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    Abstract: Explained switching power supply’s composition and the principle of work; Embarked from switching power supply’s each constituent, has analyzed the mechanism which the electromagnetism harassment produced, proposed should adopt corresponding braking measure; Discussed the issue which in the electromagnetic compatibility design needs to perform to pay attention.

    Key word: Switching power supply; Electromagnetic interference; Braking measure; Electromagnetic compatibility

Introduction

The switching power supply and the linear voltage-stabilized source compare, have the power loss to be small, the efficiency is high, the volume is small, the weight is light, the constant voltage scope wide and so on many merits, oneself is widely applied in domains and so on computer and auxiliary equipment, correspondence, automatic control, domestic electric appliances. But switching power supply’s prominent shortcoming is can have the strong electromagnetic interference (EMI). The EMI signal both has the very wide frequency range, and has certain scope, after conduction and radiation will pollute the electromagnetic environment, will create the disturbance to the communication facility and the electronic products. If processes improper, the switching power supply itself will turn a harassment source. At present, electronic products’ electromagnetic compatibility (EMC) receives day by day takes seriously, to suppress switching power supply’s EMI, enhances the electronic products the quality, causes it to conform to the EMC standard, has become the electronic products designer more and more matter of concern. This article has carried on the discussion on the high frequency switching power supply design’s in electromagnetic compatibility question.

1 switching power supply’s composition and principle of work

1.1 compositions

Switching power supply’s composition diagram as shown in Figure 1, it is composed of the following several parts:

1) the main circuit including inputs the filter, the rectification and the filter, the inversion, the output rectification and the filter;

2) controls and the protection circuit;

3) examines and the display circuit besides provides each kind of parameter which the protection circuit needs, but also provides each kind of demonstration data;

4) accessory power supply.

Figure 2

    1.2 switch voltage-stabilized source principle

Switch voltage-stabilized source electric circuit as shown in Figure 2. Figure 2 switch K by certain time-gap repeatedly connection and separation, when the K connection, inputs power source Vin supplies power through K and the filter circuit for load RL, when K separation, input power source Vin then to interrupt energy provision. Obviously, inputs the power source to provide the energy to the load is off and on, to enable the load to obtain the continual energy to provide, the switch voltage-stabilized source must have a set of stored energy installment, when the switch puts through a part energy storage, when the switch separates, to load release. In Figure 2, the electric circuit which by stored energy inductance L, filter electric capacity C2 and after flow diode D is composed, has this kind of function. (1) expressed in AB voltage mean value VAB available -like.

VAB=Vinton/T=DVin    (1)

In the formula: ton is the K breakover time;

T is the K action cycle;

D is the dutyfactor, D=ton/T.

(1) may know by the type, changes D, then changes VAB. Therefore, adjusts D along with the load and the input supply voltage’s change then to be able to cause output voltage Vo to maintain invariably. This control method is called the time ratio control (Time Ratio Control, abbreviation is TRC). According to the TRC principle, it has 3 ways:

1) the pulse width modulation (Pulse Width Modulation, abbreviation is PWM) its switching period is constant, changes the dutyfactor through the change pulse width the way;

2) the pulse-frequency modulation (Pulse Frequency Modulation, abbreviation is PFM) the breakover pulse width is constant, changes the dutyfactor through the change switch operating frequency the way;

3) the hybrid modulation breakover pulse width and the switch operating frequency are not fixed, each other can change the way, it is the above two way union.

Figure 3

2 switching power supplies produce the electromagnetic interference mechanism

The reason that the switching power supply is a very strong electromagnetism harassment source, originates in the high frequency make-and-break switch component and the output rectification diode, as well as pulse transformer and filter inductance and so on.

2.1 switching valves and electric valve

Switching valve, when electric valve high frequency make-and-break produces dv/dt, di/dt have the big spoke’s pulse, the frequency band are wide, and the overtone is rich, is a very strong harassment source.

