Abstract: Because the switching power supply the itself operational factor causes the electromagnetic interference question to be quite prominent. Obtained from the switching power supply electromagnetic interference’s model to elaborate the reason which and the type the switching power supply electromagnetic compatibility question produced, and gave commonly used has suppressed the switching power supply electromagnetic interference the measure, the filter design and the parameter choice.
Key word: Switching power supply; Electromagnetic interference; The analysis with suppresses
Introduction
In recent years, the switching power supply was high by its efficiency, the volume was small, but output stability good merit rapidly expand. But, because in the switching power supply work process’s high-frequency, high di/dt and high dv/dt cause the electromagnetic interference question to be prominent. The home had already substituted for CCIB and the CCEE authentication by the new 3C authentication, causes to be more detailed to the switching power supply in the electromagnetic compatibility aspect request and to be strict. Now, how to reduce even eliminates switching power supply’s EMI question already to become the whole world switching power supply designer as well as the electromagnetic compatibility (EMC) the designer very matter of concern. This article discussed the reason which as well as commonly used EMI the switching power supply electromagnetic interference forms suppresses the method.
1 switching power supply’s noise source analysis
The switching power supply has the electromagnetic interference most basic reason, is it high di/dt which and high dv/dt produces in the work process, they produce the surge current and the peak voltage have formed the noise source. The power frequency rectification filter use time the big condenser charge electric discharge, the switching valve high frequency work’s voltage cut, the output rectification diode’s reverse recovery current is this kind of noise source. In switching power supply’s voltage current waveform mostly for the close rectangle’s cyclic wave, for instance switching valve’s actuation profile, MOSFET leaks the source profile and so on. Regarding the rectangular wave, the cycle has from the bottom decided the profile base frequency; Two time of pulse edge rising time or the fall time had from the bottom decided these edges cause frequency component frequency value, typical value in the MHz scope, but its harmonic frequency was higher. These high frequency signals to the switching power supply elementary signal, particularly control circuit’s signal create the disturbance.
Switching power supply’s electromagnetism noise may divide into two broad headings from the noise source. One kind is the external noise, for example, the syntype which and the difference mold noise, the exterior electromagnetic radiation transmits through the electrical network to the switching power supply control circuit’s disturbance and so on. Another kind the electromagnetism noise which is switching power supply oneself produces, if the switching valve and electric valve’s electric current peak produces overtone and electromagnetic radiation disturbance.
As shown in Figure 1, in the electrical network includes the syntype and the difference mold noise have the disturbance to the switching power supply, the switching power supply while receive the electromagnetic interference also have the electromagnetic interference to electrical network other equipment as well as the load (e.g. in chart returns noise, output noise and radiated interference). Carries on when the switching power supply EMI/EMC design, on the one hand must prevent the switching power supply has the disturbance to the electrical network and the nearby electronic installation, on the other hand must strengthen the switching power supply itself to the electromagnetism harassment environment adaptiveness. Below concrete study switching power supply noise production reason and way.
1.1 power lines introduce electromagnetism noise
The power line noise is the electromagnetism harassment which in the electrical network each kind of current collector produces creates along the power line dissemination. The power line noise divides into two broad headings: Syntype disturbance, bad mold disturbance. The syntype disturbance (Common-mode Interference) defines for any year class conductor and the reference between does not hope some potential differences; The bad mold disturbance (Differential-mode Interference) defines for any two carries between the class conductor not to hope some potential differences. Two kind of disturbance’s equivalent circuit like chart 2[1] shows. Between the chart CP1 is at the beginning of the transformer, is secondary the distributed capacity, CP2 is between the switching power supply and radiator’s distributed capacity (i.e. between switching valve collecting electrode and place distributed capacity).
