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   Abstract: Has analyzed the origin which and the conflicting mode as well as to the entire converter system performance influence systematically in the 5kW constant flow invertor the accessory power supply electromagnetic interference produces. The union working practice gave a series of enhanced electrical power system electromagnetic compatibility (EMC) the design method.

    Key word: Switching power supply; Electromagnetic compatibility; Electromagnetic interference; Syntype; Bad mold

Introduction

The electromagnetic compatibility (ElectromagneticCompatibility) is refers to the electrical equipment (system, subsystem) in the common electromagnetic environment, can carry out the respective function not to reduce own performance normally together, it has included electromagnetic interference (EMI) and the electromagnetism sensitive (EMS) two aspect contents. EMI is refers to the electrical equipment to become in the electromagnetic environment the electromagnetism source of pollution, EMS is refers to the electrical product to be able in the anticipated electromagnetic environment normal work ability.

    In switching power supply’s power semiconductor device’s turn-on frequency is high (to counts MHz from several dozens kHz), the power switching valve’s high-speed switch movement, causes serious EMI inevitably. Meanwhile, the modern switching power supply’s power density enhances suddenly, the power source internal electromagnetic environment is getting more and more complex, for instance has many sub-system’s situations in the electrical power system, between many sub-system power source’s electromagnetic compatibility question even more prominent. Therefore, to enhance the high efficiency invertor’s anti-jamming and the reliability, must take electrical power system’s electromagnetic compatibility design.

1 electric circuit outline and plan introduction

This article analyzes instead stirs up-like the power source to serve as in the 5kW constant flow invertor’s accessory power supply, its output amounts to 10 groups much, requests the electrical isolation besides digital other outputs. The electrical specification see Table 1. Figure 1 is the 150W multi-channel outputs instead stirs up-like switching power supply’s structure diagram.

Table 1 accessory power supply electrical specification

Because has the high-frequency transformer to circle the system as well as the primary side and the multi-channel vice-side winding not easy couplings and so on many difficulties, in the plan choice, this power source adopted two groups instead to stir up the type DC/DC converter parallel analysis situs, the double chip electric current peak control, reduced the transformer volume. In view of the fact that each group of converter power rank is low, and considered the full use instead stirs up-like the topology simple characteristic, the actual electric circuit adopts the hard switch working, the turn-on frequency is 100kHz. Because is two groups of high-frequency transformers is mutually parallel, the primary side uses in common the rectification bridge output cocurrent bus bar voltage, therefore, besides considered that each group of converters original vice-side as well as with power frequency electrical network’s EMI question, prevents two group of converter electromagnetic interference is also one of difficult.

2 electromagnetic interference analysis

In high frequency switching power supply, because the power semiconductor device’s high-speed switch forms the electric current transient, the voltage transient (di/dt and dv/dt) are the inevitable electromagnetism noise sources. Through indicated to switching power supply’s electromagnetic compatibility analysis: Weakens the noise source, the cut-off or weakens the EMI dissemination way, reduces is disturbed electric circuit’s electromagnetism sensitive degree is easily enhances switching power supply EMC the key.

    2.1 switching power supply oneself causes EMI

Electric current transient di/dt and voltage transient dv/dt, because the origin and the disturbance way are different, produces the noise the influence way is not same to the power source, mainly includes 2 aspects.

    1) the near zone electromagnetic field radiation coupling may divide into the syntype (commonmode either CM) radiates and the difference mold (differential mode or DM) radiates two parts:

- - difference mold radiation coupling its origin is mainly transient current di/dt, the coupling way is between two electric circuit’s parasitic magnetic coupling inductance M. When the switching power supply uses in the low pressure big electric current situation, the situation is worse, some data indicated that the [2], di/dt typical value may reach 250×106A/s. But this kind of electric current transient through parasitic coupling inductance M, forms induced voltage e by the magnetic coupling way on its neighboring electric circuit. This voltage’s peak-to-peak value and the di/dt scope is proportional, namely

