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High efficiency control for a power converter LED driver Due to their superior long-time life respect to low power fluorescent lamp and to low maintenance requirements and improved luminance, high power light emitting diodes are nowadays widely used in indoor as well as outdoor applications. It is widely accepted that a switch-mode LED current regulator offers advantages in terms of efficiency in comparison with a linear-current regulation. Another operational requirement to be fulfilled concerns the brightness regulation, which strongly depends on the LED average forward current. The dimming solution here proposed is based on pulse width modulation (PWM) techniques. They have proven to be an effective approach since they are realized by switching on and off the LEDs repeatedly while the LED string current, during its on-phase, is forced to its nominal value. By Silvio Baccari, Luigi Iannelli, Francesco Vasca and Massimo Tipaldi, Department of Engineering University of Sannio, Italy A step-down switching converter LED driver with a hysteric and PWM dimming control proposed here has been conceived with the objective of driving efficiently a single or multiple series of connected LEDs. The regulation is based on a combined feedback structure. The proposed circuit can operate from 8 V up to 36 V voltage supply and can provide an adjustable output current which can even reach a value of 1000 mA. Converter dynamic model The converter topology is a step–down dc/dc buck converter without output capacitor. Internal equivalent resistances of inductance and freewheeling diode are taken into account in order to have a more realistic approximation of the current and voltage waveforms. A semi-ideal model of the switch has been considered representing a non-zero voltage drop for the ON state. Conduction losses have been included in the switch model by means of a forward state on resistance. Each LED is modeled as a series of an ideal diode, an equivalent series resistor and a voltage drop. Power converter control approach The control of the power converter is obtained by using a combined controller structure: a hysteresis closed-loop current control and a PWM open-loop that enables the switch commutations. An equivalent scheme of the controller is represented in Figure 1 where q = 1 (q = 0) corresponds to the switch ON (OFF) and δ Є [0; 1] is a sawtooth signal with period Ts.
The proposed control mechanism is mainly based on the fact that the bandwidth of the human eyes is much lower than realistic switching operating frequency. The average value of the LED current, which is the only effect perceived by the human eye, can be varied by means of the duty cycle. A typical current waveform under closed loop control is represented in Figure 1. The variable d is a square wave obtained with a PWM modulation. Note than when d = 0 the switch is always constrained to be OFF (q = 0), whereas when d = 1 the switch can be either ON (q = 1) or OFF (q = 0) depending on the hysteresis output h. The average value of d, say D, is obtained from the desired average LED current Iref, assuming that iL ≈ Imax when d = 1: D = Iref/Imax. When d = 1 the hysteresis current control is active, whereas when d = 0 the current control is disabled and the LED current goes to zero. When the hysteresis control is enabled, the inner current control loop allows the regulation of the inductor current into the interval [Imax-εoff; Imax+ εoff]. Driver implementation The basic structure of the LEDs driver is divided into two parts; the hysteretic control with driver circuitry and the PWM controller (see Figure 2).
The current regulation is based upon a low side BJT driver (2N2222A type). Connected to the switch, we have the high side sensing resistor Rs, the inductor, the load (three LEDs) and the freewheel Schottky diode to provide a path for the current stored in the inductance when the switch is OFF. The LM311 integrated circuit seems to be a good choice to implement efficiently the two comparators of the proposed control mechanism (Figure 1). The PWM controller is based on a classic bistable analog circuit. The PWM signal is produced by comparing a triangle waveform with an external DC level. The triangle signal generator is formed by two cascaded LM348 operational amplifier: one of them acts as an integrator and produces a triangle waveform from a squared waveform generated by the other operational amplifier placed inside the bistable analog circuit. The hysteresis comparator regulates the LEDs current in the hysteresis band. The LEDs current iL is sensed by the resistor Rs that has one terminal connected with the power supply (Vsupply). The measured voltage Vfb = Vsupply - RsiL is compared with a two level reference voltage Vref, whose value depends on the state of the switch (q) through the value of Vh:
Where α, β and γ can express in function some resistor values of the proposed circuit The values of the hysteric band and Imax are given by:
The connection of the hysteric and PWM controller outputs implements a wired AND (positive logic), thus allowing to switch off the BJT if one of the two LM311 outputs (the hysteresis comparator and the PWM comparator) is low. The waveforms of the main electrical variables involved in the hysteretic controller are shown in Figure 3.
Simulation results Dynamic analysis and performance evaluations have been carried out by means of the PSPICE model outcomes. A trade-off between the requirements of decreasing the ripple and of increasing the overall efficiency has to be carefully evaluated during the actual implementation of the LED driver.
Figure 4 shows the dependence of the efficiency on of the number of LEDs for different power supply voltage. The proposed circuit has shown to be a good solution for what concerns the efficiency. As matter of fact, it can reach an efficiency of 93% when Vsupply= 24 V and the load consists up to three/four 1W LEDs. Moreover, it can be shown that the load current RMS value slightly increases when the circuit is supplied by a higher voltage. This aspect is mainly due to the hysteresis region enlargement as shown in the previous formulas. It can be also observed a sudden fall for this RMS value, which comes about as soon as the source voltage value becomes too low in order to drive a specific number of LEDs with a buck converter configuration. |
