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May 27 2019

Principle and Application of DC Regulated Power Supply

I Principle of DC Regulated Power Supply

1.1 Block Diagram and Working Principle

The typical circuit block diagram of transistor series DC regulated power supply is shown in figure 1. It consists of rectifier filter circuit, series voltage regulated circuit, auxiliary power supply and protection circuit.

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Figure 1. Block Diagram of DC Voltage Regulated Power Supply Circuit

The rectifier filter circuit includes a power transformer, a rectifier circuit and a filter circuit. The DC power supply commonly used in semiconductor circuits has rated voltage values such as 6V, 12V, 18V, 24V, 30V, etc., while voltage of the grid is generally AC 220V. In order to convert the AC voltage of the power grid into the required DC voltage, first of all, the voltage must be reduced through the power transformer. Then change the alternating current into pulsating DC current through the rectifier circuit. Because the rectified voltage still has a large AC component, it must be filtered by the filter circuit, so as to get a relatively smooth DC voltage.

Although the fluctuation of the DC voltage obtained after the filter circuit is small, the value of the voltage is still unstable. The main reasons are as follows: first, the voltage of the AC power grid generally fluctuates by about ±10%. As a result, the DC voltage of the rectifier filter output will also fluctuate by about ±10%. Second, the rectifier filter circuit has internal resistance, and the voltage drop on the internal resistance will also change when the load current changes. So, the output DC voltage will change as well. Third, in the rectifier voltage regulated circuit, because the characteristics of the semiconductor devices change with the ambient temperature, the output voltage is unstable.

The voltage regulated circuit can keep the output DC voltage stable so that it does not change with the change of grid voltage, load or temperature. The series voltage regulated circuit is composed of adjustment link, comparative amplification circuit, sampling circuit, reference voltage and so on. The adjusting tube in the adjusting link is connected in series between the filter circuit and the load, so it is called the series voltage regulated circuit. The adjusting tube is equivalent to a variable resistance, if the output voltage increases, the resistance value increases accordingly, causing the output voltage to drop back; on the contrary, if the output voltage decreases, the resistance value decreases accordingly, so that the output voltage increases. In this way, by adjusting the output voltage and keeping it unchanged, the purpose of voltage stabilization can be achieved.

The sampling circuit uses the method of resistance partial voltage and the change of the output voltage is sampled according to a certain proportion, which is the sampling signal. The reference voltage is a stable and standard reference voltage. The sampling signal and the reference voltage are simultaneously added to the comparative amplification circuit for comparison, and then the difference between the two is amplified, and the amplified voltage is used to control the base injection current of the adjusting tube, so as to change the DC internal resistance of the adjusting tube and make the output voltage remain stable. In order to improve the performance of the voltage regulator, the two-stage differential amplifier is often used in the comparative amplifier circuit, which has large magnification and strong control ability. Secondly, the amplifier circuit also requires small zero drift and good temperature stability.

The rectifier filter circuit mentioned above is combined with the series voltage regulated circuit, which is also known as the main power supply. The voltage stabilization principle is as follows: if the output voltage increases due to the change of the power grid voltage or load, the sampling voltage generated by the sampling circuit also increases, and the sampling voltage is greater than the reference voltage. After the difference is magnified by the comparative amplification circuit, the emission junction voltage of the adjusting tube is reduced through the adjustment link, the base current is reduced, and the DC internal resistance of the adjusting tube is increased, the voltage drop of the tube is increased, thus the output voltage is reduced. The stability of the output voltage is maintained. Similarly, when the output voltage is reduced, through a similar process, the DC internal resistance of the adjusting tube is reduced, the voltage drop of the tube is reduced, and the output voltage will rise, so that the output voltage will remain basically unchanged.

In addition to the main power supply, DC regulated power supply generally has two groups of auxiliary power supply. The first auxiliary power supply is composed of rectifier and voltage regulator, and its output voltage is also quite stable; the second auxiliary power supply is similar to the main power supply circuit, and is also composed of rectifier filter circuit and series voltage regulated circuit, and its output voltage is very stable. The output voltage of the first auxiliary power supply is used as the power supply voltage of the protection circuit on the one hand. On the other hand, it is in forward series with the output voltage of the main power supply and the output voltage of the second auxiliary power supply. As the power supply of the last stage differential amplifier of the main power supply comparison amplifier circuit, it provides a voltage regulated power supply with higher voltage for the comparative amplifier circuit, so that the gain is larger. Therefore, the adjustment sensitivity of the series voltage regulated circuit of the main power supply is improved, and the stability of the output voltage is further improved. On the one hand, the output voltage of the second auxiliary power supply is used as the power supply of the differential amplifier of the main power supply comparison amplifier circuit. On the other hand, the stable voltage is output by the voltage division circuit and is used as the reference voltage of the main power supply comparison amplifier circuit.

