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With the global emphasis on energy issues, the energy consumption of electronic products will become more and more prominent. How to reduce its standby power consumption and improve power supply efficiency has become an urgent problem to be solved. Although the traditional linear regulated power supply has a simple circuit structure and reliable operation, it has the disadvantages of low efficiency (only 40%-50%), large volume, large consumption of copper and iron, high operating temperature and small adjustment range. In order to improve efficiency, people have developed a switching regulator power supply, which has an efficiency of more than 85% and a wide voltage regulation range. In addition, it has the characteristics of high voltage regulation accuracy and no use of power transformers. More ideal power supply. Because of this, the switching regulator power supply has been widely used in various electronic devices. This paper describes the working principle of various switching power supplies.
First, the basic working principle of the switching regulator power supply
Switching type regulated power supply control mode is divided into two types: widening type and frequency modulation type. In practical applications, the widening type is used more. In the current development and use of switching power supply integrated circuits, most of them are also Pulse width modulation type. Therefore, the following mainly introduces the widened switching power supply.
The basic principle of the widened switching regulator power supply can be seen in the figure below.
For a unipolar rectangular pulse, its DC average voltage Uo depends on the width of the rectangular pulse. The wider the pulse, the higher the DC average voltage value. DC average voltage U. Can be calculated by the formula, ie Uo=Um×T1/T
Where Um is the maximum voltage value of the rectangular pulse;
T — rectangular pulse period;
T1 — Rectangular pulse width.
It can be seen from the above equation that when Um and T are constant, the DC average voltage Uo will be proportional to the pulse width T1. In this way, as long as we try to narrow the pulse width with the increase of the output voltage of the regulated power supply, the purpose of stabilizing the voltage can be achieved.
Second, the principle circuit of the switching regulator power supply
1, the basic circuit
Figure 2: Switching the original circuit block diagram
The basic circuit block diagram of the switching regulator power supply is shown in Figure 2.
After the AC voltage is rectified and filtered by the rectifier circuit and the filter circuit, it becomes a DC voltage containing a certain pulsating component, and the voltage is converted into a square wave of a desired voltage value, and finally the square wave voltage is rectified. The filtering becomes the required DC voltage.
The control circuit is a pulse width modulator, which is mainly composed of a sampler, a comparator, an oscillator, a pulse width modulation, and a reference voltage. This part of the circuit has been integrated and has been fabricated into various integrated circuits for switching power supplies. The control circuit is used to adjust the switching time ratio of the high frequency switching element to achieve the purpose of stabilizing the output voltage.
2. Single-ended flyback switching power supply
A typical circuit for a single-ended flyback switching power supply is shown in Figure 3.
The so-called single-ended in the circuit means that the core of the high-frequency converter operates only on one side of the hysteresis loop. The so-called flyback means that when the switching tube VT1 is turned on, the induced voltage of the primary winding of the high-frequency transformer T is up and down, and the rectifier diode VD1 is in an off state, and energy is stored in the primary winding. When the switch tube VT1 is turned off, the energy stored in the primary winding of the transformer T is filtered by the secondary winding and the VD1 rectification and capacitance C, and then output to the load.
The single-ended flyback switching power supply is the lowest cost power supply circuit with an output power of 20-100W, which can output different voltages at the same time and has a good voltage regulation rate. The only drawback is that the output has a large ripple voltage and poor external characteristics, which is suitable for relatively fixed loads.
The maximum reverse voltage of the switching transistor VT1 used in the single-ended flyback switching power supply is twice the operating voltage of the circuit, and the operating frequency is between 20-200 kHz.
3. Single-ended forward switching power supply
A typical circuit for a single-ended forward switching power supply is shown in Figure 4.
This circuit is similar in form to a single-ended flyback circuit, but operates differently. When the switch tube VT1 is turned on, VD2 is also turned on. At this time, the grid transmits energy to the load, and the filter inductor L stores energy. When the switch tube VT1 is turned off, the inductor L continues to release energy to the load through the freewheeling diode VD3.
A clamping coil and diode VD2 are also provided in the circuit, which limits the maximum voltage of the switching transistor VT1 to twice the supply voltage. In order to meet the core reset condition, that is, the flux establishment and reset times should be equal, the duty cycle of the pulse in the circuit should not be greater than 50%.
