A Computer Simulation based Design and Control Solution for a Smart Current Controlled Flyback Converter for LED Lighting Applications

LED lighting technology is a widely used lighting solution owing to its numerous advantages such as high efficacy and long-lasting lifetime. The LED luminaires tend to degrade over time resulting in gradual reduction in light output continuously results in reduced performance. The paper proposes an all-inclusive optimal design and control solution for a smart constant current regulated flyback converter suited for LED applications in LED lighting systems. For accurate performance evaluation, the suggested approach incorporates advanced control techniques and makes use of computer simulations. The proposed LED driver can maintain constant illuminance by maintaining a constant drive current in the light engine. The system consists of a flyback converter stage that regulates the driving current by changing the duty ratio after an AC-DC rectifier stage. For the proposed converter, a computer simulation is executed in which a computer-generated degradation is adopted. It is observed that the converter modifies the duty ratio to maintain a constant drive current, which, in practice, translates to constant illumination. The developed technique assists industries and applications in maintaining a steady light output during their lifespan operations.


INTRODUCTION
The most common lighting technology in use today is Light emitting diode (LED) based lighting.We no longer use incandescent or tungsten bulbs since LED lighting systems are seen to be better due to their quicker startup times, more energy efficiency, and longer lifespans [10].Additionally, because of their versatility, LEDs can be employed in a wide range of applications, such as house lighting systems, hospitals, and smartphone screens.It is essential to keep working towards the better development of LED luminaires due to their superiority over other lighting technologies.However, by their very nature, LEDs have a few undesirable qualities -one of which is discussed in this work.The performance of an LED tends to degrade over time, thereby gradually decreasing its illumination capabilities.Unlike other traditional lighting sources, where the lamp maintains a constant light output until its end of life and simply burns out, LEDs' light output drops gradually [12].In fact, LEDs are said to be at their end of life when their light output reaches to 70% of their initial capacity [5].This degradation is frequently overlooked in most general lighting applications; however, specialized applications such as healthcare may require constant illumination sources, making LEDs unsuitable.
The standard method for LED control systems is to keep the LED current constant.Several researchers attempted to develop constant current/voltage/power LED for various LED luminaire applications [3], [13].The work in [1] presents a new LED control scheme to maintain a constant LED power.This constant power method can effectively avoid overheating at high input voltage.An extensive review of the various LED driver topologies and corresponding LED system level applications is described in [2].An inductorcapacitor-inductor-capacitor (LCLC) resonant network-based non isolated resonant light-emitting diode (LED) driver is given in [6].With the help of the LCLC network settings, the adopted LCLC network can optimize the semiconductor devices' rms current over the whole input and output voltage range, in addition to providing load-independent current for LED strings and a gentle switching range.Three sophisticated algorithms for advanced digital predictive current-mode control for asymmetric half-bridge LED constantcurrent drivers in presented in [9].These algorithms are based on a streamlined approach to duty cycle lost time calculation.In [11] a modified forward power factor correction converter for LED lighting applications is described.The suggested circuit topology combines a multi-winding transformer with a forward and flyback converter.A control strategy is developed and is implemented as a feedback loop to determine the pulse width modulated signals to control the operation of the switches of the converter and thus the dimming performance of LED luminaire [7], [4] .Although the literature listed above mentions constant current LED drivers, it just provides a constant current and does not account for degradation.The driver current is fixed, regardless of degradation over time.To mitigate this, a flyback converter-based LED driver has been proposed in this work, which also provides isolation of LEDs from the input power line, thereby providing additional protection.It is known that the light output in an LED luminaire is directly proportional to its drive current [8].Therefore, the strategy adopted in the proposed LED driver is to control the illuminance indirectly by controlling the drive current.The feedback control loop detects any fall in light output and suitably adjusts the drive current, thereby achieving constant illuminance in the LED luminaire.The proposed type of the control system with flyback converter can be considered advanced in comparison to the existing converters which provide constant current throughout its working despite of degradation.The proposed system employs control techniques and utilizes computer simulations for accurate performance assessment.A computer simulation is conducted, incorporating computer-generated degradation, and the results show that the converter adjusts the duty ratio to ensure a constant drive current, thereby achieving consistent illumination in practical scenarios.

