Building Green Homes Plans - Driving LEDs
Constant-Current Driver Circuit
The electronic circuit powering LEDs should have an important consideration when evaluating LED lights. Many simple driver circuits provide a constant voltage, meaning that the output current varies with the LED's voltage. A constant-voltage driver can cause early LED failure as shown in the current voltage dependence. As the LED's temperature increases, its threshold voltage drops, causing a constant-voltage driver to supply more current in response to the decreased LED voltage; this is because of the band gap decrease in semiconductor with increasing temperature. A current over a certain limit will damage the LED.
Proper LED driver circuits supply a constant DC current, holding steady as the LED voltage changes with temperature. LEDs are inherently dimmable as their brightness is a function of their current. LEDs are current-driven devices whose brightness is proportional to their forward current. Forward current can be controlled in two ways. The first method is to use the LED I–V curve to determine what voltage needs to be applied to the LED to generate the desired forward current. This is typically accomplished by applying a voltage source and using a ballast resistor. However, this method has several drawbacks. Any change in LED forward voltage creates a change in LED current.
The second preferred method of regulating LED current is to drive the LED with a constant-current source. The constant-current source eliminates changes in the current due to variations in forward voltage, which translates into a constant LED brightness. Generating a constant-current source is fairly simple. Rather than regulating the output voltage, the input power supply regulates the voltage across a current-sense resistor. The power-supply reference voltage and the value of the current-sense resistor determine the LED current. Multiple LEDs should be connected in a series configuration to keep an identical current flowing in each LED. Driving LEDs in parallel requires a ballast resistor in each LED string.
Driving LEDs with an AC Voltage
An LED under an alternative current (AC) voltage will conduct only during the positive part of the cycle, when the bias appears as forward bias. During that positive portion of the cycle the LED will conduct, so it emits light only when the voltage is higher than its threshold voltage. The period of light emission will consequently last less than half of the cycle; the brightness also might reach its peak only if the maximum of the AC voltage equals the voltage that can force enough current in the LED for its maximum brightness.
Second, even when the LED is conducting, the average voltage will be far less than the peak voltage. For a sine wave voltage, the average voltage of the positive half is about 36% lower than the peak voltage. When using an AC signal which amplitude equals the peak voltage, in the negative portion of the cycle it is necessary to compare that voltage with the maximum reverse voltage that would permanently damage the LED. This can be overcome by combining an extra diode that will impose a new threshold voltage.
Another solution is to incorporate a full-wave bridge rectifier circuit that makes both halves of the cycle positive to drive the LED when the voltage reaches its threshold value. It is also possible to operate jointly two LEDs in reverse-parallel, so that one emits light during the positive half of the cycle, and the other generates light during the negative half. This increases twice the light output, since both halves of the cycle are being used. Furthermore, using a square wave AC instead of sine wave AC would allow reaching almost 100% of brightness when using two reverse-parallel LEDs.
One can find in the market LED drivers that will convert the AC voltage into DC, specially made for LEDs, with voltage compensators to overcome the heat effects. AC LED lamps also have a built-in converter. However, the use of converters will result in some loss.
Seoul semiconductor in South Korea has developed direct AC LEDs without any additional electronic circuitry. In their approach different regions of the chip will switch ON or OFF depending on the part of the cycle. Other companies such as the Lynk Labs have developed more or less equivalent systems. The research is ongoing for new approaches to drive LEDs directly and efficiently with AC power.
Power LEDs
LED-based lights have been on the market for a few years. The first generations of LED lights were not powerful enough for general illumination. New, high-power LEDs allowed manufacturers to design LED lights with enough power to fit lighting applications.
High-power LED chips have power ratings greater than 1 W and are identifiable by a large metal pad at the bottom of the LED package. Metals are good heat conductors, this metal pad provides a direct path for heat to escape the LED, usually by means of an adjacent heat sink. An LED's lifetime is specified as the time for its light output to degrade to 70% of its initial output. Manufacturers specify 70% lifetimes between 50,000 and 100,000 h. However, to achieve the LED's predicted lifetime rating, the light must operate within the manufacturer's temperature limits. Operating LEDs at high temperatures reduces their lifetime.
To guarantee the LEDs' rated life (the expected average operating life), manufacturers accurately test their devices under drastic control conditions in order to ensure the life span they predicted. Low-quality LEDs shows a shorter lifetime due to poor process control as in the semiconductor fabrication.
As for the processor of a computer, high-power LEDs must be cooled with a heat sink combined with a fan if necessary. Because LEDs do not radiate any heat, the metal pad provides the only path for heat to leave the LED. The heat wasted in the LED propagates from the LED die through the circuit board, the metal slug, into a heat sink and then out into the surrounding air. If you find this post useful, please share it with your friends. To find out more, you can check out Building Green Homes Plans.
