# The principle of operation of a switching power supply "Exchange of haberdashery" and a circuit diagram

2023-12-02Archive

This article introduces direct single-ended switching power supply, self-excited switching power supply, push-pull switching power supply, buck switching power supply, boost switching switching power supply, and inverting switching power supply based on rich switching power supply. case analysis.

With global focus on energy issues, power consumption of electronic products will become more and more prominent. How to reduce their standby power consumption and improve power efficiency has become an urgent problem to be solved. Although traditional linear regulated power supply has a simple circuit structure and reliable performance, it has disadvantages such as 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 type regulated power supply, which has an efficiency of more than 85% and a wide voltage regulation range. In addition, it also has characteristics of high voltage regulation accuracy, and do not use a power transformer. This is a kind of ideal regulated power supply. Because of this, switching regulated power supplies have become widespread in various electronic devices. This article explains working principles of various switching power supplies.

1. The basic principle of operation of a switching regulated power supply

The connection control methods of switching regulated power supply are divided into two types: width adjustment type and frequency modulation type. In practical applications, width adjustment type is more commonly used. Most of them also have pulse width modulation. Therefore, following mainly introduces an adjustable-width switching regulated power supply. The basic principle of an adjustable width switching power supply can be seen in figure below.

For a unipolar rectangular pulse, its average DC voltage Uo depends on width of rectangular pulse, wider pulse, higher value of its average DC voltage. The average DC voltage U. It can be calculated by formula,that is, Uo=Um×T1/Twhere Um is maximum value of rectangular pulse voltage, T is period of rectangular pulse, T1 is width of rectangular pulse. It can be seen from above formula that when Um and T remain unchanged, average DC voltage Uo will be proportional to pulse width T1. So as long as we try to make pulse width narrow as output voltage of regulated power supply increases, we can achieve goal of voltage stabilization.

Secondly, circuit diagram of a switching regulated power supply

1. Basic scheme

Fig. 2. Schematic block diagram of a switching power supply

The basic block diagram of a switching regulated power supply is shown in Figure 2. After AC voltage is rectified and filtered by rectifier circuit and filter circuit, it becomes a DC voltage with a certain ripple component. The voltage is applied to high frequency converter and converted into a square wave required voltage value, and finally this square wave voltage is rectified and filtered to become 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 voltage reference circuit. At present, this part of circuit is integrated, and various integrated circuits for switching power supplies have been created. The control circuit is used to adjust switching time ratio of high frequency switching element to achieve purpose of output voltage stabilization. 2. Single-ended Switching Flyback Power Supply

A typical circuit of a single-cycle flyback switching power supply is shown in Figure 3. The so-called single-cycle in circuit means that magnetic circuit of high-frequency converter works only on one side of hysteresis loop. The so-called reverse stroke means that when switch lamp VT1 is turned on, induced voltage of primary winding of high-frequency transformer T is positive and negative, and rectifier diode VD1 is in a cutoff state, storing energy in primary winding. When switching lamp VT1 is turned off, energy stored in primary winding of transformer T is output to load after being rectified by secondary winding and VD1 and filtered by capacitor C.

The single-ended flyback power supply is lowest cost power supply. The output power is 20-100W. It can output various voltages at same time and has good voltage regulation speed. The only drawbacks are high ripple output voltage and poor external performance, which is suitable for relatively fixed loads. The maximum reverse voltage that switching tube VT1 used in single-ended flyback switching power supply can withstand is twice operating voltage of circuit, and operating frequency is in range of 20-200 kHz.

3. Single-ended direct switching power supply

A typical circuit for a single-ended direct switching power supply is shown in Figure 4. This circuit is similar in shape to a single-ended flyback circuit, but works differently. When switch VT1 is turned on, VD2 is also turned on, at this time grid transfers energy to load, and filter inductor L accumulates energy, when switch VT1 is turned off, inductor L continues to release energy to load through reverse diode VD3.