Since this circuit transmits energy to the load through the transformer when the switching tube VT1 is turned on, the output power range is large, and the power of 50-200 W can be output. The transformer used in the circuit has a complicated structure and a large volume. For this reason, the practical application of such a circuit is less.
4. Self-excited switching power supply
The typical circuit of the self-excited switching regulator power supply is shown in Figure 5.
This is a switching power supply that uses a batch oscillating circuit and is one of the most widely used basic power sources.
When the power is turned on, the starting current is supplied to the switching tube VT1 at R1, so that VT1 starts to conduct, and the collector current Ic linearly increases in L1, and a positive feedback voltage that makes the VT1 base extremely positive and emits a negative negative is induced in L2. , so that VT1 is quickly saturated. At the same time, the induced voltage charges C1. As the C1 charging voltage increases, the VT1 base potential gradually becomes lower, causing VT1 to exit the saturation region, and Ic begins to decrease. In L2, the VT1 base is induced to be extremely negative and the emission is extremely high. The positive voltage causes VT1 to turn off quickly. At this time, the diode VD1 is turned on, and the energy stored in the primary winding of the high-frequency transformer T is released to the load. When VT1 is cut off, there is no induced voltage in L2. The DC power supply input voltage is reversely charged to C1 through R1, gradually increase the base potential of VT1, make it re-conduct, and flip again to reach saturation state, and the circuit repeats the oscillation. . Here, like the single-ended flyback switching power supply, the required voltage is output from the secondary winding of the transformer T to the load.
The switch tube in the self-excited switching power supply acts as both a switch and an oscillation, and the control circuit is omitted. In the circuit, since the load is located in the secondary of the transformer and operates in the flyback state, there is an advantage that the input and the output are isolated from each other. This circuit is not only suitable for high-power power supplies, but also for low-power power supplies*
5. Push-pull switching power supply
The typical circuit of the push-pull switching power supply is shown in Figure 6.
It belongs to a double-ended conversion circuit, and the core of the high-frequency transformer works on both sides of the hysteresis loop. The circuit uses two switching tubes VT1 and VT2. The two switching tubes are alternately turned on and off under the control of the external excitation square wave signal. The square wave voltage is obtained in the secondary group of the transformer T, and the desired DC is obtained by rectification and filtering. Voltage.
The advantage of this circuit is that the two switching tubes are easy to drive. The main disadvantage is that the switching tube has a withstand voltage of twice the peak voltage of the circuit. The output power of the circuit is large, generally in the range of 100-500W.
6. Buck switching power supply
The typical circuit of the step-down switching power supply is shown in Figure 7.
When the switch VT1 is turned on, the diode VD1 is turned off, and the input rectified voltage is charged to C through VT1 and L. This current increases the energy storage in the inductor L. When the switch tube VT1 is turned off, the inductor L induces a left negative right positive voltage, and the energy stored in the inductor L is released through the load RL and the freewheeling diode VD1 to maintain the output DC voltage unchanged. The level of the DC voltage output from the circuit is determined by the pulse width applied to the base of VT1.
This circuit uses fewer components and is identical to the other two circuits described below, requiring only inductors, capacitors, and diodes.
7. Step-up switching power supply
The voltage regulator circuit of the step-up switching power supply is shown in Figure 8.
When the switch tube VT1 is turned on, the inductor L stores energy. When the switch tube VT1 is turned off, the inductor L induces a left negative right positive voltage, which is superimposed on the input voltage, and supplies power to the load via the diode VD1, so that the output voltage is greater than the input voltage, forming a step-up switching power supply.
8. Reverse switching power supply
The typical circuit of the reverse switching power supply is shown in Figure 9.
This circuit is also known as a buck-boost switching power supply. The circuit works normally regardless of whether the pulsating DC voltage before the switch VT1 is higher or lower than the stable voltage at the output.
When the switch VT1 is turned on, the inductor L stores energy, the diode VD1 is turned off, and the load RL is powered by the last charge of the capacitor C. When the switching tube VT1 is turned off, the current in the inductor L continues to flow, and a positive negative voltage is induced, and the capacitor VD1 supplies power to the load while charging the capacitor C.
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