Using PTC thermistors to control current in the driver circuit enables the use of high-output LEDs in a variety of applications
BY STEFAN BENKHOF
EPCOS AG
Munich, Germany
Advances in high-power LED technology have increased the importance of thermal aspects in the design phase. To avoid accelerated degradation of output or, worse yet, total failure, LEDs must not get too hot.
While high-power LEDs have a higher efficiency than incandescents, a high proportion of their power input generates heat rather than light. Reliable operation requires good heat sinking and allowance for high ambient temperature in the design phase. Thermal aspects must also be considered when dimensioning the drive circuit of the LEDs.
Forward current must be selected to ensure that the LED chip does not overheat. In practice, the operating current is often simply set to a low level in order to ensure that the LED does not overheat, even at very high ambient temperatures. However, operating an LED with a temperature-independent source of current has a disadvantage: at excessive temperatures, the LED is operated outside the specifications. Moreover, at low temperatures the light source is then provided with a current significantly below the maximum permissible current.
Thermistors in the LED driver
Therefore, it is often desirable to control the LED current as a function of temperature—a procedure known as derating. While some expensive LED driver ICs can fulfill this task by using internal or external temperature sensors, we want to present a much simpler aproach by using PTC thermistors in the LED driver circuit. This offers significant benefits:
Forward current is increased, as is light output at room temperature;Costs are cut because the number of LEDs can be reduced. In addition, less expensive driver ICs or even a driver circuit without integrated thermal management can be used;Design of a driver circuit without IC control that is still able to vary the LED current dependent on temperature is possible.Less-expensive LEDs can be used with more pronounced derating and a smaller safety reserve.Reliability is increased via the overheating-protection function.Simpler thermomechanical design including heat sinks is possible.
Topology #1
In one 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 Rset at the feedback pin of the IC is reached. The LED current consequently sets itself at ILED = VFB /Rset .
Fig. 1. This circuit illustrates temperature monitoring and derating with PTC thermistors.
A modification of this circuit (see Fig. 1 ) generates a temperature-dependent LED current through a PTC thermistor in the current path of the LED. 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:
ILED (T) = (VFB /Rparallel ) (1+ (Rparallel + Rseries ) / RPTC (T))
The circuit shown in Fig. 1 illustrates the resulting temperature dependence of the LED current. 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% between 0° and 40°C with a PTC thermistor, and the brightness of the LED can be boosted by about the same percentage.
Topology #2
In 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. So 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.
A modification of the standard circuit also uses a PTC thermistor (see Fig. 2 ). Although the 0603 PTC thermistors of the B59601A series used here have a resistance of R25 = 470 Ω, at the sensing temperature—which can be set in 10 increments—resistance of the component reaches 4.7 kΩ with a tolerance of ±5°C (standard series) or ±3°C (tight-tolerance series).
Fig. 2. In this circuit, measurement is not performed in the
current path. Figure 2 shows the resulting LED current as a function of the temperature, and is a simpler circuit layout than in Fig. 1. The narrowly toleranced fixed resistor Rseries dominates the total resistance at low temperatures.
Only from about 15K below the sensing temperature of the PTC thermistor does the current begin to drop owing 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, avoiding heat death.
Circuits without driver ICs
LEDs can also be operated without a driving IC. This can be illustrated by a circuit that drives a single 200-mA LED from an automotive battery—similar to popular designs for center high mounted stop lamps.
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:
ILED (T) = (Vstab – Vdiode (T) ) / Rout
Fig. 3. Here, the greatest amount of LED current flows via the PTC. Alternatively, the fixed resistor can be replaced by a combination of a radial leaded PTC thermistor B59940 C0080A070 (R25 = 2.3 Ω) and two fixed resistors (see Fig. 3 ). The resulting forward current is calculated with the equation:
ILED (T) = (Vstab –Vdiode (T)) / (Rseries + RPTC (T) Rparallel / (RPTC (T) + Rparallel ) )
In this case, the greatest amount of the LED current flows via the PTC. Therefore a larger radially leaded component needs to be selected. Smaller chip PTC thermistors would heat up themselves because of the current flowing through and would always reduce the current independent of the ambient temperature. While connecting two or more chip PTC thermistors in parallel would divide the current, there are economic 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.
The 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:
ILED min (T) = (Vstab –Vdiode (T)) /(Rparallel + Rseries )
This property is extremely important in automotive electronics, where, for example, safety requirements do not allow the lights to be switched off completely. ■
Advertisement
Learn more about EPCOS