The electronic circuit powering LEDs should have an important consideration when evaluating LED lights. Many simple driver circuits provide a constant voltage, meaning that the output current varies with the LED's voltage. A constant-voltage driver can cause early LED failure as shown in the current voltage dependence. As the LED's temperature increases, its threshold voltage drops, causing a constant-voltage driver to supply more current in response to the decreased LED voltage; this is because of the band gap decrease in semiconductor with increasing temperature. A current over a certain limit will damage the LED.
Proper LED driver circuits supply a constant DC current, holding steady as the LED voltage changes with temperature. LEDs are inherently dimmable as their brightness is a function of their current. LEDs are current-driven devices whose brightness is proportional to their forward current. Forward current can be controlled in two ways. The first method is to use the LED I–V curve to determine what voltage needs to be applied to the LED to generate the desired forward current. This is typically accomplished by applying a voltage source and using a ballast resistor. However, this method has several drawbacks. Any change in LED forward voltage creates a change in LED current.
The second preferred method of regulating LED current is to drive the LED with a constant-current source. The constant-current source eliminates changes in the current due to variations in forward voltage, which translates into a constant LED brightness. Generating a constant-current source is fairly simple. Rather than regulating the output voltage, the input power supply regulates the voltage across a current-sense resistor. The power-supply reference voltage and the value of the current-sense resistor determine the LED current. Multiple LEDs should be connected in a series configuration to keep an identical current flowing in each LED. Driving LEDs in parallel requires a ballast resistor in each LED string.
Driving LEDs with an AC Voltage
An LED under an alternative current (AC) voltage will conduct only during the positive part of the cycle, when the bias appears as forward bias. During that positive portion of the cycle the LED will conduct, so it emits light only when the voltage is higher than its threshold voltage. The period of light emission will consequently last less than half of the cycle; the brightness also might reach its peak only if the maximum of the AC voltage equals the voltage that can force enough current in the LED for its maximum brightness.
Second, even when the LED is conducting, the average voltage will be far less than the peak voltage. For a sine wave voltage, the average voltage of the positive half is about 36% lower than the peak voltage. When using an AC signal which amplitude equals the peak voltage, in the negative portion of the cycle it is necessary to compare that voltage with the maximum reverse voltage that would permanently damage the LED. This can be overcome by combining an extra diode that will impose a new threshold voltage.
Another solution is to incorporate a full-wave bridge rectifier circuit that makes both halves of the cycle positive to drive the LED when the voltage reaches its threshold value. It is also possible to operate jointly two LEDs in reverse-parallel, so that one emits light during the positive half of the cycle, and the other generates light during the negative half. This increases twice the light output, since both halves of the cycle are being used. Furthermore, using a square wave AC instead of sine wave AC would allow reaching almost 100% of brightness when using two reverse-parallel LEDs.
One can find in the market LED drivers that will convert the AC voltage into DC, specially made for LEDs, with voltage compensators to overcome the heat effects. AC LED lamps also have a built-in converter. However, the use of converters will result in some loss.
Seoul semiconductor in South Korea has developed direct AC LEDs without any additional electronic circuitry. In their approach different regions of the chip will switch ON or OFF depending on the part of the cycle. Other companies such as the Lynk Labs have developed more or less equivalent systems. The research is ongoing for new approaches to drive LEDs directly and efficiently with AC power.
Power LEDs
LED-based lights have been on the market for a few years. The first generations of LED lights were not powerful enough for general illumination. New, high-power LEDs allowed manufacturers to design LED lights with enough power to fit lighting applications.
High-power LED chips have power ratings greater than 1 W and are identifiable by a large metal pad at the bottom of the LED package. Metals are good heat conductors, this metal pad provides a direct path for heat to escape the LED, usually by means of an adjacent heat sink. An LED's lifetime is specified as the time for its light output to degrade to 70% of its initial output. Manufacturers specify 70% lifetimes between 50,000 and 100,000 h. However, to achieve the LED's predicted lifetime rating, the light must operate within the manufacturer's temperature limits. Operating LEDs at high temperatures reduces their lifetime.
To guarantee the LEDs' rated life (the expected average operating life), manufacturers accurately test their devices under drastic control conditions in order to ensure the life span they predicted. Low-quality LEDs shows a shorter lifetime due to poor process control as in the semiconductor fabrication.
As for the processor of a computer, high-power LEDs must be cooled with a heat sink combined with a fan if necessary. Because LEDs do not radiate any heat, the metal pad provides the only path for heat to leave the LED. The heat wasted in the LED propagates from the LED die through the circuit board, the metal slug, into a heat sink and then out into the surrounding air. If you find this post useful, please share it with your friends. To find out more, you can check out Building Green Homes Plans.
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