METHODOLOGY
The heart of this LED driver is a flyback converter.This is chosen because of its safety properties such as the isolation between the input and the output side.The latter becomes crucial when this LED driver is to be integrated with an AC-DC rectifier stage for commercial applications.Figure 1 is a schematic representation of a flyback converter as developed in a simulation environment.A resistive load used initially while developing the converter and this is seen in the figure as greyed out and disabled after connecting the LED load.The LED load model is custom-made to capture a real-life commercial LED non-linear performance.However, before delving into this model, it is worth noting that the input to the flyback converter has been configured as a DC voltage source for convenience of simulation.This can be replaced with the output of a rectifier stage.Table 1 shows the values of the parameters of the converter.The input voltage is chosen as 325 V since this would be the voltage level seen at the output side of an AC-DC rectifier.The LED is a non-linear device and to fully mimic its real-life behavior as accurately as possible during simulation it is decided not to model  2. This Voltage vs Current information is fed into computer simulation environment in the form of a lookup table and the simulation environment is programmed to automatically extrapolate the missing values between two discrete datapoints so that the model can function in continuous time domain.Figure 3 shows the model where there is a dependent current component, seen at the left side, which sinks an amount of current proportional to the voltage sensed at the output of the converter.This voltage sensor is seen in the middle of Figure 3 and its output voltage is translated into an equivalent amount of current to be sunk.This is being done by the lookup table which has been fed with the VI characteristics information.Once this is completed, a constant-current controller can be developed for the flyback converter as seen in Figure 1.To validate the proposed design and control solution, computer simulations are conducted using industry-standard simulation tools.This could be done with a controller design software suite integrated to the computer simulation environment.Figure 4 shows a the development of a PI controller with suitable proportional control (Kp) and integral control (Ki) values to achieve desired time domain characteristics.The Kp, Ki values are finalized for stability of the system.The controller shown in Figure 5 can achieve constant-current at the output terminals of the flyback converter.The desired current level can be set by modifying a particular block in this controller.In Figure 5, this is 0.25 A (seen at the left most corner).This flyback converter would now be able to maintain a regulated constant current output of 250 mA or 0.25 A. However, in real life, a light sensor would read its output values in voltage levels and not in terms of output current.Therefore, a small new conversion stage is introduced for simulation purposes which would give equivalent voltage levels for corresponding output current levels.This light sensor model takes in the load current as the input and maps it to a voltage level at the output side, which is shown in Figure 6 as V_Lumen_fb.With a Lux sensor, at 52 cm from the luminaire, it is experimentally found that 275 mA corresponded to 473 Lumens.This is the initial operating point of the converter simulation and can be altered if a different luminaire is used for testing.This is programmed to give a voltage level of 5V as the sensor output.After this, a new controller is designed with simulation environment as described above.This would constitute a second, outer control loop as shown in Figure 7 which operates on the sensor voltage levels.To fulfil the main objective of this work, it is needed to introduce degradation of the light output artificially in this simulation.This has been achieved with another lookup table which is shown Figure 6 with a black circle.With the simulation time as the input, the lookup table outputs a value which varies with time.This is subtracted from the sensor voltage level which is seen as a fall in light output as seen by the sensor.Physical degradation is generally modelled as linear/exponential in nature [8].Therefore, a virtual degradation is introduced as a gradual slope from 0.4 to 0.8 seconds during simulation as seen in Figure 8.After 0.8 seconds, a constant value of 1 would be subtracted from the sensor voltage level.The sensor would now output a value of only 4 volts.The controller should now adjust the duty cycle of the flyback converter to maintain the sensor voltage level at 5V.

