Abstract: Introduced one kind uses the passive power factor adjustment method to reduce the power source harmonic content the plan. The experimental result indicated that this plan cost is low, the performance is good, easy to achieve each item of EMC standard, suits in the low power power source.
Key word: Power factor adjustment; Electromagnetic compatibility; Harmonic suppression
Introduction
In order to reduce to exchanges electrical network’s overtone pollution, had already promoted some limiting current overtone standard, if the IEC100032ClassD standard, the request must take the measure to reduce inputs electrical network’s current harmonics content, enhances the power factor.
The traditional diode and the electric capacity when carries on the rectification filter to the input signal, only in the input alternating voltage’s peak value part only then has the input current, caused to have the very big current harmonics content, has disturbed the electrical network seriously, far could not meet the standard requirements. In order to cause the input current overtone to satisfy the request, must add the power factor to adjust (PFC). Compared with mature, and applies is widespread is two levels of plans, they have the respective power component and the control circuit. The PFC level causes the line electric current followed line voltage, causes the line electric current sine, very easy to achieve the high power factor, reduces the harmonic content. Especially in recent years, along with the electric power electronic technology’s rapid development, the massive electric power electronic installation’s application had the serious harmonic interference to the electrical network, brought the serious harm. Therefore various countries proposed that corresponding EMC (electromagnetic compatibility) the standard, the strict stipulation turns on electrical network’s equipment’s harmonic interference permission level. Our country carries out the 3C authentication standard, the request all electricity product must only then be possible to sell through this authentication, this standard very important part is the EMC standard.
Figure 1
1 power source parameter
Turns on electrical network’s current collector is massively after becomes the city electricity rectification the cocurrent the supplies load, what but the tradition is commonly used is the voltage does not control the rectification, is also the diode bridge-type rectification meets the big electric capacity flat wave the method. This kind of leveling circuit is one kind of misalignment component and the stored energy part’s combination, although the input alternating voltage is the sine, but the diode breakover angle is small, the input current distortion is serious, assumes the pulse shape, as shown in Figure 1.
The PFC technology is through, in does not control in the leveling circuit to join the DC/DC switch converter, the application current feedback technology, enables the input end current waveform to track the exchange input voltage profile, thus causes the input end electric current close sine. This article discusses the model Boost in the PFC circuit design electromagnetic compatibility question.
This PFC electric circuit’s technical parameter is:
The input exchanges 150~270V,50~60Hz;
The output directs current 380~400V, the ripple <5%; Power 600W;
Turn-on frequency 100kHz;
After the adjustment, power factor >0.99.
Electric circuit basic schematic diagram as shown in Figure 2.
Figure 2
2 based on UC3854 PFC principle of work
This design is the work in the inductance electric current continual pattern (CCM) under Boost electric circuit, what uses is Unitrode Corporation’s special-purpose PFC chip UC3854. This chip’s core is a simulation multiplier, its output current Imo peak-to-peak value by electricity pressure ring output decision, but the profile decided by input voltage’s sampling Iac, when the electric circuit is stable, has ImoRmo in proportion to IiRs. Because Imo is with the input voltage synchronism sine wave, therefore Ii is also a sine wave, has like this also realized PFC.
The main circuit basic parameter is: Inputs the Boost inductance L=1mH, C=470μF, the biggest input current effective value is 4A, the switching valve is IRF460, the diode to restore diode RHRP1560 quickly.
3 Boost PFC electromagnetic compatibility question
3.1 electromagnetism noise sources
This electric circuit’s main electromagnetism noise source has many kinds, what is most main is the switch power component and the variable current electric circuit the electromagnetism noise which causes in the switching process. Regardless of the electric power electronic installation is main circuit’s power semiconductor device, control circuit’s high speed integrated circuit, in the component switching process, has high di/dt, they cause the transient state electromagnetism noise through the line or primary device’s lead inductance, the frequency may reach as high as several dozens kHz even several hundred kHz, is the noticeable noise source. Below to noise source 11 analyses.
