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Jun 28 2019

Circuit Design of Linear DC Regulated Power Supply

Introduction

Linear regulated power supply refers to the DC regulated power supply when the regulating tube works in a linear state. It is a power conversion circuit and an important part of the electronic system. Its function is mainly to provide the electronic circuit with the power it needs. Electronic equipment typically requires a voltage-stabilized DC power supply to power the load. Linear regulated power supplies are widely used in electronic circuits. Although various new types of voltage regulator circuits are emerging in an endless stream, linear regulated power supplies are always irreplaceable.


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Linear DC Regulated Power Supply




Ⅰ Working Principle of Linear DC Regulated Power Supply

1.1 Working Principle of Ordinary Power Supply


1.2 Working Principle of Linear DC Regulated Power Supply


1.3 Design Indicators of Linear DC Regulated Power Supply







Ⅱ Design of the Circuit

2.1 Overall Block Diagram of DC Regulated Power Supply



2.2 Design of the Power Supply Module Circuit

2.2.1 Power Transformer

2.2.2 Rectifier Circuit

2.2.3 Filter Circuit

2.2.4 Voltage Regulated Circuit

2.3 Circuit Parameter Calculation


Ⅲ The Difference between Linear Power Supply and Switching Power Supply

3.1 Introduction of SG3525

3.1 Linear power supply

3.2 Wien Bridge Oscillator Circuit

3.2 Switching Power Supply


Ⅰ Working Principle of Linear DC Regulated Power Supply

1.1 Working Principle of Ordinary Power Supply

Nowadays, with the rapid development of electronic technology, the application fields of electronic systems have become more and more extensive, and the types of electronic devices are gradually updating and increasing. The relationship between electronic devices and people's daily work and life is also increasingly close. Any electronic device is inseparable from a safe and effective power supply, which is the power source of all power electronic devices, so it is aptly called the "heart of the circuit." In today's life, various high-tech products are increasingly demanding the technical performance indicators of power supplies.

Power supplies can be divided into AC and DC power supplies, which are an integral part of any electronic device. DC power supplies can also be divided into two categories. One type can directly supply DC current or voltage, such as batteries, solar cells, silicon photovoltaic cells, bio-batteries, etc.; the other can convert AC into the required stable DC current or voltage. These converter circuits are collectively referred to as DC regulated power supplies.

 Figure 1-1 Classification of Power


Figure 1-2 AC Voltage Conversion Circuit

A large number of semiconductor devices are used in the circuits of modern electronic devices, which typically require a DC power supply of a few volts to tens of volts in order to obtain the energy necessary for their normal operation. The DC regulated power supplies used in modern electronic equipment are mainly divided into two categories: linear regulated power supplies and switching regulated power supplies.

The linear regulated power supply is also known as the linear DC regulated power supply. Its voltage regulation performance is well, and the output ripple is small. The disadvantage is that it requires the use of a power frequency transformer with a large volume and weight, and the stability efficiency is relatively low. Switching regulated power supplies can be divided into several types according to different classification methods. According to whether the output is adjusted by other components such as adjusting elements (switching elements), the isolation can be divided into two types: non-isolated type and isolated type. According to the excitation mode of the switching element, it can be divided into self-excitation and external excitation. According to the input of the power supply, it can be divided into AC/DC and DC/DC. According to the connection form of the switching elements, it can be divided into two types: serial type and side-by-side type. Its advantages are high efficiency, small size and light weight, but there are a series of shortcomings such as complicated circuit structure and difficult maintenance technology.

The cost of both switching power supplies and linear power supplies increases as their output power increases, but the growth rates vary. In general, the cost of a linear power supply is relatively low when the output power is small. However, when the linear power supply cost is at a certain output power point, it is higher than the switching power supply, which is called the cost reversal point.

The development direction of DC regulated power supply: intelligent, digital, modular, and environmentally friendly. At the end of the 20th century, the birth of various active filters and active compensators also laid the foundation for mass production of various environmentally friendly DC regulated power products in the 21st century.

1.2 Working Principle of Linear DC Regulated Power Supply

The linear regulated power supply refers to the DC regulated power supply whose regulating tube works in a linear state. It is a power conversion circuit and is an important part of the electronic system. Its function is to provide the electronic circuit with the power it needs.

