Although a lot of switch mode power source design staff,the design complexity of power source of mode of very clear switch is mixed nuance, but a lot of companies do not have sufficient design personnel to satisfy all project demand to complete a design at all. Many design personnel will retire and leave visit job! So, how to solve this problem?

 

Above all, because design of imitate power source is undermanned,be, ask more and more numbers design personnel to undertake switch mode power source is designed so! Although most number designs personnel to know how to use simple linear manostat, but be not all designs to want beg to surrender to press (step-down mode) . In fact, it is step up mode more very (step up) , it is step-down even – step up develop attacks (step-down and union of photograph of step up mode) .

 

Apparent, a lot of electronic system manufacturer face a problem: The circuit of power source of all switch mode that how realizes systematic place to need?

 

Solve problem of design resource shortage

 

In the article, my general introduces a few fundamental that step-down manostat works, how do the tall Di/dt in including switch manostat to heat up loop and parasitism inductance bring about electromagnetism noise and switch ringing. Next we will look how to reduce high frequency noise. I still will introduce the technology of Power By LinearTM Silent Switcher® of ADI, include it how to be formed, demonstrate it how to be helped solve EMI problem, and a bit won’t affect performance. Still include SilentSwitcher parts of an apparatus how to work among them.

 

I still enclose those who summarize Silent Switcher and layout, discuss these enclosing how to improve the integral performance of step-down converter with layout. In addition, my general demonstrates how to blend in this technology our μModule® manostat, what raise SilentSwitcher parts of an apparatus thereby is compositive degree. Design design staff of the technology to not be familiar with switch mode power source, these solutions that use easily simply will be very useful.

 

Circuit of basic step-down manostat

 

The basiccest power source develop attacks one of it is step-down manostat, if pursue,1 is shown. EMI begins from tall Di/dt loop. Power supply line and laden line should not have weight of tall alternating current. Accordingly, input capacitance C2 to answer to transmit the communication heft of all and relevant electric current to C1 of output electric capacity, all voltaic communication heft end here.

 

Graph 1. Principle of synchronous step-down manostat pursues.

 

Referenced graph 1, in the open cycle that M1 is shut and M2 opens, alternating current flows in loop of solid line blue. In shutting cycle, when M1 is opened and M2 is shut, alternating current flows in loop of green dotted line. Most person understands hard, the loop both neither that produces highest EMI is loop of solid line blue, also not be loop of dotted line green. A standard-sized sheet that flows in loop of dotted line red however involves alternating current, come from 0 switch I peak value, farewell arrives 0. Loop of dotted line red points to hot loop normally, because it has highest alternating current and EMI energy.

 

Those who bring about electromagnetism noise and switch ringing is the tall Di/dt in switch manostat hot loop and parasitism inductance. Should reduce EMI and improve a function, need reduces the radiation effect of loop of dotted line red as far as possible. If we can the PC of loop of dotted line red circuit board area decreases 0, and can be bought have the ideal electric capacity of 0 impedance, can solve this problem. However, in real world, design what the engineer can do to find compromise proposal of a first-rate namely!

 

So, are these high frequency noise after all from where come? In electronic circuit, pass parasitism resistor, inductance and capacitance coupling, in switch transition, produced high frequency harmonic. Knowing is where generation noise, so how to reduce noise of high frequency switch? The traditional pattern that reduces noise is to decelerate MOSFET switch brim. Through decelerating in-house switch driver or from exterior add bumper, can come true.

 

But, this meeting reduces the efficiency of converter, because increased switch loss —— ,be in when switch manostat especially tall switch frequency (like 2MHz) when next moving. Here, why do we want to move below the frequency of 2MHz? Have a few reasons actually:

 

It allows ● use lesser (dimension) exterior component, be like capacitance and inductance. For example, every time switch frequency is double, can make inductance value and output electric capacity are worth halve.

● is in car application, below 2 MHz switch can avoid to be in AM frequency paragraph generation noise.

