I believe that everyone is familiar with the switching power supply, which is very common in various industries and domestic appliances today.Next, we will introduce the main working principles of switching power supplies to you.

1、Parallel rectification bridge

In low-power planning, the parallel connection of the rectifier bridge is rarely used, but in some cases of high-power output, if you do not want to increase the new device's single rectifier bridge current and can not meet the input power requirements, the parallel connection of the rectifier bridge is needed.The reason why the parallel connection of the rectifier bridge cannot choose the method of DC parallel connection after the rectification of the two rectifier Bridges is that the rectifier bridge is not paired, and it is generally impossible to share the current by simply relying on its own V-I characteristics.This will result in inconsistent heating of the two rectifier Bridges.It is generally thought that the two diodes in a package are very matched, and the current can be evenly divided, and the parallel connection of the rectifier bridge can be completed.

2、Floating drive

In the driver circuit planning, it is often said that the MOS tube needs to float drive, then what is floating drive?In simple terms, the S-pole of the MOS tube is not directly connected to the ground of the control IC, that is, it is not common ground.People commonly use the BUCK circuit as an example, the ground of the control IC is generally common with the ground of the input power supply, and there is a diode between the S pole of the MOS tube and the ground of the input power supply, so the drive signal of the control IC cannot be directly connected to the gate of the MOS tube, and additional drive circuit or drive IC is required.For example, transformer-isolated drives or driver chips with bootstrap circuits like the IR2110.Of course, there are other ways, that is, to use other methods to power the control IC, and then connect the control IC to the S end of the MOS tube, so that it is not floating, and the output of the control IC can directly drive the MOS tube.

3、Hysteresis comparator

In the protection circuit, in order to prevent the protection circuit from oscillating back and forth near the protection point, a certain hysteresis is generally added.1M resistance plays a hysteresis effect, assuming there is no 1M resistance, it is obvious that the VF voltage reaches a low level of 2.5V op-amp output, and below 2.5V, the op-amp output is high.After adding 1M resistance, the 6-pin level is 0.7+ (2.5-0.7) *1000/1010=2.48V at the low current level of the op amp output.When VF is lower than the 6-pin level, the 7-pin output high level (assuming that the operation amplifier power supply is 15V, the 7-pin output can be calculated according to 14V) can calculate that the 6-pin level at this time is 2.5+ (14-2.5) *10/1010=2.61V, assuming that this is an input undervoltage protection circuit, and VF is 100: When the input voltage is higher than 261V, the circuit operates normally, and when the voltage is lower than 248V, it will undervoltage protection, which enhances the anti-interference ability of the protection circuit.Generally, the current hysteresis comparator is often used: over and under voltage protection circuit, turning lamp circuit, etc.

4、Error amplifier output clamp circuit

In the planning power supply, whether it is a constant voltage source or a constant current source, as long as it is a closed-loop control, there is always an error amplifier.Before entering the closed loop, the error amplifier output voltage is the highest value.Normally, the power supply of the error amplifier is generally about 15V, then the output of the error amplifier is divided into about 14V when the loop is opened, and the input signal is increased to reach the highest value.

As an important part of switching power supply, LED switching power supply technology has developed rapidly as society and demand change.It has been used in the market for many years, and its stability and security are getting better and better. Therefore, the requirements for technology are getting higher and higher.When studying the design of led switching power supply, we first need to understand its classification.

From a large perspective, according to the drive mode, or according to the power supply mode, it can be divided into two categories:

First kind: Constant current

1.Constant current drive circuits are ideal for LED.

2.Constant current power supply The power supply circuit prohibits complete load routing.

3.The output current of the optocoupler circuit of the constant current power supply is stable, and the output AC voltage will change with the size of the load resistance value in a certain range

4.constant current power supply to the top has a larger bearing current and working voltage value.Therefore, please limit the total number of led applications.