In the series voltage regulated circuit, when it is overloaded, especially when the output is short-circuited, the input DC voltage falls almost entirely at both ends of the adjusting tube. Even if the overload phenomenon lasts for very short time, it will also cause the adjusting tube and rectifier diode to burn out immediately. Therefore, the overcurrent automatic protection circuit with fast action must be adopted. When overloaded or short-circuited, the adjusting tube is cut off by the protection circuit. At this time, the output voltage and current are basically reduced to zero, playing a protective role. This kind of protection circuit is called cut-off protection circuit.

 

1.2 Series Voltage Regulated Circuit

Figure 2 shows a series transistor voltage regulated circuit with an amplification link.

The input voltage Vi is supplied by the rectifier filter circuit. The resistance R1 and R2 form the voltage divider, which takes out part of the variation of the output voltage and adds it to the input of the amplifier composed of T1, so it is called sampling circuit. The resistor R3 and the voltage regulator Dz form a voltage stabilizing circuit to provide the reference voltage so that the emitter potential of T1 is fixed. Transistor T1 forms an amplifier to compare and amplify signals. R4 is the collector resistance of T1, and the signal output from T1 collector is directly added to the base of adjusting tube T2.

图片2.png

Figure 2. Series Voltage Regulated Circuit

If the output voltage Vo decreases due to the decrease of the grid voltage or the increase of the load current, the base potential VB1 of T1 decreases through the partial voltage of R1 and R2, and the emitter potential VE1 of T1 is stabilized by the voltage regulator Dz and remains basically unchanged, the results show that the forward voltage of T1 junction is reduced, the IC1 of T1 is decreased and the VC1 of T1 is increased. The increase of VC1 increases the IB2 and IC2 of T2, decreases the VCE2, and finally increases the output voltage Vo to close to the original value. The above voltage stabilization process can be expressed as:

 

 图片2.1.png


Similarly, when the Vo increases, the Vo is basically unchanged by the process of stabilizing the voltage. 

The comparison amplifier can be a single-tube amplifier circuit, but in order to improve its gain and output voltage temperature stability, multi-stage differential amplifier circuit and integrated operational amplifier can also be used. The adjusting tube is usually a power tube, in order to increase the value of β and make the small current of the comparison amplifier drive the power tube, or it can be a composite tube composed of two to three transistors. If the power of the adjusting tube does not meet the requirements, it can also be used in parallel with a number of adjusting tubes to increase the branch in order to expand the output current.

Because of the different useages, the connection method of the sampling circuit is also different: for the voltage regulated source, the sampling resistance is in parallel with the load, while for the current regulated source, the sampling resistance is in series with the load.

Some electronic devices require two-way power supply voltages of equal size and opposite polarity. Such a power supply voltage can be obtained by a symmetrical two-way voltage regulated circuit.

 

1.3 Auxiliary Power Supply Circuit

1.3.1 First Auxiliary Power Supply Circuit 

In the circuit shown in figure 2, the load R4 of the amplifier T1 is directly connected to the variable input voltage Vi, so the change of the input voltage is directly acted on the base of the adjusting tube T2 through R4, thereby changing the output voltage and affecting its stability. To overcome this disadvantage, a separate auxiliary power supply Vz2, can be used, as shown in figure 3. This power supply, also known as the first auxiliary power supply, is a voltage regulated circuit composed of R and Dz2. The voltage Vi1, is obtained by rectifying and filtering of the other secondary windings of the same transformer and the stable voltage Vz2, is obtained by the voltage regulated circuit. The voltage is connected in series with the Vo as the power supply for T1. Because Vz2 and Vo are quite stable, the influence of power supply voltage fluctuation on output voltage can be greatly reduced.

Because the addition of Vz2 and Vo acts as the power supply of the comparative amplifier, R4 can be selected larger than the original to improve the magnification so as to further enhance the control ability and improve the stability of the output voltage.

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Figure 3. First auxiliary power supply circuit


1.3.2 Second Auxiliary Power Supply Circuit 

In the circuit shown in figure 2, the output voltage Vo of the series voltage regulated circuit can be given by the following formula: 

图片3.1.png

It can be seen that the output voltage can be adjusted by changing the partial voltage ratio of the sampling circuit. The smaller R1 is, the smaller the output voltage Vo is. When R1 is up to 0, the output voltage is the lowest, and its value is Vomin=Vz+VBE1, that is, the minimum output voltage is still higher than the working voltage Vz of the voltage stabilizer. The output voltage can not be adjusted to zero, which is the disadvantage of this kind of circuit. In order to expand the adjustment range of the output voltage, the second auxiliary power supply can be added. As shown in figure 4, the voltage of the voltage stabilizer of this circuit is supplied by the Vi2 of another group of rectifier circuits. From the figure, it can be seen intuitively that if R1 is 0, then V = VBE1 ≈ 0. It can be seen that the second auxiliary power supply provides the possibility of adjusting the output voltage close to zero. As long as the partial voltage ratio of the sampling circuit is changed, the requirement of continuously adjustable output voltage in a large range can be realized.