IRF460 is the power field effect manages (MOSFET), belongs to the multi-child component, does not exist restores the question reverse, but his shutter speed is very high, in the switching process produces di/dt (dv/dt) may reach the very high value, affects in electric circuit’s parasitic inductance (electric capacity) on, will produce the very high transient voltage, the electric current and causes the vibration. If supposes the switching time is 10ns, the lead inductance is 500nH, in the switching process the biggest electric current may achieve 6A, then in the lead wire produces the voltage is
500×10-9× (6/10×10-9) =300V
The so big pulse voltage (electric current) will create the serious electromagnetic interference.
In the diode switch process will also have the noise. The diode clears when the electric current increases rapidly, but its pressure drop is not drops immediately, but presents a fast overshooting, then causes a wide band the electromagnetism noise. But when shutdown, because PN ties in the long base the massive surplus minority carriers to need to bond, thus produces the very big reverse recovery current, this electric current and the shutdown electric current and the shutdown speed is proportional. In high speed, big electric current situation, this reverse electrical current will be quite big, moreover when clear will superimpose on the switch electric current, will be serious when the switch component overburning. Must therefore select has restores the characteristic quickly diode, reduces the reverse recovery current as far as possible.
Figure 3
3.2 electromagnetic interference coupling way
The high frequency switching power supply creates the electromagnetism noise coupling to is disturbed the object to have two ways: Conduction way and radiation way. According to the electromagnetism noise coupling characteristic, the conduction coupling may divide into the direct conduction coupling, the common-impedance coupling and the transfer impedance coupling three kinds. In this electric circuit, conducts the coupling, the common-impedance coupling and the radiation coupling is directly should the key consideration.
The direct conduction coupling is refers to the noise and so on conductive coupling to is disturbed through the wire or the parasitic element the object, if Ldi/dt may adopt the wire coupling. Therefore, in the experiment electric circuit, should reduce the wire as far as possible the length. Certainly, the best method is the application zero current breaker (ZCS) the soft switch technology.
The common impedance conduction coupling is the noise public meets the grounding as well as in the earth network common impedance through the equipment has the public impedance coupling. If the grounding arranges improper, the grounding will receive the very big disturbance, usually might examine the peak-to-peak value to reach as high as several V burrs, the electric circuit also cannot the normal work. Therefore, should the possible arrangement earth, be as far as possible short the grounding arrangement, moreover the power and the signal separate. Like this after processes, in the grounding burr will obviously obtain suppresses.
The radiation coupling is refers to the electromagnetism noise the energy, by the electromagnetic field energy’s form, through the space radiation dissemination, the coupling to the equipment which disturbs (electric circuit). In this electric circuit, the switch and the diode will be the biggest electromagnetism noise source, the electromagnetism noise will radiate to electric circuit’s other parts. Is disturbed the electric circuit to accept the electromagnetism noise the energy to be proportional with this electric circuit return route’s area, therefore, must when arranges the electric circuit component reduces the electric circuit return route’s area as far as possible.
Figure 4
4 experimental results
This experiment’s electric circuit is based on the UC3854 Boost power factor adjuster, the working pattern for the electric current continual pattern, the output is the 380~400V DC voltage, the output is 600W.
In the experiment, wants the possible arrangement primary device layout and the grounding, reduces the lead wire length as far as possible and reduces the main circuit return route’s area, the main circuit and the control circuit separates the arrangement. Thus, the electromagnetic compatibility question may have the very big improvement. From Figure 3, chart 4 the experiment profile looked that realizes the PFC function basically, moreover the profile receives disturbs is quite small.
5 conclusions
This article through analyzes Boost the PFC electric circuit’s electromagnetic compatibility question, like the noise source, the coupling way and so on, proposed solves the method in the actual experiment, and through experimental verification.
