A Comprehensive Analysis of Resistance Capacitance Step-down Circuit and Its Application

A basic concept of resistance capacitance voltage reduction1. What is resistance capacitance depressurization?Resistance capacitance step-down is a circuit that limits the maximum working current by using the capacitive reactance generated by the capacitor under the AC signal of a certain frequency.

The capacitor actually plays a role in limiting the current and dynamically distributing the voltage between the capacitor and the load.2. Which parts does the resistance capacitance step-down circuit consist of?The resistance capacitance step-down circuit is composed of step-down module, rectifier module, voltage stabilizing module and filter module.

3. Basic design elements of resistance capacitance voltage reductionDuring circuit design, the maximum working current of the load shall be determined first, and the capacitance value shall be calculated through this current value, so as to select the appropriate capacitance.The difference between this power supply and linear transformer power supply: the resistance capacitance step-down power supply selects the capacitance through the load current; Linear transformer power supply is selected by load voltage and power.

Calculation of resistance capacitance step-down currentThe resistance capacitance step-down circuit can be equivalent to a step-down capacitor C1 and a load resistor R1, which are connected in series to divide the voltage.The capacitive reactance of capacitor C1 is ZC = - J / WC = - J / 2 FC

The impedance of resistor R1 is Zr = RThe total equivalent impedance is Z = ZC Zr = - J / 2 FC RSo I = u / z = u / (ZC Zr) = u / (- J / 2 FC R)

Because the resistance capacitance step-down power supply is only applicable to small current circuits, the selected capacitance value range is generally 0.33uf to 2.5uf, so ZC is - 1592j to - 9651j. The equivalent load impedance Zr is about 200 , which obviously has | ZC | > > | Zr |. At the same time, the voltage drop of the input power supply voltage on the load is far less than the voltage drop of the capacitor, so there are: Z ZC, vector diagram The angle is close to 90 .Thus:I=U/Z=U/Zc=U/(-j/2fC)

=220*2*f*C*j=220*2*50*C*j=j69000C

I = | I | 90 , current effective value I1 = | I | = 69000c. When half wave rectification is adopted, I1 = 0.5|i| = 34500c.Design exampleKnown conditions: load working current 15mA, Working Voltage 5V. Find the capacitance of step-down capacitance?

Half wave rectification mode is adopted. According to the calculation formula I1 = 0.5|i| = 34500c, C = 0.43uf. Therefore, a capacitor of 0.47uf is selected here. In turn, it can be verified that the supplied current I1 = 34500c = 16.2ma, and the excess current flows through the voltage stabilizing tube.Advantages of resistance capacitance voltage reduction:Small volume; Low cost.

Disadvantages of resistance capacitance voltage reduction:Non isolated power supply, unsafe;It cannot be used for high-power load;

Not suitable for capacitive and inductive loads;Not suitable for dynamic loads.2 Basic principle of resistance capacitance voltage reduction

1. Principle of capacitor charging and dischargingA capacitor is a passive device that stores energy in the form of an electric field. The essence of capacitor charging and discharging process is the process of two conductive parallel plates acquiring and releasing electrons.Capacitor charging:

When the electric field intensity E in the capacitor is less than the external power supply voltage u at both ends of the capacitor, the capacitor begins to charge. At this time, the positive electrode of the capacitor keeps losing electrons, the negative electrode keeps getting electrons, and the internal electric field E keeps increasing until it is equal to the external voltage u, and the charging ends.Capacitor discharge:When the electric field intensity E in the capacitor is greater than the external power supply voltage u at both ends of the capacitor, the capacitor begins to discharge. At this time, the positive electrode of the capacitor keeps getting electrons, the negative electrode keeps losing electrons, and the internal electric field E keeps weakening until it is equal to the external voltage u, and the discharge ends.

