Light Emitting Diodes have come a long way from simply being cheap and inexpensive indicator lights on a myriad of electronic appliances. Today they are powerful source of illumination for a wide range of room, signage, displays and decorative lighting applications.
LEDs have been gaining importance over incandescent and fluorescent lamps for their capability to provide an equivalent amount of light for a significantly reduced intake of energy. Energy is one of the biggest debates of this century and is soon expected to become one of the most important issues concerning designers across the planet.
There are many potential benefits for lamp manufacturers to use LEDs. However, there are also many vendors trying to get in early on the LED action, so there is a pressing need for product differentiation. Also, with energy conservation and human labor costs being the prime design concerns, large lighting installations are almost expected to be ‘intelligent’.
The ability of a lamp to be able to communicate with a ‘parent’ controller, to monitor its own condition, modify its mode of operation based on this monitoring, and even ensure movement to a safe state during faults are all examples of what the next generation LED lamp is expected to be. This article will explore a few of these ‘intelligent’ options suited for LED lamps and the steps involved in achieving them.
The input voltage to an LED drive system is usually DC. The supply is either produced by an AC-DC converter working off the line or from a bus. Apart from providing the power for the LED drive, this supply will also be used to power the controller in the system (after converting to 5V or 3.3V as suited to the controller).
As shown in Figure 1, above, this controller power supply will usually be designed such that it will start operating when the input supply is a little above the required output voltage. For instance, a 5V regulator will start operating when the input reaches 6-7 volts. However, the steady state level of such a supply could be 24V supplying a string of 5-6 LEDs with 1A per string.
Once the controller powers up, it assumes that power is available and turns the LED drive system on (assuming it is configured as such), which will then try to draw the full power. If the input has reached only 10V by this time, the amount of current required from the input supply would be much higher than under steady state conditions, and it could collapse due to the sudden draw. The excess current draw could also surpass the ratings of the cable, connectors, and any other components on the power supply input, potentially causing permanent damage to the system.
In order to avoid this situation, the system should implement an ‘under-voltage lockout’ feature. The hardware for this involves a resistor divider setup that steps down the input voltage to a range that is tolerable by the controller’s inputs. The input is connected to a comparator internally.
The behavior inside the controller (firmware) should be designed such that the power section is turned on only when the input voltage has crossed the threshold that is deemed reasonable for operation.
Moreover, rather than turn on the power system as soon as the comparator switches, the firmware must poll the output of the comparator to check that the condition is consistent (since the comparator is a piece of combinational logic) and then turn the power system on. Figure 2 below shows the hardware schematic (simplified) that implements this feature.
Figure 2: Load (LED) monitoring
The load here has a constant current that is regulated through the LEDs. While it is true that the current regulation system is inherently monitoring the load, the purpose is to ensure that the correct load current is flowing. LEDs are prone to damage, which often shows up as open circuits or short circuits.
These kinds of faults can also be caused by loose wires, connectors, assembly issues on PCBs, and so forth. A short circuit on the channel could also be caused due to damage to the MOSFET (which plays the role of the switch).
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