D kind an exists possibly defect that amplifier switch develop attacks, it is its give out electromagnetism radiate easily, may violate facility of circumjacent other electron. Can pass method of wave of exterior and passive filter to disturb this kind delay to be decreased somehow, but the cost that this meeting raises end item, take an area and complexity. The article will be discussed certain the in-house circuit that is used at easing EMI problem devises a method.

 

Rate of the first margin is controlled

 

For enlarge frequency signal, d kind the output of amplifier (or all sorts of output, configure with what differ) in course of two power source (it is positive electrode and ground connection normally) between alternant switch, its frequency is the highest frequency frequency that requires enlarge 10 times or taller (the likelihood is 300kHz or taller) .

 

Switch signal passes modulation, pass thereby simple, it is loudhailer itself sometimes those who include is low connect filter to restore frequency signal. This switch changes average rate very fast —— perhaps is 2ns or shorter —— include the high frequency energy that displaying consequently. This meeting is brought about interrelate radiate of EMI of wire cable generation, do not have in signal method especially low connect filter, and below the case with the lead very clear length between amplifier and loudhailer (perhaps exceed 1cm) .

 

Decrease EMI radiant with Yu Huan the changeover rate that a method is output of bring down amplifier (Slew Rate) . The graph shows a case in be time domain 1 times, trace of its upper part has the rise of 2ns and decay time, and the rise that lower part trace has 20ns and decay time.

 

Those who change rate reduce (the factor here is 10) to D kind the radiation energy that amplifier produces is having the effect that showing. Graph the 2 spectrum that showed two kinds of weaveform, right now D kind output is in become silent (without frequency, occupy sky comparing =50% ) , switch frequency is 333kHz. Can see perforative the major spectrum between 30MHz~1GHz, its are high frequency (HF) content decreases make an appointment with 20dB. FM broadcast is contained to receive electronic facility in the bag (88MHz ~ 108MHz) mobile phone or wireless Internet circuit (700MHz ~ 2.7GHz) in the system, this can reduce EMI considerably, reduced the risk that affects systematic performance possibly thereby.

 

 

Graph 2 showed margin rate is controlled clearly (Edge Rate Control, ERC) the dominant position that the technology reduces EMI, nevertheless cost was to increase loss.

 

It is D above all kind the efficiency advantage that amplifier offers, basically come from be opened at outputting parts of an apparatus to leave completely from beginning to end or shut completely, because this outputs the P= VI of instantaneous dissipation power in parts of an apparatus, basically maintain in all time for 0 (unlike AB kind amplifier, the VI product of its power parts of an apparatus never is 0) .

 

In when switch is changed, introduce every time (or increase) time span, meantime V≠0, at the same time load current I≠0, bring about piece on power comsumption increases moderately, bring efficiency consequently reduce. Next, level of output of ERC of a blame is a large inverter only substantially (the likelihood includes direct or the delay of short circuit impulse current is decreased) , and level of an ERC output includes add circuit, can adjust go up to pull and issue those who pull parts of an apparatus to touch hair voltage, so that be on output end,generation expects, suffer pilot to change rate. Depend on uses method, this raised chip area (cost) use up with electric current (reduce efficiency) .

 

As a whole, as a result of the efficiency cost that add ERC and produces the likelihood is 1% ~ 2% .

 

 

Clock of frequency of the 2nd enlarge

 

The margin rate that afore-mentioned discussion is controlled (ERC) it is an effective method, but abate the EMI that arises in limits of 30MHz above frequency (the restriction that also accepts code of FCC of be confined to) , and D kind what the main carrier frequency that amplifier switch outputs falls in 30MHz the following limits with its is relevant strange second harmonic (square wave) , not quite good use this technology to handle. The graph shows what appear for this frequency band 3 times by traditional, the D that did not revise kind the energy that amplifier output produces.

 

 

To reduce D kind the foundamental tone in outputting spectrum and overtone aiguille height, a few frequency modulation —— can be joined in the clock circuit of amplifier perhaps modulation index is controlled in ±5% , won’t affect the quality of an enlarge frequency signal. The character that is aimed at modulation signal source has a lot of choices, a groovy course of action is to use contain repeat frequency (whole pattern repeats frequency) pseudo-random mode, its are exceeded highest anticipate frequency signal frequency (it is 20kHz normally) a proper surplus, this can prevent to produce a likelihood to fall into the tone of frequency frequency band.

 

Pursue 4 showed with the graph 3 show identical D kind output, but its contain ±5% modulation, frequency repeats to fall to come true by pseudo-random alignment in 40kHz mode.

 

 

Graph 5 showed a picture 3 with the graph the picture after 4 color overlay, more the difference that showed by enlarge the arteries and veins when frequency is brought clearly. Can see inside whole spectrum limits, of fiducial clock frequency strange second harmonic was restrained near 10dB.

 

 

The 3rd unilateral modulation

 

Can use a kind of additional means to reduce EMI, through amending modulation plan, become enough when amplitude of signal of frequency base band big when, allow unilateral to differ cent or bridge-type D kind output is right stop switch (graph 6) . This allows retrorse output substantially, last to switch all the time, so that undertake comprehensive modulation, output signal holds in odd interval till its are highest peak value.

 

This plan, inside time of very large proportion (depend on frequency source material) , only an output is in switch, consequently EMI (inside that time) reduced an in part. This increased a dominant position, because door of power parts of an apparatus and other parasitism capacitance charge the fixed switch loss that discharge brings,decreased. It still shortened the time that output changes an aspect in ERC, as above, this changeover has a few efficiency cost. The whole that the defect of this technology is amplifier is forward gain meets some Xu Jiang is low, no less, overall harmonic is lack fidelity (Total Harmonic Distortion, THD) also have a few addition with noise. The D that contain and did not contain unilateral modulation kind if output spectrum pursues 7.

 

 

 

 

The 4th conclusion

 

D kind amplifier is used at portable apparatus normally, because of its power efficiency exceeds traditional AB kind amplifier. D kind main drawback of the technology is its inherent EMI, can produce adverse effect to circumjacent electron equipment. Had appeared now a few effective IC design a technology, can alleviate greatly EMI problem, and need not the exterior component with additional responsibility.