Circuit of wave of power source filter

 

The output voltage of rectification circuit is not dinkum dc, observe the output of rectification circuit from oscillograph, differ with dc very big, larger pulsatile part is contained in weaveform, call ripple. To obtain idealer volts d.c. , need benefit appliance has store can the reactance sex component of action (like capacitance, inductance) filter wave circuit of composition comes filter exports the pulsatile part in voltage except rectification circuit in order to obtain volts d.c. .

 

Commonly used filter wave circuit has passive filter wave and active filter wave two kinds big. The main form of passive filter wave has capacitance filter wave, inductance filter wave and wave of double entry filter (include L model, the) such as wave of LC filter wave, LCπ filter wave and RCπ filter. The main form of active filter wave is wave of active RC filter, also be called electronic filter. The bulk of the pulsatile composition in dc expresses with pulsatile coefficient, this value is greater, criterion the filter wave effect of filter is poorer.

 

Pulsatile coefficient (S)= outputs voltage communication heft base ／ outputs wave maximum the dc heft of voltage

 

Half commutate outputs the pulsatile coefficient of voltage to be S=1. 57, the outputs voltage pulsatile coefficient S≈O of complete wave commutate and bridge-type commutate. 67. After using circuit of C filter wave to Quan Bo and bridge-type commutate circuit, its are pulsatile coefficient S=1 ／ (4(RLC ／ T-1) . (T sheds the cycle of pulsatile voltage continuously for what rectification outputs. (T sheds the cycle of pulsatile voltage continuously for what rectification outputs..

 

Circuit of resistor filter wave

 

Circuit of RC-π filter wave, add composition of circuit of wave of filter of one class RC again on the foundation of capacitance filter wave essentially. If pursue circuit of 1(B)RC filter wave. If use S to express C1 the pulsatile coefficient of two upright voltage, output voltage the pulsatile coefficient S=(1/ωC2R)S of two end.

 

By the analysis knowable, the action of resistor R is will leftover ripple voltage drop fall in resistor two end, finally by C2 again bypass is dropped. In ω value particular case falls, r heals big, c2 heals big, pulsatile coefficient heals small, namely filter wave effect is better. And when R value increases, the dc on resistor is pressed fall can increase, increased the in-house loss of direct current source so; If increase the capacitance of C2, can increase the bulk of capacitor and weight again, implementation rises not actual also. This kind of circuit is used at load current to compare small situation commonly.

 

Circuit of inductance filter wave

 

Be opposite according to reactance sex component hand in, what shed impedance continuously is different, if the basic form of the filter wave circuit that comprises by capacitance C and inductance L place pursues 1 is shown. Because capacitor C is right,spread open a way continuously, small to communicating impedance, so C paralell connection is in laden two end. Inductor L is small to dc impedance, big to communicating impedance, because this L should establish ties with load.

 

(A) capacitance filter wave (B) wave of filter of C-R-C or RC-π resistor is pulsatile coefficient S=(1/ωC2R”)S”

 

(C) wave of L-C inductance filter (D) π filter wave or call C-L-C filter wave

Graph the basic form of circuit of wave of 1 passive filter

 

When shunt-wound capacitor C is inputting voltage to lift, charge to capacitor, can store partial energy in capacitor. And when should inputting voltage to reduce, capacitance two upright voltage discharge with exponential rule, can release the energy of memory. Pass load of direction of filter wave report to discharge, the output tension that gets on load is more flowing, had smooth wave effect. If use inductance filter wave, when inputting voltage heighten, with the inductance L of laden series connection medium electric current increases, accordingly inductance L will store energy of partial magnetic field, when electric current is reduced, release energy again, make load current becomes flowing, accordingly, inductance L also has smooth wave effect.

 

Use Chu Nengyuan the electric current of inductor L cannot choppy characteristic, establish ties in the laden loop of rectification circuit an inductance, make weaveform of output electric current relatively flowing. Because inductance is right the impedance that flows continuously is small, the impedance of communication is big, because this can get better filter wave result and dc loss is small. Defect of inductance filter wave is bulk big, cost is high.

 

Circuit of wave of filter of bridge-type commutate inductance is shown 2 times like the graph. If the graph is shown 2 times,the weaveform of inductance filter wave pursues. According to the characteristic of inductance, when outputting voltaic happening to change, induction of L lieutenant general gives to oppose electromotive force, the electric horn that makes rectification is in charge of increases, its direction will prevent voltaic happening change.

 

Graph circuit of wave of 2 inductance filter

 

In bridge-type commutate circuit, when U2 half Zhou Shi, d1, D3 is electric, the electric current in inductance is less than lag U2 90° . Begin to drop after U2 exceeds 90° , on inductance turn over electromotive force to conduce to D1, D3 continueing electric. Be in when U2 when losing half weeks, d2, D4 is electric, voltage of transformer deputy edge adds two end of D1, D3 entirely, cause D1, D3 slants instead and end, right now, the electric current in inductance offers by D2, D4. Of the electric current in the symmetry as a result of bridge-type circuit and inductance successional, 4 diode D1, D3; The electric horn θ of D2, D4 is 180° , this and circuit of capacitance filter wave are different.