    2.2 high-frequency transformers

The switching valve load for the high-frequency transformer primary coil, in the switching valve breakover instantaneous, the primary coil produces very big flowing rapidly, and presents the high surge peak voltage; In switching valve separation instantaneous, as a result of primary coil’s leakage flux, causes part of energies not to transmit the sencondary coil, but through the collecting electrode electric circuit’s in electric capacity, the resistance forms has peak’s damped oscillation, superimposes on the shutdown voltage, forms the shutdown voltage peak, produces with the primary coil connection when same magnetization surge current transient, this noise will conduct to the input, the out-port, will form the conduction harassment, heavy has the possibility to penetrate the switching valve.

Moreover, the high-frequency transformer primary coil, the switching valve and the filter electric capacity constitutes the high frequency switch current loop will possibly produce the big space radiation, will form the radiation harassment. If the electric capacity filter capacity insufficiency or the high-frequency response are not good, on electric capacity’s high frequency impedance will cause the high-frequency current to conduct by the difference norm type to the alternating current supply in forms the conduction harassment. What needs to pay attention, the diode rectification electric circuit produces in electromagnetism harassment, rectification diode reverse recovery current|di/dt| far compared to after flow diode reverse recovery current|di/dt| is much bigger. Studies as the electromagnetism harassment source, the rectification diode reverse recovery current forms the harassment intensity is big, band width. But, the rectification diode produces the voltage jump is smaller than far when the power switching valve breakover and the shutdown produces voltage jump. Therefore, does not count the rectification diode to produce|dv/dt| sum|di/dt| influence, but regards the leveling circuit an electromagnetism harassment coupling channel’s part to study is also may.

2.3 stray parameter influence coupling channel’s characteristic

In the conduction harassment frequency band (<30MHz), the most switching power supply harassment’s coupling channel is may use the electric network to describe. But, in switching power supply’s any actual primary device, like the resistor, the capacitor, the inductor and even the switching valve, the diode contains has the stray parameter, and studies the frequency band is wider, of equivalent circuit’s order is higher, therefore, will be much more complex coupling’s including between various primary devices stray parameter and primary device’s switching power supply’s equivalent circuit. When high frequency, the stray parameter is very big to the coupling channel’s characteristic influence, distributed capacity’s existence into electromagnetism harassment channel. Moreover, when the switching valve power is big, the collecting electrode generally must add on the radiator fin, between the radiator fin and switching valve’s distributed capacity when high frequency cannot neglect, it can form the syntype harassment which conducts face the spatial radiation harassment and the power line.

3 electromagnetic compatibility design

3.1 input end filter’s design

The switching power supply produces the noise contains the syntype noise and the bad mold noise. The syntype disturbance is because carries between the class conductor and the earth potential difference produces, its characteristic is two on-line miscellaneous news voltage is the same electric potential cocurrent; But misses the mold disturbance is because carries between the class conductor the potential difference to produce, its characteristic is two on-line miscellaneous news voltage is the same electric potential is reverse. Usually, on the line disturbing voltage’s these two kind of components are simultaneously exist. Therefore should add the filter in the power source input end, the filter impedance should with the source impedance unbalance, the unbalance be fiercer, the weaken which realizes is more ideal, the insertion loss characteristic which obtains is better. That is, if the noise source interface resistance is the low impedance, with it docking EMI filter’s input impedance should be the high impedance (for example inductance quantity very big series inductance); If the noise source interface resistance is the high impedance, then the EMI filter’s input impedance should be the low impedance (for example capacity very big shunt capacitance). Because the line impedance is not balanced, two kind of components will transform mutually in the transmission, the situation will be very complex. The typical EMI filter have contained the syntype miscellaneous news and the difference mold miscellaneous news two part of suppression circuits, as shown in Figure 3.

In chart: The bad mold suppresses electric capacity Cx1, Cx 20.1~0.47 μF;

The bad mold suppresses inductance L1, L2 100~130 μH;

The syntype suppresses electric capacity Cy1, Cy2 <10000pF;

The syntype suppresses inductance L 15~25mH.

