High power LED strings (such as those used in car headlights) require constant current, that is, regardless of the fluctuation of the ambient temperature and input voltage, the current remains stable. Current stability can ensure that the brightness level of the LED string is constant, which helps to extend its service life. There are several solutions for driving power LEDs. The more complete discrete solution is to combine precision parallel voltage regulators and power bipolar transistors, and configure with current absorbers.

? The schematic diagram of discrete LED driving circuit.

Using linear regulation at the output stage of the boost converter, the 24 V power supply voltage can be converted to 72 V to provide the required high voltage for two parallel LED strings. The base terminals of two parallel 100 V NPN transistors are controlled by an adjustable precision parallel voltage regulator to work in the linear region. The resistor in the feedback loop determines the size of the load current, so the total current consumption in the brightest working mode is 1 A.

Work stably and reliably using BJT.

By using an infrared camera to inspect the circuit (Figure 2), it can be seen that even at the brightest setting, the temperature of the BJT is far below the maximum allowed limit. Compared to integrated solutions, this discrete solution also provides more flexibility for thermal management on the PCB. When the input voltage fluctuates to 1V, the brightness and temperature of the LED string will not change, indicating that the solution is robust. Even when the ambient temperature rises to 50℃, the circuit can still operate reliably.

? There is a certain relationship between PCB and the thermal images obtained by infrared cameras.

It adopts DPAK package, with excellent thermal performance.

Nexperia's MJD series of power bipolar transistors in DPAK package have excellent thermal performance, making them ideal for this application. The solution uses two MJD31C high power 100V 3A NPN bipolar transistors compliant with automotive standards, but for applications requiring more precise linear regulators, the higher gain MJD31CH-Q can be used as a replacement for the MJD31CA. 

The base-emitter currents of these devices are controlled by the TLVH431 A-grade precision parallel voltage regulator, with a maximum bias current of 70 mA. Another advantage of linear regulation at the output stage is that it does not introduce additional electromagnetic interference (EMI), which is also a key consideration in automotive environments.

Sustainable Operation

Achieving robust and continuous operation in high-power LED strings requires that the current remain stable regardless of temperature and input voltage fluctuations. The discrete high voltage current absorber topology uses Nexperia (NXP Semiconductors) MJD series of power bipolar transistors and precision TLVH shunt regulators to ensure LED brightness remains constant with a simple, cost-effective solution.