After accessing 220V/50Hz AC, the 220V AC high voltage is converted to a low voltage output to the rectifier bridge through the transformer. After the rectification of the rectifier bridge, the DC motor with large pulsation is output and connected to the filter circuit. Next,using the characteristics of the voltage across the capacitor of the energy storage element can not be abrupt, filtering out the AC component in the output voltage of the rectifier circuit, retaining its DC component, and finally obtaining a smooth output voltage.


 Figure 1-3 Output Waveform of Each Module

1.3 Design Indicators of Linear DC Regulated Power Supply

Grid Adjustment Rate

It indicates the relative change in the output voltage of the regulated power supply when the input grid voltage changes by ±10% from the rated value, and is sometimes expressed in absolute value. Generally, the grid regulation rate of the regulated power supply is equal to or less than 1%, 0.1%, or even 0.01%.

Voltage Regulation Coefficient

The voltage regulation coefficient has two kinds. One is absolute voltage regulation coefficient and other is relative voltage regulation coefficient. The absolute voltage regulation coefficient indicates the ratio of the output DC variation ∆UO of the regulated power supply to the input grid voltage variation ∆Ui when the load is constant, that is

Formula 

It indicates how much the output voltage changes due to the input grid voltage variation ∆Ui. Therefore, the absolute value of the absolute voltage coefficient K is the smaller the better. The smaller K is, the smaller the ∆UO caused by the same ∆Ui is, that is, the more stable the output voltage is.

However, the voltage regulation factor is more important in the regulated power supply. The relative voltage regulation coefficient S indicates the ratio of ∆UO, the relative change in the output DC voltage UO of the regulator to the relative change ∆Ui of the input grid voltage Ui when the load is constant,that is

Formula 

The voltage regulation coefficient usually refers to the relative voltage regulation coefficient S, not the absolute voltage regulation coefficient K.

Output Resistance (also called equivalent internal resistance or internal resistance)

Under nominal grid voltage, the load current change ∆IL causes the output voltage variation ∆UO and the output resistance

Formula 

Ripple voltage

— Maximum Ripple Voltage

The absolute value of the ripple (including noise) of the output voltage at rated output voltage and load current is usually expressed as the value of peak-peak or rms.

— Ripple Coefficient γ

The ratio of the effective value Urms of the output ripple voltage to the output DC voltage UO at the rated load current

Formula 

Ripple Voltage Suppression Ratio

The ripple voltage suppression ratio is the ratio of the ripple voltage Ui in the input voltage to the ripple voltage UO in the output voltage at a specified ripple frequency (for example, 50 Hz)

Formula 


Ⅱ Design of the Circuit

2.1 Overall Block Diagram of DC Regulated Power Supply

 Figure 2-1 Overall Block Diagram of the DC Regulated Power Supply


2.2 Design of the Power Supply Module Circuit

2.2.1 Power Transformer

The power transformer is a step-down transformer that converts the AC voltage of the 220V/Hz grid into a low AC voltage that meets the needs and sends it to the rectifier circuit. The transformer's transformation ratio is the ratio of the primary voltage to the secondary voltage, which is determined by the voltage output from the secondary side of the transformer.

The main parameters of the transformer are:

1. Transforming ratio: The transformer's transformation ratio is the ratio of the primary voltage to the secondary voltage.

2 Rated power: It is the output power that the transformer can work continuously at the specified frequency and voltage without exceeding the specified temperature rise.

3 Efficiency: It is the ratio of output power to input power, which reflects the loss of the transformer itself.

4 No-load current: When the transformer is under no load at the working voltage (the secondary current is zero), the current flowing through the primary coil is called the no-load current. A transformer with a large no-load current has large loss and low efficiency.

5 Insulation resistance and electric strength: Insulation resistance refers to the resistance between the transformer coils, between the coil and the core and between the leads. The electric strength is the voltage that the transformer can withstand within the specified time. It is an important parameter for the safe operation of the transformer, especially the power transformer.