Insertion loss’s definition as shown in Figure 4, when has not met the filter, the supply oscillator output voltage is V1, when after filter turning on, obtains the supply oscillator in the filter out-port the voltage is V2. If the supply oscillator output impedance and the receiver input impedance are equal, is 50Ω, then filter’s insertion loss is

When design, must make the syntype filter circuit and the difference mold filter circuit’s resonance frequency is lower than switching power supply’s operating frequency obviously, generally must be lower than 10kHz, namely

Figure 5 is the difference mold and the syntype disturbance schematic drawing.

In actual use, because the equipment produces the syntype and the difference mold’s ingredient is dissimilar, therefore, the filter circuit may increase or the reduction filter element suitably. After the concrete electric circuit’s adjustment must undergo the EMI experiment, generally can have satisfaction result, installs when the filter circuit certainly must guarantee the good earth, and the input end and the out-port must isolate good, otherwise, does not have filter’s effect.

Figure 6 is two kind of filter circuits, their filter effect like chart 7 the experimental curve shows.

3.2 radiate EMI the braking measure

Must reduce the radiated interference, may apply the voltage cushion circuit, like in the switching valve both sides parallel RCD cushion circuit, or the electric current cushion circuit, like connects 20~80 μH inductance on switching valve’s collecting electrode.

The power switching valve’s collecting electrode is a strong harassment source, switching valve’s radiator fin receives on the collecting electrode, guaranteed that between the collecting electrode and the radiator fin in the electric current inflow main circuit which produces as a result of the distributed capacity. In order to reduce between the radiator fin and cabinet’s distributed capacity, the radiator fin should be far away from the cabinet as far as possible, if has condition, may use has the shield measure radiator fin. The rectification diode should use restores the electric charge to be small, and restores the time to be short reverse, if the Short base tube, should better select restores reverse assumes softly characteristic. Moreover, may reduce the disturbance in the Short base tube two cable muff magnetism bead and the parallel RC absorption network, the resistance, electric capacity’s value may be several OMEGA and several thousand pF, the electric capacity lead wire should be as far as possible short, reduces the lead inductance.

    The load current is bigger, the diode restores reverse the time is also longer, then the peak electric current’s influence is also bigger. Uses many diodes parallel to share, may reduce the short circuit peak electric current the influence.

The switching power supply must shield, uses the module type entire seal structure, generally with 1mm above thickness galvanization steel plate, the shielding layer must earth good. Between the radio impulse transformer at the beginning, is secondary adds shielding layer joining-up place, may the barrage jamming electric field coupling. Radio impulse transformer, output filter inductance and so on magnetic elements add on the screening can, may limit the magnetic line of force in magnetic resistance small shield in vivo.

For example, surpasses standard limiting value 20dB to the radiated interference some switching power supply, used following somewhat the measure which easy to realize in the laboratory to make the improvement:

1) in all rectification diode both sides parallel 470pF electric capacity;

2) in switching valve G input end parallel 50pF electric capacity extremely, forms a RC low pass filter with the original 39Ω resistance;

3) in each output filter electric capacity (electrolysis electric capacity) on parallel 0.01μF electric capacity;

4) on rectification diode base pin a set of small magnetism bead;

5) improves the shield body’s earth.

After the above improvement, this power source may through the radiated interference test limiting value request.

3.3 conduction harassment solution

Switching power supply’s conduction harassment through inputs the power line to disseminate outward, both has the difference mold harassment, and to have the syntype harassment. The conduction harassment’s test frequency range is 0.15~30MHz, limiting value request like table 1 arranges in order.