Like Figure 2 (a) shows, switching valve V1 when becomes the closure condition by the breakover, its collector voltage suddenly rises for the high voltage, this voltage will cause syntype electric current Icm2 to the CP2 charge and syntype electric current Icm1 to the CP1 charge, distributed capacity’s charge frequency will be switching power supply’s operating frequency. Then in the line the syntype electric current overall size is (Icm1 Icm2). Like Figure 2 (b) shows, when V1 breakover, bad mold electric current Idm and signaling current IL the return route circulation which along the wire, the transformer primary, the switching valve composes. May know by the equivalent model, syntype interference current not through grounding, but through input power line transmission. But misses the mold interference current through the grounding and the input power line return route transmission. Therefore, we establish time the power line filter must consider that assumes one’s post the mold disturbance and the syntype disturbance difference, uses the difference mold or the syntype filter element in its route of transmission suppresses their disturbance, achieves the best filter effect.
Figure 2
1.2 input current distortion creates noise
Switching power supply’s input uses the bridge-type rectification, the electric capacity filter eliminator supply generally. As shown in Figure 3, is not having the PFC function input level, as a result of the rectification diode’s misalignment and the filter electric capacity’s stored energy function, causes diode’s breakover angle changes is small, the input current i becomes time to be very short, the peak value very high periodic peak electric current. This kind of distortion’s electric current materially besides contains the fundamental wave component also to include the rich higher harmonic component. These higher harmonic component pours into the electrical network, causes the serious overtone pollution, other electric appliances creates the disturbance to the electrical network on. For the controlling switch power source as well as realizes the high power factor to electrical network’s pollution, the PFC electric circuit is the indispensable part.
1.3 switching valves and the transformer produce disturbance
The main switch tube is switching power supply’s core component, simultaneously is also a noise source. Its operating frequency is direct and electromagnetic interference intensity correlation. Elevates along with switching valve’s operating frequency, the switching valve voltage, the electric current cut speed speeds up, its conducted interference and the radiated interference also increase along with it. In addition, on the main switch tube the counter-parallel clamp diode restores the characteristic not to be good reverse, or voltage peak absorbing circuit’s parameter choice not, when will also create the electromagnetic interference.
In the switching power supply work process, by the primary filter big electric capacity, the high-frequency transformer primary coil and the switching valve constituted a high-frequency current ring circuit. This ring circuit can have the big radiated noise. In the switch return route switching valve’s load is the high-frequency transformer primary coil, it is a perceptual load, therefore, time switching valve make-and-break will present the peak noise in high-frequency transformer’s primary both sides. Light creates the disturbance, the heavy puncture switching valve. Between the main transformer winding’s distributed capacity and the leakage inductance also cause the electromagnetic interference important attribute.
1.4 output rectification diode produces disturbance
Ideal diode when withstanding reverse voltage cuts off, will not have the reverse electrical current to pass. But time actual diode forward breakover, in PN ties the electric charge to accumulate, when diode withstanding reverse voltage, in PN will tie the electric charge which will accumulate to release and to form reverse recovery current, it will restore to factors and so on zero time and junction capacitance concerns. The reverse recovery current will have the intense high frequency damped oscillation in the transformer leakage inductance and under other distribution parameter’s influence. Therefore, the output rectification diode restores the noise also to become in reverse the switching power supply a main noise source. May through in the diode both sides parallel RC buffer, suppress it to restore the noise reverse.
1.5 distributions and the stray parameter cause switching power supply noise
Between switching power supply’s distributed parameter is the most disturbance intrinsic factors, between the switching power supply and at the beginning of radiator’s distributed capacity, the transformer is secondary the distributed capacity, the original vice-side leakage inductance is a noise source. Between the syntype disturbance is through the transformer at the beginning, is secondary the distributed capacity as well as between the switching power supply and radiator’s distributed capacity transmission. And transformer winding’s distributed capacity and the high-frequency transformer winding structure, the fabrication technology concern. May circle the system craft and the structure through the improvement, increases between winding’s insulation, to use Faraday to shield and so on methods to reduce distributed capacity between winding’s. But between the switching power supply and radiator’s distributed capacity and switching valve’s structure as well as the switching valve install the way to concern. Uses has the shield insulating gasket to be possible to reduce between the switching valve and radiator’s distributed capacity.
