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Resistor-capacitor step-down circuit and application design

November 30, 2020

First, the basic concept of resistance and pressure reduction

1, what is resistance and pressure?

Resistor-to-capacitor step-down is a circuit that limits the maximum operating current by using capacitive reactance generated by a capacitor at a certain frequency of AC signal.

The capacitor actually plays a role in limiting the current and dynamically distributing the voltage across the capacitor and load.

2. What parts of the RC capacitor circuit consist of?

Resistor-capacity step-down circuit consists of buck module, rectifier module, voltage regulator module and filter module.

3, resistance and capacity buck basic design elements

When designing a circuit, it is necessary to determine the maximum operating current of the load and calculate the capacitance value of the capacitor through this current value so as to select an appropriate capacitor.

Here is the difference with the linear transformer power supply: The resistive-capacitive step-down power supply is selected by the load current; the linear transformer power supply is selected by the load voltage and Power Transformer.

Resistor-capacitor buck current calculation

Resistor-capacitor step-down circuit can be equivalently composed of a step-down capacitor C1 and a load resistor R1, and the resistor and the capacitor are connected in series.

The capacitive reactance of capacitor C1 is Zc=-j/wC=-j/2πfC

Resistance of resistor R1 is Zr=R

The total equivalent impedance is Z=Zc+Zr=-j/2πfC+R

So I=U/Z=U/(Zc+Zr)=U/(-j/2πfC+R)

Because the resistive-capacity step-down power supply is only suitable for small-current circuits, the capacitance range of the selected capacitor is generally from 0.33 UF to 2.5 UF, so Zc is -1592 j to -9651 j. The equivalent load impedance Zr is about 200Ω, obviously there is |Zc|>>|Zr|, and the voltage drop of the input power supply voltage on the load is also much smaller than the voltage drop of the capacitor, so there are: Z≈Zc, vector The angle θ is close to 90°.

Therefore:

I=U/Z=U/Zc=U/(-j/2πfC)

=220*2π*f*C*j

=220*2π*50*C*j

=j69000C

I=|I|∠90°, the current effective value I1=|I|=69000C. When the rectification mode uses half-wave rectification, I1=0.5|I|=34500C.

Design example

Known conditions: load operating current 15mA, operating voltage 5V. Seeking buck capacitance?

Using half-wave rectification, according to the formula I1=0.5|I|=34500C, C=0.43uF. Therefore, the capacitor of 0.47uF is used here, which in turn can verify that the supplied current I1=34500C=16.2mA, and the excess current flows through the regulator.

Resistance and pressure reduction advantages:

Small size; low cost.

Disadvantages of resistance and pressure reduction:

Non-isolated power supply, insecure;

Cannot be used for high power loads;

Not suitable for capacitive and inductive loads;

Not suitable for dynamic loads.

Second, the basic principle of resistance and pressure reduction

1, the principle of capacitor charging and discharging

A capacitor is a passive device that stores energy in the form of an electric field. The nature of the charge-discharge process of the capacitor is the process of acquiring and releasing electrons from two conductive parallel plates.

Capacitor charging:

When the electric field strength E in the capacitor is less than the external power supply voltage U across the capacitor, the capacitor starts to charge. At this time, the positive electrode of the capacitor continuously loses electrons, and the negative electrode continuously gains electrons. The internal electric field E is continuously enhanced until it is equal to the external voltage U, and charging ends.

Capacitance discharge:

When the electric field strength E in the capacitor is greater than the external power supply voltage U across the capacitor, the capacitor begins to discharge. At this time, the positive electrode of the capacitor continuously gains electrons, the negative electrode continuously loses electrons, and the internal electric field E continuously weakens until it is equal to the external voltage U, and the discharge ends.

DC charge discharge process of capacitor

As shown in the above charging process, seek C1 voltage rushed to 1V time:

Since V0=0V, Vt=1V, V1=5V, R=10K, C=0.1uF, T=10000*0.1*0.000001*Ln(5/4)=223uS

Capacitor AC charge discharge process

DC charging and discharging of the capacitor is completed at once, and AC charging and discharging is a process that repeats itself.