- - syntype radiation coupling its origin is mainly transient voltage dv/dt, the coupling way is distributed capacity C between two electric circuit’s. Is similar with di/dt, dv/dt must be bigger than the switch motion level far. Applies in the switching power supply in the high-pressured undercurrent situation situation is worse, dv/dt typical value [2] may achieve 10×109V/s. The voltage transient forms the induced current through the parasitic coupling electric capacity in its neighboring electric circuit i. That is, the induced current, its source is the high frequency electric field, may voltage which exists for any electrical node or on the electric circuit primary device transient. Similarly, this kind of node or the primary device have parasitic capacity Cd to earth E, the induced syntype electric current flows to the earth through Cd, and flows through the power source input end interface resistance to form the ring circuit finally. The induced current i peak-to-peak value and dv/dt are proportional, namely

From said essentially, the stray parameter forms the inductive character and the capacitive character direct conduction coupling by the primary device ‘OR’ circuit wiring in belongs to the near field electromagnetic field radiation coupling, may sum up mostly as the above two types.

when 2) the common impedance conduction coupling two electric circuits (m and n) between the existence has the common impedance, on return route m conduction current’s change will cause the return route n voltage change. The common impedance including in the equipment security and the earth network’s common impedance (common impedance is mainly public resistance as well as electrical segment parasitic inductance). Common-impedance coupling’s essence belongs to the direct conduction coupling, the noise source is di/dt, creates disturbance performance for difference mold voltage e, namely

em=Zm, ni; en=Zn, mi    (3)

2.2 external environment to switching power supply’s EMI

Mainly comes from 2 aspects.

1) from the electrical network in each kind of high frequency overtone and the transient state noise, mainly conducts the coupled mode to enter the switching power supply and to carry on the disturbance to the electric circuit normal work, usually is also called the transient state disturbance.

The transient state disturbance performance the surge voltage which, the ringing voltage, the spark discharge on-line appears for the alternating current and so on instantaneous unwanted signal, its characteristic is when the function the auxiliary pole is short, but the voltage amplitude is high, the transient state energy is big, many in time domain scope to its description and analysis. In the international electrician committee formulates in the standard, the surge voltage and ringing voltage typical value peak value [5] is Vp=3000V. If the coupling to the input filter electric capacity, surpasses the MOSFET source, drain electrode fixed pressure resistance value VDS(limit), will penetrate the MOS tube or causes other harm through the transformer coupling to the out-port.

2) takes a high efficiency invertor’s sub-system, the accessory power supply will also receive the high frequency electromagnetism noise radiation which the invertor main power electric circuit will send out. The radiating capacity very possibly enters the accessory power supply through many kinds of ways, disturbs the electric circuit normal work.

3 EMC design countermeasure

When carries on the EMC design, considered instead stirs up-like the multi-channel output power source electromagnetic interference as well as the power source including two group of subsystem’s characteristics, the EMC design should pass through in the experiment, the design, the debugging throughout, considered in advance before the item design the measure, in after the experiment meets the question, adopts target-oriented measure, as well as after comparison other plans.

3.1 weaken the difference mold radiation coupling

As shown in Figure 2, the coupling way the noise magnetic field which produces through the noise source with is disturbed for parasitic magnetic coupling inductance M the return route to occur the magnetic flux hinge to form. Supposes the noise magnetic field the magnetic-flux density is B, passes through a closed area is the S return route, in this return route induced disturbing voltage e, namely

In the formula: B and S are vectors.

The convolution (1) and the type (4) easy to push, parasitic magnetic coupling inductance M with disturbance return route area S which involves becomes the proportional relationship. In Figure 2, the existence has the big di/dt return route mainly to include transformer input side ring circuit S0, two side ring circuit S1 and S2 as well as actuation ring circuit S3.

The careful design circuit wafer walks the line, reduces the above return route as far as possible regarding the area. Approaches the transformer primary side and the MOS tube as far as possible high frequency decoupling electric capacity Cd0, Cd1 approaches the load as far as possible, reduces in order to regarding area S0 and S2, and requests Cd0 and Cd1 is low ESR and the low ESL capacitor.

The power source out-port to the load lead wire should as far as possible short, moreover multi-channel output each groups must use the twisted pair line, because, twists in the link the electromotive force direction which produces on the identical conductor to be opposite neighboring, counter-balances mutually, this to electromagnetic interference good inhibitory action.