DC charging and discharging process of capacitorAs shown in the charging process above, calculate the time when C1 voltage reaches 1V:Because V0 = 0V, VT = 1V, V1 = 5V, r = 10K, C = 0.1uF, t = 10000 * 0.1 * 0.000001 * ln (5 / 4) = 223us

AC charging and discharging process of capacitorThe DC charging and discharging of capacitor is completed at one time, while the AC charging and discharging is a repeated process.Full wave rectifier circuit

Half wave rectifier circuitFunction and selection of componentsF1: fuse, with overcurrent protection function, 400ma250v model is selected.

Rv1: varistor, surge protection, generally 10d471k model.C1: step down capacitor, which uses large capacitive reactance to limit the total current of the circuit. Polyester capacitor (CL21), polypropylene capacitor (CBB21) and safety regulation capacitor (x2) are commonly used , the capacitance value depends on the load demand. The larger the capacitance, the more unsafe the circuit is. When designing this circuit, if the capacity exceeds 2.5uf under 220VAC power supply and 4uf under 110VAC power supply, it should give up resistance capacitance step-down and consider other circuits. Here, 0.56uf safety capacitor (x2) is selected to provide 19ma current.

R2: discharge resistance, which provides a discharge circuit for capacitor C1 after power failure, so as to prevent the superposition of residual voltage on C1 capacitor and grid voltage from forming a high-voltage impact on subsequent devices when the power plug is plugged in and out quickly or the plug is in poor contact, and prevent personal injury caused by contact with human body after unplugging the power plug. Generally, the time for C1 voltage attenuation to 37% after power failure shall be less than 1s, because t = RC * LN[ (v0-v1) / (vt-v1)], so t = RC, r = t / C, R

R1: current limiting resistance, which is mainly used to prevent the rectifier diode from being damaged by the high-voltage impact generated during the first power on and when the power plug is quickly plugged in or out or the plug is in poor contact. If the capacitor C2 just touches the wave crest during the first power on, it will be in a short-circuit state at the moment of power on (first-order zero state response) At this time, the AC power supply is directly applied to R1 and rectifier tube, and there is 220VAC * 1.414 = 311vdc instantaneous DC voltage on R1. If C1 charge is not discharged during power on, this voltage may be higher. Therefore, R1 should select a resistance with strong current impulse resistance and high voltage resistance. R1 resistance should not be too small or too large. If the resistance is too small, the impulse current is large, and the resistance is too large, the power consumption of the whole circuit will increase. Rectifier diode The peak current of 1n400x series is generally large. For example, the peak current of 1n400x series is 50a, so the R1 resistance is generally between 10-50 .

DZ1: zener diode, 1n4733 is selected, and the regulated voltage VZ is 5.1V. The maximum regulated current iz of DZ1 must be greater than the maximum charge and discharge current of capacitor C1.R5: RC filtering with capacitors E1 and C2 to reduce ripple.D1: rectifier diode, used for half wave rectification, 1N4007.

D2: rectifier diode, half wave rectifier, 1N4007.E1: electrolytic capacitor, which filters the voltage after voltage stabilization and provides electric energy for the load in the half cycle of power off. Before the second half cycle of power supply comes, E1 must ensure that the voltage provided for the load cannot be attenuated too much. Here, 1000uf25v model is selected. T = RC * ln [(v0-v1) / (vt-v1)] = 10ms, so the attenuated voltage Vt = 4.8V.C2: chip capacitor, filtering function, 0.1uF.

R6: discharge resistance, which provides a discharge circuit for E1 after power failure, generally 5 10K.R7: equivalent load.Pictures of main components

Primary fuseSelf recovery fuseVaristor

Metallized polyester film capacitor (CL21)Metallized polypropylene capacitor (CBB21)X2 safety capacitor (CBB62 / MKP)

3 Application of resistance capacitance voltage reductionBecause of its small size and low cost, resistance capacitance voltage reduction is suitable for low-power and low current loads. Common applications include electric energy meter, low-power LED lamp drive, small household appliances and temperature controller.LED lamp drive

Application of small household appliancesFan controllerElectric heating controller

Coffee machine