The generation of ripple of switch power source

 

Our ultimate goal is to want to reduce output ripple to sustainable level, reaching the means of settlement with this the most primary purpose is the generation that should avoid ripple as far as possible, want to understand reason of the sort of ripple of switch power source and generation above all.

 

As the switch of SWITCH, the electric current in inductance L also fluctuates up and down in the virtual value that exports electricity. Also can appear in output end so a ripple that is the same as frequency with SWITCH, the ripple that says commonly points to this namely. It and the capacity that output capacitance and ESR are relative. The frequency of this ripple and switch power source are same, for a few to hundreds of KHz.

 

Additional, SWITCH is chosen commonly bipolar transistor or MOSFET, no matter be which kinds, guide in its when connect and ending, can have a rise time and decay time. Can appear in circuit at that time a frequency with SWITCH ascendant decay time is same times odder perhaps the noise of frequency, it is a few MHz commonly. Same diode D is restoring the instant reversely, its equivalent circuit is the series connection of resistor capacitance and inductance, can cause resonance, the noise frequency of generation also is a few MHz. These two kinds of noise are called high frequency noise commonly, amplitude should be gotten greatly than ripple normally much.

 

If be transformer of AC ／ DC, besides two kinds of afore-mentioned ripple (noise) beyond, still have AC noise, frequency is the frequency that inputs AC power source, for left and right sides of 50 ～ 60Hz. Still one is plant in all modular noise, because the power parts of an apparatus of power source of a lot of switch uses crust to serve as radiator,be, the equivalent capacitance of generation brings about. Because oneself make car electron research and development, to hind contact of two kinds of noise is less, so of short duration is taken no account of.

 

The measurement of ripple of switch power source

 

Main demand: Use oscillograph AC coupling, 20MHz carries wide restriction, unplug the ground wire of probe.

 

1, AC coupling is the volts d.c. of take out overlay, get accurate weaveform.

 

2, opening 20MHz bandwidth to restrict is the interference that prevents high frequency noise, prevent to measure the result that gives a mistake. Because amplitude of high frequency composition is bigger, eliminate answers when measuring.

 

3, unplug the ground connection of oscillograph probe is placed, annulus of use ground connection is measured, it is to reduce interference. A lot of branches do not have ground connection annulus, if the error allows,also measure with the ground connection clip of probe directly. But judging whether should this factor consider when qualification.

 

Still having a bit is to should use 50Ω terminal. The horizontal stroke says on the data of river oscillograph, 50Ω module is eliminate DC composition, measure AC part accurately. But very rare oscillograph matchs this kind of special probe, most circumstance is to use mark to match 100KΩ to be measured to the probe of 10MΩ , the influence is not clear about temporarily.

 

The basic note when measuring switch ripple is above. If oscillograph probe is not dot of direct contact output, should use double skeining thread, or means of 50Ω coaxial cable is measured.

 

When measuring high frequency noise, the allpass of use oscillograph is taken, arrive for hundreds of million commonly GHz level. Other with afore-mentioned identical. May different company has different test method. In the final analysis the first test result that should be clear about oneself. The 2nd good is approbated to the client.

 

About oscillograph:

 

Because some numbers oscillograph disturbs the account with memory deepness, cannot correct measurement gives ripple. Should change at this moment oscillograph. This respect occasionally although a few old imitate oscillograph bandwidth has only, but it is good that expression should compare digital oscillograph.

 

Peaceful gram company has special departure to measure two kinds of afore-mentioned ripple (noise) software, can read reference material 5. Same, about the ground connection of oscillograph, the relevant knowledge that power source tests, also can look.

 

The inhibition of ripple of switch power source

 

To switch ripple, theoretic mixing is constant presence actually. The practice that restrain normally or reduces it has 5 kinds:

 

1 increase inductance and wave of filter of output electric capacity

 

According to the formula of switch power source, size of wave motion of the electric current inside inductance and inductance value become inverse ratio, output ripple and value of output electric capacity become inverse ratio. Increase inductance value and value of output electric capacity to be able to reduce ripple so.

