LED brightness control requires a driver that provides a constant, regulated current. To achieve this goal, the driver topology must be able to generate a large enough output voltage to forward bias the LEDs. So, if the input and output voltage ranges overlap, what should we do? The converter may sometimes need to gradually reduce the input voltage, and sometimes it may be necessary to raise the output voltage. This is common in applications that have a wide range of "dirty" input power sources, such as in-vehicle systems. Several topologies in this buck/boost operation work well, such as a SEPIC or 4-switch buck-boost topology. These topologies typically require a large number of components, which increases the material cost of the design. However, because they provide a positive output voltage, they are often considered an acceptable choice. However, we should not ignore the negative output voltage converter, which can provide an alternative solution.
Figure 1 shows a schematic of an inverting buck-boost circuit that drives three LEDs in a constant current configuration. This circuit has many advantages. First, it uses a standard buck controller to minimize cost and help reuse system level as much as possible. If necessary, the circuit can be easily modified to use an integrated FET buck controller or a synchronous buck topology for greater efficiency. This topology uses the same number of power stage components as a simple buck converter, thereby achieving the lowest component count of the switching regulator and the lowest overall cost relative to other topologies. Since the output of the LED itself is light, it may have little to do with the system level of the LED's negative (rather than positive) voltage bias, making it a circuit design worth considering.
The LED current is adjusted by sensing the voltage across resistor R1 and using it as feedback to the control circuit. The controller ground pin must be the reference voltage for the negative output voltage to allow this direct feedback to work properly. If the controller is the reference voltage for system grounding, a level shifting circuit is required. This "negative grounding" imposes some limitations on the circuit. The power MOSFET, diode, and controller must be rated for higher than the sum of the input and output voltages.
Second, externally connected controllers (eg, turn-on operations, etc.) require level shifting of signals from system ground to controller ground, requiring more components. For this reason alone, the best way is to remove or minimize unnecessary external controls.
Finally, the voltage and current stresses on the power devices applied to the inverting buck-boost topology are greater than the 4-switch buck-boost topology, reducing the associated efficiency, but the efficiency is comparable to SEPIC. Even so, this circuit can achieve 89% efficiency. By fully synchronizing the circuit, we can also increase efficiency by 2% to 3%.
Turning the converter on and off quickly by shorting the soft-start capacitor C5 is an easy way to adjust the brightness of the LED. Figure 2 shows the PWM input signal and the actual LED current. This PWM dimming method is more efficient because the converter is turned off and it consumes very little power when the SS pin is shorted. However, this method is also relatively slow, because each time the converter is turned on, the output current must be gradually increased in a controlled manner, which produces a nonlinear, finite time lag before the output current rises. . At the same time, it also reduces the minimum on-time duty cycle to 10% to 20%. In some LED applications that do not require high speed and 100% PWM regulation, this approach may be sufficient.
This inverting buck-boost circuit provides engineers with another way to drive LEDs. The use of low-cost buck converters and a small number of components make it an ideal alternative to high-complexity topologies.
High temperature thermocouple slip ring with German and Japanese imports of key materials, can be used in high temperature environment rotate 360 degrees to transmit current and thermocouple signal, can be long-term stability in 100 ~ 250 ℃ high temperature environment, it is mainly used for hot roller, the high temperature environment, such as heating device, suffered long-term complex field practice test, transmission performance is stable and the quality.
High Temperature Slip Ring,Taidacent Slip Ring,Slip Ring Capsule,Fiber Brush Slip Ring
Dongguan Oubaibo Technology Co., Ltd. , https://www.sliproub.com