How much do you know about LED driver, and do you know all these parameters?

LED is one of the most sensitive devices of semiconductor devices. Because of its negative temperature characteristics, it needs to be stable and protected in the process of application, resulting in the concept of drive. LED applications cover almost all fields of electronics applications, and its luminous intensity, light color, on-off control and other changes are almost unpredictable. Therefore, LED drivers have become almost one-to-one servo devices, making the members of this device family diverse. From this point of view, it is necessary to have a deep understanding of the parameters of LED driver. Let's have a deep understanding together!

1, How much do you know about LED driver power?

■ Input voltage range

The user sees that the marked input voltage range on the power supply is 85-265VAC, but it is 100-240VAC in actual use. In fact, the so-called tightening ± 10% test (IEC60950 tightening+6% - 10%) will be carried out during the safety certification, so the voltage range defined in the power supply specification will not have problems in use; The label on the power supply is to meet the safety regulations and ensure that the user can input the power correctly.

■ Power factor (PFC)

PFC (Power Factor Correction) power factor correction is mainly to improve the ratio of effective power and apparent power at the input end of the power supply. Generally, for models without PFC lines, the power factor at the input end is only 0.4~0.6, while for models with active PFC lines, it can reach more than 0.95, and the correlation formula is as follows:

Apparent power=input voltage × Input current (VA)

Effective power=input voltage × Input current × Power factor (W)

From the perspective of environmental protection: the power plant of the power company must produce more power than the apparent power, and its power generation unit can stably supply the market power demand, while the actual use of power is the effective power. If the power factor is 0.5, it means that the generator unit can safely supply the demand of 1W of electric energy only when it sends more than 2VA of power, and its energy operation efficiency is poor. On the contrary, if the power factor is improved to 0.95, as long as the generating unit of the power company sends more than 1.06VA of power, the demand for supplying 1W of electric energy will be no problem, and the energy operation efficiency is better.

■ Protection function

Overvoltage/over-current/overload/over-temperature fault protection refers to the protection action that occurs when the power supply is unable to work normally due to the change of internal and external conditions such as input power supply, load, environment, cooling circuit or device failure, which threatens the safety of the power supply.

OVP: Over Voltage Protection. A feature of the switching power supply circuit is to protect the switching power supply and load in case of abnormal high voltage at the output end.

Undervoltage protection: when the power supply voltage of the protected line is lower than a certain value, the protector will cut off the line; When the power supply voltage is restored to the normal range, the protector will automatically switch on.

OCP: Over Current Protection. In the DC switching power supply circuit, in order to protect the regulating tube from being burnt when the circuit is short circuited and the current increases. The basic method is that when the output current exceeds a certain value, the adjusting tube is in the reverse bias state, thus cutting off and automatically cutting off the circuit current.

Short Circuit Protection: limit the output current of the switching power supply to a safe value in case of short circuit to protect the switching power supply from damage.

OTP: overheat protection circuit (Over Temperature Protection. The high integration and light weight and small volume of the switching voltage regulator in the DC switching power supply greatly improve the power density per unit volume. Therefore, if the requirements of the components in the power supply device for their working environment temperature are not improved accordingly, the circuit performance will deteriorate and the components will fail prematurely. Therefore, the overheating protection circuit should be set in the high-power DC switching power supply.

There are several ways to protect (act):

1. Restart (the power supply will return to normal after being disconnected and connected again. There are two types: automatic and manual);

2. Hiccup (intermittent output);

3. Fold back limiting (a method that can linearly reduce the output current to the normal value when the load is close to short circuit);

4. Constant current lim (or constant current lim) can limit the output current from increasing indefinitely due to overload or short circuit of the load. Even if the load has short circuit, it will not cause equipment shutdown and power damage).

5. Downtime overcurrent/overload/overvoltage/over-temperature fault usually refers to the dangerous state when the output current/power/(or input) voltage and radiator temperature exceed the protection threshold above the rated value of the power supply.

