LED balancing circuit

TN0025

2/7

1

LED balancing circuit

This circuit is used to balance the current in two or more parallel strings of series loads

where the individual loads vary in voltage, because loads have been opened or shortened.

The circuit contains one bipolar transistor and one resistor per string, a voltage reference

(two diodes shown in Figure 1) and a resistor for the reference. The current passing through

each load (Note: a 'load' is one string of LEDs. See Figure 1 which shows LED loads D1 and

D2, D3 and D4, and D5 and D6 in series) is also the current through one transistor.

Figure 1 also shows the power supply for an example using LED loads D1 plus D2, D3 plus

D4, D5 plus D6 and a power supply output voltage, with a constant output current, providing

number of LEDs can be used.

current through R1 is the voltage V1 divided by the resistor R1. The total LED current is

approximately the sum of the current through the emitter resistors R1, R2 and R3.

The current through each string remains approximately equal, even if the LED voltage varies

or if one is shorted. This is due to the relatively constant voltage of two reference diodes (D7

and D8, in series) will determine the current through the emitter resistors. For example if

LED (D1) in one string is open, the current through the string (D1 and D2) is zero. The base

to emitter junction of Q1 behaves like a diode and a small amount of current passes through

less current flows through the other strings (see Equation 10)

Figure 1.

LED balancing circuit

●

Equation 1

I

Re

V

–

Re

--------------------

=