Table 1 conduction harassment limiting value table

In the 0.15~1MHz frequency range, the harassment mainly by the syntype form existence, in the 1~10MHz frequency range, the harassment form is the difference mold and the syntype coexistence, above 10MHz, harassment form mainly by altogether membrane primarily. The bad mold harassment’s production is mainly as a result of the switching valve work in on-off state, when the switching valve clears, winds through power line’s electric current linearity rise, the switching valve shuts off when the electric current sudden change is zero, therefore, winds through power line’s electric current for the high frequency triangle pulsating current, includes the rich high frequency harmonic component, along with frequency elevation, this harmonic component scope getting smaller, therefore the difference mold harassment elevates along with the frequency reduces, output circuit’s filter circuit as shown in Figure 8, electric capacity C5 and the inductance L3 composition low pass filter, the bad mold conduction harassment mainly has the low frequency band.

Has the syntype harassment primary cause is the power source and the earth (protectorate) between has the distributed capacity, the electric circuit Chinese side wave voltage high frequency harmonic component spreads to the earth through the distributed capacity, with the power line constitution return route, has the syntype harassment. As shown in Figure 8, L, N are the mains input, C1, C2, C3, C4, C5, L1, the L2 composition input the EMI filter, DB1 are the rectification bridge, VT2 are the switching valve, the switching valve installs when the radiator, switching valve’s D extremely and the radiator is connected, forms a coupling capacity with the radiator, like Figure 8 C7 shows, the VT2 work in on-off state, its D voltage extremely will be the high frequency square-wave, the square-wave frequency for switching valve’s turn-on frequency, in square-wave various subharmonics through the coupling capacity, L, the N power line constitution return route, will have the syntype harassment. The power source and the earth distributed capacity is quite scattered, estimates with difficulty, but looking from Figure 8, VT2 D extremely and between the radiator coupling capacity’s function is biggest, from DB1 to the inductance L3 between voltage is 100Hz, but from L3 to VD1 and the VT2 D extremely segment’s voltage is the square-wave voltage, includes the massive higher harmonics. The next L3 influence is also quite big, but L3 and cabinet’s distance is quite far, the distributed capacity is much smaller than between the switching valve and radiator’s coupling capacity, therefore, we mainly consider between the switching valve and radiator’s coupling capacity.

    3.4 earth technology application

“the earth ” has the equipment internal signal ground and the equipment meets the earth, both concept is different, the goal is also different. “” the classics definition is ” takes the electric circuit either the system datum equipotential or the plane “.

3.4.1 equipment’s signal grounds

Equipment’s signal ground, possibly is or the metal takes the signal together by an equipment’s in spot the earth reference point, it has provided a public reference potential for equipment’s in all signals. In here introduced that floats and the mix earth, moreover, but also has the single-point earth and accepts after checking.

1) floats uses floats the place the goal is the electric circuit or the equipment and the public earth system, possibly or causes the circulation public wire to isolate. Floats may also cause during the different electric potential the electric circuit coordination becomes easy. Realizes the electric circuit or the equipment floats the place the method to have the transformer isolation and the electro-optical isolation. Floats the place the biggest merit is the resistance to interference is good. Floats the place the shortcoming is because not with public connected, causes the static electricity in both to accumulate easily, when the electric charge accumulates after the certain extent, possibly causes the fierce static discharge, but becomes the broken link very strong harassment source. A trade-off decision is in floats with public between bridges a resistance number very big bleeder, with releases the electric charge which accumulates. The attention control release resistance’s impedance, the too low impedance will affect the equipment leakage current the qualification.

2) mixes the earth mix earth causes the earth system when low frequency and high frequency presents the different characteristic, this in the wide band sensitive electric circuit is necessary. The electric capacity has the high impedance to low frequency and the cocurrent, can therefore avoid between two modules the place ring circuit formations. When will direct current and the radio frequency separates, each sub-system’s cocurrent through the 10~100nF capacitor receives the radio frequency ground, these two farms should have the low impedance in a spot to connect, junction elected in highest turn over speed di/dt signal existence spot.

3.4.2 equipment meet the earth

In the project practice, besides considered earnestly the equipment internal signal ground, usually also equipment’s signal place, the cabinet and the earth in the same place, takes continually by the earth equipment’s earth reference point. The equipment meets the earth the goal is:

1) assures the equipment operators person’s safety;

2) in the draining engine case accumulates the electric charge, avoids the electric charge accumulation causing the engine case electric potential to elevate, causes the electric circuit work not to be unstable;

3) avoids the equipment causing the equipment under the outside electromagnetic environment function to change to the earth electric potential, causes the equipment work not to be unstable.

