Abstract: In view of switching circuit electromagnetism harassment question. Elaborated the power switching valve produces the electromagnetism harassment mechanism, as well as electromagnetism harassment production reason and dissemination way. Proposed the choice appropriate operating frequency, the electric circuit primary device, the cushion circuit, the power factor corrective network, the shield, the filter network and the earth technology reduce the switching circuit electromagnetism harassment the measure and the method. The practice and the experimental proof, these measures and the method to reduce switching circuit’s electromagnetism harassment to have the tangible effect.
Key word: Switching power supply; Electromagnetism harassment; Suppression
Foreword
Along with the modern science technology’s development, the switching power supply is widely applied in each kind of electronic installation or the system. Switching power supply performance quality, immediate influence equipment or system’s normal operation. The switching circuit is switching power supply’s core, the switching circuit passes, breaks the process under high frequency to have the large scale voltage jump, namely produces dv/dt has the great scope pulse, the frequency band is wide, and the overtone is rich, is the switching power supply electromagnetism harassment primary factor. Suppresses switching circuit’s electromagnetism harassment to become enhances the switching power supply performance the main way.
1 switching circuit electromagnetism harassment (EMD) production and dissemination way
The switching power supply mainly by a leveling circuit, the switching circuit, two leveling circuits, the feedback trigger control electric circuit is composed. The switching circuit is switching power supply’s core, mainly (L1 and L2) is composed of power switching valve S and the high-frequency transformer, as shown in Figure 1.
1.1 switching circuits have the electromagnetism harassment reason
The power switching valve S load is high-frequency transformer primary coil L1, is the perceptual load. In the S breakover instantaneous, L1 produces very big flowing rapidly, and presents the high surge peak voltage in its both sides; Separates in S instantaneous, as a result of the L1 leakage flux, causes part of energies not to transmit from L1 to secondary coil L2, will preserve in the leakage inductance this part of energies with the drain electrode electric circuit’s in electric capacity, the resistance will form has peak’s damped oscillation, will superimpose on the shutdown voltage, will form the shutdown voltage peak, this kind of voltage will produce with the L1 connection when same magnetization surge current transient, this noise will conduct to inputs the out-port, will form the conduction harassment, heavy has the possibility to penetrate in the switching valve and this electric circuit other primary devices.
L1, S and the filter electric capacity C1 constitution’s high frequency switch current loop will possibly produce the big space radiation, will form the radiation harassment. If the electric capacity C1 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.
Figure 1
1.2 electromagnetism harassment dissemination way
In switching power supply’s electromagnetism harassment foreign performance for conducted interference and radiated interference. The conducted interference may divide into the syntype (CommonMode–CM) to disturb and the difference mold (DifferentialMode–DM) disturbs. As a result of the switching circuit stray parameter’s existence as well as the switch component’s high frequency clear and the shutdown, cause the switching power supply (prompt delivery discharge net side) to have the big syntype disturbance and the bad mold disturbance in its input end.
The syntype disturbance’s characteristic is the disturbance size and the direction is consistent, exists in power source any relative earth, or the median line to the earth between. The syntype disturbance is also called the longitudinal mode disturbance, not to be asymmetrical disturbs or the earth disturbance. Is carries between the fluid and the earth disturbance.
Switching circuit in high frequency situation, because dv/dt is very high, stimulates between the transformer coil, as well as parasitic capacity between the switching valve and radiator fin’s, thus has had the syntype disturbance. Like Figure 1 the dashed line shows, the syntype interference current from has high dv/dt switching valve S to embark passes through distributed capacity Ci to flow through the earth radiator fin and the grounding, again by the high frequency LISN network (by two 50Ω resistances equivalent) class feedback inlet road.
The bad mold disturbs the characteristic is the size is equal, the direction is opposite, exists between the power source phase line and the median line and the phase line and the phase line. The bad mold disturbance is also called often the mold disturbance, the transverse mode disturbance or the symmetrical disturbance. Exerts in carrying between the fluid the disturbance.
In switching circuit’s electric current in high frequency situation mean switch change, thus in the input, on the output filter electric capacity produces very high di/dt, namely induced the disturbing voltage in the filter electric capacity’s equivalent inductance or the impedance. Will have by now the bad mold disturbance, like Figure 1 the solid line showed. Therefore selects the high grade filter electric capacity (equivalent inductance or impedance is very low) may reduce the bad mold disturbance.
The syntype disturbance showing disturbance is from the radiation or the crosstalk coupling to the electric circuit, but misses the mold disturbance to explain that the disturbance stems from the identical strip power circuit. Usually these two kind of disturbances are simultaneously exist, because the line impedance is not balanced, two kind of disturbances mutually will also transform in the transmission, the situation will be very complex.
2 switching circuit electromagnetism harassment suppression
2.1 choice appropriate switching power supply electric circuit analysis situs and operating frequency
Switching power supply’s operating frequency the harassment intensity which produces with it is closely related. Not only the low switching power supply operating frequency may reduce the harassment the high frequency component, its conduction will harass and radiates the harassment the dissemination efficiency also greatly to reduce.
