More and more application must adopt EMI standard, manufacturer just wins commercial resale approval. Switch power source means interior of parts of an apparatus to have electronic switch, EMI can produce radiation through it.

 

The article will introduce the origin of the EMI in switch power source and the method that reduce EMI or technology. The article still will reveal power source module to you (controller, high side and low side FET and inductor are enclosed for an organic whole) how to help reduce EMI.

 

Above all, must respect physical law. According to maxwell equation group, alternating current can produce electromagnetism field. This kind of phenomenon all can appear in every conductance body, its oneself contains a few capacitance that can form oscillating circuit and inductance. This oscillating circuit with specific frequency (F=1/(2*π*sqrt(LC)) ) electromagnetism can radiation arrives in the space. This circuit acts as electromagnetism can emitter, but also can receive electromagnetism can appropriative receiver. Aerial design is for the biggest change transmit or receive energy.

 

But like be not every application to should resemble aerial, and this kind of design may produce negative effect. For example, design of switch step-down power source is used at be taller voltage changeover inferior voltage, but they also acted as at the same time (harmful) electromagnetic wave emitter, the likelihood disturbs other applying, disturb AM frequency for example paragraph. This kind of effect calls EMI.

 

Move to ensure the function is normal, it is very important that utmost ground reduces EMI source. International Special Committee on Radio Interference (CISPR) defined all sorts of standards, if regard a car as the CISPR 25 with electric fiducial application, and the CISPR 22 that is aimed at IT equipment.

 

Where is the EMI radiate that how reduces power source design? A kind of method is to use a metal power source of complete screen switch. But in great majority application, as a result of the reason of cost and space, this kind of method cannot come true. A kind of better method is decrease and optimize EMI source. A lot of document already bat around this one special subject; The article recommended two kinds of way.

 

Let us review the main source of the EMI in switch power source, and why power source module can help you reduce EMI easily.

 

 

Just as its name implies, switch power source is to use those who have transition. Their action is input voltage with hundreds of kilohertz are opened to the frequency of a few conspicuous and be being shut. This brought about fast electric current to change (DI/dt) change with fast voltage (DV/dt) . According to maxwell equation group, alternating current and voltage generation are handed in change electromagnetism field. These electric magnetic field diffuse from its origin radial, their intensity follows a distance and reduce.

 

Graph 1. The EMI that comes from switch power source can be mixed to load advocate power source produces an effect.

 

Graph 2. Way of annulus of critical electric current is formed between input end, switch and input capacitor.

 

Graph 3. Reduce annulus road section to conduce to reduce EMI

 

Magnetic field and the electric part that electric field can violate application (for example, printed circuit board [the cupreous trace on PCB] , resemble aerial same) the noise with the additional generation on online road, can bring about again so produce EMI (see a picture 1) . Actually the changeover of a few made of baked clay power can enlarge the radiation limits of EMI.

 

Graph 4. Bring a foot to arrange conduce to reduce annulus road area. Zun Tu: Those who optimize bring a foot to arrange; Right graph: Blame optimize distribution, cannot form good position almost.

 

The electric flow that radiant electromagnetism can have shed with its (I) accumulate with annulus road surface (A) into direct ratio. The area that reduces alternating current and voltage annulus way conduces to reduce EMI (see a picture 2 with the graph 3) .

 

With a view to brings a foot to arrange (see a picture 4) can help you design good position better through reducing area of road of tall DI/dt annulus. For example, switch node can cause tall electric current to change (DI) change with tall voltage (DV) . It is OK to bring a foot goodly to arrange detached noise is sensitive bring foot and noise to cite a base. Leave to joint is nodded and be started bring a foot to should be far from noise as far as possible sensitive model feedback cites a base. In addition, the input brings foot and ground connection to bring crural Ying Xianglin. Simplified so the wiring on PCB and the placement that input capacitor.

 

Graph the 5 improvement that showed converter of LMR23630 SIMPLE SWITCHER® evaluate module (EVM) . Two inputs capacitor is apart from an input to bring a foot to make an appointment with 2.5 centimeters. Arrange so, it is for imitate bad position, because of section of voltaic annulus road (graph 5 medium red are rectangular) express a requirement and proposal than data want big. Graph 5 medium elliptic gules appearance express the switch node between converter and inductor. It is good that the annulus road surface between IC and inductor accumulates Yue Xiaoyue.

 

Graph 5. The input makes foot and input annulus road surface is accumulated between capacitor (gules rectangle) larger wrong layout give a demonstration. Section of road of the 2nd annulus is formed between IC and inductor (elliptic gules appearance) .