As shown in Figure 4, has high frequency parasitic characteristic [2] under the high frequency work’s part, has the influence to its active status. When high frequency work the wire turned the emission line, the electric capacity to turn the inductance, the inductance to turn the electric capacity, the resistance to turn the resonant circuit. Observation chart 4 the frequency response curve may discover, when frequency Gao Shige parts’ frequency characteristic has had the quite big change. In order to guarantee switching power supply’s when high frequency work stability, designs the switching power supply must consider part’s in high frequency work time characteristic fully, the choice use high-frequency response quite good part. Moreover, when high frequency, the wire parasitic inductance’s induction reactance obviously increases, as a result of the inductance uncontrollability, causes it to turn an emission line finally. Also has become in switching power supply’s radiation noise source.
2 switching power supply EMI braking measure
The electromagnetic compatibility three essential factors are the noise source, the coupling circuit and the sensitive body, suppresses the above any item to be possible to reduce the electromagnetic interference question. Switching power supply work when high voltage big electric current high frequency on-off state, it causes the electromagnetic compatibility question is quite complex. But, still conformed to the basic electromagnetic interference model, may obtain from three essential factors to seek suppresses the electromagnetic interference the method.
2.1 suppress in the switching power supply each kind of electromagnetism noise source
In order to solve the input current waveform distortion and reduce the current harmonics content, the switching power supply needs to use the power factor to adjust (PFC) the technology. The PFC technology causes the current waveform followed voltage waveform, adjusts the approximate sine wave the current waveform. Thus reduced the current harmonics content, improved the bridge-type rectification electric capacity filter circuit’s input characteristic, simultaneously also enhanced switching power supply’s power factor.
The soft switch technology is reduces the switch component loss and the improvement switch component electromagnetic compatibility characteristic important method. When switch component clear and shutdown will produce the surge current and the peak voltage, this will be the switching valve has the electromagnetic interference and the switching loss primary cause. Uses the soft switch technology causes the switching valve when the zero potential, zero electric current carries on the switch to transform may suppress the electromagnetic interference effectively. The use cushion circuit absorption switching valve or the high-frequency transformer primary coil both sides’ peak voltage can also improve the electromagnetic compatibility characteristic effectively.
The output rectification diode restores the question to be possible reverse through to connect a saturated inductance on the output electric valve to suppress, as shown in Figure 5, saturated inductance Ls and diode serial operation. The saturated inductance’s magnetic core is with has the rectangular BH curve magnetic material to make. With the material which the magnetic amplifier uses is the same, this kind of magnetic core does the inductance has the very high magnetic conductivity, this kind of magnetic core has a section of close vertical linear area in the BH curve and very easy to enter saturated. In actual use, when output rectification diode breakover, causes the saturated inductance work under the saturated condition, is equal in section of wires; When the diode shuts off restores reverse, causes the saturated inductance work under the inductance characteristic condition, has hindered the reverse recovery current large scale change, thus has suppressed it to the exterior disturbance.
Figure 5
2.2 cut-off electromagnetic interference route of transmission–Syntype, difference mold power line filter design
The power line disturbance may use the power line filter filtration, switching power supply EMI filter basic electric circuit as shown in Figure 6. Reasonable effective switching power supply EMI filter should the difference mold disturbance and the syntype disturbance have the strong inhibitory action to the power line on. CX1 and CX2 are called the bad mold electric capacity in Figure 6, the L1 named syntype inductance, CY1 and the CY2 named syntype electric capacity. The bad mold filter element and the syntype filter element separately has the strong damping action to the difference mold and the syntype disturbance.
Syntype inductance L1 is by circles opposite to the identical magnetism link, the number of windings same two winding constitution. Usually the use ring-like magnetic core, the leakage is small, the efficiency is high, but winds thread the difficulty. As soon as when the city net power frequency electric current for enters one obsolete in two winding averages, produces the magnetic field counter-balances exactly, causes the syntype inductance to the city net power frequency electric current any hindrance function, may not have the loss transmission. If in the city net includes the syntype noise current through the syntype inductance, this kind of syntype noise current is the same direction, when flows through two windings, produces the magnetic field synchronism superimposition, causes the syntype inductance to present the big induction reactance to the interference current, from this played has suppressed the syntype disturbance the role. The L1 inductance quantity and the EMI filter’s nominal current I related, the concrete relations see also Table 1 to arrange in order.
