Abstract: Through to Buck the converter electric circuit’s EMC analysis, explained in the electromagnetic compatibility technologies and so on filter, earth, cushion as well as reasonable PCB design in switching power supply’s application.
Key word: Switching power supply; Electromagnetic compatibility; Electromagnetism harassment; Electromagnetic interference; Coupling channel
Introduction
The switching power supply recently effectively controlled the output voltage and the electric current size through the change switch component’s breakover. Usually it works under several dozens kHz above turn-on frequency, when switch breakover, it will wind through surge current Cdv/dt; When the switch separates, its both sides will produce surge voltage Ldi/dt, will form the strong electromagnetism harassment source. Along with the semiconductor switch component’s unceasing development, the turn-on frequency will enhance to the MHz magnitude, causes the electromagnetism harassment to be more serious. Therefore, must use the corresponding measure, strengthens switching power supply’s electromagnetic compatibility (EMC).
The electromagnetic compatibility is refers to, in does not lose the useful signal cabin to contain the information under the condition, information and disturbance coexistent ability. The electric power electronic installation under its use environment, withstands from exterior electromagnetic interference at the same time also disturbs to the environment release. When design manufacture electric power electronic installation, should consider electric power electronic installation when work produces the electromagnetism harassment to other electronic installation’s movement which works in the identical environment does not have the adverse effect, simultaneously will come from the external environment the electromagnetic interference not to affect the electric power electronic installation the work.
1 Buck system’s electromagnetic interference
Following unifies the Buck converter to come the reason which and the condition the concrete discussion electromagnetic interference produces, thus discovers the method which suppresses and eliminates. Figure 1 is the Buck converter’s principle structure drawing.
The main circuit mainly by power switching valve S, Short base diode D, filter electric capacity C, inductance L, resistive load Ro as well as non-feeling sampling resistance RL is composed. This electric circuit’s basic parameter is the input end is the 36V lead-acid battery, the output request is the 10A constant flow, the turn-on frequency is 50kHz. The control chip uses SG3525, the actuation chip uses TLP250. The accessory power supply uses counter-violently. The main circuit choice appropriate closed loop parameter is important one step, the appropriate closed loop parameter may cause the electric circuit to be stable, produces small EMD.
Figure 2 is this system’s electromagnetic compatibility schematic drawing, unifies electromagnetic environment which and interaction situation this chart analysis system locates. Obviously, the electromagnetic interference both may occur in the system, and has the possibility to occur between the system.
May see from Figure 2, any kind of EMI is composed of three parts: The harassment source, the coupling way and is harassed the body. The harassment source produces the disturbance enters after the coupling way is harassed the body, if the disturbance level will surpass is harassed body’s sensitive degree to affect its normal work to constitute the disturbance. Compares with the digital circuit, as a result of the switching power supply power switching valve’s high-speed switch movement, the jamming intensity which it produces is big; The harassment source mainly concentrates in the power switch component as well as with it connected high-frequency transformer on; The turn-on frequency is not high, mainly disturbs the form is the conducted interference and the near field disturbance. Generally solves EMD in view of 3 aspects: Suppresses the harassment source, the cut-off disturbance way and the enhancement is harassed body’s antijamming ability.
Thus it may be known the Buck main harassment source is the switching valve and the power diode. Because the turn-on frequency is high, the transmission energy is also big, will therefore produce the very high burr in the switching process.
Because designs switching power supply’s geometry size is smaller than the 30MHz electromagnetic field correspondence wave length far, therefore, what electromagnetic interference main consideration is the conducted interference. MOSFET the voltage peak which and the vibration produces in the switching process mainly through the wire parasitic inductance and the parasitic capacity disturbance are harassed the body, the switching process are quicker, the peak is higher, the vibration is more obvious, the disturbance is stronger.
2 electromagnetic interference braking measure
Mainly from the filter, the earth, the absorption, PCB panel several aspects carries on the analysis and the design.