2.2 choice appropriate electric circuit primary device
In the switching circuit, the switching valve is a core, the different brand’s switching valve radiation harasses the phase difference to be possible to reach 15~20dB. In the switching circuit another key component is the pulse transformer, the pulse transformer the influence displays to the electromagnetic compatibility in two aspects: One is the primary coil and sencondary coil distributed capacity Cd, one is the pulse transformer’s leakage. Through adds the electric shield level between the primary coil and the sencondary coil and draws out the earth, should meet the grounding to approach switching valve’s emitter electrode to meet the direct-current input as far as possible 0V (hotly), like this may reduce distributed capacity Cd greatly, thus reduced at the beginning, the secondary electric field coupling harassment. In order to reduce the pulse transformer’s leakage, may choose the seal magnetic core (for example ring-like), seals up the magnetic core to be smaller than the aperture magnetic core’s leakage. May also through suppress the slippery magnetism in the pulse transformer outsourcing high magnetic conductivity shielding material. Thus reduced through the leakage radiation harassment.
2.3 increase the passive cushion circuit
The cushion circuit not only may suppress when the clear di/dt, time limit shutdown dv/dt, but also has the electric circuit to be simple, cost low characteristic, thus obtained the widespread application.
(As shown in Figure 2(a)) or DRC in switching circuit’s S both sides parallel RC (as shown in Figure 2(b)) the absorbing circuit, may absorb the high surge peak voltage which the S connection and the separation instantaneous produce, reduces electromagnetism harassment which the switching circuit produces.
in 2.4 leveling circuits adds the power factor to adjust (PFC) the network
As for a leveling circuit, the most remarkable harassment is the leveling circuit to exchanges electrical network’s harassment as the overtone source, causes to exchange electrical network’s waveform distortion, the power factor is somewhat low. In order to solve this problem, may join the ready-made power factor in a leveling circuit to adjust (PFC–PowerFactorCorrection) the module, may enhance through the compensation the power factor to 0.99 above.
2.5 increase the filter network
The filter are the barrage jamming one effective action, particularly is coping with switching power supply’s conducted interference and certain spoke animal resembling a fox? The aspect, has the tangible effect. The electromagnetic interference (EMI) the filter are take can suppress the electromagnetic interference effectively as the goal filter, may divide into the holding wire EMI filter, the power source EMI filter, the printed wiring board EMI filter, to reflect the EMI filter, to isolate the EMI filter and so on.
2.6 earths
2.6.1 earth functions
The electronic installation has two kind of earths generally, one kind is the safe earth, soon the chassis earth, protects staff’s safety; Another kind is the working earthing, provides a datum electric potential to the circuitry, simultaneously may also direct the high frequency interference. But, not the correct working earthing instead will increase the disturbance, for instance altogether grounding disturbance, loop jamming and so on.
2.6.2 earth types and earth request
1) the single-point earth working earthing selects the different earth method according to the operating frequency, when operating frequency <1MHz uses the single-point earth, the grounding length (L/m) and the cross-sectional area (S/mm2) the relations are S>0.83L.
2) accepts after checking when operating frequency >30MHz uses accepts after checking. The operating frequency may use the mix earth type between the above both.
3) floats floats is electric circuit’s place with the earth non-conductor connection. 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 this electric circuit the big terrestrial electricity performance influence, the resistance to interference is not been good. Its shortcoming is because equipment 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 in the equipment with the public between potential difference, but becomes the broken link very strong harassment source.
4) shields electric circuit’s shield body, namely shields with the shielding material the electromagnetism radiant, and will shield the body earth, reduces the electromagnetic radiation the disturbance. As soon as shields in vivo the electric circuit grounding only to be able to accept after checking shields the body, but do not make the returns conductor using the shield body.
as soon as 5) electric cable’s shielding layer regarding the multiple shield electric cable, each shielding layer should in accept after checking, each shielding layer should insulate mutually. When the cable length is bigger than the work signal wave length 0.15 time, uses the gap work signal wave length 0.15 time to accept after checking the type, if cannot realize, then should at least the shielding layer both sides earth.
2.7 electric circuit primary device installment must be reasonable
The electric circuit primary device installs should cause the input to exchange and outputs the direct-current plug to separate and to be far away. The wiring separates strictly, the simplification passage of current’s way, reduces the intercross disturbance, simultaneously causes the input, the output wiring to be far away from the electrostatic field and the electromagnetic field noise production source. Moreover, everything includes the big electric current, the voltage rate of change component (power transistor and switch diode and so on) the electric circuit should the rational distribution. The reduction has high di/dt, the dv/dt wiring as far as possible, reduces the noise radiant the coverage. The power transistor and the switch diode and the radiator assemble when together, with the copper plate shield transistor, reduces, because the parasitic capacity causes noise propagation.
3 conclusions
Suppresses switching power supply’s harassment develops an application switching power supply’s important topic, therefore, in reduces and suppresses the EMD aspect, besides above antijamming measure and method, but must use the advanced semiconductor design technique, the magnetic material, the inductance part technology, the switch component technology effectively fully, like phase shifting type entire bridge ZVS-PWM technology; Zero potential transition (ZVT) and zero electric current transition (ZCT) PWM technology; Power factor adjustment control technology and so on. The practice proved that these measures and the method to reduce switching circuit’s harassment to have the tangible effect.
