- Constant current drive circuit
When evaluating LED lamps, LED driving circuit is an important factor. Many simple drive power supplies only provide a constant voltage output, which means that the output current varies with the LED voltage. A constant voltage drive can cause early failure of the LED. As the temperature of the LED increases, the threshold voltage decreases, causing the constant voltage driving power supply to provide more current in response to the decrease of the LED voltage; This is because the band gap of the semiconductor decreases with the increase of temperature. If the current exceeds a certain limit, the LED will be damaged.
LED is suitable for constant current driving circuit. When the voltage of LED changes with temperature, it can maintain the stability of LED. Since the brightness of LED is a function of current, its performance also changes with current.
LED is a current driver whose brightness is proportional to the forward current. The forward current can be controlled in two ways. The first method is to use the 1-V curve of led to determine the expected forward current according to the voltage. This is usually achieved by using a voltage source and current limiting resistor. However, this method has some defects. Any change of led forward voltage will cause the change of LED current.
The second method to control the LED current (also the preferred method) is to use a constant current source to drive the LED. The constant current source eliminates the current change caused by the change of forward voltage, so as to maintain the stable brightness of LED. The composition of the constant current source is quite simple: it does not regulate the output voltage, but the voltage on the current sensing resistance. The reference voltage of the constant current source and the resistance of the current sensing resistance determine the current of the LED. When multiple LEDs are working, they should be connected in a suitable way to ensure that the current flowing through each LED is the same. Driving LEDs in parallel requires a current limiting resistor on each LED branch.
- AC driven LED
Under AC drive, the LED can be turned on only when the positive half of a cycle is forward biased. During the conduction of the positive half cycle led, the LED will light up only when it is higher than the threshold voltage. Therefore, its luminous duration is less than half a cycle; Only at the maximum value of AC voltage can the LED reach the maximum optical output current and its luminous intensity reach the maximum.
Secondly, even if the LED is on, the average voltage will be much lower than the peak voltage. For a sine wave voltage, the average voltage in the positive half cycle is about 36% lower than its peak voltage. When an AC signal with an amplitude equal to the peak voltage works in the negative half cycle, the peak voltage must be lower than the maximum reverse breakdown voltage of the LED, otherwise the LED will be permanently damaged. This problem can also be solved by connecting an additional diode to impose a new threshold voltage.
Another solution is to combine the full wave bridge rectifier circuit, so that the LED can be driven when the voltage reaches the threshold voltage in both positive and negative cycles. Two LEDs connected in reverse parallel can also work in the AC state. One LED emits light in a positive half cycle and the other emits light in a negative half cycle. Since the positive and negative parts of a cycle are fully utilized, the optical output is also doubled. In addition, using rectangular AC wave instead of sinusoidal AC wave can make the brightness of two reverse parallel LEDs almost reach 100%. At present, LED drivers sold in the market, especially those dedicated to LED, can convert AC voltage into DC voltage, and are equipped with voltage compensators that can overcome thermal effects. AC LED lights also have a built-in driver. However, the use of drives also introduces some additional losses.
Seoul semiconductor company in South Korea has developed an LED that can be directly used for AC without any additional circuit conversion. In their scheme, different areas of the chip are “on” or “off” according to the positive and negative cycles. Other companies, such as lynk laboratories, have more or less developed similar systems. This research is still exploring to find new ways to use AC source directly to drive led effectively.
- High power LED
LED lights were already on the market a few years ago. The first generation of LED lamps was not enough for general lighting. The new high-power LED enables manufacturers to design high-power LED lamps to meet the lighting needs. The rated power of high-power LED chip is greater than 1W, and can be identified by the large metal pad at the bottom of LED package. Metal is a good thermal conductor. This metal pad is usually closely connected with a heat sink to provide a direct heat dissipation channel for LED. The lifetime of an LED is defined as the time when its light output is reduced to 70% of its initial light output. The manufacturer defines that 70% of the service life is usually 50000 ~ 100000 H. However, in order to achieve LED life expectancy, LEDs must be used under the temperature limits given by the manufacturer. Using LEDs at high temperatures will reduce their service life.
In order to ensure the rated life (expected average service life) of LEDs, manufacturers will measure and control devices under strict conditions to ensure that their predicted life is achieved. Poor quality LED will lead to short service life due to poor process control in semiconductor manufacturing. Just like the processor of a computer, if possible, a high-power LED can be dissipated by a heat sink with a fan. Since the LED does not radiate any heat, the metal pad is the only way for the led to dissipate heat. The heat generated by the LED chip enters the heat sink through the internal circuit and metal substrate, and then disappears into the surrounding air.
Read more: What are LED materials and their evolution?