Also in circuit there is a limiting coil and diode VD2, which can limit maximum voltage of switching tube VT1 to twice supply voltage. To fulfill condition of resetting magnetic circuit, that is, time of establishment and reset of magnetic flux must be equal, therefore, duty cycle of pulse in circuit cannot be more than 50%. Since this circuit transfers power to load through transformer when switching lamp VT1 is turned on, output power range is large, and 50-200W of power can be output. The transformer used in circuit has a complex design and large volume, for this reason there is little practical application of this circuit.

4. Switching regulated power supply with self-excitation

A typical diagram of a switching regulated power supply with self-excitation is shown in Figure 5. This is a switching power supply consisting of oscillatory circuits of periodic action, as well as one of main power supplies widely used at present.

When power supply is connected, R1 provides inrush current to VT1's switch lamp so that VT1 starts conducting and its collector current Ic increases linearly in L1 and induces in L2 to make VT1's base positive and emitter very negative Positive feedback voltage causes VT1 to saturate very quickly. At same time, induced voltage charges C1. As charging voltage of C1 increases, base potential of VT1 gradually decreases, causing VT1 to go out of saturation and Ic starts to decrease. turning off VT1, then VD1 diode turns on, and energy stored in primary winding of high-frequency transformer T is released into load. When VT1 is off, there is no induced voltage in L2 and DC input voltage charges C1 through R1 in reverse, gradually increasing base potential of VT1, causing it to turn on again, flipping again to reach saturation, and circuit keeps oscillating like this. Here, as in a single-cycle flyback switching power supply, secondary winding of transformer T supplies required voltage to load. The switching tube in self-excited switching power supply plays dual role of switching and generating, and control circuit is also missing. In circuit, since load is located on secondary side of transformer and operates in flyback mode, it has advantages of mutual isolation between input and output. This circuit is suitable not only for high power supply, but also for low power supply.

5. Push-pull switching power supply

A typical circuit of a push-pull switching power supply is shown in Figure 6. It belongs to two-way conversion circuit, and magnetic circuit of high-frequency transformer operates on both sides of hysteresis loop. The circuit uses two switching lamps VT1 and VT2, and two switching lamps are turned on and off alternately under control of an external excitation square wave signal, and a square wave voltage is obtained in secondary group of transformer T, which is converted into required DC current by rectification and filtering.

The advantage of this circuit is that two switching tubes are easy to control, but main disadvantage is that switching tube's withstand voltage must be twice circuit's peak voltage. The output power of circuit is relatively high, typically in range of 100-500 watts.

6. Step-down switching power supply

A typical circuit of a step-down switching power supply is shown in Figure 7. When VT1 switch lamp is turned on, VD1 diode is turned off, and input rectified voltage is charged to C through VT1 and L, and this current increases energy reserve in inductor L. When VT1 switch tube is turned off , inductor L induces a negative left and positive voltage, and releases energy stored in inductor L through load RL and shunt diode VD1 to keep output constant voltage constant. The level of DC voltage output by circuit is determined by width of pulse applied to VT1 base.

This circuit uses few components. As with other two circuits below, only inductors, capacitors, and diodes should be used.

7. Boost switching power supply

The voltage regulator circuit of switching boost power supply is shown in Figure 8. When switch lamp VT1 is turned on, inductor L stores energy. When switch lamp VT1 is turned off, inductance L induces a left negative and a right positive voltage, which is superimposed on input voltage and supplies power to load through VD1 diode, so that output voltage is greater than input voltage, forming a switching boost power supply. 8. Reversible power supply

A typical circuit of a reverse switching power supply is shown in Figure 9. This circuit is also called a switching power supply. Regardless of whether pulsating DC voltage in front of switch lamp VT1 is higher or lower than stable output pin voltage, circuit can work normally.

When switch lamp VT1 is turned on, inductor L stores energy, diode VD1 turns off, and load RL is powered by last charge of capacitor C. When switch tube VT1 is turned off, current in inductor L continues to flow and induces a voltage of upper negative and lower positive, which supplies power to load through diode VD1 and simultaneously charges capacitor C. The above represents basic working principle and various types of pulse width modulated switching power supply circuits. In practical applications, there will be various actual control circuits, but despite no matter what, they are all designed on their basis. .