Thus it can be seen, the equipment meets the earth besides is to the personal security, the equipment security consideration, also suppresses the important means which the harassment occurs.

3.5 shield technologies

Suppresses the switching power supply to have harassment radiation another method is the shield, the goal is shuts off the electromagnetic wave the dissemination way, will use electricity the magnetic shield method solution electromagnetic interference question not to interfere with electric circuit’s normal work. Its using electricity conductance good material carries on the shield to the electric field, carries on the shield with the magnetic conductivity high material to the magnetic field. In order to prevent the pulse transformer’s magnetic field revelation, may use closed ring-like becomes the magnetic shield, moreover, but must carry on the electric field shield to the entire switching power supply. The shield should consider that radiates and ventilates the question, in the screening test air vent should better be the circular porous, in satisfies under the well ventilated condition, hole quantity may be many, each hole’s size must be as far as possible small. The connecting point must weld, guarantees the electromagnetism circuit’s continuity, if uses the screw retention, pays attention to the bolt spacing to be short. Screening test’s introduction, the lead-out wire place must take the filter measure, otherwise, these will become the harassment transmitting antenna, will reduce the shield effect seriously. If to the electric field shield, the screening test must certainly earth, otherwise, will not have the shield effect; If to the magnetic screen, the screening test cannot earth. Sets at the type switching power supply’s outer covering to non-inserting outside certainly to carry on the electric field shield, otherwise, very difficultly through radiation harassment test. Regarding the switching power supply, mainly completes the cabinet shield, the high-frequency transformer shield, the switching valve and the rectification diode’s shield, uses the electro-optical isolation technology. The power switching valve and the output diode usually have the big power loss, to radiate often needs to install the radiator or the immediate packing on the power source ledger wall. The component installs when needs to use the heat conduction performance good insulating piece to carry on the insulation, this caused between the component and the ledger wall and the radiator has produced the distributed capacity, switching power supply’s ledger wall is alternating current supply’s grounding, thus to exchanged through between the component and ledger wall’s distributed capacity the electromagnetic interference coupling the input end to produce the syntype disturbance, solved this question means is uses between two insulating pieces to clamp a shield piece, and received the shield piece the cocurrent ground, cut off the radio-frequency interference to the input electrical network dissemination way. In order to suppress the radiation electromagnetic interference which the switching power supply produces to other electronic installation’s influence, may defer to completely to the magnetic screen method processes the screening can, then the entire screening can and system’s cabinet and the place connection is a body, can carry on the effective shield to the electromagnetic field. The power source certain part and the earth connected may play the barrage jamming the role. For example, the electric shield level earth may suppress the change electric field the disturbance; The electromagnetic screen uses the conductor may not earth in principle, but does not earth the shield conductor often strengthens the electrostatic coupling to have so-called ” negative electric shield ” the effect, therefore still take earthed as well, like this enables the electromagnetic screen simultaneously to display the electric shield the function. Electric circuit’s common reference point and the earth are connected, may provide the stable reference potential for the signal channel. Therefore, in after system’s safekeeping of security grounding, the shield meet the grounding and the public reference grounding form the ground strap respectively, with the earth is finally connected.

3.6 primary device layouts and print circuit wafer wiring technology

Switching power supply’s radiation harassment and in passage of current’s electric current size, circuit’s ring circuit area, as well as power frequency’s square product is proportional, namely radiation harassment E∝IAf2. Is the circuit size is smaller than the frequency wave length far using this relations’ premise.

The above relationship indicated that reduces the passage area is reduces the radiation harassment the key, is openly expresses turns off light source primary device each other to be closely packed. In the primary circuit, requests the input end electric capacity, the transistor and transformer each other approaches, and the wiring is compact; In the secondary circuit, requests the diode, the transformer and the out-port electric capacity each other drawing close.

