Overcome the challenges of driving parallel LED strings
An LED is similar to a standard diode by virtue of being a current-driven device. It has an I-V curve in which the current and voltage are non-linear and a small change in its forward voltage can translate into a large current change. Since the LED current in nearly proportional to the LED’s luminous flux, it is important in applications such as TVs to control the current accurately. But not all applications necessarily require high accuracy for brightness matching of the LEDs. If the LEDs are driven in a single string, there is inherent matching because each LED has the same current level. As the number of LEDs in use increases, paralleling strings becomes necessary, and a choice must be made as to how to control the current in each string.
A typical white LED can have a forward voltage of 3.3V with as much as a 20% variation at its rated current. If 10 LEDs are used in series, it’s possible that one string may require 33V to adequately drive it, while a second string requires 39.6V at the same current. If these two strings are wired in parallel, the lower voltage string pulls significantly more current than intended and the second significantly less. The probability that all the LEDs in one string would fall at the high end of its forward voltage specification is rather small and this probability decreases as more LEDs are used.
In reality, balancing between these two strings is much better, but there could still be a difference of several volts. To help this situation, LED manufacturers use binning to sort parts into groups that accurately match the LEDs forward voltage (Vf) drops (as well as flux and wavelength) to allow better performance. Figure 1A shows a simple, low-cost implementation for paralleling two strings. A fixed-voltage source and a simple resistor to set the current level is all that is necessary.
The voltage across one sense resistor can be regulated by an external control circuit to adjust the output voltage higher or lower to accurately control the LED current. While this regulates the LED current in one string, it does not necessarily do a good job for the second. It can actually make the current in the second worse, as in the case where the control loop increases the output voltage for the regulated string, but the second string has the lower voltage drop of the two.
As in standard diodes, the forward drop of LEDs decreases with increasing temperature. If one string gets significantly hotter than the other, its forward drop decreases and it begins to draw more current. This added dissipation heats it further, increasing its current and possibly leading to LED failure due to this thermal runaway. This situation requires that the voltage driving the strings is current regulated and is constant. Additionally, all LEDs should be mounted on a common heatsink to keep the operational temperature between them as equal as possible.
Thermal runaway isn’t a problem when the strings are driven by a constant voltage, but the current matching between strings can be quite poor. Since each string is independent of the other (that is, the current in one is not directly regulating the current in the other), fault tolerance is good when driven by a voltage source, but poor when current is regulated in one string (via Vfb). In this situation, if an LED opens in the regulated string, the voltage driving the strings is commanded higher by the control circuit and eventually causes overvoltage in the unregulated string, leading to failure. While adequate when driven by a voltage source without feedback, the circuit of Figure 1A doesn’t provide accurate current matching in the LED strings for more demanding applications.
Figure 1. The current mirror (B) offers advantages over simple resistor (A) current regulation.
Figure 1B implements a current mirror to regulate the currents in both strings. The first string uses voltage feedback (Vfb) from sense resistor Rs1 to regulate its current and relies on Vbe of Q1 and Q2 matching to set the same voltage across Rs2. With the same sense resistor voltage and value, the same current is forced to flow in the second string. The regulation accuracy largely depends on the matching between the Vbe voltages of Q1 and Q2. For this reason, a dual transistor with both components on the same die helps reduce temperature, processing and lot variations.
This circuit gives reasonable accuracy, but base current mismatch and the Vbe to Rs ratio introduce errors that make it less than perfect. The larger the Vfb voltage is relative to Vbe, the lower the errors will be, but with increasing power dissipation. Adding base resistors in series with Q1/Q2 may also help with accuracy.