Abstract: Through instead stirred up-like the power factor compensating circuit to explain in the single stage power factor compensating circuit’s electromagnetic compatibility question, has analyzed the production mechanism which in the single stage power factor compensating circuit harassed, has given the electromagnetic compatibility the design, finally proposed several other reduction electromagnetic interference method.
Key word: Electromagnetic interference; Electromagnetic compatibility; Power factor adjustment
Introduction
The electromagnetic compatibility (EMC) is refers to the electronic installation or the system in its electromagnetic environment can the normal work, and any thing constitution cannot withstand to this environment in electromagnetism harassment ability. More and more uses the low power loss, the high velocity, the high integration rate LSI electric circuit along with the electronic products, but made these installments to be easier than to receive the electromagnetic interference at any time in the past the threat. But at the same time, high efficiency electrical appliances and office automation equipment’s increase, as well as mobile communication, beeper widespread application and so on, also increased the electromagnetism harassment source greatly. These changes force the people to take the electromagnetic compatibility the important technical question to pay attention. Specially the European Economic Community requests the product electromagnetic compatibility to integrate the technical laws and regulations, enforces the 89/336/EEC instruction, namely the stipulation the electricity and the electronic products must meet the EMC requirement from January 1, 1996, after and Canada pasted the CE symbol, since has been able to sell in the European Economic Community market, has urged various countries’ government from the international trade angle, takes the electromagnetic compatibility technology.
The switching power supply has the volume to be small, the weight is light, efficiency high merit, and in the market had the switching power supply integration control module, this caused the power source design, the debugging to simplify many, therefore, in the majority electronic installations (for example computer, television and each kind of control system) obtained the widespread application. However, switching power supply oneself produces each kind of noise actually caused it to become a very strong electromagnetism harassment source. These harassments along with turn-on frequency’s enhancement, the output increase, but strengthens obviously, posed the latent threat to electronic installation’s normal operation. Therefore, only then enhances switching power supply’s electromagnetic compatibility, can enable the switching power supply to have the strict request situation in these to the power source noise target to use.
Electromagnetic compatibility including two aspect meanings.
(1) electronic installation or system interior each part and subsystem, a system interior each equipment and even neighboring several systems, in they produce electromagnetic environment and, in they locate in the outside electromagnetic environment, can according to the original design request normal operation. In other words, they should have certain electromagnetism sensitivity, guaranteed that they have certain interference rejection to the electromagnetic interference (Immunity of a Disturbance).
(2) this equipment or system produce the electromagnetism noise (Electromagnetic Noise-EMN) must limit in certain level, causes the electromagnetic interference which creates by it not to send to it around the electromagnetic environment causes the serious pollution and affects other equipment or system’s normal operation.
It is well known, the constitution electromagnetic interference has three essential factors, namely: Harassment source (noise), noise coupling way and noise receiver (by interferometric technique). Therefore, the broad electromagnetic compatibility design’s duty is needs to weaken the harassment source the energy, the isolation or weakens the noise coupling way and enhances the equipment to the electromagnetic interference resistance. Below take instead stirs up-like (Flyback) the electric circuit as the example, the discussion low power single stage PFC electric circuit’s electromagnetic compatibility design.
1 harassment source analysis
As shown in Figure 1, in the low power DC/DC converter, the main harassment source is the electromagnetic induction noise and the misalignment switching process noise. This is because in the power transfer network’s rectification second-level tube and the power switching valve produce in the work process the voltage and the electric current jump, and between the stray parameter which through the high-frequency transformer, in the stored energy inductance coil as well as electric circuit’s part layout and the component itself brings the interaction creates.
