Detailed explanation of linear DC stabilized power supply: circuit diagram analysis of linear DC stabilized power supply design

　　Linear mode refers to a DC regulated power supply in which the regulating tube works in a linear state (that is, it works in the amplification zone). Just like a transistor, which has three working states: amplification, saturation, and cutoff, the adjustment tube works in a linear state, which can be understood this way: RW is continuously variable, that is, linear. But in the switching power supply, it is different. The switching tube only works in two states: on and off: the on resistance is close to very small; the off resistance is very large and close to infinity. The tube working in the switching state is obviously not a linear state. Therefore, DC regulated power supplies can be divided into linear mode DC power supplies and switching mode DC power supplies.

　　Linear DC power supply (Linear power supply) first reduces the voltage amplitude of AC power through a transformer, and then rectifies it through a rectifier circuit to obtain pulsed DC power, which is then filtered to obtain a DC voltage with a tiny ripple voltage. To achieve high-precision DC voltage, it must be stabilized by a voltage stabilizing circuit. Voltage stabilization process The voltage stabilization process is a core of the voltage stabilization power supply, so I will briefly explain it here. It will be very complicated to explain in detail, but as long as we know a rule, it will be very convenient to analyze.

　　Voltage stabilization process

　　For example, the output voltage ↑ → the base voltage of the error amplifier tube ↑ → the base current of the error amplifier tube ↑ → the collector current of the error amplifier tube ↑ → the base current of the adjustment tube ↓ (where does the reduced base current go? It is affected by the error The collector of the amplifier tube is shunted, and the equivalent resistance of the tube is adjusted ↑→output voltage ↓, completing the purpose of adjustment. Vice versa, ↑ changes ↓. Mastering this rule will be very helpful for understanding this concept.

　　Since the adjustment tube is equivalent to a resistor, it will generate heat when current flows through the resistor. Therefore, the adjustment tube working in a linear state will generally generate a large amount of heat, resulting in low efficiency. This is one of the most important shortcomings of linear regulated power supplies. However, the advantages of linear regulated power supplies are incomparable to switching power supplies: fast adjustment speed, small ripple, and low interference. It is these advantages that make linear regulated circuits widely used in digital circuits, CPU power supplies (in home appliances), signal processing, etc. It is widely used in circuits with high power quality requirements.

　　Basic working principle

　　The working process of the main circuit of the linear DC power supply is that the input power is first subjected to preliminary AC voltage stabilization by the pre-stabilizing circuit, and then converted into DC power through isolation rectification by the main working transformer, and then under the intelligent control of the control circuit and single-chip microprocessor controller Finely adjust the linear adjustment component to output a high-precision DC voltage source.

　　Linear DC power supply products can be widely used in scientific research, universities, laboratories, industrial and mining enterprises, electrolysis, electroplating, charging equipment, etc.

　　Linear DC regulated power supply design circuit diagram analysis

　　1. Overall block diagram of DC regulated power supply

　　2. Design of voltage-stabilized power module circuit

　　2.1. Power transformer

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

　　The main parameters of the transformer are:

　　① Transformation ratio: The transformation ratio of a transformer is the ratio of primary voltage to secondary voltage.

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

　　③Efficiency: It is the ratio of output power to input power, which reflects the transformer's own loss. ④ No-load current: When the transformer is at a lower operating voltage than the secondary side without load (the secondary current is zero), the current flowing through the primary coil is called no-load current. Transformers with large no-load currents have large losses and low efficiency.

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

　　The function of the power transformer Tr is to convert the 220V AC voltage of the power grid into the rectifier and filter circuit required

　　2.2. Rectifier circuit

　　The rectifier circuit converts AC voltage into pulsating DC voltage. Then the larger ripple component is filtered out by the filter circuit, and a DC voltage with smaller ripple is output. Commonly used rectifier and filter circuits include single-phase half-wave rectifier and filter, bridge rectifier and filter, etc.

　　Half-wave rectification: Taking advantage of the unidirectional conductivity of the diode, it only outputs the positive voltage part of the AC component. The circuit is very simple and easy to implement, and the number of diodes used is also very small. However, because it utilizes half a cycle of the AC voltage, the output voltage is low, the AC component is large, and the efficiency is low. Therefore, this kind of 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 is composed of four diodes. Its composition principle is to ensure that the direction of the voltage and current on the load does not change throughout the entire cycle of the secondary side voltage of the transformer. It implements a full-wave rectification circuit and fully utilizes the negative half-cycle of the secondary output voltage. Therefore, when the effective value of the secondary voltage of the transformer is the same, the average output voltage is twice that of the half-wave rectification circuit.

　　Therefore, after comprehensive consideration, this circuit design uses a single-phase bridge rectifier circuit.

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

　　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 uses 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 and filter circuit.

　　2.4. Voltage stabilizing circuit

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

　　3. Calculation of circuit parameters

　　(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. Based on the above analysis, a transformer with a secondary voltage of 16V and a power of 8W can be purchased.

　　Select rectifier diodes and filter capacitors.

　　(3) Since the circuit form is bridge rectifier capacitor filtering, the filter capacitor value is calculated through the reverse peak voltage and operating current of each rectifier diode.

　　(4) Selection of resistor

　　Technology Zone Typical voltage topology applications in automotive systems How to convert an adjustable DC-DC into a numerically controllable switching regulated power supply How to wire a three-terminal voltage regulator tube and its method Description Three-terminal voltage regulator tube model Voltage stabilization value and detailed information ADI The LT8603 with powerbyLinear™ can accept 42V input voltage

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