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Figure 4. Second auxiliary power supply circuit

 

1.3.3 The Protection Circuit of Series Voltage Regulated Circuit 

The protection circuit of series transistor voltage regulated circuit can be divided into two types: current limiting circuit and cut-off circuit.


Current Limiting Protection Circuit

When the output current exceeds a certain value, the protection circuit begins to work, so that the adjusting tube is in an incomplete cut-off state, and the output current and output voltage decrease accordingly to achieve the purpose of protecting the power supply. The protection circuit is relatively simple, and when the output is overloaded or short circuit is eliminated, the voltage regulated circuit will automatically resumes operation.

图片5.png 

Figure 5. Current Limiting Protection Circuit

 

The part surrounded by the dashed line shown in figure 5 is the more common current limiting protection circuit. T3 is called a protective tube. The output voltage passes through R5 and R6 partial voltage, and the voltage on R6 provides reverse bias voltage to T3 base. R7 is the detection resistance, and its resistance value is small. The voltage drop of the output current on R7 provides a positive bias voltage to the T3 base.

Under normal circumstances, the reverse bias on R6 exceeds the forward bias on R7, so T3 is in the cut-off state, which has no effect on the operation of the voltage regulated circuit.

When the output current is too large due to overload, the forward voltage drop of R7 increases and T3 is in the conduction state, so that the voltage at both ends of T3 tube decreases, and the positive voltage of emission junction of adjusting tube T2 decreases, and the current of adjusting tube decreases. The output current and voltage are reduced, which can achieve the purpose of protecting the adjusting tube.

The necessary condition for this kind of protection circuit to maintain the conduction of T3 is that the output current produces positive bias voltage through R7, so the output current can only be reduced to a certain extent, but the adjusting tube can not be cut off. When the cause of the output overload is excluded, it can automatically return to the normal state. The advantage of this kind of circuit is simple and reliable, but the disadvantage is that the adjusting tube still consumes a lot of power when it is overloaded.


Cut-off Protection Circuit

The cut-off protection circuit is when the load is overloaded or short-circuited, the adjusting tube is cut off by the protection circuit, and the output voltage and current are basically reduced to zero, thus achieve the purpose of protecting the circuit. The cut-off protection circuit is a little more complicated. It can be divided into two situations: one is that the work can be resumed automatically, and the other is that when the fault is removed, the reset button should be pressed or the AC power supply must be cut off and restart in order to make the regulated power supply return to normal operation.

图片6.png

Figure 6. Cut-off Protection Circuit


The part surrounded by the dotted line shown in fig. 6 is a cut-off protection circuit. In the figure, the resistance R8, the voltage stabilizer Dz2 and the partial voltage resistance R4 and R5 provide the base voltage for the protective tube T3, and the output voltage Vo supplies the T3 emitter voltage through the resistance R6 and R7 partial voltage. The detection resistance R is connected between R7 and R5. The output current Io flows through it to produce a voltage drop. The polarity of the voltage on R5, R7 and R is shown in the figure. It can be seen that the emission junction voltage added to the protective tube T3 is

VBE3=(VR5+VR-VR7


When the voltage regulated circuit is working normally, the Io is in the rating and the VR=IoR is small, which makes VR5+ VR < VR7, then the VBE3 is negative, and the T3 emitter junction is cut off reliably because of the reverse bias. The protection circuit does not work and has no effect on the normal operation of the voltage regulated circuit.

When the output current Io exceeds the rated value, the voltage on R makes T3 turn on, and the collector voltage VC3 decreases, that is, the VB2 of adjusting tube T2 decreases, so that it tends to cut off. When the VCE2 increases, the output voltage Vo decreases. As a result, the voltage VR7 on R7 decreases. The T3 tube is further turned on, and the Vo is further reduced, forming a positive feedback process, so that the adjusting tube T2 is quickly cut off, and the output voltage and current are close to zero. At this time, T3 is turned on and T2 is cut off by the voltage VR5 on R5, which achieves the purpose of protection.


II Application of DC Regulated Power Supply

The use of DC regulated power supply is very simple. When it is in use, attention should be paid to the polarity of the required DC voltage. If a positive voltage needs to be output, the output end of the DC voltage regulated power supply "-"  connects with the "ground" end of the electrical equipment, and the terminal "+" should be connected to the required positive voltage terminal. If you need to output negative voltage, you need to reverse the above wiring method. Before electrifying, measure it with a multimeter to check whether the output voltage meets the requirements, so as to avoid excessive voltage damage to the electrical equipment.