Figure 8

    When design print circuit wafer, should place as far as possible the incident cross-correlation primary device in the same place, because avoids the primary device leaving too far creates the disturbance which the print line is excessively long brings; Furthermore the input signal and the output signal will lay aside as far as possible nearby the lead wire port, will avoid the disturbance which will produce because of the coupling way. On the board, abuts separately the load class wire the cloth in board both sides, and tries to cause it maintains parallel, because the parallel abutting load class conductor produces the exterior magnetic field tends to counter-balances mutually. The practice proved that the board primary device arrangement and the wiring design have the enormous influence to the switching power supply EMC performance, in high frequency switching power supply, because on the board both has the level for ±5V~±15V small signal pilot wire, and has the high-voltage power supply bus bar, meanwhile has some high frequency power switch, the magnetic elements, how to arrange the primary device position reasonably in the board limited space, immediate influence to electric circuit in various primary devices own anti-jamming and electric circuit work reliability. Moreover, prohibited that two print holding wires parallel walk the line. If parallel walks the line to be avoidless, may recover through the following methods:

1) adds a grounding between two holding wires, by plays the shielding effect;

2) spreads out between two parallel holding wire’s as far as possible, reduces two between electromagnetic fields the influences;

3) causes the direction of current which two parallel holding wires wind through to be opposite. Electromagnetic coupling between the wiring is carries on through the electric field and the magnetic field, therefore in wiring time, should pay attention to the electric field and magnetic field coupling suppression. Suppresses the method to the electric field to include:

1) increases the space distance as far as possible, causes the capacitive coupling to be smallest;

2) uses the electric shield, the shielding layer must earth;

3) reduces the sensitive line’s input impedance.

Suppresses the method to the magnetic field to include:

1) reduces the harassment source and the sensitive electric circuit’s ring circuit area;

2) increases the space distance, causes mutual inductance between the coupling harassment of source and the sensitive electric circuit’s is as far as possible small;

3) the easiest to use harassment source and the sensitive electric circuit assume the right angle wiring, with the aim of reducing between greatly the line the coupling.

Moreover, through the analysis printed wiring’s characteristic impedance, selects the printed wiring the laying aside way, the length, the width as well as the layout way. The simple root wire’s characteristic impedance is composed of DC resistance R and self inductance L, print line l is shorter, DC resistance R is smaller; Simultaneously increases the print line the width and thickness may also reduce DC resistance R. Print line length l is shorter, self inductance L is smaller, moreover increases the print line width b also to be possible to reduce self inductance L. But the many print line’s characteristic impedance besides is composed of DC resistance R and self inductance L, but also has the mutual inductance M influence, mutual inductance M besides receives prints the line the length and the width influence, prints between the line to be away from is also playing the vital role, increases two spacings to be possible to reduce the mutual inductance. In view of the above phenomenon, when design print circuit wafer, should reduce the power line and the grounding impedance as far as possible, because the power line, the grounding and other print line have the inductance, when the supply current change is big, will have the big pressure drop, but the grounding pressure drop will be forms the common impedance disturbance the important attribute, therefore should reduce the grounding as far as possible, may also overstrike as far as possible the power line and the grounding line. In the two-sided board designs, cleans out possibly overstrikees outside the power line and the grounding line, but should also installs the high-frequency response good decoupling electric capacity between the grounding and the power line.

4 conclusions

Must raise the turn-on frequency, enhances the switching power supply product the quality, the electromagnetic compatibility question must the key consideration. This article is in analyzed the disturbance to produce the mechanism as well as to undergo the massive practices in the foundation, proposed effective braking measure. Has the switching power supply electromagnetic interference factor to be also many, suppresses the electromagnetic interference also to have the massive work to do. When design, but must from eliminate the harassment source and is harassed between equipment’s coupling and the radiation, the cut-off electromagnetic interference’s dissemination way embarks, causes switching power supply’s electromagnetic interference to fall the bottom.