■ Surge current

The switching power supply will have a short (1/2~1 power cycle, EX: 60Hz power supply 1/120~1/60 seconds) large current (about 20~60A according to the product design, please refer to the product specification) at the moment of power transmission of the input power supply. After the product is started, the normal current input will resume. It will occur at the moment of power transmission at the power input end each time. This is a normal phenomenon and will not cause damage to the power supply. However, it is not recommended to continuously turn on/off the power supply. In addition, it should be noted that if multiple power supplies are used to start at the same time, it may cause the tripping action of the protection switch of the system power distribution. It is recommended to delay the start of multiple power supplies or use the remote control function of the power supply product to delay the start of the product in sequence.

■ Output voltage accuracy

Output voltage accuracy refers to the difference between the actual output voltage and the rated output voltage. This error is the superposition value of line stability and load stability. Generally, the parameter+/- 1% line stability refers to the percentage of output voltage deviation from rated voltage when the input voltage changes between the maximum and minimum values of the allowable range. Load stability refers to the percentage of output voltage deviation from rated voltage when the output load current changes between the maximum and minimum values of the allowable range.

2, Why use constant current power to drive LED lamps?

The life of LED refers to the time of light decay. The constant current drive controls the current of LED, ensures that the junction temperature of LED chips is not too high, and prevents the abnormal aging of semiconductor chips, packaging materials, and fluorescent materials. The luminous intensity of LED will not decrease too fast (i.e. light decay). The use of other types of power supply can not control the constant current of LED, and its temperature rise is not easy to be controlled, resulting in the occurrence of light decay.

The luminous intensity of LED is proportional to the current. Therefore, the LED driver power supply needs to have constant current output characteristics to ensure that the LED can obtain stable luminous intensity and long life guaranteed by the manufacturer during use. In order to ensure constant current driving LED, the LED must be connected in series to ensure that the current of each LED in the circuit is equal and constant. When the power demand of LED lamp sets is increasing, the number of LED series is increasing, and the voltage demand is proportional to the number of series. As a result, the voltage is higher and higher, and the safety is easy to be a problem. The manufacturing and use requirements will be more strict, which will bring higher costs and difficulties to the power supply. Therefore, the drive of high-power LED has the requirement of low-voltage drive.

The power supply of CV+CC can work at constant voltage or constant current.

Efficiency: the ratio of total output power to active input power expressed in percentage. That is, efficiency=output power/input power * 100%.

Rated power: refers to the maximum output power of the power supply (product of voltage V and current A).

EMC: Electromagnetic compatibility (EMC) refers to the ability of the equipment or system to operate in accordance with the requirements in its electromagnetic environment without generating unbearable electromagnetic interference to any equipment in its environment. EMC includes EMI (electromagnetic interference) and EMS (electromagnetic tolerance). The so-called EMI refers to the harmful energy conducted or radiated by switching power supply. EMS refers to the ability of switching power supply not to be affected by the surrounding electromagnetic environment in the process of performing its functions.

Ripple: Because DC stable power supply is generally formed by AC power supply through rectification and voltage stabilization, it is inevitable that there are some AC components in the DC stable quantity. This AC component superimposed on the DC stable quantity is called ripple.

Ripple and Noise, Output: The amplitude of the AC voltage output by the switching power supply within the specified bandwidth, usually expressed in millivolt peak-to-peak or RMS value.

Total Harmonic Distortion (THD). It refers to the additional harmonic part of the output signal (harmonic and its frequency doubling component) that is more than the input signal when the signal source is used for input, usually expressed as a percentage. Generally speaking, the total harmonic distortion at 1000Hz is the smallest, so many products take the distortion at this frequency as its indicator. Therefore, when testing the total harmonic distortion, a 1000Hz sound is emitted to detect it. The smaller the value, the better.

Overshoot and undershoot: overshoot means that the first peak or valley value exceeds the set voltage - the highest voltage for the rising edge and the lowest voltage for the falling edge. Downstream refers to the next valley or peak. Excessive overshoot can cause the protection diode to work, leading to premature failure. Excessive undershoot can cause false clock or data errors.

Temperature, Operating Ambient: The switching power supply can have reasonable electrical indicators and stable operating temperature range. Unless otherwise specified, do not think that the switching power supply can output full power in the whole temperature range, nor does it mean that the switching power supply can maintain the same electrical index in the whole operating temperature range.

PWM: Pulse Width Modulation: a voltage adjustment method used by switching power supply, which refers to controlling the output only by changing the width of pulse sequence.

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