In order to make the electrical equipment work properly and not to affect the stable and reliable operation of the electrical equipment because of the poor performance of the DC power supply, it is best to test it simply before acting on the regulated power supply. The main contents of the test are: the regulation range of the output voltage, the stability degree, the ripple voltage and the overcurrent protection and so on.


III Maintenance of DC Regulated Power Supply

3.1 Maintenance Procedures

3.1.1 Preliminary Surface Examination 

All kinds of voltage regulated power supplies are generally equipped with overload or short circuit protection fuses and input and output terminals. You should first check if the fuses are fused or loose, whether the terminals are loose or short-circuited to the ground, and whether the gauge needle of the voltage indicator is jammed. Then open the casing cover to see if the power transformer has scorched taste or mildew, and whether the resistance and capacitance have obvious damage phenomena, such as scorching, mold breaking, liquid leakage, cracking and so on.


3.1.2 Measurement of Rectifier Output Voltage 

In all kinds of regulated power supply, there is one or more groups of rectified output voltage. If these rectified output voltage has a group of abnormal, then the regulated power supply will have a variety of faults. Therefore, when overhauling, it is necessary to first measure whether the rectified output voltage is normal or not.


3.1.3 Test Electronic Device 

If the output of the rectifier voltage is normal and the output voltage stabilization is abnormal, it is necessary to further test whether the performance of the adjusting tube, amplifying tube, etc., is good, whether the capacitance has breakdown, a short circuit or an open circuit. If a device with damaged or variable value is found, normally the regulated power supply can be restored to normal after updating.


3.1.4 Check the Working Point of the Circuit 

If the rectifier voltage output and related electronic devices are normal, the operating point of the circuit should be further checked. For the transistor, there should be a certain working voltage between the collector and the emitter, and the bias voltage between the base and the emitter should meet the requirements and ensure that it works in the amplification area.


3.1.5 Analysis of Circuit Principle 

If the voltage of an operating point of a transistor is found to be abnormal, there are two possibilities: one is that the transistor is damaged; the other is caused by the damage of other components in the circuit. At this time, it is necessary to carefully analyze the causes of the problems according to the circuit schematic diagram, and further identify the damaged and variable components.

3.2 Common Examples of Troubleshooting of Voltage Regulated Power Supply

3.2.1 There is voltage stabilization but no voltage regulation 

When using a regulated power supply, it is usually to start preheating, then adjust the output voltage "rough adjustment" potentiometer, observe whether the voltage regulation function and adjustment range are normal, and finally adjust to the required power supply voltage value, and connect to the load. If the voltage is normal when there is no load, but after the load is connected, the output voltage decreases. If the possibility of external circuit fault is eliminated, the fault is that the voltage regulated power supply has no voltage regulated effect.

During maintenance, the on-off condition between the collector and the emitter of the high-power adjusting tube can be measured by a multimeter. If no problem is found, the rectifier diode can be further inspected for damage. As long as a rectifier tube is damaged, the full-wave rectifier becomes a half-wave rectifier. When there is no load, the large capacity filter capacitor can still provide enough rectifier output voltage to ensure the voltage regulation function of the stable voltage output. After connecting to the load, the rectified output voltage immediately decreases, and the voltage at the stable voltage output terminal also decreases and lose the function of stabilizing the voltage.


3.2.2 The output voltage is too high, and there is no function of voltage regulation and voltage stabilization 

Under the condition of no load, the output voltage of transistor DC regulated power supply is larger than the specified value, and there is no voltage regulation and voltage stabilization, the fault may occur because of the following reasons:

— The collector and emitter of one of the composite adjustment tubes is breakdown or short-circuited, and the rectifier output voltage is directly added to the voltage regulation output through the short circuit transistor, and is not controlled by voltage regulation and voltage stabilization.

— The collector or emitter of the sampling amplifier tube is on or off, and the composite adjusting tube is directly under the negative voltage of the auxiliary power supply Dz, and the base current is very large, so that the internal resistance between the emitter and the collector of the adjusting tube becomes very small. The voltage of the rectifier output is added directly to the stable voltage output terminal.


3.2.3 The voltage output of each gear is very small and there is no voltage regulating effect 

The failure may occur because of the following reasons:

— There is no rectifying voltage output from the main rectifier

— The voltage of the auxiliary power supply Dz is zero, resulting in the adjustment tube not working.

— The c-e of the sampling amplifier tube breaks through the short circuit in reverse, resulting in the adjustment tube not working.


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