RESULTS AND DISCUSSION
The Flyback converter in Figure 1 when used with the controller in Figure 5 is capable of maintaining a constant current at the load side.This is verified by simulation and the results are shown in   last section, the flyback converter's input is fed from a DC voltage source than from a AC-DC rectifier stage.However, to verify the converter's working even with a rectifier, a AC-DC stage is included and the model is simulated.The load current waveform can be seen in Figure 10 with the desired average output current set as 250 mA.It is observed that there are periodical spikes in the output waveform even though the averaged output load current comes out to be approximately 250 mA.These spikes are introduced because of the nature of the rectifier and the peaks of the AC sinusoids.This is further confirmed since the spikes appear at a frequency of 100 Hz which is exactly the frequency of appearance of the sinusoids in a full-bridge rectifier's output waveform.These spikes don't produce any flicker or visible changes, and can largely be ignored since they were seen to be lasting only for about 1.5 milliseconds.For the final step of validation of the proposed LED driver's model, the flyback converter in Figure 1 is run with the full controller as seen in Figure 7. Theoretically, this converter should adjust the duty ratio to maintain a constant level of 5V as the sensor voltage level.This is verified by simulation and the load current waveform is seen in Figure 11.The waveform of the load current, initially once the converter is in steady-state, averages at about 275 mA which corresponds to 5V level at the sensor, as discussed above.There is a slope seen in the output load current between 0.4 and 0.8 seconds.This exactly corresponds to the degradation that is introduced as a gradual slope as shown in Figure 8.The converter is seen to be adjusting its output drive current since it perceives the fall in sensor voltage level as a fall in light output.After 0.8 seconds, we see a slightly higher average current level than before 0.4 seconds since the voltage level at the sensor now only reads 4V instead of 5V.The controller accounts for this by increasing the output current, thereby trying to maintain the 5V.Thus, the developed LED driver senses a fall in light output of the luminaire and accordingly adjusts the drive current to maintain a constant illuminance.

CONCLUSION
The work proposed to address the pertinent issue of continuous light output deterioration experienced by LED lighting solutions throughout operation An approach based on software simulation is utilized to create a power electronic converter that is intended to provide a consistent light output over the course of its lifetime.The smart approach suggested adjusting the driving current to the light engine using a PI-based control strategy so that the decrease in light output could be readily compensated for, maintaining a constant light output across the light engine's lifetime.The findings of the computer simulation, which includes computer-generated degradation, demonstrate that the converter modifies the duty ratio to maintain a steady drive current and consistent lighting in realworld situations.The selection of a flyback converter may have several restrictions, such as big size, weight, voltage stress, and EMI, which can be overcome through good design and mitigation measures.Furthermore, the usage of a lookup table can be eliminated by using actual sensors, and the hardware implementation of the proposed approach is the future focus of the research investigation.Overall, employing such a novel control strategy for LED drivers will help all the consumers and, most importantly, the applications that require constant output from the luminaire up to the end of life.

Figure 1 :
Figure 1: Flyback converter with an LED load.

Figure 2 :
Figure 2: VI Characteristics of the LED luminaire.

Figure 3 :
Figure 3: The non-linear LED Load model.Lookup table is circled in black.

Figure 4 :
Figure 4: Development of PI controller to implement control strategy via simulation environment.

Figure 5 :
Figure 5: Controller with load current feedback.

Figure 6 :
Figure 6: Modelling the light sensor.The degradation component is circled in black.

Figure 7 :
Figure 7: A second controller forming an outer sensor voltage level-based control loop.

Figure 8 :
Figure 8: Introduction of degradation over time.

Figure 9 .
Figure 9.The graphs in red are of the load current and it is seen that once the converter reaches steady-state, the average load current is maintained exactly as set by the user, barring minimal ripple content.The top half of the figure is when the converter is run with the desired output current as 250 mA and the bottle half is when it is run with 150 mA as the desired output.As mentioned in the

Figure 10 :
Figure 10: Load current of Flyback converter with a rectifier stage.

Figure 11 :
Figure 11: Response of the converter due to degradation.

Table 1 :
Parameters of the proposed flyback converter it as a linear device.Therefore, the luminaire's VI characteristics is studied experimentally and is used to develop the LED load's nonlinear model.The VI characteristics of the LED luminaire has been plotted in Figure