In design of switch power source, we often use the RC circuit that to series connection of a resistor a capacitance makes, RC circuit function can affect product function and stability directly. Article general can reduce switch one kind to run loss already for everybody introduction, and the RC circuit of the leakage move that can drop transformer and aiguille voltage.

Power source of high frequency switch closes in switch canal when, the loss that the jackknife of voltage and electric current causes is the body of loss of switch power source, in the meantime, because parasitism inductance and parasitism capacitance exist in circuit, when power switch canal closes, overvoltage also can appear in circuit and generation oscillation. If aiguille voltage is exorbitant, can damage switch is in charge of. In the meantime, the existence of oscillation also can make output ripple increase. Close loss and aiguille voltage to reduce, need is in charge of circuit of amortize of RC of two upright paralell connection in switch in order to improve the performance of circuit.

Graph 1

What the graph shows 1 times is turn over circuit of power source of the switch that stimulate type simply, can see from inside the graph RC circuit is in those who pursue medium to had appeared 6 times from RaCa—RfCf, the position of every RC circuit is different, motive is different also. Of article introduction is a graph 1 in RbCb, the RC that RcCc forms draws circuit. Circuit of these two RC is in the graph main effect is:

Decrease guide connect or involve loss;

Reduce voltage or voltaic aiguille;

Can improve EMI character secondhand.

When design RC draws circuit, we must know a few of network of whole power source important parameter, input electric current of voltage of voltage, input electric current, aiguille, aiguille to wait for instance. In the graph 1 is shown when Q1 closes, source pole voltage begins to rise to 2Vdc, and capacitance Cb restricted source pole (the ascendant speed of D) voltage, reduced at the same time ascendant voltage and drop voltaic jackknife, thereby bring down the loss of switch canal Q1. And be in next time before switch closes, cb must will already full voltage is put, discharge method is Cb, Rb, Q1.

Graph 2 switch are in charge of source pole (D) Vds voltage is undee

Graph 2-A is denotive it is the Vds voltage weaveform that switch is in charge of Q1 to did not add RC to draw circuit, see apparently in the graph, when switch is in charge of Q1 to disconnect, vds voltage rises quickly to apogee, accompany this concussion to drop after that, concussion frequency is 20MHZ.

Graph 2-B is denotive it is the Vds voltage weaveform that RC added to draw circuit on switch canal, opposite with graph 2-A, after adding RC to draw circuit, switch canal disconnects the instant, vds voltage rises gentlier, and high frequency concussion won’t arise when rising to highest voltage to decline, EMI character also is met preference.

In perceptual load, the instant that switch parts of an apparatus closes, if right now the magnetic flux of perceptual load is 0, according to be stupefied second law can produce electromotive force of one each feeling, put magnetic field to outside demit store can, for simple for the purpose of, use RC to draw return circuit commonly, waste this part energy with the means of heat energy.

Design RC absorbs loop parameter, need defines magnetic field first store can size, in stimulating transformer instead, magnetic field store can put by two percentages decline, among them major it is to pass mutual inductance to provide energy to 2 side, only leakage move part wants to be handled through RC loop, need measures excitation inductance, mutual inductance and leakage feeling are worth, get the value of initiative electric current of RC loop again.

The extraction of R is worth, with switch can susceptive instantaneous is pressed instead, than value of initiative electric current; This value is too little dynamic power comsumption is too big, bring the value is too great short of to protect the action of switch;

The extraction of C is worth, need to be satisfied in clamp can store below n magnetism can half, and contented and certain DV/dt, c closes amortize, r opens current limliting, the block value of resistor is basic and OK according to:

This formula calculates R=(sqrt(Llk/Cj))/n, power chooses according to actual condition, c chooses between 102——103 commonly, what still need to consider EMI while the consideration absorbs the effect is phasic when choosing C and from the back the stress of ripple electric current that outputs capacitance, have:

C=(Ip*Tf)/(2*2*Vdc)

Ip: ? Lu archives Si?

Tf: ? Does Man of Si of  Ke  mirror neon of astounded of agitate of archives of land of Jia Gu Di to run quickly?

Vdc: ? Cong Yun is troubled by stretch tight Jian Miao?

R=Ton(min)/(3C)

Ton(min): ? Plutonium of Long of  to bring up > does male discipline ㄊ run quickly?

Summary

 

What give out according to above is formulary, can very choose to give appropriate RC to draw circuit conveniently. But when be being designed, the function index that should design according to whole power source, get truly appropriate parameter through debugging ability actually. Occasionally, to reach function target of the system, the efficiency with particular sacrifice also is necessary. Anyhow, when design RC absorbs circuit parameter, must consider function and efficiency integratedly, choose appropriate RC parameter finally.