Full-wave rectifier circuit

Half-wave rectifier circuit

The role of various components and options

F1: Fuse, for over-current protection, 400mA250V type.

RV1: varistor, from surge protection, generally use 10D471K model.

C1: Step-down capacitor, using a larger capacitive reactance to limit the total current in the circuit. Commonly used polyester capacitor (CL21), polypropylene capacitor (CBB21), safety capacitor (X2), the capacitance value depends on the load requirements, the greater the capacity of this capacitor, the more unsafe the circuit, in the design of this circuit, if the 220VAC power supply Under the capacity of more than 2.5uF, the capacity exceeds 4uF in the case of 110VAC power supply, because other circuits should be considered in order to give up resistance and voltage. The 0.56uF safety capacitor (X2) is used here to provide 19mA.

R2: Discharge resistance, providing a discharge circuit for the capacitor C1 after power off, to prevent the residual voltage on the C1 capacitor and the grid voltage from superimposing on the subsequent device to form a high-voltage shock and preventing the plug from being pulled out when the quick plugging of the power plug or the plug is bad. Contact with the human body causes injury to personnel. Generally, the time for the C1 voltage to decay to 37% after power off should be less than 1 second because T=RC*Ln[(V0-V1)/(Vt-V1)], so T=RC, R=t/C,R <1/C. Here, three 390K 0805 chip resistors (shared voltage and power) are used.

R1: Current-limiting resistor. This resistor is mainly used to prevent damage to the rectifier diode due to the high-voltage surge generated when the power is first applied and when the power plug or plug is rapidly connected or unplugged. When capacitor C2 is powered on for the first time, if it just hits the peak, C2 is short-circuited (first-order zero-state response) at the moment of power-on. At this time, AC power is directly applied to R1 and the rectifier. There is 220VAC*1.414=311VDC on R1. Instantaneous DC voltage, this voltage may be higher if C1 charge is not discharged at power up. Therefore, R1 should select a resistor with strong current-impact resistance and high-voltage resistance. The R1 resistor should not be too small or too large. The resistance is too small and the inrush current is large. The resistance is too large and the power consumption of the entire circuit increases. The peak current of the rectifier diode is generally relatively large, such as 1A400X series peak current is 50A, so generally take R1 resistance between 10-50Ω.

DZ1: Zener diode, select 1N4733, voltage regulator Vz is 5.1V. The maximum regulated current Iz of the DZ1 must be greater than the maximum charge and discharge current of the capacitor C1.

R5: RC filter with capacitors E1 and C2 reduces ripple.

D1: rectifier diode, half-wave rectification, use 1N4007.

D2: Rectifier diode, half-wave rectification, 1N4007.

E1: Electrolytic capacitor, which filters the regulated voltage, and provides power to the load during the half cycle of the power off. Before the next half cycle of power supply, E1 must ensure that the voltage provided for the load can not be attenuated too much, 1000uF25V model is used here. Since T=RC*Ln[(V0-V1)/(Vt-V1)]=10mS, the attenuated voltage Vt=4.8V.

C2: SMD capacitor, filtering effect, choose 0.1uF.

R6: Discharge resistance, providing a discharge circuit for E1 after power off, generally 5~10K.

R7: Equivalent load.

The main components of the picture

One time blow fuse

Resettable fuse

Varistors

Metallized Polyester Film Capacitors (CL21)

Metallized Polypropylene Capacitors (CBB21)

X2 safety capacitor (CBB62/MKP)

Third, resistance and pressure reduction applications

Resistor-capacitor buck is suitable for low-power and small-current loads due to its small size and low cost. Common applications include energy meters, low-power LED light drivers, small appliances, and thermostats.

LED light driver

Small appliances application

Fan controller

Electric heater controller

Coffee machine

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