Working Principle and circuit diagram of switching power supply

The working principle of the switch power supply in this paper is the analysis of the principle of the switch power supply engineers of Electronic enthusiasts. Based on the analysis of a wide range of switch power supply cases, this paper introduces the single-positive excited switch power supply and Self-excited switch power supply, push-pull switch power supply, Buck switch power supply, boost switch power supply and reverse switch power supply.

With the world's attention to energy problems, the energy consumption of electronic products will become more and more prominent. How to reduce its standby power consumption and improve power supply efficiency becomes an urgent problem. Although the traditional linear regulated power supply has simple circuit structure and reliable operation, it has low efficiency (only 40%-50%), large size, large copper and iron consumption, high operating temperature and small adjustment range. To improve efficiency, a power supply with on/off voltage can reach 85% or more, and the voltage regulation range is wide. In addition, it also has the characteristics of high voltage regulation precision, no use of power transformers, etc, it is an ideal regulated power supply. Because of this, switching type regulated power supply has been widely used in a variety of electronic equipment, this paper describes the working principle of various types of switching power supply.

I. basic working principle of On/Off regulated power supply
Switch-type regulated power supply control methods are divided into two types: Width Adjustment and frequency modulation. in actual application, the Width Adjustment type is used more, in the current development and use of the switch power supply integrated circuit, the vast majority of also for the pulse width modulation type. Therefore, the following describes the adjustable-width switch regulated power supply.
For the basic principle of the adjustable-width switch regulated power supply, see.

For a single polar rectangular pulse, the average DC voltage uo depends on the width of the rectangular pulse. The wider the pulse, the higher the average DC voltage value. Average DC voltage U. It can be calculated by formulas,
That is, UO = Um × t1/T
In the formula, um is the maximum voltage value of the rectangular pulse, T is the Rectangular Pulse cycle, and T1 is the rectangular pulse width.
As can be seen from the above formula, when um and t remain unchanged, the average DC 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, we can achieve the goal of stable voltage.

Ii. principle circuit of On/Off regulated power supply
1. Basic Circuit

Figure 2 basic circuit diagram of switching power supply
Diagram 2 shows the basic circuit of the on/off regulated power supply.
After the AC voltage is rectified and filtered by the rectification circuit and the filter circuit, it becomes a dc voltage containing a certain pulsating component. The voltage is converted into a square wave of the required voltage value, finally, the square wave voltage is converted into the required DC voltage by rectifying and filtering.
The control circuit is a pulse width modulation. It consists of sampling devices, comparator, oscillator, pulse width modulation, reference voltage, and other circuits. This part of the circuit has been integrated, made into a variety of switching power supply integrated circuit. The control circuit is used to adjust the switching time ratio of the high-frequency switch component to achieve the goal of stable output voltage.

2. Single-ended anti-excited switch power supply
The typical circuit 3 of the single-ended anti-exciting switch power supply is shown in. The so-called single-ended circuit means that the core of the high-frequency converter only works on one side of the hysteresis loop. The so-called anti-excitation refers to when the switch vt1 is turned on, the induction voltage of the high-frequency transformer t primary winding is positive and negative, the rectification diode vd1 is in the cutoff state, and energy is stored in the primary winding. When the switch end at vt1, the energy stored in the T primary winding of the transformer is output to the load through the secondary winding, vd1 rectification, and capacitor C filtering.

The single-ended anti-exciting switching power supply is a power supply circuit with the lowest cost. The output power is 20-100 W. It can output different voltages at the same time and has a better voltage adjustment rate. The only drawback is that the output ripple voltage is large and the external characteristics are poor, which is suitable for relatively fixed loads.
The maximum reverse voltage of the switch vt1 used by the single-ended anti-exciting switch power supply is twice the operating voltage of the circuit, and the operating frequency is between 20-kHz.

3. Single-positive excited switch power supply
The typical circuit 4 of the single-positive excited switch power supply is shown in. This type of circuit is similar in form to a single-ended anti-exciting circuit, but its operation is different. When the switch vt1 is turned on, vd2 also
When the power grid transmits energy to the load, the filter inductance L stores energy. When the switch vt1 ends, the inductance L continues to release energy to the load through the continued flow diode vd3.