2.1 filter
As a result of battery existence certain internal resistance, in addition in the input terminal lead wire’s parasitic inductance and the interface resistance, will cause a series of high frequency ripples in the input end. In order to cause the input end to become satisfies the request the constant pressure source, must enter the terminal in the electric circuit to add on the EMI filter, also has suppressed the outside to electric circuit’s disturbance, also has prevented the electric circuit to battery’s disturbance. Usually, and filters the high frequency ripple in the input terminal parallel electrolysis electric capacity the electric capacity. The electrolysis electric capacity mainly filters the low frequency ripple, filters the high frequency ripple the electric capacity to use the CBB electric capacity.
As a result of the out-port to current waveform’s request, must reduce the output ripple the size, therefore, also needs and large capacity filters the high frequency ripple in the out-port parallel large capacity electrolysis electric capacity the CBB electric capacity.
Moreover, needs to integrate the chip the decoupling filter electric capacity to carry on the science the disposition. Each integrated chip meets has the decoupling filter capacitor, recharges in each time the switching process, so that is the chip power supply, decoupling condenser’s value in 470pF~1000pF, uses the porcelain piece or is the CBB electric capacity generally, uses in filtering the higher mode ripple. The decoupling filter capacitor must abut the integrated circuit installment, makes every effort the shortest capacitor lead wire and the smallest transient current return route area. Simultaneously must lay aside the overall decoupling condenser on the entire integrated chip PCB board, comes by the power source to charge, and should install in the power source bus bar enters the PCB board the place.
In what the system is easiest to disturb is the electric current sampling resistance, but sampling electric current precision immediate influence electric circuit’s output target. Sampling electric current’s profile as shown in Figure 3. Because the sampling resistance receives the switching valve breakover and the shutdown disturbance, therefore, needs to carry on the filter to the sampling resistance signal, in this electric circuit the use RC second-order passive filter, electric circuit following shown in Figure 4.
2.2 earths
A system’s earth mainly has the security, the signal, the cabinet and the shield place. Here only discusses this system’s public place connection method.
1) the earth system must have the very low common impedance, causes in the system each group electric current, produces the direct conduction noise voltage through this common impedance to be smallest. 2) in high-frequency current’s situation, guaranteed that “the signal place” right “the earth” has the low syntype voltage, enables to adopt the radiation noise which “the signal place” produces to be lowest.
3) guaranteed the grounding and the holding wire constitute the galvanic circle has the smallest area, avoids constituting “the earth return” by the grounding, causes the external interference magnetic field to pass through the bad mold disturbing voltage which this return route produces to be smallest, simultaneously, also avoids by the terrestrial electricity potential difference causing the oversized earth current through the earth return, creates the conducted interference.
In this system uses the mix earth and floats the spatial earth way. The main power uses floats the way, with the aim of reducing the common impedance and big electric current passing. The control system interior connects first earths, then again simple point and main power connection. The driving circuit uses the light pair isolation technology to actuate switching valve’s breakover and the shutdown.
2.3 cushions
After having used the above measure, discovered that MOSFET in the switching process the burr was still high, this is because the electric circuit average electric current has been quite big, the very small parasitic inductance can also cause the very big burr.
Cushion circuit’s goal is the transient state noise which produces to the switching valve carries on suppresses. What uses is carries on the switching valve both sides and the previous R-C-D network suppresses, it may slow down between switching valve’s drain electrode and the source voltage climbing rate, as shown in Figure 5.
After adds the cushion circuit to the switching valve, may obtain the quite ideal switch profile, as shown in Figure 6.
In shutdown process, because the power diode can have the reverse restoration, this is an important harassment source.
The RC cushion circuit is solves the power diode to restore the question reverse the commonly used method. Power diode which works under high frequency, must consider the stray parameter. Figure 7(a) is the electric circuit model, D0 is the ideal diode, Lp is the lead inductance, Cp is the junction capacitance, Rp is the shunt resistance (high resistance number), Rs is the lead wire resistance. As shown in Figure 7(b), after electric capacity C and resistance R series, parallel to power diode D on. When diode reverse shutdown, in parasitic inductance energy to parasitic capacity charge, meanwhile through cushion resistance R to the cushion electric capacity C charge. In the similar energy’s situation, the cushion electric capacity is bigger, the above voltage is smaller; When diode forward breakover, C through the R electric discharge, the energy major part consumes on R.