 

Graph weaveform of circuit of wave of 3 inductance filter pursues

 

Foregone bridge-type rectifies those who shed circuit diode to guide current flow angle is 180° , rectification output voltage is half half sine wave, its average is about. Circuit of inductance filter wave, of diode guide current flow angle also is 180° , when the resistor of oversight inductor L, the voltage average that exports on load also is. If consider the dc block R of filter wave inductance, criterion the voltage average that circuit of inductance filter wave exports is

 

 

The electric current that should notice circuit of inductance filter wave must want enough big, namely RL cannot too big, should satisfy WL>>RL, right now IO (AV) usable computation leaving type

 

 

Because the dc resistance of inductance is little, communication impedance is very big, because this is straight the loss after shedding heft to pass inductance is very small, but to communicating heft, mix in WL on after cent is pressed, very big heft of one part communication descends on inductance, reduced the pulsatile ingredient in outputting voltage consequently. Inductance L heals big, RL heals small, criterion filter wave effect has healed, so inductance filter wave applies to load current bigger and the circumstance with bigger change. After using inductance filter wave, lengthened the electric part that rectification provides, had avoided big impulse current thereby.

 

Detailed of principle of capacitance filter wave is solved

 

1. The circumstance when carrying for nothing

 

Use capacitance filter wave when circuit, output end carries for nothing, if graph 4(a) place is shown, set initiative when capacitance voltage UC is 0. After receiving power source, be in when U2 half Zhou Shi, charge to capacitor C through D1, D3; Should be in of U2 when losing half weeks, charge to capacitor C through D2, D4, charge time constant is

 

(A) circuit pursues (B) undee graph

Graph circuit of wave of filter of capacitance of commutate of the bridge-type when 4 sky carry

 

Of the dc resistance that winding of transformer deputy edge includes in type and diode guide electrify block. As a result of general very small, capacitor charges the maximum of alternating voltage U2 very quickly, if weaveform pursues 2 (B) always. After this, u2 begins to drop, because circuit outputs end to did not receive load, capacitor does not have discharge loop, UC of capacitance voltage value is so changeless, right now, UC ＞ U2, diode two end bear retrorse voltage, be in off-state, the output voltage of circuit, circuit output maintains constant value. Actually circuit always should take certain load, the case that has load is as follows.

 

2. Take the case when carrying

 

Graph 5 gave out the working circumstance after circuit of capacitance filter wave is taking resistor load. Put through after alternating current source, diode guides, rectification power source charges to capacitance at the same time and provide electricity to load, the weaveform that outputs voltage is sine form. In hour, when reaching U290° peak value namely, u2 begins to drop with sine rule, whether does diode close right now, depend on diode susceptive is be being returned to voltage is retrorse voltage.

 

Set first after achieving 90° , diode closes, so only filter wave capacitance discharges to load with exponential rule, keep constant load current thereby. But the rate that rule of the index after 90° drops is rapid, and the rate that sine wave drops is minor, there is diode of period of time to still bear after exceeding 90° so to voltage, diode guides. As the fall of U2, the fall of sine wave rate is rapidder and rapidder, the fall of UC rate is slower and slower.

 

So the some after exceeding 90° is nodded, pursue for example the T2 hour in 5(b) , diode begins to bear retrorse voltage, diode closes. After this has the form that capacitor C discharges with exponential rule to load to provide electricity only, till half weeks sine wave comes to the next, u2 exceeds UC again, be like the T3 hour in graph 5(b) , diode is heavy electric.
 

 

The discharge time constant of above process capacitor is

 

Capacitance filter wave is ordinary load current is lesser, can satisfy the requirement with bigger Td, the discharge that outputs voltage weaveform so paragraph gentlier, ripple is lesser, output is pulsatile coefficient S is small, output is average voltage UO(AV) is big, have better filter wave character.

 

(A) circuit pursues (B) undee graph

Graph circuit of wave of filter of commutate of the bridge-type when 5 belts carry

 

Circuit of above filter wave has a general character, that needs very great capacitance capacity gift to satisfy a requirement namely, come so high capacity capacitance has very big short-circuit current very much in the instant that append report, this electric current is right rectification diode, transformer impact is very big, so common now practice is thermal resistor of the power of one NTC is added to maintain a balance before rectification, because NTC thermal resistor is below normal temperature resistance is very great, after appending report, lift as temperature, resistor block value is reduced quickly, this circuit makes soft starting circuit. Defect of this kind of circuit is: After cutting off the power, inside hot time constant, NTC thermal resistor did not restore 0 power resistance, the open that shoulds not be frequent so.

 

Why is the voltage when is filter wave capacitance taking load added to lift after rectification? Because increase the tension that filter wave measures,this is it is the peak value voltage that contains pulsatile part, adding load hind is average, computation: Theory of =1.414× of peak value voltage outputs voltage

 

Active filter wave – wave of electronic circuit filter

 

Itself of resistor filter wave has a lot of contradiction, cost of inductance filter wave tall, common line often uses reason circuit of active filter wave, if circuit pursues 6. The ejaculation that it is circuit of wave of filter of the π that comprises by C1, R, C2 RC and composition of T of transistor of active parts of an apparatus extremely output implement the circuit that join and becomes. By the graph 6 knowable, the voltaic IR=IE ／ that has shed R (1+β)=IRL ／ (1+β) . The electric current that has shed resistor R is the 1/(1+β) of load current only. Can use bigger R so, cooperate better with obtaining filter wave effect with C2, in order to make the pulsatile composition of the voltage of two end of C2 is reduced, the voltage that outputs two end of voltage and C2 is basic and equal, because this exports the pulsatile part of voltage,also got cutting down.

 

From RL laden resistor two end look, amount to of the R of filter wave component of base loop, C2 arrives shoot extremely loop, was equivalent to R reducing (1+β) times, and C2 increased (1+β) times. The capacitance C2 that needs so is the 1 ／ β that general RCπ filter needs electric capacity only, for instance the dc of transistor puts big coefficient β=50, if what need capacitance capacity to be 1000μF with general RCπ filter, if use electronic filter, so capacitance needs 20μF to satisfy a requirement only. Use this circuit to be able to choose greater resistance achieves similar filter wave result with lesser capacitance, in the power source that is used at equipment of a few small-sized electrons widely accordingly.