Switching power supply is a type of power supply that uses modern power electronics technology to control the time ratio of the on and off of the switching transistor, maintaining a stable output voltage. Switching power supply is generally composed of pulse width modulation (PWM) control IC and MOSFET. With the development and innovation of power electronics technology, switch mode power supply technology is also constantly innovating. At present, switch mode power supplies are widely used in almost all electronic devices due to their small size, lightweight, and high efficiency, and are an indispensable power supply method for the rapid development of the electronic information industry today.

Main purpose

Switching power supply products are widely used in industrial automation control, military equipment, scientific research equipment, LED lighting, industrial control equipment, communication equipment, power equipment, instruments and meters, medical equipment, semiconductor refrigeration and heating, air purifiers, electronic refrigerators, LCD displays, LED lights, communication equipment, audio-visual products, security monitoring, LED light bags, computer cases, digital products and instruments.

Basic Composition

Switching power supply is roughly composed of four parts: main circuit, control circuit, detection circuit, and auxiliary power supply.

1. Main circuit

Impulse current limiting: Limit 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 that exists in the power grid and hinder the feedback of the clutter generated by the machine back to the power grid.

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

Inverter: Convert rectified DC power into high-frequency AC power, which is the core part of high-frequency switching power supply.

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 terminal, compared with the set value, and then the inverter is controlled to change its pulse width or frequency to stabilize the output. On the other hand, based on the data provided by the testing circuit, various protection measures are provided to the power supply by the control circuit after identification by the protection circuit.

3. Detection circuit

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

4. Auxiliary power supply

Implement software (remote) startup for power supply, providing power supply for protection circuits and control circuits (PWM chips, etc.).

Main categories

In the field of switch mode power supply technology, people are developing related power electronic devices and switch frequency conversion technology at the same time. The two mutually promote and drive the development of switch mode power supplies towards the direction of light, small, thin, low-noise, high reliability, and anti-interference with an annual growth rate exceeding two digits. Switching power supplies can be divided into two categories: AC/DC and DC/DC.

Micro Low Power Switching Power Supply

Switching power supplies are moving towards popularization and miniaturization. Switching power supplies will gradually replace transformers in all applications in daily life. The application of low-power micro switching power supplies should first be reflected in digital displays, smart meters, mobile phone chargers, and other aspects. At present, the country is vigorously promoting the construction of smart grids, and the requirements for energy meters have been greatly increased. Switching power supplies will gradually replace transformers in the application of energy meters.

Reverse series switching power supply

The difference between a reverse series switching power supply and a general series switching power supply is that the output voltage of this reverse series switching power supply is a negative voltage, which is exactly opposite in polarity to the positive voltage output of a general series switching power supply; And because the energy storage inductor L only outputs current to the load when the switch K is turned off, under the same conditions, the current output of the inverted series switching power supply is twice as small as that of the series switching power supply.

working principle

The working process of a switching power supply is quite easy to understand. In a linear power supply, the power transistor is operated in linear mode. Unlike a linear power supply, a PWM switching power supply operates the power transistor in on and off states. In these two states, the voltage ampere product applied to the power transistor is very small (low voltage and high current when conducting; high voltage and low current when turning off)/the volt ampere product on the power device is the loss generated on the power semiconductor device.

Compared with linear power supplies, the more effective working process of PWM switching power supplies is achieved through "chopping", which means chopping the input DC voltage into a pulse voltage with an amplitude equal to the input voltage amplitude.

The duty cycle of the pulse is adjusted by the controller of the switching power supply. Once the input voltage is chopped into AC square waves, its amplitude can be increased or decreased through a transformer. By increasing the number of secondary windings of the transformer, the output voltage value can be increased. Finally, these AC waveforms are rectified and filtered to obtain a DC output voltage.

The main purpose of the controller is to maintain stable output voltage, and its working process is similar to that of a linear form controller. That is to say, the functional blocks, voltage reference, and error amplifier of the controller can be designed to be the same as those of a linear regulator. Their difference is that the output (error voltage) of the error amplifier needs to go through a voltage/pulse width conversion unit before driving the power transistor.

Switching power supplies have two main working modes: forward conversion and boost conversion. Although there is little difference in the arrangement of their various parts, the working process varies greatly, and each has its own advantages in specific application scenarios.

Working conditions

1. Switch: Power electronic devices operate in a switch state rather than a linear state

2. High frequency: Power electronic devices operate at high frequencies rather than low frequencies close to the power frequency

3. DC: Switching power supplies output DC instead of AC

main features

1. Small size and light weight: Due to the absence of power frequency transformers, the volume and weight are only 20-30% of linear power supplies.

2. Low power consumption and high efficiency: Power transistors work in a switching state, so the power consumption on the transistor is low and the conversion efficiency is high, generally 60-70%, while linear power supplies only have 30-40%.