Second kind: Voltage regulator tube type

1.After clarifying various main parameters of the regulated power supply circuit, the output is a fixed operating voltage, and the output current is changed with the adjustment of the load.

2.Although the regulated power supply circuit is not afraid of the load guide, it is forbidden to completely short-circuit the load.

3.Through the rectifier after the working voltage transition will be very easy to harm the LED chromaticity.

4.Please be sure to add a suitable resistor, so that each string of regulated power supply circuit driver LED display information chromaticity can be symmetrical.

According to the design structure of the circuit, it can be divided into six categories.

1.Capacitor buck mode: This mode is susceptible to power supply voltage fluctuations, low efficiency and reliability. Since the capacitor reduces the voltage, the instantaneous current flowing through the LED is very large due to the charge and discharge when the flash is used, which is easy to damage the chip.

2. Resistor blood pressure reduction method: this method to electricity is very low efficiency, credibility is not high, the net weight of the appearance design or focus, the advantage is small size, in general, the switching power supply application of such methods is less.

3.Basic transformer blood pressure reduction method: this kind of switching power supply will be very few applications.Although the volume of the design is small, the net weight is heavy, the efficiency is low, and the credibility is not high.

4.Electronic transformer step-down mode: the disadvantage of this mode is low power efficiency, voltage range is not wide, ripple interference is large, the market is less used for this type of power products.

5.RCC buck mode switching power supply: the advantage of this mode is high efficiency and widely used. 【LED switching power supply 】 The disadvantage is that the vibration frequency of the control mode is discontinuous, the switching frequency is not easy to control, the abnormal load adaptability is poor, and the load voltage ripple coefficient is relatively large, which is not recommended.

6.PWM control mode switching power supply: This mode power supply is recommended for design. It consists of four parts, namely, the input rectifier filter part, the output rectifier filter part, the PWM voltage regulation control part and the switching energy conversion part.The basic working principle is that when the input voltage, internal parameters, external load changes, the control circuit through the controlled signal and the reference signal difference for closed-loop feedback, adjust the pulse width of the main circuit switching devices, so that the output voltage or current of the switching power supply is stable (that is, the corresponding voltage regulation power supply or constant current power supply). The advantages are high efficiency and stable output voltage and current.This design type of power supply has complete protection measures.

Finally, what are the main working principles of switching power supply?

1、Matters needing attention when choosing switching power supply.

1) Select the appropriate input voltage specification.

2）Choose the right power. In order to increase the life of the power supply, we can choose 30% more output power rated models.

3）Consider load characteristics. If the load is a motor, bulb or capacitive load, the current is large when the machine is turned on, and the appropriate power supply should be selected to avoid overload. If the load is motor should be considered when shutdown voltage backflow.

4）In addition, it is necessary to consider the working environment temperature of the power supply, and whether there is additional auxiliary heat dissipation equipment. In the case of high ring temperature, the power supply needs to reduce the output. The decrement curve of ring temperature on output power.

5) Select various functions according to the application requirements:

Protection functions: Over voltage protection (OVP), over temperature protection (OTP), over load protection (OLP), etc.

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

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

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

2、Precautions for using switching power supply

1）Before using the power supply, it is necessary to determine whether the input and output voltage specifications are consistent with the nominal value of the power supply.

2）Before powering on, check that the leads of the input and output are connected correctly to avoid damaging the user's equipment.

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

4) To ensure the safety of use and reduce interference, ensure that the ground terminal is reliably grounded.

5) Multi-output power supply is generally divided into main and auxiliary output, the main output characteristics are better than the auxiliary output, under normal circumstances, the main output current is large.In order to ensure the output load adjustment rate and output dynamics, it is generally required that each channel carry at least 10% load.If the auxiliary road does not use the main road, the main road must add appropriate false load. For details, please refer to the specifications of the corresponding model.

6) Attention please:Frequent switching of power supply will affect its life.

7) Working environment and load degree will also affect its life.