To design an LED drive power supply, it is necessary to master the working principle of the LED. The brightness of LED is mainly related to UF and IF. Small changes in UF can cause large changes in I, which in turn cause large changes in LED brightness. To keep the LED in the best brightness state, it needs to be driven by a constant current power supply. The LED drive power supply is a power converter that outputs a constant current instead of a constant voltage. In any case, the LED drive power supply must output a constant and stable current, and the ripple current must be controlled within a certain range.

 When selecting the LED driving power supply, the UF value of the LED at the operating temperature should be determined, and the appropriate range of the output voltage (working voltage) of the driving power supply should be selected according to the number of connected LEDs and their series and parallel relationships. The total voltage obtained by multiplying the maximum UF value of the LED by the number of LEDs in series is the maximum operating voltage of the LED, which is more than 5V lower than the maximum output voltage of the power supply. The total voltage obtained by multiplying the LED minimum UF value by the number of LEDs in series is the minimum LED operating voltage, and the minimum operating voltage is more than 5V higher than the minimum output voltage of the power supply. That is to say, the working voltage range of the LED driving power supply is the total LED working voltage (3.2V×the number of LEDs in series)+5V~the total working voltage of the LED (3.2V×the number of LEDs in series)-5V. The total working power of the LED drive power is about 90% of the total power of the power supply.

 

In order to make the white light LED get a good application and obtain a higher use efficiency, a suitable LED driving power supply must be used to meet the working requirements of the LED. The requirements for the LED driver power supply are as follows.

1) As a special power supply for LED power supply, LED driving power supply should have the characteristics of simple circuit structure, small size and high conversion efficiency.

(2) The output electrical parameters (including output current, output voltage, etc.) of the LED drive power supply should match the parameters of the LED, and should have a high constant current precision control function and a suitable voltage limiting function.

(3) The LED drive power supply must have a dimming function with good linearity to meet the dimming requirements of different LED applications.

(4) When the LED is open-circuited, short-circuited or the driving circuit is faulty, the LED driving power supply should be able to provide corresponding protection for itself, the LED and the user.

(5) The LED drive power supply should meet the relevant electromagnetic compatibility requirements.

 

According to the application requirements of different occasions, the LED driver can use constant current (CC) power supply, constant voltage (CV) power supply, and constant current and constant voltage (CCCV) power supply.

1) Constant current power supply

The characteristics of the constant current power supply are as follows.

(1) The current output by the constant current power supply is constant, and the output DC voltage will vary within a certain range according to the size of the load resistance. The smaller the load resistance value, the lower the output DC voltage; the larger the load resistance value, the higher the output DC voltage.

(2) The circuit using the constant current power supply allows the load to be short-circuited, but it is strictly forbidden to open the load.

(3) It is an ideal way to use a constant current power supply to drive the LED, but the price is relatively high.

(4) The maximum voltage that the constant current power supply can withstand limits the number of LEDs used.

 

2) Constant voltage power supply

The characteristics of the constant voltage power supply are as follows.

(1) After the parameters in the constant voltage power supply are determined, the output voltage is fixed, and the output current will change with the increase or decrease of the number of loads.

(2) The constant voltage power supply is not afraid of the open circuit of the load. The current constant voltage power supply has the functions of short circuit, overcurrent and overload protection.

(3) If a constant voltage power supply is used as the LED driving power supply, it is necessary to connect a resistor in series in the series branch to make the brightness of each LED consistent.

(4) The brightness of the LED is affected by the voltage change from rectification.

 

3) Constant current and constant voltage power supply

The characteristics of constant current and constant voltage power supply are as follows.

(1) The constant current and constant voltage power supply will try to keep the output voltage stable when the load changes, and the output current must be less than the constant current value.

(2) When the constant current and constant voltage power supply is in constant voltage output, the constant current power supply is in a resting state and does not interfere with the output voltage and output current.

(3) When the constant current and constant voltage power supply is in constant current output, the constant voltage power supply is in a resting state and does not interfere with the output voltage.

 

 The following factors need to be considered when designing LED lighting fixtures.

(1) Output power, mainly related to LED forward voltage, current, LED connection method, LED package, LED power, etc.

(2) Driving power supply, the main types are AC-DC power supply, DC-DC power supply, AC power supply, and the driving power supply must be certified by 3C.

(3) Functional requirements, such as dimming requirements, dimming methods, lighting control, etc.

(4) Intelligent control, such as sensors, WiFi, ZigBee and Bluetooth, etc.

(5) Other factors such as energy efficiency, power factor, size, cost, troubleshooting, standards, reliability, and mechanical connections, installation, repair/replacement, life cycle, logistics, etc.

 

 At present, there are three main driving power sources for indoor LED lighting fixtures: built-in power supply, external power supply, and switching power supply.

 LED bulb lights, LED fluorescent lights, and LED corn lights are all built-in power supplies. The LEDs of LED bulbs are connected in series, and the number of LEDs in series does not exceed 20. The internal current of LED lighting fixtures with built-in power supply cannot exceed 280mA.

The LED corn lamp drive power is different from the LED bulb lamp drive power, because the LED corn lamp working environment is outdoor, and its drive power must be treated with glue.

 The potting glue that is suitable for electrical properties and hardness must be selected for the potting treatment of the LED corn lamp driving power supply.

 Advantages of LED corn lamp drive power supply: It has the functions of open circuit, short circuit, overload, over temperature, over voltage and under voltage protection, and has its own fuse circuit to ensure the safety of the product; it is suitable for working in high humidity and high dust environments ; Can pass high temperature test, aging test, efficacy test and short circuit test.

The LED fluorescent lamp driving power supply is long strip, it adopts wide voltage AC 100~277V design, the maximum output power is 25W, high stability, high reliability, and has short circuit, overcurrent, open circuit protection functions.

The external power supply is used for lamps with separate power supply and lamps, including LED ceiling lamps, LED down lamps, LED grill lamps, etc. The external power supply does not add the built-in power supply to the shell, but reasonably designs the relevant PCB according to the size of the shell, and must pass the relevant certification, such as 3C certification.

The switching power supply is a constant voltage power supply, which can supply power to LED cabinet lights or MR16 spotlights. Its working voltage is mostly DO12V or DC 24V. The size of the power is related to the number of LEDs driven, and its line loss should also be considered.