Special integrated switching power supply circuit diagram principle

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Overview:
1 Introduction Compared with the traditional switching power supply, the special integrated switching power supply has the characteristics of novel circuit, strange function, advanced performance and wide application fields. The special integrated switching power supply mainly includes the following four types: composite switching power supply, constant voltage/constant current switching power supply, and cut-off output switching power supply constant power output type switching power supply. They are all designed with TOPSwitch or TOPSwitch-II series monolithic switching power supply integrated circuits, and are equipped with corresponding peripheral circuits (including control loops). They have different output characteristics and application fields to meet the needs of different users.

2 circuit design of composite switching power supply
2.1 The design of the composite switching power supply is well known. The output stability of the linear integrated regulator is very high, the ripple voltage is small, but the power supply efficiency is low, and a heavy power frequency transformer is needed. The single-chip switching power supply is highly efficient, and the power frequency transformer can be omitted, but the voltage stability is low and the ripple voltage is high. If the two are skillfully combined, the single-chip switching power supply is used as the pre-stage voltage regulator, and then the linear voltage regulator is used for secondary voltage regulation, so that the advantages and disadvantages can be avoided and the advantages can be complemented to form an ideal high-efficiency and precise stability. Pressure power supply. The wiring integrated voltage regulator has the following three options:
(1) With 7805 series three-terminal fixed type integrated voltage regulator, the circuit is simple and the cost is low;
(2) With LM317 type three-terminal adjustable integrated voltage regulator, the output voltage can be continuously adjusted, and the voltage regulation performance can be further improved;
(3) With low-dropout regulators such as LM2937 (fixed output) and LM2991 (adjustable output), the efficiency of the hybrid switching power supply can be further improved.
The two-stage voltage regulator combined switching power supply can achieve a voltage regulation rate and a load regulation rate of ±0.1%.
According to the need, it can also add automatic protection functions such as undervoltage protection, overvoltage protection, AC power failure protection and soft start for the composite switching power supply.
2.2 Design Example of Composite Switching Power Supply The circuit of the composite switching power supply with power-down protection function is shown in Figure 1. The circuit uses a TOP209 single-chip switching power supply for pre-stage regulation, and then uses the LM2937 three-terminal low-dropout regulator for post-stage regulation. The LM2937's input-to-output differential voltage (UI-UO) ≤0.6V at rated load is only 1/6 to 1/3 of the LM7805, which can significantly reduce the power consumption of the regulator and improve power efficiency. The LM2937 has an output voltage of 5V and a maximum output current of 0.5A. The AC power-down protection circuit consists of VT1, VT2, C7, and R4 to R6. Among them, R5, R6 and VT2 are used to detect the AC voltage, the threshold voltage must be lower than 85V, and now take 70V. Once the UI is <70V, VT2 is immediately turned off, and the DC voltage charges C7 through R4. When the voltage across C7 is higher than the total voltage drop of the emitter junction of Darlington VT1, VT1 is turned on, causing the voltage at the control terminal of TOP209 to become low. At the same time, C5 discharges and turns off the power. Under normal circumstances, UI>70V, because VT2 is on, the voltage across C7 is always 0V, so the circuit does not work. It is not difficult to see that the power supply time after power failure is proportional to the time constant τ=R4·C7. Set the power supply time by changing the τ value.

Figure 1 A hybrid switching power supply with power-down protection

Figure 2 Circuit of 15V, 2A precision constant voltage / constant current switching power supply

This switching power supply is suitable as an auxiliary power source for energy-saving computers (also known as "green" PCs). After the AC power is turned off, the auxiliary power supply continues to supply power for a period of time, so that the microcomputer can store the data; when the power failure time exceeds the set time, the auxiliary power supply is automatically cut off.

3 Circuit design of precision constant voltage / constant current switching power supply

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More about the device LM2937 data: integrated circuit query network http://

More about the device TOP209 information: integrated circuit query network http://

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