Circuit Analysis: Classic Integrated DC Regulated Power Supply Circuit

　　Any electronic device cannot work without a DC power supply . Transistors and integrated circuits need a DC power supply to work properly. There are two main ways to provide DC power: dry batteries and regulated power supplies . Dry batteries have advantages such as stable output voltage and easy to carry, but their disadvantages of low capacity and short life are also very obvious. The DC regulated power supply can convert 220V AC power into a steady stream of stable DC power. It consists of four circuits: voltage transformation, rectification, filtering, and voltage stabilization. The reference circuit is shown in Figure 1.

　　Voltage Transformation

　　The output voltage of a voltage-stabilized power supply is generally determined according to the needs of the instrument and equipment. Some instruments and equipment require several different voltages at the same time. The output voltage of a single voltage-stabilized power supply can be adjusted within a certain range. When the adjustment range is large, it can be divided into several gears. Therefore, the AC power needs to be converted into a voltage of appropriate amplitude through a power transformer before it can be rectified and transformed. According to the needs, the secondary coil of the transformer is generally more than two groups. Select a suitable transformer to convert the 220V±10% high-voltage AC power into the required low-voltage AC power. To meet the needs of power supply power and output voltage, the selection of the transformer should follow the following principles:

　　(1) Under the condition of 220V±15%, reliable and stable output should be ensured. In general projects, the DC voltage after transformation, rectification and filtering can be determined according to the following conditions:

　　First, it is necessary to consider that the integrated voltage regulator circuit generally requires the minimum input-output voltage difference; second, it is necessary to consider that the bridge rectifier circuit consumes the voltage drop of two diodes in the forward conduction; third, a certain margin must be left. If the output voltage is too high, it will increase the heat dissipation, and if it is too low, it will be unstable when the output is low voltage, so the DC voltage is determined accordingly.

　　(2) The transformer　　should have a current margin of more than 20%.

　　It converts sinusoidal alternating current into pulsating direct current, which is mainly achieved by using the unidirectional conduction principle of diodes. The rectifier circuit can be divided into half-wave rectification, full-wave rectification and bridge rectification. Most power supplies use bridge rectifier circuits, which are composed of 4 diodes. Each diode involves two parameters when working:

　　First, the current must meet the needs of the power supply load current. Since the four diodes in the bridge rectifier circuit work alternately every two, the working current of each diode is half of the load current. Second, the reverse withstand voltage must be greater than the possible maximum peak value.

　　(1) Current load ID>?I1

　　(2) The reverse withstand voltage is the peak value of the transformer's maximum output VD> V2.

　　Filtering

　　The function of filtering is to filter the pulsating DC into a DC voltage with a certain ripple. Capacitors, inductors and other devices can be used. In practice, large-capacity electrolytic capacitors are often used for filtering.

　　In the figure, C2 and C4 are low-frequency filter capacitors. According to the relevant formulas in the experimental principles and the changes in the power grid, their capacitance and withstand voltage can be designed and calculated, and the nominal value and withstand voltage of the capacitor and the capacitor model (generally several hundred to several thousand microfarads) can be selected.

　　C1 and C3 are high-frequency filter capacitors used to eliminate high-frequency self-excitation to make the output voltage more stable and reliable. They are usually in the range of 0.01μF~0.33μF.

　　(1) The withstand voltage of the low-frequency filter capacitor should be greater than the highest voltage in the circuit, and a certain margin should be left;

　　(2) The selection of low-frequency filter capacitor C2 should meet the following requirements: C2 ≥ RL1(3~5)2T;

　　RL is the load resistance, T is the period of the input AC power. For the filter capacitor after integrated voltage stabilization, a few hundred microfarads can be selected appropriately;

　　(3) In engineering, the low-frequency capacitor C2 can also be used to determine the capacity of the filter capacitor after rectification based on the value of the load current, that is:

　　C2≥(IL/50mA)×100uF.

　　Voltage stabilization

　　The DC voltage after rectification and filtering is an unstable DC voltage that contains ripples and changes with the fluctuation of the AC power supply voltage and the change of the load. The unstable voltage will cause the equipment to work unstably, and sometimes even fail to work properly. For this reason, a voltage stabilization circuit should be added after filtering to ensure the stability of the output voltage. The voltage stabilization methods include a voltage stabilization circuit composed of discrete components and an integrated voltage stabilization circuit.

　　The voltage stabilization methods of the voltage stabilization circuit composed of discrete components include series stabilization, parallel stabilization and switching stabilization, among which the most commonly used is the series stabilization method.

　　(1) Series voltage regulator circuit

　　The working block diagram of the series voltage regulator circuit is shown in Figure 2. It consists of a sampling circuit, a reference voltage circuit, a comparison amplifier circuit and an adjustment circuit.

　　(2) Integrated voltage regulator

　　With the widespread use of integrated process technology, the voltage stabilizing circuit is also integrated on a chip, called a three-terminal integrated voltage stabilizer, which has the advantages of being safe, reliable, convenient and low-priced.

　　Three-terminal regulators can be divided into four categories according to the output voltage:

　　① Fixed output positive regulator 7800 series, such as 7805, the voltage regulation value is +5V.

　　② Fixed output negative regulator 7900 series.

　　③ Adjustable output positive voltage regulators LM 1 1 7, LM 2 1 7, LM317 and LM123, LM140, LM138, LM150, etc.

　　④Adjustable output negative voltage regulator LM 1 3 7, LM 2 3 7, LM337, etc.

　　DC regulated power supplies generally require the installation of insurance and heat sinks

　　The fuse is an important device used to ensure the safe operation of the power supply. The fuse can automatically cut off the circuit when the power supply output is short-circuited or overloaded, thereby ensuring the safety of the power supply. The heat sink is used to dissipate heat from the integrated voltage regulator circuit to ensure that the temperature of the integrated voltage regulator does not exceed the standard value when it is working.