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Circuit design, circuit diagram, and working principle of eight types of switching power supplies

Article Source：Kinri Energy | Author：Kinri Energy | Issuing Time：2024.04.16
Today I will share with you a detailed explanation of the circuit design and working principle diagram of a switching power supply.

1.Introduction to Switching Power Supply

Switching power supply, also known as switching power supply or switching converter, is a high-frequency electrical energy conversion device and a type of power supply. Its function is to convert a precise voltage into the voltage or current required by the user through different forms of architecture.


switching power supply

2. Basic composition of switch mode power supply

(1) Main circuit

Impulse current limiting: limits the impulse current on the input side at the moment the power is turned on.

Input filter: Its function is to filter out the clutter existing in the power grid and feedback the clutter that hinders the generation of the machine back to the power grid.

Rectification and filtering: Directly rectifying the AC power supply of the power grid into smoother DC power.

Inversion: Transforming rectified DC power into high-frequency AC power, which is the core part of high-frequency switching power supplies.

Output rectification and filtering: Provide stable and reliable DC power supply according to load requirements.

(2) Control circuit

On the one hand, samples are taken from the output end, compared with the set value, and then the inverter is controlled to change its pulse width or frequency to ensure stable output. On the other hand, based on the data provided by the test circuit and identified by the protection circuit, control circuits are provided to provide various protection measures for the power supply.

(3) Measuring circuit

Provide various parameters and instrument data that are currently in operation in the protection circuit.

(4) Auxiliary power supply

Implement software (remote) startup of the power supply to provide power for the protection circuit and control circuit (PWM and other chips).

3. Detailed explanation of the switch power supply schematic

The basic working principle of switch type regulated power supply

The switching power supply is connected to the control mode, which can be divided into two types: the width modulation type and the frequency modulation type. In practical applications, the width modulation type is widely used. In the current development and use of switching power supply integrated circuits, the vast majority are also pulse width modulation type. Therefore, the following will mainly introduce the width adjustable switch stabilized power supply.


Basic switching power supply block diagram

For unipolar rectangular pulses, the average DC voltage Uo depends on the width of the rectangular pulse. The wider the pulse, the higher the average DC voltage value. The average DC voltage U can be calculated by the formula, i.e

Uo=Um x T1/T

●  Um is the rectangular pulse * high voltage value
●  T is the rectangular pulse period
●  T1 is the rectangular pulse width.

From the above equation, it can be seen that when Um and T remain constant, 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 as the output voltage of the voltage regulator increases, we can achieve the goal of stabilizing the voltage.

4. Principle Circuit of Switching Power Supply - Schematic Diagram of Switching Power Supply

(1) Basic circuit


Basic circuit diagram of the implemented switching power supply

After the AC voltage is rectified and filtered by the rectification circuit and filtering circuit, it becomes a DC voltage with a certain pulsating component. This voltage enters the high-frequency converter and is converted into a square wave of the required voltage value. Finally, this square wave voltage is rectified and filtered to become the required DC voltage.

The control circuit is a pulse width modulator, which mainly consists of a sampler, comparator, oscillator, pulse width modulation, and reference voltage circuits. This part of the circuit has been integrated and made into various integrated circuits for switching power supplies. The control circuit is used to adjust the switching time ratio of high-frequency switching components to achieve stable output voltage.

(2) Switching Power Supply Schematic - Single ended Flyback Switching Power Supply

The typical circuit of a single ended flyback switching power supply is shown in the following figure. The so-called single ended in the circuit refers to the magnetic core of the high-frequency converter working only on one side of the hysteresis loop. The so-called flyback refers to when the switching transistor VT1 is turned on, the induced voltage of the primary winding of the high-frequency transformer T is positive and negative, and the rectifier diode VD1 is in the cut-off state, storing energy in the primary winding.

When the VT1 switch is turned off, the energy stored in the primary winding of transformer T is output to the load through the secondary winding, VD1 rectification, and capacitor C filtering.


Single ended flyback switching power supply

Single ended flyback switching power supply is a low-cost power supply circuit with an output power of 20-100W, which can simultaneously output different voltages and has good voltage adjustment rate. The only drawback is that the output ripple voltage is relatively large, the external characteristics are poor, and it is suitable for relatively fixed loads.

The maximum reverse voltage borne by the switching transistor VT1 used in a single ended flyback switching power supply is twice the circuit operating voltage value, and the operating frequency is between 20-200kHz.

(3) Schematic diagram of switching power supply - single ended forward switching power supply

The typical circuit of a single ended forward switching power supply is shown in the following figure. This circuit is similar in form to a single ended flyback circuit, but operates differently. When the switching transistor VT1 is turned on, VD2 is also turned on. At this time, the power grid transmits energy to the load, and the filtering inductor L stores energy; When the VT1 switch is turned off, the inductor L continues to release energy to the load through the freewheeling diode VD3.


Single ended forward switching power supply

There is also a clamp coil and diode VD2 in the circuit, which can limit the maximum voltage of the switching transistor VT1 to twice the power supply voltage. To meet the reset conditions of the magnetic core, that is, the magnetic flux establishment and reset time should be equal, so the duty cycle of the pulse in the circuit cannot exceed 50%. Due to the fact that this circuit transfers energy to the load through a transformer when the VT1 switch is turned on, it has a wide output power range and can output a power of 50-200W. The transformer used in the circuit has a complex structure and a large volume, which is why its practical application is limited.