Table 1 inductance quantity scope and nominal current relations
|
Nominal current I/A |
The inductance measures L/mH |
|
1 |
8~23 |
|
3 |
2~4 |
|
6 |
0.4~0.8 |
|
10 |
0.2~0.3 |
|
12 |
0.1~0.15 |
|
15 |
0.0~0.08 |
In actual use syntype inductance two inductance windings, because will circle the system craft the question to have the inductance differential value, but this inter-species difference value is used happen to does the bad mold inductance. Therefore, in the electric circuit did not need to establish the independent bad mold inductance again generally. The syntype inductance’s differential value inductance and electric capacity CX1 and CX2 constituted ∏ filter. This kind of filter have the good weaken to the difference mold disturbance.
Besides the syntype inductance, Figure 6 electric capacity CY1 and CY2 also use for to filter the syntype disturbance. Syntype filter’s weaken when low frequency mainly has an effect by the inductor, but when high frequency the majority has an effect by electric capacity CY1 and CY2. The electric capacity CY choice needs to act according to the actual situation to decide, because electric capacity CY meets between the power line and the grounding, the withstanding voltage is quite high, therefore, needs to have the high pressure resistance, to leak the electric current characteristic lowly. Computation electric capacity CY leaks the electric current the formula is
ID=2πfCYVcY
In the formula: ID is leaks the electric current;
f is the mains frequency.
Generally installs on mobile equipment’s filter, its exchange leaks the electric current should <1mA; If to install, in stationary position, and earths on equipment’s power source filter, its exchange leaks the electric current should <3.5mA, the medical supplies to stipulate leaks the electric current to be smaller. Because considered leaked the electric current the security specifications, the electric capacity CY size has been restricted, generally was 2.2~33nF. The electric capacity type generally for the porcelain piece electric capacity, in the use should pay attention when the high frequency work capacitor CY and the lead inductance resonance effect.
The bad mold noise remover usual use low-pass filtering part constitution, the simple is a filter electric capacity meets the input filter circuit which forms between two power lines (e.g. Figure 6 electric capacity CX1), so long as the electric capacity choice is suitable, can the high frequency interference inhibitory action. This electric capacity to high frequency interference impedance really bottom, therefore between two power line’s high frequency interferences may through it, it be very high to the power frequency signal impedance, therefore does not affect to the power frequency signal’s transmission. This electric capacity’s choice main consideration pressure resistance value, so long as satisfies the power line’s pressure resistance rank, and can withstand the voltage surge which may expect then. In order to avoid the impact harm which the discharging current causes, CX electric capacity capacity not suitable oversized, generally in 0.01~0.1 μF between. The electric capacity type is the ceramic electric capacity or the polyester film electric capacity.
2.3 use shields reduce the electromagnetism sensitive equipment’s sensitivity
Suppresses the radiated noise the efficacious device is shields. May use the electric conductivity good material to carry 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 transformer’s magnetic field revelation, causes at the beginning of the transformer the secondary coupling to be good, may use the closed magnetism ring-like to become the magnetic shield, like pot magnetic core’s leakage flux obviously compared to E small many. Switching power supply’s ties, the power line should use has the shielding layer wire, prevents the exterior disturbance coupling as far as possible to the electric circuit. Or use EMC parts and so on magnetism bead, magnetism link, filter the power source and holding wire’s high frequency interference, but, must the attention signal frequency not to be able to receive the EMC part’s disturbance, is also the signaling frequency must in filter’s pass band. The entire switching power supply’s outer covering also needs to have the good shield characteristic, the connecting point must meet the EMC stipulation shield requirement. Through the above measure guaranteed that the switching power supply will already not be produced the exterior electromagnetic environment’s disturbance to the exterior electronic installation the disturbance.
3 conclusions
Now in switching power supply volume getting smaller, under power density more and more major tendency. The EMI/EMC question has become a switching power supply stable key aspect, is also the aspect which easiest to neglect. Switching power supply’s EMI suppresses the technology to hold the very important position in the switching power supply design. The practice proved that the EMI question considered more early, Vietnam solves early, the expense is smaller, the effect to be better.
