In December last year, the Republic of Ireland has announced that it would abandon the incandescent lamp in 2009 due to its poor energy efficiency. Higher efficiency alternatives like CFL lamps or high efficiency LEDs are readily available, but attention must be paid to the actual design to confirm that it is as environmentally friendly as possible. Environmentally friendliness does not just cover the components themselves but the number of components used and their lifetime – things that are meant by the term “Ecodesign”. Longer lifetime of electronic equipment, resulting in less waste, is achieved by reducing the number of components and increasing their durability. This article explores the design of a primary side regulated (PSR) off-line ballast for LED with constant current output. The low component count and long lifetime makes the design cost-effective and environmental friendly at the same time.
The majority of electronic ballasts for high power LEDs operate in constant current mode. Due to the V-I characteristic of a LED, some kind of current limiting element is mandatory for stable operation. Consequently, the most popular approach is to put a number of LEDs in series and drive them with a current source.
The traditional approach of implementing a constant output current power supply is to measure the load current – for example, with a shunt resistor – and feed this signal back to the PWM controller. Unless one doesn’t care about power dissipation and efficiency, the signal generated by the shunt is too small and has to be amplified in some way. This can be done by a simple single stage BJT amplifier or an integrated operational amplifier.
The BJT has the virtue that there is a built in “reference voltage”, the forward voltage VBE of the base-emitter diode. But the latter is not very accurate and has a considerable negative temperature coefficient. If the BJT amplifier is used in cost-effective applications like mobile phone chargers, consequently there is some kind of compensation for this temperature coefficient of VBE, as for example, the PTC THR1 in Figure 1.
The schematic further shows that there are additional things to consider when designing a current source PSU. At no load current, the output voltage would rise to an unacceptable high value. Hence, there is an additional voltage regulation loop that is implemented with the reference/error amplifier KA431. Finally, if the output has to be isolated from the mains input, an optocoupler is needed in the feedback path. This optocoupler is an often overseen component that can limit the lifetime of a PSU.
While in consumer applications a lifetime of 10,000h is excellent and most applications don’t need such a long one, the situation in lighting applications is different. Without a doubt, one expects the ballast to live at least as long as the light source itself. But since in a lot of applications electric lighting may be used a significant part of the day, one doesn’t want to replace the electronics each time the light source is defective but expects a lifetime of up to 50,000h from the ballast.
Electrolytic capacitors are electronic components with very short lifetime. On the other hand, fluorescent lamp ballasts feature a remarkably long lifetime. Both the electrolytic capacitors’ and the optocouplers’ lifetimes are reduced by high temperatures. Unlike fluorescent lamps, LEDs must be cooled and often the complete luminary is used as a heatsink. As a result, the ambient temperature of the ballast and in turn, that of the optocoupler, will be quite high. Thus, it would be advantageous to design LED ballasts without optocouplers. For constant voltage output, there is a well known solution: the primary side regulated flyback.
A primary side regulated PSU works without any direct feedback path from the secondary, accordingly reducing part count and cost while increasing reliability considerably. This topology has recently been extended to constant current output.
The actual ballast is designed around the FAN102, a dedicated PSR flyback controller with patented constant current regulation circuit. The schematic of the ballast is shown in Figure 2 and looks quite unspectacular at first glance. Nevertheless, the ballast can deliver up to 17V, enough to drive up to four LEDs in series at a current level of up to 700mA from a universal mains input. The output current can be selected to be 350mA or 700mA by jumper J101 that changes the value of the current sense resistor. If a bigger transformer is used (EF25 core instead of EF20) and the current sense resistors R102 and R103 are adjusted, even 1A output is easily possible.
A detailed description of the operation of the ballast, that is available as a completely assembled evaluation board with three white high power LEDs attached, is given in the following section. C101 and LF101 together with C1 form an EMI filter followed by the rectifier bridge D101 and the filtering capacitor C101. At startup, C105 is charged via R110 and 112 to the start voltage of the controller. When the latter is reached, oscillation starts and the MOSFET is controlled by a PWM gate signal. The topology of the PSU is that of a flyback. When the MOSFET is in the off state, D201 at the secondary of the transformer is conducting and the energy stored in TR101 is transferred to C201 and the load. R101, C106 and D102 form the well known clamping network that limits the voltage spikes due to the energy stored in the leakage inductance of the transformer. In steady state, the controller is supplied from a separate winding of the transformer. The voltage of this winding is rectified by D104 and filtered by C108. A simple linear regulator consisting of R111, D105 and Q102 limits the voltage at the VDD pin of the controller to 24V maximum. This is necessary since the ballast is designed to operate even with a single LED connected to the output – for instance, voltages down to 2.8V in the worst case. The voltage across the supply winding of the transformer will vary with the same ratio of 17V/2.8V = 6.07. Since the minimum operating voltage of the FAN102 may be 7.25V, the maximum VDD would be 44V, which would destroy the device.
The voltage of the supply winding is used to do the regulation of both output current and voltage. In case of high load resistance, the PSU is not in constant current but constant voltage mode. The FAN102 uses quite an elaborate method of regulating the output voltage tightly. The voltage of the supply winding that has almost the same waveform but lower voltage level as the drain of the MOSFET is scaled by the divider R109 and R108, slightly filtered with C107, and fed to the VS pin. The voltage is internally sampled, and the zero crossing of the current through D201 is determined by monitoring the rate of change of VS since the sample at this point gives the best estimate and hence regulation of the output voltage.
As load current increases, the output goes from constant voltage into constant current mode. To understand how the latter mode works, a bit of math is indispensable. In DCM the output current IO of a flyback is (Figure 3).
Using the transformer winding ratio , the formula for the output current can be written with peak primary side current:
Finally, the peak primary current is given by and the concluding equation is:
Obviously the first factor is constant for a given design. VCS is the measured voltage at the CS pin and tdis is determined in the same step the zero crossing of ID201 is determined. In order to achieve a constant output current, the feedback loop has to regulate the on time of the MOSFET such that the product is kept constant.
From the equation, it’s clear that the output current for a given design is inversely proportional to the sense resistor RCS – for instance, double the value gives half the output current. Taking a closer look at the schematic diagram will show that there is only one electrolytic capacitor: C102. For universal input, a capacitance value around 22µF is necessary. However, if the input is limited to European power line voltage, the latter can be made as small as 6.8µF, a value that is readily available as film capacitor. As mentioned earlier, replacing the electrolytic capacitor results in a ballast with extraordinary long lifetime, possibly longer than that of the LED itself.
The component count can be reduced even further by using the FPS device, incorporating the controller and a 600V/1A MOSFET in one package. It goes without saying that the reliability of the ballast will be increased at the same time.
For today's designers of electronic devices, it is highly important to keep the environmental impact of their designs in view. This does not only concern energy efficiency of the application, but reducing electronic waste as well. By developing long lifetime and low component count, products with high efficiency are a step closer to this objective.
Click here for the illustrations: Figure 1, Figure 2, Figure 3
|