(4) Schematic diagram of switch power supply - self-excited switch regulated power supply

The typical circuit of a self-excited switch regulated power supply is shown in the following figure. This is a switch mode power supply composed of intermittent oscillation circuits and is currently one of the widely used basic power supplies.


Self-excited switch regulated power supply

After connecting to the power supply, starting current is provided to the switching transistor VT1 in R1, causing VT1 to start conducting. The collector current Ic increases linearly in L1, and a positive feedback voltage is induced in L2 that makes the base of VT1 positive and the emitter negative, causing VT1 to quickly saturate.

At the same time, the induced voltage charges C1. As the charging voltage of C1 increases, the base potential of VT1 gradually decreases, causing VT1 to exit the saturation zone and Ic to decrease. In L2, a voltage is induced that causes the base of VT1 to be negative and the emitter to be positive, causing VT1 to quickly cut off. At this time, diode VD1 conducts, and the energy storage in the primary winding of the high-frequency transformer T is released to the load. At the cut-off time of VT1, there is no induced voltage in L2, and the DC power supply input voltage is then reverse charged to C1 through R1, gradually increasing the base potential of VT1, causing it to conduct again, flip again to reach saturation state, and the circuit oscillates repeatedly like this.

Here, just like a single ended flyback switching power supply, the secondary winding of transformer T outputs the required voltage to the load. The switching transistor in a self-excited switching power supply plays a dual role in switching and oscillation, eliminating the need for control circuits. Due to the load being located in the secondary of the transformer and operating in the flyback state, the circuit has the advantage of isolating the input and output. This circuit is not only suitable for high-power power sources, but also for low-power power sources.

(5) Schematic diagram of switch mode power supply - push-pull switch mode power supply

The typical circuit of a push-pull switching power supply is shown in Figure 6. It belongs to a dual ended conversion circuit, and the magnetic core of the high-frequency transformer works on both sides of the hysteresis loop. The circuit uses two switching tubes VT1 and VT2, which alternately conduct and cut off under the control of an external excitation square wave signal. The square wave voltage is obtained in the secondary system of transformer T, which is rectified and filtered to obtain the required DC voltage.


Push-pull switching power supply

The advantage of this circuit is that the two switching tubes are easy to drive, but the main disadvantage is that the withstand voltage of the switching tubes needs to reach twice the peak voltage of the circuit. The output power of the circuit is relatively high, generally within the range of 100-500W.

(6) Schematic diagram of switch mode power supply - step-down switch mode power supply

The typical circuit of a step-down switching power supply is shown in the following figure. When the switching transistor VT1 is turned on, diode VD1 is cut off, and the input rectified voltage is charged to C through VT1 and L. This current increases the energy storage in inductor L. When the switching transistor VT1 is turned off, the inductor L senses a voltage that is negative on the left and positive on the right. The energy stored in the inductor L is released through the load RL and the freewheeling diode VD1, maintaining the output DC voltage unchanged. The output DC voltage of the circuit is determined by the pulse width applied to the VT1 base.


Step-down switching power supply

This circuit uses fewer components and, like the other two circuits introduced below, only requires the use of inductors, capacitors, and diodes to achieve it.

(7) Schematic diagram of switch mode power supply - step-up switch mode power supply

The voltage stabilizing circuit of the step-up switching power supply is shown in the following figure. When the VT1 switch is turned on, the inductor L stores energy. When the VT1 switch is turned off, the inductor L induces a voltage of left negative and right positive, which is superimposed on the input voltage. The voltage is supplied to the load through diode VD1, causing the output voltage to be greater than the input voltage, forming a step-up switching power supply.


Pressure generating switching power supply

(8) Switching Power Supply Schematic - Reverse Switching Power Supply

The typical circuit of a reverse switching power supply is shown in the following figure. This type of circuit is also known as a step-down switching power supply. Regardless of whether the pulsating DC voltage before the VT1 switch is higher or lower than the stable voltage at the output terminal, the circuit can operate normally.


Reverse switching power supply

When the switching transistor VT1 is turned on, the inductor L stores energy, the diode VD1 is cut off, and the load RL is powered by the last charging charge of the capacitor C. When the switching transistor VT1 is turned off, the current in inductor L continues to flow and induces positive and negative voltages, which supply power to the load through diode VD1 and charge capacitor C.

5. Analysis of the Development Direction of Switching Power Supply

The high-frequency of switching power supplies is the direction of its development. High frequency makes switching power supplies miniaturized and enables them to enter a wider range of application fields, especially in high-tech fields, driving the development of switching power supplies. Every year, with a growth rate of more than two digits, it develops towards the direction of light, small, thin, low noise, high reliability, and anti-interference.

Switching power supplies can be divided into two categories: AC/DC and DC/DC. DC/DC converters have now achieved modularization, and the design technology and production process have become mature and standardized both domestically and internationally, and have been recognized by users. However, the modularization of AC/DC, due to its own characteristics, encounters complex technical and manufacturing problems in the process of modularization.

In addition, the development and application of switch mode power supplies are of great significance in energy conservation, resource conservation, and environmental protection. The power electronic devices used in switch mode power supplies mainly include diodes, IGBTs, MOSFETs, and transformers.
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