Light-emitting diodes (LEDs) have developed greatly in recent years: from leading a niche existence as pure indicator lamps to high-power LEDs with a light output currently exceeding 100 lumens. Lighting costs with LEDs will soon drop to levels similar to those of classic cold cathode fluorescent (CCFL) lamps. This makes them of increasing interest for automotive lighting, as LED sources in and on buildings, as well as for LCD panel backlighting in notebook PCs or TVs.
Advances in high-power LED technology have increased the importance of the thermal aspects in the design phase. Like all other semiconductors, LEDs must not get too hot, in order to avoid an accelerated degradation in output or, in the worst case, total failure. Although high-power LEDs have a higher efficiency than incandescent lamps, a high proportion of their power input nevertheless generates heat rather than light. Reliable operation thus demands good heat sinking and the taking into account of high ambient temperature in the design phase already.
Thermal aspects must also be considered when dimensioning the drive circuit of the LEDs: their forward current must be selected to ensure that the LED chip does not overheat, even at maximum ambient temperatures. This is done by reducing the maximum acceptable current as the temperature rises, a procedure known as derating. LED manufacturers include the derating curve in their specifications. A curve of this kind is shown in Fig. 1:
| ||FIGURE 1: DERATING CURVE OF LEDs|
Red: At a maximum operating temperature of 80 °C a constant current must be limited to 370 mA. The maximum light efficiency is not reached at lower temperatures.
Green: When the operating voltage is applied as a function of temperature, the light efficiency can be increased over a large temperature range.
Black: LED derating curve
Operating an LED with a temperature-independent source of current has the disadvantage that at excessive temperatures the LED is operated outside of the specifications. Moreover, at low temperatures the light source is then provided with a current significantly below the maximum permissible current (red curve in Fig. 1). Controlling the LED current by using PTC thermistors in the LED driver circuit as illustrated in the green curve of Fig. 1 is a major improvement. Among other things, this offers the following benefits:
- Increasing the forward current and thus the light output at room temperature.
- Cost savings, as the number of LEDs can be reduced, using less expensive driver ICs or even a driver circuit without integrated thermal management.
- The possibility of designing a driver circuit without IC control that is still able to vary the LED current dependent on temperature.
- The ability to use less expensive LEDs with more pronounced derating and a smaller safety reserve.
- Reliability is increased by the overheating-protection function.
- The thermo-mechanical design including heat sinks is simpler.
Most driver topologies for LEDs have the following in common: the forward current through the LED is set via a fixed resistor (Fig. 2). As a rule, the current flowing through the LED ILED depends on this resistance Rout in accordance with ILED ~ 1/Rout. As Rout does not change with temperature, the LED current is also temperature-independent.
| ||FIGURE 2: CONVENTIONAL DRIVING OF LEDs|
|In these drive circuits, the current flowing through the LED is independent of the temperature. Thus, the derating required at high temperatures does not take place.|
Thermal management of the LED current can be achieved by replacing the fixed resistor with a circuit that is temperature-dependent itself. The following diagrams illustrate how standard circuits can be improved with a PTC thermistor.
Example 1: constant-current source with a feedback loop
Circuit 1 in Fig. 2 shows a frequently used driver topology. The constant-current source contains a feedback loop. The LED current is changed until the IC-specific feedback voltage VFB across the setting resistor at the feedback pin of the IC is reached. The LED current consequently sets itself at ILED = VFB / Rout.
Figure 3 shows a modification of this circuit: it generates a temperature-dependent LED current through a PTC thermistor. This circuit is matched to a special combination of driver IC and LED by the correct selection of PTC thermistor, Rseries and Rparallel. The LED current is calculated from the following equation:
| ||FIGURE 3: TEMPERATURE MONITORING AND DERATING WITH PTC THERMISTORS|
In this case, the PTC is in the current path of the LED.
The actual current curve as a function of the temperature corresponds to the LED specifications (Rseries = 910 Ω, Rparallel = 18 Ω).
The circuit shown in Fig. 3 illustrates the resulting temperature dependence of the LED current (Fig. 3). In comparison to a constant-current source dimensioned for a maximum operating temperature of 60 °C, the LED current can be increased by up to 40 percent between 0 °C and 40 °C with a PTC thermistor and the brightness of the LED can be boosted by about the same percentage.