The function of the power transformer Tr is to convert the 220V AC voltage of the grid into the AC voltage Ui , which is required by the rectifier filter circuit. The power ratio of the secondary side of the transformer to the primary side is


Formula 


In the above formula, η refers to the efficiency of the transformer.

 Figure 2-2 Power Transformer Circuit

2.2.2 Rectifier Circuit

The rectifier circuit converts the AC voltage into a pulsating DC voltage. The filter circuit filters out the large ripple component and outputs a DC voltage with a small ripple. Commonly used rectification and filtering circuits include single-phase half-wave rectification filtering and bridge rectification filtering.

Half-wave rectification: Using the unidirectional conductivity of the diode, only the positive voltage portion of the AC component is output, the circuit is very simple, and the number of diodes used is also small. However, since it utilizes a half cycle of the AC voltage, the output voltage is low, the AC component is large, and the efficiency is low. Therefore, this circuit is only suitable for places where the rectified current is small and the pulsation requirements are not very high.

Single-phase bridge rectifier circuit: it consists of four diodes and its principle is to ensure that the direction of the voltage and current on the load does not change during the entire period of the secondary voltage of the transformer. It realizes the full-wave rectification circuit, and fully utilizes the negative half cycle of the secondary side output voltage. Therefore, when the rms value of the secondary side of the transformer is the same, the average value of the output voltage is twice that of the half-wave rectifying circuit.

Therefore, considering this, the circuit design in this article uses a single-phase bridge rectifier circuit.

 Figure 2-3 Single-phase Bridge Rectifier Circuit


When the terminal 1 of the transformer is positive and the terminal 2 is negative, the diodes VD2 and VD4 are subjected to a forward voltage and turned on, and VD1 and VD3 are subjected to a reverse voltage and are turned off. At this time, the terminal 1 of the transformer flows through the RL through VD4, and then returns to the terminal 2 through VD2. When the  terminal 1 is negative and the terminal 2 is positive, the diodes VD1 and VD3 are turned on, VD2 and VD4 are turned off, and the current flows from the terminal 2 through the VD3 through the RL, and then returns to the terminal 1 through the VD1.

2.2.3 Filter Circuit

The filter circuit can filter out most of the AC components in the output voltage of the rectifier circuit, thereby obtaining a relatively smooth DC voltage. This circuit selects a capacitor filter circuit to meet its requirements. Each filter capacitor C must satisfy 2/) 5~3 (*TCRL=, where T is the period of the input AC signal, and RL is the equivalent load resistance of the rectifier filter circuit.

2.2.4 Voltage Regulated Circuit

The function of the voltage regulated circuit is to ensure that the output DC voltage is stable and does not change with changes in the AC grid voltage and load. It satisfies the change of load current by adjusting the current flowing through the zener tube itself, and cooperates with the current limiting resistor to convert the current change into a voltage change to adapt to the fluctuation of the grid voltage. Commonly used integrated regulators are fixed three-terminal regulators and adjustable three-terminal regulators. This circuit requires an output of ±5V/1A, ±12V /1A and ±15V/1A. Therefore, the fixed three-terminal regulator LM7805CT, LM7905CT, LM7812CT, LM7912CT and LM7815CT, LM7915CT are used. The circuit design is very simple. The simplest circuit external components needed is only a fixed resistor and a potentiometer, and the operation is stable. The chip has transition, overheating and safe working area protection, and the maximum output current also meets the requirements.

Figure 2-4 Rectification and Filter Circuit 


2.3 Circuit Parameter Calculation

(1)Determine the minimum input DC voltage Ui,min of the voltage regulator circuit


Formula 

Substituting each indicator,calculating and getting the result: Ui,min ≥13.23V

(2) Determine the voltage, current and power of the secondary side of the power transformer.

The output current is less than 0.5A, and the output voltage is less than 12V. From the above analysis, a transformer with a secondary voltage of 16V and a power of 8W can be purchased.

Select rectifier diode and filter capacitor.

(3) Because the circuit form is bridge rectifier capacitor filtering, the filter capacitor value is obtained by the inverse peak voltage and operating current of each rectifier diode.

(4) Selection of resistance

Formula 

R2 = 750 ohms, then R3 can be a 1K sliding varistor.

