Light-emitting diodes (LEDs) are often used with a voltage source from a power-supply output voltage. The output voltage increases until the current through the LEDs reaches the value of the constant-current source from the power supply. Also, a maximum output voltage is limited by the power supply.
LEDs are used individually, and when more light is required, they are stacked in series with the same current passing through each one. The voltage across the string increases with each LED added. After several LEDs are added in series, the power supply may not be able to handle the higher output voltage required. Also, a change of output voltage could be a problem for the power supply if a range in the number of LEDs is required, which would cause too wide a range in the output voltage and auxiliary circuits.
Without a circuit, the variation of the parallel loads could cause most or all of the current to pass through the path of the load with the lower voltage, with a small amount of current passing through the path of the load with the higher voltage. The result would be a variation in light output and could cause LED failure from excessive current. The objective is to achieve the same current through each parallel string of series loads, even though the voltage across each load is not the same.
The circuit in Fig. 1 can be used to balance the current in two or more parallel strings of series loads where the individual loads vary in voltage, if one load is open or if one load is shorted. The circuit contains one bipolar transistor and resistor per string, a voltage reference (two diodes shown) and a resistor for the reference. The current passing through each load is also the current through one transistor, with load being defined as one string of LEDs, such as D1 + D2 in series.
The Fig. 1 example includes LED loads D1 + D2, D3 + D4, D5 + D6 and a power-supply output voltage, with a constant output current, providing the circuit input voltage (VIN). With each string containing the same number of LEDs, any number of LEDs can be used.
The voltage at V1 is the reference voltage (VR) minus the base-to-emitter voltage (VBE1). The 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 one is shorted, due to the relative constant voltage of the reference (two diodes in series D7 and D8), which will determine the current through the emitter resistors.
If an LED (D1) in one string is open, the current through the string (D1 and D2) is zero. The base-to-emitter junction of Q1 will behave like a diode and a small amount of current will pass through R1. The reference voltage will drop, the other base-to-emitter junction voltages will drop and less current will flow through the other strings (Eq. 6).
For operation, with RE representing an emitter resistor:
For one open LED:
Power Limit Circuit
A power-limit circuit can be added to the circuit in Fig. 1 to limit the overall power when there is a large voltage variation for parallel strings of LEDs, with the result shown in Fig. 2. The voltage variation can result from one string containing LEDs with low-voltage drop while another string contains LEDs with high-voltage drops. Or voltage variation can occur if an LED in one string is shorted.
In Fig. 2, the power-limit circuit is formed by R11, R12, R13, R14, Q11 and Q12. If there is a low drop in the LED string D1 + D2, the voltage at V1 (Q1 collector) will be high and Q11 will turn on through R11 and R12. The voltage at VR will drop and the current in both LED strings will drop. The power-supply source will try to supply the required current, with its constant-current source, and will raise the output voltage up to the power-supply output voltage limit. VIN will increase, V1 will increase, and Q11 will latch on and disable the LED balancing circuit until VIN is removed.