Usage Guide

Output calculation

Due to the high efficiency of switch mode power supplies, which can generally reach over 80%, the maximum absorption current of the electrical equipment should be accurately measured or calculated in the selection of its output current, so that the selected switch mode power supply has a high performance price ratio. The usual output calculation formula is:

Is=KIf

In the formula: Is - rated output current of the switching power supply;

If - maximum absorption current of electrical equipment;

K - margin coefficient, generally taken as 1.5 to 1.8;

Grounding

Switching power supplies generate more interference than linear power supplies. Electrical equipment sensitive to common mode interference should be grounded and shielded. According to EMC restrictions such as ICE1000, EN61000, FCC, etc., switching power supplies should adopt EMC electromagnetic compatibility measures. Therefore, switching power supplies should generally be equipped with EMC electromagnetic compatibility filters. For the HA series switching power supply from Lide Huafu Technology, the FG terminal should be grounded or connected to the user's casing to meet the electromagnetic compatibility requirements mentioned above.

protection circuit

Switching power supplies must have protection functions such as overcurrent, overheating, and short circuit in their design. Therefore, a switching power supply module with complete protection functions should be preferred in the design, and the technical parameters of its protection circuit should match the working characteristics of the electrical equipment to avoid damage to the electrical equipment or switching power supply.

wiring method

L: Connect to 220V AC live wire

N: Connect 220V AC neutral wire

FG: Grounding

G: Ground for DC output

+5V: Port for outputting+5V points

ADJ: It adjusts the output voltage within a certain range. The rated voltage output on the switching power supply is originally fixed at the factory, that is, the nominal rated output voltage. Setting this potentiometer allows users to adjust the output voltage within a smaller range according to actual usage, and generally does not require adjustment.

matters needing attention

1. Precautions when choosing a switch power supply

1) Select appropriate input voltage specifications;

2) Choose the appropriate power. To extend the lifespan of the power supply, models with an additional 30% output power rating can be selected.

3) Consider the load characteristics. If the load is a motor, light bulb, or capacitive load, and the current is high at the moment of startup, a suitable power source should be selected to avoid overload. If the load is a motor, consideration should be given to voltage backflow during shutdown.

4) In addition, it is necessary to consider the working environment temperature of the power supply and whether there are additional auxiliary cooling devices. The power supply needs to reduce its output in high ambient temperatures. The derating curve of output power under ambient temperature.

5) Select various functions according to the application requirements:

Protection functions: overvoltage protection (OVP), over temperature protection (OTP), overload protection (OLP), etc.

Application functions: signal function (normal power supply, power failure), remote control function, telemetry function, parallel function, etc.

Special functions: Power factor correction (PFC), uninterruptible power supply (UPS)

6) Select the required safety regulations and electromagnetic compatibility (EMC) certifications.

2. Precautions for using switch mode power supply

1) Before using a power supply, ensure that the input and output voltage specifications match the nominal value of the power supply being used;

2) Before powering on, check if the input and output leads are connected correctly to avoid damaging the user's device;

3) Check whether the installation is firm, whether the installation screws are in contact with the power board components, measure the insulation resistance between the shell and the input and output to avoid electric shock;

4) To ensure the safety of use and reduce interference, please ensure that the grounding terminal is reliably grounded;

5) Multi output power supplies are generally divided into main and auxiliary outputs, with the main output having better characteristics than the auxiliary output. In general, the main output has a larger output current. To ensure the output load adjustment rate and output dynamics, it is generally required that each channel carry at least 10% load. If auxiliary roads are used instead of main roads, appropriate false loads must be added to the main roads. Please refer to the specifications of the corresponding model for details;

6) Please note: Frequent switching of the power supply will affect its lifespan;

7) The working environment and load capacity can also affect its lifespan& nbsp;

Common faults

The fuse is blown

In general, a blown fuse indicates a problem with the internal wiring of the power supply. Due to the high voltage and high current operation of the power supply, fluctuations and surges in the grid voltage can cause an instantaneous increase in the current inside the power supply, leading to the melting of fuses. The key should be to check the rectifier diode, high-voltage filtering electrolytic capacitor, inverter power switch tube, etc. at the input end of the power supply, and check whether these components have breakdown, open circuit, damage, etc. If the fuse is indeed blown, you should first check the various components on the circuit board to see if their surfaces have been burnt or if there is electrolyte overflow. If the above situation is not found, use a multimeter to measure whether the switch tube has a breakdown or short circuit. It is important to note that when a component is found to be damaged, it should not be replaced and turned on directly. This may cause damage to the replaced component due to other high-voltage components still having faults. It is necessary to conduct a comprehensive inspection and measurement of all high-voltage components in the above circuit before completely eliminating the fault of fuse blowing.

No DC voltage output or unstable voltage output

If the fuse is intact, there will be no output of DC voltage at all levels under load. This situation is mainly caused by the following reasons: open circuit and short circuit in the power supply, overvoltage and overcurrent protection circuit failure, auxiliary power supply failure, oscillation circuit not working, excessive power load, breakdown of rectifier diodes in the high-frequency rectification and filtering circuit, leakage of filtering capacitors, etc. After measuring the secondary components with a multimeter and ruling out the breakdown of high-frequency rectifier diodes and load short circuits, if the output is zero at this time, it can be confirmed that there is a fault in the control circuit of the power supply. If there is partial voltage output, it indicates that the front-end circuit is working normally, and the fault lies in the high-frequency rectification and filtering circuit. The high-frequency filtering circuit is mainly composed of rectifier diodes and low-voltage filtering capacitors to output DC voltage. The breakdown of rectifier diodes will cause the circuit to have no voltage output, and the leakage of filtering capacitors will cause faults such as unstable output voltage. By using a multimeter to statically measure the corresponding components, damaged components can be detected.

Poor power load capacity

Poor load capacity of power supply is a common fault, which usually occurs in old or long-term power supplies. The main reasons are aging of various components, unstable operation of switch tubes, and failure to dissipate heat in a timely manner. Special attention should be paid to checking whether the voltage regulator diode is overheating or leaking electricity, whether the rectifier diode is damaged, and whether the high-voltage filter capacitor is damaged.