Regulator power estimation

When the input AC voltage increases by 10%, the regulator input DC voltage will reach its maximum

Formula 

Therefore, the maximum voltage difference that the regulator is subjected to is:

Formula 

The maximum power consumption is:

Formula 

Therefore, a heat sink with a heat dissipation of 16W is selected, but considering that the voltage drop will be larger after the load is applied, a high-power heat sink is used.  

DC regulated power supply is an essential instrument for electronic experiments. In order for an electronic circuit to perform its functions, it must be powered by a DC power source. The DC regulated power supply provides a continuously adjustable DC voltage for the electronic circuit, and should also have functions such as voltage and current display, alarm, and overcurrent protection. It also has many other functions. For example, it can be used for the maintenance of various aging electronic equipment, such as aging PCB board, home appliances, various IT products, CCFL, lamp, etc.; it is suitable for testing electronic aging components that require automatic timing, power-off, and automatic cycle counting; it can also be used for electronic components performance testing or routine testing. DC regulated power supply can also be widely used in national defense, scientific research, colleges and universities, laboratories, industrial and mining enterprises, electrolysis, electroplating, DC motors, charging equipment, etc.

In addition, linear DC regulated power supplies can be used for mobile phone and computer maintenance. In the maintenance of the computer, the direct output of 5V and 12V is very reliable, and the adjustable output must be payed attention to before connecting the circuit. Whether it can work or not is not the most important. If the voltage is too high, the circuit will burn out, which is not good, and it may also cause a series of safety problems. In the process of repairing the mobile phone, because the change of the boot current needs to be observed in the judgment of the power-on failure of the mobile phone, it is sufficient to use a current meter of up to 1A.


Ⅲ The Difference between Linear Power Supply and Switching Power Supply

Regarding the circuit structure, whether it is a linear power supply or a switching power supply, it depends on the specific occasion and should be  reasonably adopted. The two circuits,with its own characteristics, are widely used both at home and abroad. Linear power supplies are widely used because of their high accuracy and superior performance. Switching power supply has reduced the size and weight of the power frequency transformer due to the elimination of the cumbersome power frequency transformer. It is also widely used in many applications where the output voltage and output current are relatively stable.

3.1 Linear power supply

(1)The main circuit of the linear power supply is as follows:


 Main Circuit of Linear Power Supply

Main Circuit of Linear Power Supply

The linear power supply is actually connecting a high-power triode (actually multiple parallel) at the output of the thyristor power supply. The control circuit can control the output current of the triode by outputting a small current to the base of the triode. The power supply system is regulated once more on the basis of the thyristor power supply, so the voltage regulation performance of the linear regulated power supply is 1-3 orders of magnitude better than that of the switching power supply or the thyristor power supply. However, the power transistor (also known as the regulating tube) generally occupies 10 volts. Each output of 1 amp is required to consume 10 watts of power inside the power supply. For example, The 500V 5A power supply has a loss of 50 watts on the power tube, which accounts for 2% of the total output power. Therefore, the efficiency of the linear power supply is slightly lower than that of the thyristor power supply.

(1)Principle of linear power supply:

Linear power supplies mainly include power frequency transformers, output rectifier filters, control circuits, and protection circuits. 
The basic principle of the linear power supply: After the AC is stepped down into a low-voltage alternating current through a power frequency transformer, the direct current is formed by rectification and filtering, and finally the stable low-voltage direct current is output through the voltage regulated circuit. The adjustment components in the circuit operate in a linear state.

(3) Advantages and disadvantages of linear power supply

 Advantage: 

(1) The strength of the linear power supply is that the structure is relatively simple, the output ripple is small, and the high frequency disturbance is small.

(2) The biggest advantage brought by the simple structure is that it is easy to maintain. The difficulty of repairing a linear power supply is often much lower than that of the switching power supply.

(3) Ripple is an AC component superimposed on the DC stability. The smaller the output ripple, the higher the purity of the output DC, which is an important indicator of the quality of the DC power supply.

(4) DC power with excessive ripple will affect the normal operation of the transceiver. Now advanced linear power supply ripple can reach the level of 0.5mV , and the average product can achieve the level of 5mV.