 

Same, output ripple and the relation that output capacitance: Vripple=Imax/(Co×f) . Can see, increase value of output electric capacity to be able to reduce ripple.

 

Normally practice, to outputting capacitance, use aluminium electroanalysises capacitance in order to achieve the goal of high capacity. But electroanalysis capacitance is restraining effect of side of high frequency noise is not very good, and ESR is bigger also, the meeting is so shunt-wound by it capacitance of a pottery and porcelain, will make up for aluminium to electroanalysis the inadequacy of capacitance.

 

In the meantime, when switch power source works, the voltage Vin that inputs end is changeless, but electric current changes along with switch. Input power source to won’t provide electricity well at this moment, standing by electric current to input end normally (with BucK model for exemple, it is SWITcH around) , shunt-wound capacitance will provide electricity.

 

This kind of practice is opposite above the effect that reduces ripple is finite. Because bulk is restricted, what inductance won’t do is very big; Output electric capacity raises fixed rate, do not have apparent effect to reducing ripple; Increase switch frequency, can increase switch loss again. It is so when the requirement is stricter, this kind of method is not very good. Wait about the principle of switch power source, design manual of power source of OK and referenced of all kinds switch.

 

Wave of filter of 2 2 class, add filter of one class LC again namely

 

LC filter is more apparent to the inhibition of ripple of a confusion of voices, choose right inductance electric capacity to make filter wave circuit according to wanting the ripple frequency of eliminate, can reduce ripple very well commonly.

 

Sampling dot is chosen before LC filter (Pa) , output voltage can be reduced. Because any inductance have resistance of a dc, when electric current is outputted, can have on inductance pressure fall generation, the output voltage drop that causes power supply is small. And this is pressed falling change along with output electric current.

 

Sampling dot is chosen after LC filter (Pb) , such output voltage are the voltage that we hope to get. But introduced an inductance and a capacitance in interior of power source system so, possible meeting brings about a system to be not stabilized. About systematic stability, a lot of data have the introduction, here was not written in detail.

 

After 3 switch power source is outputted, receive LDO filter wave

 

This is to reduce the way with ripple and the most efficient noise, output tension is constant, do not need to change original feedback system, but also be cost highest, the method with highest power comsumption. An any LDO have an index: Noise is restrained than. It is a frequency – DB curve.

 

To reducing ripple. The PCB wiring of switch power source is very crucial also, this is very the problem of hertz hand. Have engineer of technical PCB of switch power source, to high frequency noise, because frequency is tall amplitude is bigger, hind although class filter wave has certain effect, but the effect is not apparent. This respect has technical research, simple way is go up in diode and capacitance C or RC, or series connection inductance.

 

4 go up in diode and capacitance C or RC

 

Diode high speed guides when ending, want to consider parasitism parameter. During diode restores reversely, equivalent inductance and equivalent capacitance become oscillator of a RC, produce high frequency vibration. To restrain this kind of high frequency oscillation, need to be in diode C of capacitance of two upright paralell connection or RC amortize network. Resistor takes 10Ω-100 Ω commonly, capacitance takes 4.7pF-2.2nF.

 

Shunt-wound on diode capacitance C or RC, its take a value to want to try check ability to decide through relapsing. If choose undeserved, can cause more serious oscillation instead.

 

To the word with strict requirement of high frequency noise, can use soft switch technology. About soft switch, a lot of books introduce technically.

 

Inductance is received after 5 diode (EMI filter wave)

 

This also is commonly used inhibition the method of high frequency noise. Be aimed at the frequency that produces noise, choose appropriate inductance component, can restrain noise effectively likewise. Those who need an attention is, the rated electric current of inductance should satisfy real requirement. Compare simple way, explain in detail no longer.

 

Brief summary

 

Above is about ripple of switch power source, a few content of summary, if can add some of weaveform better. Although the likelihood is not quite complete, but already enough to general application. Restrain about noise, actual in not certain and all application, important is the design requirement according to oneself, for instance product volume, cost, development cycle, choose appropriate method.