Use of a single current loop to drive multiple LED strings
This keeps the junction temperature constant, which in turn improves reliability and also ensures that the intensity/chromaticity of different emitters is matched. At low current levels following a linear approach is acceptable. However, when LEDs require larger currents, use of a switching power supply design is likely to be needed. Control of a single LED string is relatively straightforward, but many lighting systems need many LEDs and multiple LED strings, which presents a number of challenges.
There are several important considerations to be aware of for a multiple string design. One of the most vital factors that must to taken into account is how to handle an LED string entering an open circuit state. This is because, with current mode control, all of the current meant to be flowing through the opened string will now be forced to split up into the remaining strings. This increases the current in the LEDs affecting brightness and reducing operational lifespan.
Typically when using a switching regulator in constant current mode, the current is measured by a small sensing resistor located at the bottom of the LED string. The voltage of this sensor is fed back to the regulator’s control loop. However, if there are multiple LED strings to take care of, then how does one control the loop? Some engineers will continue to use the one sense resistor for all the LED strings, and simply accept the risk that an open string may pose. Others will put Zener diode clamps across each of the LEDs so that if an LED string does open up the string current then flows through the Zener clamp instead (a methodology that is illustrated by Figure 1). This is effective, but at the same time very costly to implement, since it requires placing a Zener diode for each and every LED. Another option is to employ separate switchers and control loops for each LED string, but again this is an expensive strategy.
Figure 1: Schematic of Zener diode clamping LED strings
Instead of using multiple control loops for each string, a single control loop will suffice if a sensing resistor is added to the bottom of each LED string (see Figure 2). By placing PNP transistors at the top of the sense resistors and tying their collector outputs together, the sense voltage can then be sent to the controller’s feedback loop. A separate NPN transistor is also added in series with the PNP transistor’s base in order break the current path when the PNP transistor is placed in reverse bias operation. This occurs when an LED string opens up and the PNP transistor’s emitter junction gets pulled to ground while its collector has the feedback voltage on it.
When a current is flowing through the LED string, the NPN transistor turns on, which will subsequently pull the PNP transistor’s base resistor to ground, turning this on too. The outputs of these PNP transistors are tied together and sent to the feedback of the switching regulator. When both PNP transistors are on, the two sensing resistors are basically in parallel, telling the loop that it needs enough current for these two strings.
In the event that an LED string opens up, the current through the other LED string(s) will not be affected in any way. The PNP and NPN transistors for the open LED string automatically turn off, removing the feedback voltage of that string from the control loop. The total output current of the switcher will drop, but the currents being passed through each of the remaining strings will stay at a constant level, unchanged by the open string.
Figure 2: Schematic of an LED string arrangement utilizing an NCP3063 regulator
By using a device such as ON Semiconductor’s NCV3063 (a 1.5A switching regulator) it is possible to create a constant current switching regulator that can control multiple LED strings with just one control loop (see Figure 3). The regulator requires a 1.25V feedback voltage, which is the same voltage that will be developed across the sense resistors. This voltage will turn on the NPN transistors in the above scheme, which then turns on the PNP transistors allowing the sense voltage to be sent to the system’s control loop. If an LED string opens up, the faulty string is automatically eliminated from the circuit’s feedback and the system’s current is adjusted in response. In this case the device’s high voltage input of up to 40V and operating temperature range of -40Â°C – 125Â°C makes it robust enough to be applied in heavy duty industrial or automotive environments.
One of the additional benefits of implementing the scheme described here is that there is no limit to how many strings can potentially be added in parallel. This allows greater design flexibility for the lighting system. The differences in current between multiple strings will be dependent mainly on the variations in the forward voltage drop of the LED emitters themselves. Even though there is likely to be some disparity in the forward voltage characteristics of these devices, these variations converge quickly to the average forward voltage drop as the LED string length increases, so this can be treated as negligible in most large format lighting designs. Other variation factors, such as the tolerance of the sensing resistors and the saturation voltage of the PNP transistors have proved to be very small in laboratory measurements. The affect on the optical output of each emitter can be considered imperceptible to the human eye, and in reality is not detrimental to system performance.