Example 2: Constant-current source with setting resistor not connected in series with the LED
Circuit 2 in Fig. 2 shows another popular version of the constant-current source: the current is determined by a resistor connected to the driver IC. In this case, however, the setting resistor is not connected in series with the LED. The ratio between Rset and ILED is given in the IC specification. Thus, using a series resistance of 20 kΩ a driver IC of the TLE4241G type from Infineon Technologies results in an LED current of 30 mA. Fig. 4 shows a modification of the standard circuit also containing a PTC thermistor. Although the PTC thermistors of the B59601A* series used here (size 0603) have a resistance of R25 = 470 Ω. At the sensing temperature, which can be set in ten-degree increments, resistance of the component reaches 4.7 kΩ with a tolerance of ±5 °C (standard series) or ±3 °C (tight-tolerance series).
| ||FIGURE 4: TEMPERATURE RECORDING WITHOUT SHUNT MEASUREMENT|
In this circuit, the measurement is not performed in the current path. The circuit layout is simpler than in the example shown in Fig. 3.
The resulting LED current as a function of the ambient temperature.
Fig. 4 shows the resulting LED current as a function of the temperature. The narrowly toleranced fixed resistor Rseries dominates the total resistance at low temperatures. Only from about 15 K below the sensing temperature of the PTC thermistor does the current begin to drop due to the increasing resistance of the PTC thermistor. A current of about 23 mA is reached at the sensing temperature (total resistance Rtotal = Rseries + RPTC = 19.5 kΩ + 4.7 kΩ = 24.2 kΩ). The steep rise of the PTC resistance at even higher temperatures leads quickly to shutdown and thus avoids heat death.
Example 3: Simple driver circuit without IC
As circuit 3 in Fig. 2 shows, LEDs can also be operated without a driving IC. This is illustrated by a circuit that drives a single 200-mA LED from an automotive battery. To avoid the fluctuations of the supply voltage, a voltage stabilizer generates a stable supply voltage Vstab of 5 V. The LED is operated at Vstab and the current set via a resistance element Rout connected in series to the LED. In this type of circuit, the temperature-independent forward current is obtained from the following equation, where VDiode is the forward voltage of a single LED:
Alternatively, the fixed resistor can be replaced by a combination of a radial leaded PTC thermistor of the type B59940C0080A070 (R25 = 2.3 Ω) and two fixed resistors as shown in Fig. 5. The resulting forward current is calculated with the equation:
| ||FIGURE 5: TEMPERATURE-COMPENSATED DRIVING WITHOUT IC|
In this case, the greatest amount of the LED current flows via the PTC (Ordering Code: B59940C0080A070).
The current as a function of the temperature.
As a significant amount of the LED current flows through the PTC thermistor itself, a larger radial-leaded component was selected. A significantly smaller chip PTC thermistor would heat up itself because of the current flowing through it and would always reduce the current independent of the ambient temperature (Fig. 5). While connecting two or more chip PTC thermistors in parallel would divide the current, there are limits to this concept.
The current is essentially set to the desired value by suitable selection of the two fixed resistors. These resistors also play an essential role in improving the circuit by keeping the tolerance of the resulting LED forward current low. This is particularly important in the normal operating temperature range in which the PTC thermistor itself still has a high resistance tolerance. The second parallel fixed resistor also ensures that the PTC does not switch the LED off completely even at extreme excess temperatures. So the current never drops below the value calculated from:
This property is extremely important in automotive electronics, for example, where the safety requirements do not allow the lights to be switched off completely.
| ||BACKGROUND: TEMPERATURE DEPENDENCE OF LED|
Like all semiconductors, LEDs have a maximum permissible junction temperature that must not be exceeded, in order to avoid premature aging or total failure. The maximum permissible forward current must drop as the ambient temperature rises if the junction temperature is to remain below the critical value. However, the forward current at a specified ambient temperature can be increased if a heat sink is used. The light output of the LEDs declines as the junction temperature of the chip rises. This effect occurs principally with red and yellow LEDs, whereas white LEDs show a lower temperature dependence. The light efficiency increases in line with the forward current. However, a high thermal resistance of the LED mounted between the junction layer and the surroundings can reduce or even reverse this effect because the emitted light declines as the junction temperature rises.
In addition, the dominant wavelength of the emitted light increases at a typical rate of +0.1 nm/K as the junction temperature rises and the forward voltage of the LED increases in line with the temperature.
Table: Overview of current PTC thermistors for excess temperature protection ( Table as PDF)