(5) The linear power supply does not have equipment operating under high frequency conditions. If the input filtering is done well, there is almost no high frequency interference/high frequency noise.

 Disadvantages: 

(1) The transformer is bulky and heavy, so the size and weight of the filter capacitor are also large, and the voltage feedback circuit is operating in a linear state, and a voltage drop is generated on the adjustment tube. When outputting a large operating current, the power consumption of the adjusting tube is too large, the conversion efficiency is low, and a large heat sink should be installed.

(2) This power supply is not suitable for the needs of computers and other equipment, and will be gradually replaced by switching power supplies.

3.2 Switching Power Supply

(1)The main circuit of the switching power supply is as follows:

Main Circuit of Switching Power Supply 

Main Circuit of Switching Power Supply


It can be seen from the circuit that the AC is converted to 311V high voltage after rectification and filtering. After the power switch tube K1~K4  works in an orderly manner, it becomes a pulse signal and is added to the primary of the high-frequency transformer. The height of the pulse is always 311V. When K1 and K4 are turned on, the 311V high-voltage current flows into the primary of the main transformer through K1, and flows out through K4 to form a positive pulse at the primary of the transformer. Similarly, when K2 and K3 are turned on, the 311V high-voltage current flows back into the primary of the main transformer via K3, and flows out through K2, forming a reverse pulse at the primary of the transformer. In this way, a series of forward and reverse pulses are formed in the secondary of the transformer, and the DC voltage is formed after rectification and filtering. When the output voltage Uo is high, the pulse width is wide. When the output voltage Uo is low, the pulse width is narrow, so the switch tube is actually a device for controlling the pulse width.

(2) Principle of switching power supply: 

The switching power supply mainly includes an input power grid filter, an input rectification filter, an inverter, an output rectification filter, a control circuit, and a protection circuit. 
Their function is:  
— Input grid filter: Eliminate interference from the power grid, such as the start of the motor, the switch of the electrical appliance, the lightning strike, etc., and also prevent the high-frequency noise generated by the switching power supply from spreading to the power grid.  

— Input rectifier filter: rectify and filter the input voltage of the grid to provide DC voltage for the converter. 

— Inverter: It is a key part of the switching power supply. It converts the DC voltage into a high frequency AC voltage and acts to isolate the output from the input grid.   
— Output rectification filter: Rectify and filter the high-frequency AC voltage outputted by the converter to obtain the required DC voltage, and also prevent the interference of high-frequency noise on the load at the same time. 

— Control circuit: Detect the output DC voltage and compare it with the reference voltage for amplification. The pulse width of the oscillator is modulated to control the converter to maintain the output voltage stable. 
— Protection circuit: When the switching power supply is short-circuited due to an overvoltage or overcurrent, the protection circuit stops the switching power supply to protect the load and the power supply itself.   
The switching power supply rectifies the alternating current into direct current, converts the direct current into alternating current, and rectifies and output the required direct current voltage. In this way, the switching power supply saves the transformer in the linear power supply and the voltage feedback circuit. The inverter circuit in the switching power supply is completely digitally adjusted, and can also achieve very high adjustment accuracy.  

The basic principle of the switching power supply: the input terminal directly converts the alternating current into a direct current, and then the switching tube is used to control the on and off of the current under the action of the high frequency circuit. With the help of an inductor (high-frequency transformer), a stable low-voltage direct current is output. 
(3) Advantages and disadvantages of switching power supply:   
 Advantage: 

Small size, light weight (20 to 30% of volume and weight of linear power supply), high efficiency (generally 60 to 70%, and linear power supply is only 30 to 40%), strong anti-interference, wide output voltage range, modular.   
 Disadvantages: 

Due to the high frequency voltage generated in the inverter circuit, there is some interference to the surrounding equipment. Therefore, the functions of shielding and grounding are required. The alternating current is rectified to obtain direct current. In order to obtain a stable DC voltage, a stable circuit is selected.

Power supply is an important part of circuit design, and the stability of the power supply largely determines the stability of the circuit. Linear power supplies and switching power supplies are two common types of power supplies. The principle is very different and determines the difference between the two applications.

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