In other words, in the electric circuit produces all disturbance question’s root, is the power switching valve and the high frequency rectification diode, in fast opens breaks di/dt which and dv/dt in the process produces. Therefore, in circuit design’s initial period, namely carries on the electric circuit plan time the choice should begin to consider the EMC question. In various aspects conditions are ripe and in the permission situation, should use (phase shifting entire bridge electric circuit which regarding the main switch tube’s design the soft switching circuit e.g. in the middle of power electric circuit uses widely and so on), not only like this may reduce switching valve’s switching loss enormously, moreover is helpful in reduces in electric circuit’s di/dt. But is not higher is better in turn-on frequency’s choice, but is must select the appropriate frequency. Also, constitutes the return route by the switching valve and the high frequency diode as well as the output capacity to be as far as possible small, because the return route small parasitic inductance was small. Instantaneous will produce very big di/dt in the switching valve and the high frequency diode clear and the shutdown, if the parasitic inductance big will induce the very high voltage, has like this formed a big harassment source. Moreover, high frequency diode in shutdown time will have the situation which will restore reverse, this also will be a very big harassment source. We must pay attention weaken it, in order to avoid interferes with electric circuit’s normal work, for this reason may give the high frequency diode to string together a small inductance, suppresses the high frequency diode’s reverse recovery current. But this inductance cannot be big, because di/dt is very big under high frequency, also will introduce a harassment, must therefore compromise.
Another produces the electromagnetic induction noise main harassment source is the pulse transformer. In instead stirs up-like in the electric circuit, because the original vice-side winding coupling coefficient is not 1, the transformer has certain leakage inductance Ls. When the switch shuts off, Ls produces counter electromotive force - Lsdi/dt will cause the switching valve to leak between the source the voltage to present overshooting. This is because on Ls energy–The leakage flux cannot to vice-side carry on the release through the transformer coupling, therefore, this part of energies constitute a weaken LC resonance together with switching valve’s parasitic capacity Cs and input power source Vin, superimposes on the shutdown voltage, forms the shutdown voltage peak. This peak noise in fact is a peaked pulse, besides creates the disturbance, heavy has the possibility to penetrate the switching valve. Moreover it is one kind of conductive electromagnetic interference, also affects transformer’s primary, will also cause the disturbance conduction returns electrical power distribution system, will create the input side electrical network’s electromagnetic interference, will thus affect other current collector’s security and the economical movement.
Is the same with the switching valve, in the middle of electric circuit’s pulse transformer also has the high-frequency the di/dt change, also to the space radiation high frequency electromagnetic wave, will disturb other primary devices and the equipment. Therefore must the idea shield transformer these high frequency electromagnetic waves.
Speaking of the electromagnetic field, the electric field component and the magnetic field component always simultaneously exist. Therefore, when shields the electromagnetic field, must performs to at the same time the electric field and the magnetic field to shield. The high frequency electromagnetic screen’s mechanism is mainly passes through the metallic shield body based on the electromagnetic wave to have the wave reflection and the wave absorption mechanism. Electromagnetism Extensive to when shields the body surface surface, the reason that will have the wave reflection, its primary cause is the electromagnetic wave impedance and the metallic shield body’s characteristic impedance is not equal, both value difference is bigger, the reflection causes the loss is bigger. Reflected wave also with frequency related, the frequency is lower, the reflection is more serious. But electromagnetic wave in penetration shield body time produces the absorption loss is mainly causes by the electromagnetic wave in shield body induced turbulent flow. The induced turbulent flow may have a counter magnetic field to counter-balance the original disturbance magnetic field, simultaneously, the turbulent flow is shielding in vivo flowing to produce the heat generation loss.
2 electromagnetic compatibility design
Electromagnetic compatibility design including electric circuit choice, primary device’s choice, filter, shield, earth, layout and so on.
2.1 soft switch technologies
Choice zero potential switch, zero current breaker resonance technology or other soft switch technology. In the zero potential resonance converter, on the power switch’s voltage waveform is the standard sine, dv/dt is small; In zero electric current accurate resonant transformation, winds through the power switch’s electric current is the standard sine, di/dt is small, like this may reduce the EMI level. Because, the interference spectrum is narrow, and concentrates nearby the resonance frequency, easy filter’s design.
Wants special attention derate switch’s di/dt and dv/dt and reduces the rectification diode noise cushion circuit’s design.