 

Reduce radiation, also can use filter and screen, but this needs more exterior component and circuit board area. Still can use exhibit frequency (SSFM) technology, but can make inside prescribed limit systematic clock shakes so. SSFM conduces to requirement of contented EMI standard. EMI energy is distributinged to be on frequency domain by break up. Although place of common switch power source is chosen switch frequency can be normally outside AM frequency Duan Zhi (530 KHz comes 1.8 MHz) , but in AM frequency paragraph inside, still do not accord with strict car EMI possibly to ask without the switch harmonic of modulation. Add SSFM function to be able to reduce AM frequency apparently paragraph inside the EMI in etc area.

 

Perhaps use the Silent Switcher technology of ADI, this technology can satisfy afore-mentioned all requirements:

 

● is efficient

Frequency of ● tall switch

Radiate of ● low electromagnetism (EMI)

 

Silent Switcher technology

 

Silent Switcher parts of an apparatus need not decelerate switch margin rate, solved the balance problem between EMI and efficiency. How can you just come true so? The consideration uses LT8610, if A pursues 2 left place is shown. The sheet that this is input of supportive 42 V piece (interior has FET) synchronous step-down converter, can provide the output electricity that is as high as 2.5 A. Ask an attention, its top left corner has an input to cite a base (VIN)

 

Graph 2. How be LT8610 changeover ——LT8614 of Silent Switcher parts of an apparatus.

 

But, LT8610 and LT8614 (those who support 42V input is odd a synchronous step-down converter, can provide the output electricity that is as high as 4 A) photograph comparing, we can see, LT8614 has two VIN to bring foot and two ground connection to cite a base in enclosed other a side. This is very important, because it is the one share that implementation exceeds low noise switch!

 

How to make switch manostat is had exceed low noise

 

How to achieve this goal? In chip the VIN of another side and ground connection are brought two inputs capacitance is placed to be able to eliminate magnetic field between the foot. Highlight in slide showed this, pursue in the principle and demonstrate board the position that goes up to all point to capacitance with gules arrowhead, if pursue,3 are shown.

 

Graph 3. LT8614 graph, indication filter capacitance finds a place for in IC the VIN of another side and ground connection cite a base between.

 

LT8614 detail

 

LT8614 includes Silent Switcher function. Use this function, we can reduce parasitism inductance through using cupreous column to install chip to enclose. In addition, still have retrorse VIN, ground connection and input electric capacity, can eliminate magnetic field (law of applicable right hand) in order to reduce EMI radiate.

 

The long bond line that because do not need to use type of solder line bond to assemble technical place,asks, won’t produce great parasitism resistance and inductance, can reduce thereby enclose parasitism inductance. The retrorse magnetic field that two inputs that distributing symmetrically heat up loop to arise is mutual and quits, and electric loop does not have only magnetic field.

 

We undertake manostat of LT8614 Silent Switcher and current and advanced switch manostat LT8610 comparative. In GTEM room, demonstrate to the standard of two parts of an apparatus board used voltage of identical load, identical input and identical inductance to have a test. We discover, have very good EMI function look to compare with use LT8610, 20dB still can rise when use LT8614, managing the area with more high frequency more difficult rate especially. In integral design, compare with photograph of other and sensitive system, the filter wave that LT8614 switch power source needs fewer, distance is shorter, can realize simpler and compact design thereby. In addition, inside time domain, the performance that LT8614 nods the brim in switch part is good.

 

Graph 4. Function of LT8614 radiate EMI can satisfy the strictest CISPR 25 Class 5 to limit a demand.

