LED lighting power supply topology of-LED lighting, power supply – the electronics industry
Editor's note: With the LED Decline in production costs, the use of increasingly widespread, covering applications ranging from handheld devices to the car, to architectural lighting. China Lighting net technical papers? LED technology
Introduction As LED production costs drop, the use of increasingly widespread, covering applications ranging from handheld devices to the car, to the architectural lighting. LED high reliability (more than 50,000 hours of life), relatively high efficiency (> 120 lm / W) and near-instantaneous response capabilities make it an attractive light. And incandescent bulbs than 200mS response time, LED will light within a short response time of 5ns. Therefore, at present they have Car Brake lights in the industry widely adopted. China Lighting net technical papers? LED technology
Drive LED Drive LED is not without challenges. Adjustable constant current intensity needed to drive LED, and no matter how the input voltage must be to maintain the current constant. This is merely connected to the incandescent bulbs Battery Power compared to its more challenging.
LED has a forward VI characteristics similar to the diode. Below the threshold voltage LED (white LED turn-on voltage threshold of approximately 3.5V), the pass through the LED current is very small. Higher than the threshold value, the current form of forward voltage will increase exponentially. This allows the LED setting for the series resistance with a voltage source, one with a warning note: This model is only a single work of DC current, to be effective. If the LED in the DC current change, then the resistance of the model should be immediately changed to reflect the new operating current. Under a large forward current, LED devices in the power dissipation will heat, this will change the forward voltage drop and dynamic resistance. LED impedance in determining the full account of thermal environment is very important.
When driving through the buck regulator when the LED, LED is often based on the selected output Filter Arrange to transfer the inductor AC ripple currents and DC currents. This will not only enhance the LED current in RMS amplitude, but also increases its power. This would increase the service life of LED junction temperature and a significant impact. If we set a limit 70% of the light output as the LED's life, then the LED's life span will be 74 degrees Celsius degree from 15,000 hours to under 63 degrees Celsius under 40,000 hours. LED's power consumption by the LED resistor multiplied by the RMS current squared plus the average current multiplied by the forward voltage drop to determine. As the junction temperature can be determined by the average power consumption, so even a large ripple current on the impact of power consumption is not great. For example, in the buck converter is equal to DC output current (Ipk-pk = Iout) of the peak to peak ripple current will increase by no more than 10% of the total power loss. If the loss is far more than the above level, then the need to reduce the AC ripple current from the power supply in order to remain the same junction temperature and working life. A very useful rule of thumb is decreasing by 10 degrees Celsius junction temperature, Semiconductor Life will triple. In fact, due to inhibition of the inductor, so most designs will tend to lower the ripple current. In addition, LED in the peak current shall not exceed the manufacturer's maximum security under the current rating.
Topology options Table 1 shows the information helps to choose the best LED driver Switch Topology. In addition to these topologies, you can also use a simple current limiting resistor or linear regulator to drive the LED, but such methods are usually waste too much power. All relevant design parameters including input voltage range, the number of LED driver, LED current, isolation, EMI suppression, and efficiency. Most of the LED driver circuit topologies belong to the following types: buck, boost, buck – boost, SEP IC And flyback topology.
Table 1, the LED power supply topology options
Figure 1 shows the three basic power topology example. The first diagram shows the buck regulator output voltage applied to the case of the total is less than the input voltage. In Figure 1, the buck regulator will turn on time by changing the MOSFET to control current into the LED. Current sensor can measure the voltage across the resistor was, where the LED series resistor should be. This method, the important design challenge is how to drive MOSFET. From the cost point of view, needs a floating gate drive recommended, N-channel field-effect Transistor (FET). This requires a driver transformer or a floating driver circuit (which can be used to maintain the internal voltage higher than input voltage).
Figure 1 also shows the optional step-down regulator (buck # 2). In this circuit, MOSFET to drive on the ground, thus significantly reducing the drive circuit requirements. The FET circuit current choice or by monitoring the LED current sensing resistor in series LED current sensing. The latter requires a level shift circuit to get the power supply ground information, but this would complicate the simple design. In addition, Figure 1 also shows a boost converter, the converter output voltage is always greater than can be used when the input voltage. As the MOSFET on the ground to drive and current sense resistor also use ground reference, this type of topology design up is easy. The circuit of a gap in the short-circuit during the current through the inductor will be no restrictions. You can fuse or circuit breaker in the form of increased electronic fault protection. In addition, some more complex topologies can provide such protection.
Figure 1 a simple topology for the buck and boost LED power supply
Figure 2 shows the two step-down – step-up circuit, the input voltage and output voltage is low when compared to high use. Compromise between the two has the same characteristics (which can compromise the current sensing resistor and the location of the two gate drive buck topology shown). Figure 2 buck – boost topology shows a ground referenced gate drive. It requires a level shift of the current sense signal, but the reverse buck – boost circuit with a ground referenced current sensing and gate level shifter driver. If the control IC and the negative output and current sense resistor and LED exchangeable, then the reverse buck – boost circuit can be a very useful way to configure. Appropriate control IC, the output current can be measured directly, and the MOSFET can be driven directly.
Figure 2 buck – boost topology can be adjusted higher or lower than the input voltage Vout
The buck – boost approach is a flaw in the current very high. For example, when the same input and output voltage,
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