 

Graph the EMI radiate that 6 medium graph showed LMR23630 converter, the annulus road area that forms between VIN, GND and input capacitor only among them is different. The capacitor in good position stands by an input to bring foot and ground connection to cite a base as far as possible (annulus road area as far as possible small) . And input of distance of capacitor of the input in bad position cites a base 2.5 centimeters, form a larger annulus road area thereby.

 

Graph 6.LMR23630 converter inputs capacitance layout to be affected to EMI radiant.

 

Graph 6 in the EMI radiate that the red line of graph represents bad position. Blue line expresses to use the EMI radiate of the good position of identical EVM. Revise area of an annulus road to be able to produce tremendous effect. The EMI radiation level of LMR23630 converter can reduce 20 DBμV/m above.

 

Graph 7. The interior of module of different type power source is comprised. Below these two kinds of circumstances, inductor all is located in the coping of IC chip.

 

Accordingly, when using step-down converter or module of step-down power source to undertake designing, how placing input capacitor should be one of first consideration elements. Power source module still has the following advantage: The area of crucial annulus road between inductor and IC has been optimized too. Inductor is in enclose interior and integrated circuit join (see a picture 7) . This kind places means to be able to be in enclose interior to form a lesser annulus road section. Accordingly, need not be in wiring of noise switch node printed circuit board on.

 

The screen in power source module among them most inductor, come from the electromagnetism radiate of coil in order to prevent. In special the place that stands by inductor can produce voltage of tall electric current to change, and open field of bit more articulatory one part electromagnetism get screen, the coping that inductor is located in down-lead frame (see a picture 7) .

 

 

Fast twinkling changes can bring about switch node to produce ringing, produce EMI thereby. Below certain circumstance, converter can join to start cite a base. Set a resistor and series of the capacitor that start can increase rise time (Dt) , reducing EMI while loss efficiency.

 

Graph 8. Add the resistor that start the effect of node of LMR23630 converter switch. EMI radiate is inferior, but because switch loss is higher, because this efficiency is reduced somewhat.

 

Graph the 8 EMI radiate that showed LMR23630 EVM scan. After undertaking changing to layout, the input capacitor is put in be apart from bring a foot to arrange 2.5 centimeters of far places, with imitate bad position, how will the placement that reveals the capacitor that start affect EMI character. It is easier to put a capacitor that start to may compare complete change position more in the design. When you are being designed, the proposal considers the capacitor that start go in from beginning to end, in order to have untimely needs. If do not have, you can use 0Ω resistor to reduce the space on PCB.

 

Can reduce the resistor that start and series connection of the capacitor that start EMI spectrum. The emissive meeting in certain frequency limits is reduced amount to 6dB. Graph 8 still showed efficiency balances a circumstance. The resistor of use 30.1Ω shortens rise time Dt, thereby loss of efficiency 1% above.

 

See power loss can show this more. Be fully loaded with (3A) power loss increases 2.1W from 1.9W. When power loss exceeds 10% , may bring about medicinal powder hot issue.

 

Foot and ground connection are brought in switch node place between the foot diode can reduce a small-sized Xiaoteji to restore electric current reversely, the DI of ringing of switch node electric current in reducing synchronous converter thereby, but such meetings increase stock detailed account (BOM) cost. Or, you can add an amortize network, include to be located in switch node and ground connection among them between additional enclose capacitance and resistor greatly. Bumper can waste the energy of switch node ringing, but the ringing frequency that needs to know add component and correct computation. This kind of method can reduce the efficiency of switch power source likewise.

 

The parasitism inductance in voltaic method and capacitance

 

To synchronous step-down converter, every IC framework can produce the noise of different intensity, expression is EMI radiate. But find this one very hard from inside data table. Most number did not offer EMI to pursue according to the watch, because component of PCB layout, BOM is mixed,other factor can is opposite EMI character produces an effect. Lucky word, EVM user guideline can offer this EMI character of specific design to pursue. But if the layout of your design and EVM is mixed,BOM does not match, the EMI character of the application that you design may have very big difference. Power source module simplified layout, realized fast and handy design, because you need only,consider rule of a few experience. For example, reduce the trace in ground connection plane or cut amount as far as possible; When necessary, its the design is keep parallel with voltaic direction (graph 9) .

 

The cut in graph 9.PCB and trace can affect electric current, because this also can affect radiation EMI.

 

 

Shorten as far as possible noise is sensitive node, be far from noise node. For example, from resistor cent controls a network to feedback (FB) the long trace that cites a base is OK and appropriative aerial captures the noise that electromagnetism radiate disturbs (graph 10) . This kind of noise can be introduced FB cites a base, cause output end to produce additional noise, make parts of an apparatus not stable even. When the overall arrangement that devises switch step-down adjuster, considering all these inside is a challenge.

