Migration from incandescent lighting to solid state alternatives is continuing, but if LED adoption is to reach applications such as replacement lamps in the domestic environment or automotive interiors, new driver and control techniques are needed, writes Steve Sheard, ON Semiconductor
Supplying LED arrays with a constant forward current is the preferred approach for maintaining consistent luminosity across multiple emitters. It also safeguards against LEDs being harmed by over driving.
One way of regulating an array’s current is placing a resistor in series with it. There are, unfortunately, drawbacks with this, as resistors have relatively large power dissipation – causing unwanted heat to be generated and lowering overall efficiency.
Furthermore, series resistors cannot offer protection against supply voltage variations. This can affect the array’s light output and potentially leave it exposed to damage from high voltage transients.
Use of constant-current regulators (CCR), based on a self-biasing transistors, is an effective way to regulate LED array currents, avoiding the inherent disadvantages of series resistors. Also, because CCRs have a negative current v temperature characteristic, they can protect against thermal runaway as ambient temperature rises.
High-power LEDs are far more operationally efficient than incandescent lamps, but nevertheless a percentage of the energy supplied is converted into heat. If this is not combatted the heat produced will cause temperature rises in the LEDs, resulting in decreased luminosity and a change in the emitted colour. Also LED liufetimes could be reduced by operation at high temperatures over a long period.
For CCR regulation of an LED array to prove totally successful, issues of thermal management must be taken into account throughout the system design. The thermal effects on the array’s emitters, plus the various passives supporting it need to be considered, but even the CCR itself must be included in this as it dissipates power when conducting the LED driving current.
How the LED’s light output changes with time in relation to its initial output defines the effective lifespan of an LED lamp. This lifespan is determined by defining a point at which luminosity goes below a certain acceptable level (normally 70% of the LED’s initial light output).
LED suppliers will, in many case, publish graphs describing lumen output, temperature and drive current for a given emitter type. This data enables engineers to predict the lifespan of specified LEDs and calculate the optimum current/operating temperature to ensure the required light output over a target period.
The heat the LED array generates is a key factor in determining the lamp assembly temperature and greatly influences the surrounding ambient temperature if the lamp is in a confined space. Hence, in addition to calculating the lifetime of the LEDs, engineers must also ensure that surrounding electronic components are capable of matching the lamp’s projected lifespan.
The thermal effects on electrolytic capacitors must be considered for example. High operational temperatures will tend to dry the electrolyte, causing loss of capacitance. Capacitors may need derating as a result, in order to ensure the minimum required capacitance over temperature and time.
CCR thermal management
The LED drive current flowing in the CCR has the potential to lead significant self heating of this device – so the thermal performance of the system around the CCR must be scrutinised. A key parameter is the degree heat sinking (area and thickness of copper on the PCB, whether to utilize thermally enhanced substrates, etc). For a series circuit, the maximum power dissipated (PD) by the CCR is determined by:
PD = (Vsource – (VLEDS + VRPD)) x IREG
Where VRPD is the voltage across the reverse-battery protection diode.
With respect to thermal design, worst case values should be used when calculating PD. That is, the highest Vsource, lowest LED VF, and highest target Ireg.
Given a maximum supply voltage of 16Vdc and minimum LED forward voltage VF of 2.0Vdc per LED, 0.2Vdc Schottky diode forward voltage, the maximum power dissipated in the CCR can be derived as follows:
PD = (16V – (3 x 2V) + 0.2V) x 30mA = 294mW
For an ambient temperature of 85 °C, Figure 2 shows the required PD can be met using 100 mm2 of 1oz copper with a SOT-223 package CCR.
Two CCRs connected in parallel allow further increases in current handling. By splitting the total power between two CCRs, the thermal effect is spread over a larger area. (Vsource – VLEDS ) x IREG determines the power dissipated by each CCR.
Combining the highest Vsource, lowest LED VF and highest target IREG to give the worst case scenario, an 18V source driving two LEDs with a VF of 3.5V and 350mA IREG requires a maximum dissipation of:
PD = (18V ? (2 x 3.5V)) x 0.350A = 11V x 0.35A = 3.85W per CCR
If the ambient temperature is 85°C, the desired dissipation cannot be achieved with a standard FR4 PCB, so a copper metal-clad PCB may be required.
By employing CCRs it is possible to accurately regulate LED arrays while reducing design complexity. However, power dissipated by the CCR will impact on system performance, so effort needs to be put into the thermal aspect of the design.
Engineers must also look at the PCB’s thermal characteristics in relation to the LEDs and calculate the optimum combination of CCR package and substrate metallisation to prevent the CCR from being placed in danger of overheating.