LED Technology
Figure 3: Using a current mirror to balance LED string currents
forward voltages are mismatched. The balancing circuit must also continually compensate for changes in forward voltage caused by changes in the operating temperature and by aging of the LEDs. Fortunately, there is a very simple transistor circuit that will do this job admirably. It is called a current mirror and “reflects” the current flowing through one reference transistor onto the current flowing through a second transistor. As long as the transistors are reasonably well matched in terms of their Vbe values , the currents will also be reasonably well matched.
In tests using RECOM’s 700mA LED driver and two strings of 350mA Osram LEDs, the currents flowing through the two strings were matched to an accuracy of about 87% over the entire input voltage range of the driver from 16VDC to 36VDC. The LED currents were stable as the LEDS warmed up and no thermal runaway was observed. It is important that the two transistors are both at the same temperature so a copper clamp was used to thermally connect both transistors together to keep their Vbe voltages stable. In addition, if any of the LEDs in String 1 fail, the current to ALL of the LEDs is disconnected. Thus the LEDs on String 2 are automatically protected against being over-driven. However, this circuit is still not ideal. The
currents are not perfectly matched and if any of the LEDs in String 2 fail, then all of the 700mA source current will still flow through the first string and destroy it. Figure 4 shows the final version of the
current balancing circuit. The addition of 1.5 Ohm resistors in the emitter paths
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The circuit suggestion given in Figure 4 can theoretically be extended to any number of LED strings by adding an NPN transistor and emitter resistor to each additional string and tying the transistor bases together. The current flowing through the reference transistor will be faithfully mirrored by all of the other transistors. However, considering that LEDs are high reliability illumination sources and the driver and associated components need to be equally reliable to get the maximum lifetime out of the system, it is recommended that the circuitry be kept as simple as possible and restricted to only one or two strings per driver.
RECOM Electronic |
www.recom-international.com
Paul Cheeseman is Business Development Manager, Sales Northern Europe, RECOM Electronic
makes the circuit less sensitive to small Vbe changes and balances the currents in the two strings to 99% accuracy.
Figure 4: A current mirror with over-current protection
The addition of a small signal transistor
as a current monitor protects the LEDs from being over-driven in the case of any LED failures. If LED1-LED5 fail open circuit, then the current in the second string falls to zero as before. However, if LED6 –LED10 fail, then the current increases in the first string until the voltage developed across the 1.5 Ohm emitter resistor reaches around 0.7V, thus turning on the BC337 transistor and pulling the base voltage of the power transistor to ground and limiting the current. With the component values given in the circuit, the measured current limit was 445mA with String 2 open circuit.
Components in Electronics February 2012 13
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