    3.2 weaken the syntype radiation coupling

Reference chart 3, (2) may result in by the type, syntype electric current Icm1 and Icm2 peak-to-peak value and two electrical nodes①With②Place dv/dt and to PE (safely) distributed capacity Cd1 and the Cd2 product is proportional. In chart node①Is the MOS tube drain electrode and the transformer primary side junction, the node②And outputs diode’s junction for the transformer two sides. Syntype electric current radiation intensity and syntype electric current regarding return route area related, i.e., the electromagnetic radiation intensity and the current loop area is proportional, here ring circuit area expressed with the shaded area. Therefore, weakens the syntype radiation coupling to obtain from 3 aspects, namely reduces dv/dt; Reduces the distributed capacity; Reduces the syntype current loop area.

Node①And②Existence very great voltage transient, thus in node①And②Place the wiring must take as far as possible the small area, reduces the distributed capacitance value. The distributed capacity is the pF level generally, thus (<1MHz) its impedance influence is remarkable in the low frequency band, needs the filter to carry on the weaken to the syntype electric current, the syntype impedance coil inductance value takes 10~100mH generally.

Installs on the MOS tube’s radiator, because the surface area is very big, it to node①The distributed capacity must consider. May know by Figure 4, uses the shield method to clamp the copper foil between the radiator and the MOS tube, causes Ck1 which and Ck2 original distributed capacity Ck turns connects mutually, thus reduced the distributed capacity. Radiator and transformer magnetic core similar existence voltage transient, radiator and magnetic core shield distinction nearby with node③And④The connection, with suppresses the radiator and magnetic core’s voltage transient, and reduces the syntype electric current the coupling way.

    3.3 weaken the common impedance conduction coupling

Weakens the common impedance conduction coupling, is the careful wiring avoids two electrical return route’s common impedance part. And grounding laying down, must follow “the simulation especially more importantly partially and the numerical part separates, the power partially and the control section separates” the principle in the reality laid down has adopted “astroid” the form, as shown in Figure 5, avoids using ring-like. So-called “astroid” is refers to the different return route to walk the line alone, finally collects to O. O spot usually is the decoupling electric capacity or the filter electric capacity’s negative pole. For instance in controls around the chip, actuates return route’s place (the O1 spot) to embark alone from the control chip decoupling electric capacity, connects the input end filter electric capacity place (the O2 spot), but the MOS tube source power partial lead wires also directly turn on the O2 spot. The voltage and the current feedback signal’s grounding alone turns on astroid selects O1.

Figure 5 L1 and L2 respectively meet the input terminal be the voltage feedback and current feedback’s grounding. Cd1 is the control (actuation) electric circuit’s power source decoupling electric capacity, Cd0 is inputs the filter electric capacity.

3.4 weaken the exterior electromagnetic field disturbance

Rational design EMI filter. The EMI filter except can weaken the switching power supply to electrical network’s EMI outside, but can also weaken the partial transient state disturbance which the electrical network introduces. What needs to stress, increases (PE) to weaken the syntype electric current safely, suppresses the outside transient state disturbance to be very essential.

As shown in Figure 6, (2.2nF) and varistor (VSR) carries on the clamp in exchange coil in end parallel high frequency CBB electric capacity Ca to the transient voltage.

Besides takes the partial shield measure to the electric circuit, has also used the overall screening can in the debugging process, reduces the accessory power supply subsystem to the outside electromagnetic radiation disturbance. Turns on screening can’s input, the output lead (shielded wire) to be as far as possible short, and must earth properly.

    3.5 weaken the voltage transient and the electric current transient

May know from the above analysis, the EMI intensity is proportional with dv/dt and di/dt. But restores and so on reverse by the transformer leakage inductance and the diode the voltage which, the electric current overswing and the ringing causes compares switching period unusual narrow, will create strong broadband the transient state electromagnetism noise. Therefore, in the experiment process, target-oriented to electric circuit various part of voltages, the electric current overswing and the ringing carried on has suppressed.

3.5.1 aim at the switching valve

1) considered that reduces speed the MOS tube’s shutter speed, adopts increases the gate to drive extremely? The resistance, reduces the drive current to realize. But must pay attention moderately, because the shutter speed is slower, the MOS tube’s turn-on time, the turn-off time correspondingly lengthen, the switching loss increases along with it, will create the switching valve overheated, will cause the converter efficiency to reduce.

2) uses the RCD cushion circuit, absorbs peak which the transformer primary side leakage inductance produces, reduces the MOS tube’s stress, simultaneously reduces EMI. When the switching valve shuts off, comes up the transformer leakage inductance energy transfer to electric capacity C, then by resistance R this part of energy consumption. Figure 7 (a) and Figure 7 (b) respectively be joins around the absorbing circuit the switching valve to leak the source voltage waveform, the experimental result indicated that this electric circuit may further absorb the voltage peak which leakage inductance Lp (line parasitic inductance) and the switching valve junction capacitance forms.