2.2 filter
The filter are the barrage jamming one effective action, particularly is dealing with the conducted interference aspect, has the tangible effect. Wants to weaken the conducted interference, EMI level control in below related EMC standing operating procedure limit level. Besides suppresses the harassment source, the most effective method is installs the EMI filter in the switching power supply input and the output circuit. In the filter circuit, selects puts on the heart electric capacity, three end electric capacities, the ferrites magnetism link, can improve electric circuit’s filter characteristic. EMI filter as shown in Figure 2.
This kind of EMI filter both can suppress the syntype disturbance and to be able to suppress the bad mold disturbance. It is the switching power supply EMI filter’s basic network structure, L1 and L2 are circle on the identical magnetism link two independent coils, the number of windings are the same, have the same direction end of the same name, calls it the syntype inductance coil or the syntype coil. L3 and L4 are the independent bad mold suppress the inductance, C1, C2 and C3 are capacitors. If enters this filter termination the noise source, the head end joins by the interferometric technique, then L1 and C1, L2 and C2 separately constituted low pass filters between two pair of independent port’s, uses for the syntype EMI signal which suppresses on the power line to exist, causes it weaken, and is controlled in the very low level. L3 and L4 form the independent difference mold suppressed the inductance and electric capacity C3 has composed a low pass filter, uses for the bad mold EMI signal which suppressed on the power line to exist.
The suitable design or the choice appropriate filter, and install the filter are the antijamming technology important components correctly, the specific measures are as follows.
1) in the alternating current input end addition power source filter, its circuit diagram as shown in Figure 2. And L3, L4 and C3 use in suppressing the bad mold noise, L1, L2, C1 and C2 use in suppressing the syntype noise. All power source filter must earth, because filter’s syntype shunt capacitor must in the earth time only then has an effect. Earth method generally besides filter and metal shell docking, but must use the thick wire is connected the filter outer covering and equipment’s ground point, the ground impedance smaller filter effect is better. Moreover, the filter should install as far as possible are approaching the power source entrance, avoids the unwanted signal from the input end conductive coupling to the out-port.
2) outputs the filter in power source out-port Canada. Installs the high frequency electric capacity, enlarges the output filter inductance the inductance quantity and the filter electric capacity’s capacity, may suppress the bad mold noise. If is parallel many electric capacities, the effect will be better.
In uses filter’s time, we must pay attention to the following several spots.
1) the filter must have the good shield, shields the body and power source good joining.
2) inputs the filter to install is inputting the port place, outputs the filter to install in the output port place, and is far away from the internal electromagnetic emission very strong inductor, the power switch and so on. If possible, as far as possible takes an independent part and the power source connects reasonably.
3) filter’s input, the output line cannot overlapping, should use the shielded wire or establish the shielding layer mutually.
4) the filter internal part, own must carry on the good electromagnetic screen and earth processing, in order to avoid winds through the filter earth lead the short-circuit current to create the harmful electromagnetic radiation.
5) the filter inductance’s ferrite core should better use the pot or annular, if uses the other shape coca short circle or the magnetic shield. The coil uses the monolayer or the sectional winding, when the undercurrent may use the honeycomb cell to circle the system the multilayer winding. The associate coil cannot adopt the double thread and circle, should be the symmetrical two independent coils.
6) uses the high-frequency response good capacitor elected.
2.3 earths
Must pay attention to electric circuit’s earth question, because the common-impedance coupling mainly carries on through the public impedance. If the earth has not processed, possibly to the electric circuit introduction very big place disturbance, will thus enable the electric circuit the normal work. Take the Boost electric circuit as the example, if MOSFET S earths extremely has not processed, i.e. G extremely, S extremely, between the PWM signal and the place constitutes the earth return is very big, the electric circuit cannot the normal work, sometimes the PWM signal is unable to actuate MOSFET, this has introduced a very big disturbance through the public impedance for the Boost electric circuit. Therefore, in the use general board comes cloth electric circuit’s time, must pay attention to these details, S extremely and between the place wire must be as far as possible short. When uses the general board, uses the thick line between two points to take the grounding as far as possible, but also has, can link in the together place should link as far as possible in the same place, the ground point is as far as possible thick a spot, but may also overstrike as far as possible the grounding width, reduces the loop resistance. If the grounding is very thin or the ground point is very small, the earth electric potential changes along with the electric current change, causes the anti-noise performance to go bad. When uses the general board, but must pay attention to the power electric circuit’s place to the signal place disturbance.