 

Increase of Silent Switcher parts of an apparatus further

 

Although LT8614 has excellent performance, but the pace that we did not stop to improve. Then, LT8640 step-down manostat uses Silent Switcher framework, aim utmost ground reduces EMI/EMC radiate, offer below the frequency that is as high as 3 MHz at the same time efficient. It uses 3 Mm×4 Mm QFN is enclosed, use sheet of compositive power source piece type construction, provide all and necessary circuit function at the same time, form PCB to take up jointly the solution with the least space. Transient state answers function very outstanding still, any load (arrive from 0 electric current full electric current) the output voltage ripple when under 10 MV P-p. LT8640 allows to undertake below high frequency rate tall VIN is changed to low VOUT, the shortest switch guides connect time to be 30 Ns.

 

To improve EMI/EMC, LT8640 can work in the mode that exhibit frequency. This function adjusts clock with the trigonometry frequency modulation of 20% . When LT8640 is in mode of the modulation that exhibit frequency, function of use triangle frequency modulation is in RT set value and make an appointment with prep above to should be worth 20% between adjust switch frequency. Modulation frequency is 3 KHz about. For example, set when LT8640 when be 2 MHz, the frequency below 3kHz rate will differ to 2.4MHz from 2MHz. When choosing mode of the job that exhibit frequency, operation of sudden mode Burst Mode® can be banned with, parts of an apparatus will hop in pulse move below mode or compulsive and successive mode.

 

However, although we have a specification in manual of Silent Switcher data, if offerred principle graph and layout proposal, and the input capacitance is put in the positional —— that stands by two side of IC as far as possible to a few clients still can make mistake. In addition, our in-house engineer also spent too much time to solve PCB layout problem of the client. Accordingly, our design personnel put forward to solve the framework of optimal solution ——Silent Switcher 2 of this problem.

 

Silent Switcher 2

 

Use technology of Silent Switcher 2, we need only capacitance compositive enclose in new LQFN inside: —— of IntVCC and VIN capacitance, step up capacitance is stood by as far as possible bring a foot to place. The advantage is will all hot loop and ground connection layer are included, reduced EMI thereby. Exterior element is less, solution size is smaller. In addition, we still eliminated PCB layout sensitivity.

 

If the graph is shown 5 times, the graph has the principle that can see LT8640 and LT8640S why to differ. And sale breach is to be those who include in-house electric capacity is compositive degree of taller new version coronal with “S” suffixal. Because it compares generation more “ quiet ” !

 

Graph 5. LT8640S is one has higher electric capacity compositive parts of an apparatus of spent Silent Switcher 2.

 

Technology of Silent Switcher 2 rose to come loose hot property. The ground contact of many large size that LQFN holds chip to seal mount is bare solder dish conduce to enclose and PCB comes loose hot. Because we eliminated line of tall resistor bond, because this still improved changeover efficiency. The EMI function of LT8640S satisfies peak value of CISPR 25 Class 5 of radiation EMI function easily to limitation asks and have larger abundant amount.

 

Next: All component are as compositive as manostat of Silent Switcher 2 μModule

 

Silent Switcher technology is so conspicuous, we choose to blend in its our μModule manostat product line. All component are compositive in small size is enclosed in, offerred for the user simple and reliable, high-powered the solution with density of tall power source. LTM8053 and LTM8073 are almost compositive the miniature module manostat of all component, only a few capacitance and resistor accept outer ministry.

 

Graph 6. LTM8053 Silent Switcher 2 μModule.

 

Summary

 

The place on put together is narrated, silent Switcher function and advantage will make your switch mode power source designs easier contented CISPR 32 and CISPR 25 to wait for requirements of all sorts of the standard that fight a confusion of voices. As a result of,they can accomplish this effectively is the following character:

 

● can be in be more than 2 MHz switch efficient transition has below frequency, the impact that and be opposite changes efficiency is the least.

Capacitance of ● in-house bypass reduces EMI radiate and offer more compact solution to occupy board space.

● used technology of Silent Switcher 2 to basically eliminate the sensitivity of PCB layout.

The optional modulation that exhibit frequency conduces to ● reduce noise susceptibility.

● uses Silent Switcher parts of an apparatus to be able to save PCB area already, can reduce needs number of plies again.