 

Express 1. The noise in step-down converter is sensitive the give typical examples of node and noise node.

 

Graph 10. Bring FB the resistor voltage divider on the foot to stand by FB to bring a foot to place as far as possible from beginning to end.

 

The advantage of module depends on noise sensitive node and noise node maintain in lowermost limit, thereby the odds that utmost ground reduces wrong position. What want an attention exclusively is the trace that holds FB to cite a base as far as possible short.

 

 

There is a lot of come using to adjust in switch step-down converter the knob of EMI, but use implementation optimal program may be not quite convenient still. Find optimal configuration to be able to spend a large number of valuable design time. Power source module includes FET and inductor already, this makes found and finish the power source design that has good EMI character to become simple and quick. The most crucial when use step-down module undertakes designing is a few exterior component set pattern, this conduces to raise EMI character significantly.

 

 

The source that preamble says to understand the EMI in switch power source and how to reduce EMI. Now, article general through comparing converter is mixed use identical integrated circuit (IC) the measurement between power source module result, will demonstrate module how to be helped reduce EMI radiate. Both the SIMPLE SWITCHER product line that all comes from TI, converter is LMR23630, power source module is LMZM33603, use LMR23630 IC. Change through making a part to the EVM of two parts of an apparatus, in order to acquire same BOM number, what because this result depends on only,pick a part (converter or power source module) with layout. Two kinds of EVM all are had optimize distribution goodly. Later, place capacitor in the position that is far from an input to cite a base, made undesirable overall arrangement.

 

The function of LMR23630 converter

 

 SHAPE * MERGEFORMAT

 

Converter – LMR 23630

 

Good position

 

Capacitor is stood by

 

Capacitor is far from

 

Without capacitor

 

Frequency [MHz]

 

Good position

 

Small capacitor is stood by

 

Small capacitor is far from

 

Without small capacitor

 

N [DBµV/m]

 

Class of CISPR 22 A3M

 

Graph 11. Have the EMI radiate of the LMR23630 converter of layout of different input capacitance.

 

Graph spectrum of 4 kinds of when indicated different design position 11 different EMI. Design layout is arranged to bad from actor (be similar to a graph 5, just part each measure) . Measure for the first time (good position / blue line) when, did not make to the layout of EVM change (all input capacitor in good position special stand by an input to cite a base) . Measure the 2nd times (small capacitor is stood by / red line) when, two 4.7μF capacitor all is placed in be apart from an input to cite a base 2.5 centimeters of place. The small capacitor of 0.22μF special stand by an input to cite a base. In the 3rd (small capacitor is far from / green line) and the 4th (without small capacitor / violet line) second when measuring, small capacitor is apart from an input to cite a base respectively 2.5 centimeters, move completely next except.

 

You can be in graph 11 be pleasant to the eye to place special key to what input capacitor. small input capacitor is far from an input to bring the foot is placed or move its completely except can violate standard of class of CISPR 22 A3M. It is OK that will small capacitor stands by an input to bring a foot to place utmost ground reduces area of road of high frequency annulus. Small capacitor but filter divides high frequency noise, and the capacitor of larger electric capacity but filter divides low frequency noise.

 

Of power source module capacitor of a small input is included normally in enclosing. Let us watch the performance of module of the power source when position is bad.

 

The function of module of LMZM33603 power source

 

Graph the 12 EVM position that indicated power source module, arrange to bad from actor likewise. Blue line expresses to did not change the EMI radiate of EVM. Red line and green line represent bad position, among them a line has two 4.7μF to input capacitor, be located in PCB bottom lower part (red line) . The capacitor of green line is apart from an input to bring a foot to make an appointment with 3.5 centimeters (graph 13 in highlight elliptically with red show) . Graph the EVM after thick wire still showed 13 medium red to change, and the area of crucial annulus road that forms between capacitor of VIN, input and ground connection. EMI character becomes poor, but do not violate standard of class of CISPR 22 A3M.

 

The EMI radiate of module of power source of graph 12.TI LMZM33603 is characteristic

 

The bad position give typical examples of module of power source of graph 13.TI LMZM33603.

 

 

Graph 14 right in individual chart LMR23630 converter (red line) with module of LMZM33603 power source (blue line) made contrast. Both all have similar bad position, capacitor of all and exterior input is far from an input to cite a base.

 

Apparent, the EMI radiate character of module of LMZM33603 power source wants excel LMR23630 converter. Although two kinds of layout all faulty, but power source module can check through CISPR, and converter cannot pass a test.

 

 

Graph 14. The EMI that compares TI LMR23630 converter and module of LMZM33603 power source is characteristic.