2.4 transformer’s designs
To reduce transformer’s electromagnetism noise as far as possible, must make its primary side winding and the vice-side winding’s coupling coefficient as far as possible close 1, thus reduces the leakage flux, achieves reduces the leakage inductance the goal. This needs to work hard in transformer’s design, causes the primary side winding and the vice-side winding approaches as far as possible, simultaneously must approach as far as possible with the magnetic core, such leakage flux will reduce to is smallest. According to this principle, the best winding is the primary side and vice-side overlapping and circles, like this can achieve causes the leakage inductance smallest goal. But in the practical application, the transformer must consider the original vice-side between the high-pressured isolation, what therefore actual middle applies is ” the sandwich ” contains filling the winding (as shown in Figure 3), namely circles a primary side, circles one vice-side, rerolls a primary side, or vice-on the one hand, a primary side, the final level vice-on the other hand, this can cause the original vice-side between the coupling to be better, reduces the leakage inductance, reduces, because the leakage inductance causes electromagnetic induction noise. (design wire line diameter time, besides must consider through electric current size and skin effect, but must argue vigorously lets wire flatten each, but do not present sparse 2-3 circles phenomenon, only then like this, the original vice-side coupling effect can go a step further enhancement). Figure 4 has given the experiment oscillogram, may know from Figure 4, with contained filling the winding to circle the system the transformer, on the MOSFET vibration small many.
In order to reduce transformer’s radiated interference, manufactures transformer’s shield strata time, often uses the method is wraps a light copper sheet in transformer’s winding and the magnetic core outside surface. For can reduce the Xiayuan vice-side the distributed capacity, but may also in transformer’s at the beginning, between the secondary add an electric shield. The concrete method is after circling makes the primary winding, wraps a 0.02~0.03mm thick thin copper sheet, the copper sheet beginning end and the terminal must have 3~5mm long to overlap (overlap section to mutually insulation). In order to guarantee that the electric shield achieves the anticipated goal, the key is reduces from the technological design leaks electric capacity Cs and the ground impedance Z size, following shown in Figure 5.
3 other reduction electromagnetic interference method
3.1 primary device’s layouts
When designa circuit, usually harasses the source and is harassed the electric circuit as a result to receive the working condition the limit to avoid with difficulty. By now, should place as far as possible the incident cross-correlation primary device in the same place, because avoids the component 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.
3.2 radiator fin’s installments
Considered the possibility worsens the electric circuit running status the power component to give off heat the question, may give the power component to install the radiator fin, both can radiate, and may reduce the electromagnetism noise. In order to cause the power switching valve and the radiator can have the good heat conduction, often smears the heat conduction performance good insulation silica gel between the power switching valve and the radiator (to guarantee that has good insulation, also in middle pad insulation spacer, prevents radiator fin frequently charged). These silica gel and the insulation spacer were equal in connected coupling capacity Ck between the power tube and the radiator fin. The electromagnetism noise which therefore, the power switching valve when will be opening will break produces to adopt the Ck coupling fast to the radiator fin above. From prevents between each switching valve’s noise mutual crosstalk angle to consider that should better be each power is effective an independent radiator fin, but is not several power switching valve public radiator fin.
4 conclusions
Receives day by day along with electronic products’ electromagnetic compatibility takes seriously, to suppress switching power supply’s EMI, enhances the electronic products the quality, causes it to conform to the related standard, already became the people more and more matter of concern. This article to instead stirred up-like the power factor compensating circuit’s harassment source to carry on the analysis, simultaneously has given the corresponding solution, moreover, but also electric circuit’s electromagnetic compatibility the design has carried on the detailed analysis regarding this.