Through after power diode and R-C cushion circuit, may obtain the quite ideal clear and the reverse shutdown profile, as shown in Figure 8.
2.4 PCB panels
On the printed wiring board primary device’s laying aside and the wiring design have the enormous influence to the switching power supply EMC performance, in high frequency switching power supply, because on the printing plate both has the low level small signal pilot wire, and has the high-pressured big electric current power line, meanwhile has some high frequency power switch, the magnetic elements. How to arrange the primary device position reasonably in the printing plate limited space, as well as reasonably wiring immediate influence to electric circuit in various primary devices own anti-jamming and electric circuit work reliability.
Through the analysis printed conductor’s characteristic impedance, selects the wiring reasonably the laying aside way, the length, the width as well as the entire distribution. The simple root wire’s characteristic impedance is composed of DC resistance R and self inductance L.
In the formula: l is the printed conductor length;
b is the printed conductor width.
May see printing line l to be shorter, DC resistance R is smaller, self inductance L is also smaller; Simultaneously increases the printing line the width also to be possible to reduce DC resistance R and self inductance L.
Many printing line characteristic impedance besides DC resistance R and self inductance L, but also has mutual inductance M.
In the formula: s is two between distances.
As shown in the above analysis, when design printed wiring board, should reduce the power line and the grounding impedance as far as possible. Because the power line, the grounding and other printing line have the inductance, when the supply current change is big, will have the big pressure drop, but the grounding pressure drop will be forms the common impedance disturbance the important attribute, therefore should reduce the grounding as far as possible, will widen the power line and the grounding as far as possible.
Cocurrent power supply system’s reality equivalent circuit as shown in Figure 9.
In the cocurrent feed circuit produces the transient state noise voltage, stems from the power source load current the sudden change δiL. This electric current change is the instant, because produces transient voltage’s peak-to-peak value δuL is the power source power supply transmission line characteristic impedance Z0 function.
In order to reduce δuL, must make Z0 to be as far as possible low, (4) may know by the type, requests Lt to be as far as possible small, Ct is as far as possible big. In order to reduce Lt and increase Ct, the power supply bus bar application rectangular section’s wire, and enables two bus bars to approach as far as possible, with two as far as possible wide flat printing lines. What because uses is the two-sided printing plates, the power line and the grounding parallel wiring, causes two power flow direction of current to be opposite, may reduce effectively induces the magnetic flux. Simultaneously also other plus and minus holding wire distinction cloth in printing plate’s both sides, tries to cause two to carry fluid wire each other maintains parallel, because parallel abutting, the load fluid conductor produces the exterior magnetic field tends to counter-balances mutually.
Regarding primary device’s laying aside, because switching power supply’s radiated interference (E) with passage of current in electric current (I) size, circuit’s ring circuit area (A), as well as power frequency (f) the square and so on three products are proportional, namely E=I · A · f2. Is the circuit size is smaller than far using this relations’ premise the frequency wave length (this electric circuit meets condition).
Using the above relationship, reduces the passage area is reduces the radiated interference the key. In this Buck electric circuit, should cause the input end electric capacity, the switching valve, the power diode each other abutting, and the wiring is compact; Simultaneously causes the out-port power diode, the inductance, the output capacity, the sampling resistance each other abutting.
Moreover, when wiring causes switching valve’s drain electrode segment to be as far as possible short, thickly, reduces the wire the parasitic inductance. Chose appropriately filters the high frequency electric capacity (in prototype to use the CBB electric capacity), and enabled it to approach MOSFET as far as possible the drain electrode, the electric capacity lead wire is as far as possible short, reduced the wire inductance.
Principle which obtains according to the above analysis, actual design’s prototypical main power PCB board as shown in Figure 10.
3 conclusions
Not only the switching power supply electromagnetic compatibility design’s goal, is the product which causes to design can in certain electromagnetic interference environment the normal work, moreover also causes the electromagnetism harassment which the product own produces not to interfere with other equipment’s work. This article through each aspect’s analysis, summarizes designs the Buck electric circuit electromagnetic compatibility the principle, and makes according to this principle experiments the prototype, thus explained in the practice foundation these principle’s feasibility and the accuracy, have provided the very good experience for the later switching power supply’s electromagnetic compatibility design.