3.5.2 aim at the transformer two side after flow return route

Nearby after flow diode D parallel RC absorbing circuit, simultaneously with the after flow diode D series connection may saturated magnetic core inductance Ls, as shown in Figure 8. But saturated magnetic core coil in through normal current time the magnetic core is saturated, the inductance quantity is very small, will not interfere with the electric circuit normal work; Once the electric current wants the counter current to be obsolete, the magnetic core coil will have the very big counter electromotive force, the impediment reverse electrical current rise, therefore, can suppress effectively it with the diode D series diode’s reverse surge current. One kind of small magnetism link, may wrap directly in diode’s positive electrode lead wire, the use is very convenient.

Figure 9 (a) and Figure 9 (b) respectively is the after flow electric circuit takes around the corresponding measures the after flow diode voltage waveform, obviously to suppresses the voltage overswing effect to be obvious.

    3.5.3 aim at transformer’s leakage inductance

In instead stirs up-like in the analysis situs, may the transformer equivalent for the ideal transformer and primary side initiation inductance parallel. In order to transmit the enough power, the transformer must increase the air gap, in order to magnetic circuit cumulative energy, thus magnetism leaks is generally big. When the reality circles the system transformer, adopted the sandwich winding to reduce the leakage inductance. By group of transformers is the example, most in one is the primary side winding, second, third are vice-side, most outside one was still a primary side. This kind of winding strengthened the original vice-side coupling degree, reduced the transformer leakage inductance, like this may reduce on switching valve’s peak voltage, will have the syntype disturbance by the radiator return route also large scale to reduce. The sandwich winding’s shortcoming was the primary side winding passed through the middle vice-side winding from the inner layer to the outer layer, the axial-tab terminal side insulating ability reduced greatly in the transformer, regarding bore the high-pressured experiment is disadvantageous, thus multipurpose to insulating ability request not high situation.

3.6 feedback element’s adjustment

In the electric circuit debugging, the feedback element adjusts very important, EMI often is causes the feedback element characteristic to be bad, the electric circuit presents the vibration the primary cause. What because uses is the electric current peak control, feeds back including the voltage and the current feedback. For instance, the RC filter network which increases in the electric current sampling resistance end, is a low-pass filtering network, the oscilloscope observation, around the increase, the switching valve clear instantaneous electric current burr reduced approximately 3/4. But the voltage feedback started also to use the RC differential pressure filter network, namely in the resistance bleeder network’s earth resistance side parallel filter electric capacity, Rong Zhiyue was the PI adjustable ring capacitance value 1/10.

Debugs in the process, once occurred, because earths improper, causes in control section and the main power between has the distributed syntype impedance, causes the voltage out-port’s syntype noise to conduct into the control chip through the syntype impedance the place, causes the dutyfactor to lose, load relative regulation not higher question. After the out-port turning on syntype EMI filter, the situation to have a new look greatly, vibration vanishing. Obviously is obvious in the out-port turning on syntype filter function. Passes through in the PCB making lithograph plates to the grounding improvement, after using astroid spreads out on the floor, then greatly reduced the syntype noise conduction way, even if does not turn on this filter, undergoes the feedback element own PI adjustment, the converter also similarly tends to be stable.

4 conclusions

The above some methods aim at a concrete power source, from reduces the noise source and the cut-off disturbance way conducts the analytical study. What because the electric circuit analysis situs adopts is the hard switching circuit, the EMI question is the question which its comparison solves with difficulty. What must explain, although the soft switch working to reduces switching valve’s electric current and the voltage stress effect compared with the hard switch working is remarkable, but because realizes soft switch’s way existence many kinds of ways, some way introduction active and passive primary device in specific active status and under itself stray parameter influence, also will become the EMI source, similarly will be noticeable. Whether therefore to use may reduce the switch stress the soft switching circuit, must the related test result confirm.

Moreover, because this switching power supply has two groups of parallel situations, between each group has the unknown disturbance, is actually the syntype or misses the mold, needs quite to examine in the experiment. Moreover the control circuit has not used the synchronous mode, the synchronized switch phase does not cause the mutual disturbance even more may not expect, waits for further studying.