The Clamp coil and diode vd2 are also provided in the circuit, which can limit the maximum voltage of the switch vt1 between two times the power supply voltage. To meet the core Reset conditions, that is, the magnetic flux establishment and
The reset time should be equal, so the duty cycle of the pulse in the circuit cannot be greater than 50%. This type of circuit transmits energy to the load when the switch vt1 is turned on, so the output power range is large and the output power is 50-200 W. The transformer used in the circuit has a complex structure and a large volume. Due to this reason, this circuit has less practical application.

4. Self-activated switch regulated power supply
The typical circuit 5 of the Self-excited switch regulated power supply is shown in. This is a type of switching power supply consisting of a intermittent oscillator circuit, and is also one of the most widely used basic power supplies.

When the power supply is connected, the start current is provided to the switch vt1 at R1 so that vt1 starts to turn on. The collector current IC increases linearly in L1, And the vt1 base pole is determined in L2, transmit a very negative positive feedback voltage, so that vt1 is quickly saturated. At the same time, the induction voltage is used to charge C1. With the increase of the C1 charging voltage, the base potential of vt1 gradually decreases, causing vt1 to exit the saturation zone and the IC to decrease, in L2, the voltage of vt1 base is induced to be extremely negative and the emission pole is positive, so that vt1 can quickly end. In this case, the diode vd1 is turned on and the energy storage in the T primary winding of the high-frequency transformer is released to the load. At the end of vt1, there is no induced voltage in L2, And the DC power supply input voltage is recharged to c1 through R1, which gradually increases the base potential of vt1 so that it can be turned on again and switches to saturation, the circuit oscillates repeatedly. Here, the voltage required by the transformer T's secondary winding is output to the load, just like the single-ended anti-exciting switch power supply.
The switch in the Self-excited switch power supply plays a double role of switch and oscillation, and also saves the control circuit. Because the load in the circuit is located at the secondary of the transformer and is working in the anti-exciting state, it has the advantages of isolating the input and output. This type of circuit is not only applicable to high-power supply, but also small-power supply.

5. push-pull switch power supply
The typical circuit 6 of the push-pull switch power supply is shown in. It is a double-ended conversion circuit, and the core of the high-frequency transformer works on both sides of the hysteresis loop. Two vt1 and VT2 switches are used in the circuit. The two switches are turned on and off under the control of the square wave signal when the two switches are activated. The square wave voltage is obtained in the transformer T sub-system, the DC voltage is converted into the required DC voltage by rectifying and filtering.

The advantage of this circuit is that the two switches are easy to drive, and the main disadvantage is that the voltage of the switch must reach twice the peak voltage of the circuit. The output power of the circuit is large, generally within the range of 100-500 W.

6. step-down Switching Power Supply
The typical circuit 7 shows the step-down switching power supply. When the switch vt1 is turned on, the diode vd1 ends, and the transformer's Rectification voltage is charged to C through vt1 and L. This current increases the energy storage in the inductance L. When the switch end at vt1, inductance L senses the voltage of the left negative and right positive. The energy stored in inductance L is released by the load RL and the continued-flow diode vd1, so that the output DC voltage remains unchanged. The DC voltage output of the circuit is determined by the pulse width added to the vt1 base electrode.

This type of circuit uses fewer components. It is the same as the other two types of circuits described below, and can be achieved by using inductance, capacitance, and diode.

7. Boost switch power supply
The voltage regulator circuit 8 of the boost switch power supply is shown in. When the switch vt1 is turned on, the inductance L stores energy. When the switch end at vt1, inductance L is used to sense the voltage of the left negative and right positive. The voltage is superimposed on the input voltage, and is powered by the diode vd1 to the load, so that the output voltage is greater than the input voltage, forming a boost switch power supply.

8. inverted switch power supply
The typical circuit of the inverted switching power supply is shown in section 9. This type of circuit is also known as the lifting pressure switch power supply. The circuit works normally regardless of whether the pulsating DC voltage before the switch vt1 is above or below the stable voltage at the output end.

When the switch vt1 is turned on, the inductance L stores energy, the diode vd1 ends, and the load RL is powered by the last charging charge of capacitor C. When the switch end at vt1, the current in inductance L continues to flow, and generates a positive voltage from the upper and lower sides. The diode vd1 powers the load and charges the capacitor C at the same time.

The above describes the basic working principle and various circuit types of the Pulse Width Modulated switch regulated power supply. In practical application, there will be a variety of practical control circuits, but no matter what, it is also developed on these basis.

Working Principle and circuit diagram of switching power supply