better multiple capacitors in parallel.

c output rectifier diode using multiple diodes are connected in parallel to share the load current, select the rectifier diode reverse recovery current was soft features appropriate to reduce the rate of the opening of the switch, to reduce the high-frequency transformer leakage inductance and ensure that it is not saturated are effective means for suppressing noise.

d the primary side of the high-frequency transformer, the secondary side switch CE poles, as well as the output rectifier diodes have an RC network absorption. Suppress voltage spikes and current surges. Saturable the amorphous magnetic ring to reverse surge current suppression diode in series with the output rectifier diode branch better. As shown in Figure 3.
 

Figure 3 RC absorption Examples of use of the network and the toroid


e. row of printed boards, minimize the area of ​​the high frequency loop, to shorten the high-frequency signal line. It should also be noted in the wiring of the machine:

Switching power supply input AC power line and output DC power lines close together, not to bundle, as far as possible from the noise source.

Output DC power cable is best to use twisted pair, should be at least traces close together.

The input and output power of the power supply line should be as far away as possible from the control signal line driving circuit.

f. minimize the distributed capacitance CI switch between the collector and the heat sink. A low dielectric constant insulating mat can be selected and properly thicken the thickness of the gasket. When necessary, is inserted between the insulating pad thin copper used as an electrostatic shield.

g. ground

Power ground to safety, another purpose is to consider the electromagnetic compatibility problems. A good grounding system can play a significant role in reducing electromagnetic interference.

For security reasons grounding, generally referred to as the safety, the power supply metal shell and connected to earth. Considering the electromagnetic compatibility problems, you should first understand the concept of the signal, ground loop interference.

Signal: the signal current flows back to source low-impedance path. The ground loop interference: local online a large current flows, will produce a voltage drop due to the impedance of the ground is not zero, the voltage will generate a current in the two circuits on the connecting cables. Due to the unbalanced nature of the circuit, each of the wires on the current, it will produce a differential mode voltage, circuit causing interference. Such interference due to ground loop currents caused, so called ground loop interference.

Grounding problem:

1) to minimize lead inductance of the high frequency impedance.

2) to increase the impedance of the ground loop, the use of the shield between the initial level of isolation transformers or optocouplers transmission signal, in order to reduce ground loop interference.

3) two unit circuit is best not to share a single power supply and the same stretch of ground.
Amplifier shielding shell, transformer shield well grounded.

(2) structural measures: shield

The shield is one of the important means to solve the problem of electromagnetic compatibility, the purpose is to cut off the route of transmission of electromagnetic waves. Most of electromagnetic compatibility problems to solve electromagnetic shielding. The greatest benefits will not solve the problem of electromagnetic interference electromagnetic shielding affect normal circuit operation.

The shield divided electrical shielding, magnetic shielding and electromagnetic shielding.

The switching power supply is mainly to do the shielding and control of the shield of the housing, the shielding of the high frequency transformer, the switch and the rectifier diode, the driving circuit shielding, and by various methods to improve the shielding performance