“To avoid thermal breakdown, LED lighting system designers should consider the thermal characteristics of the components. This is especially important in applications such as automotive lighting, where higher ambient temperatures and longer operating times cause components to age rapidly.
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To avoid thermal breakdown, LED lighting system designers should consider the thermal characteristics of the components. This is especially important in applications such as automotive lighting, where higher ambient temperatures and longer operating times cause components to age rapidly.
Advances in automotive lighting technology (increasing drive currents and smaller and smaller package sizes) make optimizing thermal design both difficult and necessary. Higher drive currents raise the junction temperature to the point where heat dissipation cannot be adequately optimized. Therefore, it is necessary to create a way to reduce the LED current when the temperature is too high.
Most automotive LED drivers have current dimming capabilities. However, dimming control circuits are usually controlled by complex analog or digital circuits, which often take up a lot of space in the final application and increase the overall system cost. This article presents a simple circuit solution based on NTC resistors (negative temperature coefficient) that linearly regulates the output current with respect to temperature.
Figure 1 The MPQ2489 LED driver IC uses the DIM pin for PWM and analog dimming. Source: MPS
Figure 1 Circuit The circuit is designed to maintain a stable nominal output current in the driver at temperatures below 70°C. If the circuit exceeds the temperature threshold, the output current is quasi-linear with temperature, reducing the output current to avoid thermal breakdown, which reaches the minimum current value when the LED reaches its maximum rated temperature of about 120°C.
induction circuit
As an example, this article uses the MPQ2489-AEC1, a 60V, 1 A automotive-grade step-down LED driver, shown in Figure 1. The driver implements both PWM and analog dimming, although only the latter is used in this application. To use the analog dimming function, a DC voltage of 0.3 to 2.5V must be applied to the DIM pin. This voltage can linearly regulate the LED current between 250 mA and 1.1 A (Figure 2). When the DC voltage is between 0.3 and 1.25V, a current between 250 and 550 mA is produced.
Figure 2 This analog dimming curve is generated by the MPQ2489-AEC1 buck LED driver. Source: MPS
The temperature is sensed using an NTC thermistor (TDK’s NTCG164BH103JTDS), which is implemented in a voltage divider resistor. Changes in the NTC resistance cause the voltage at the output of the divider to change depending on temperature. This shifts the voltage on the DIM pin, changing the output current.
The nominal voltage applied on the DIM pin is set by the 1.25V reference voltage. This ensures a stable input voltage at temperatures below the 70°C threshold. In addition, the supply voltage of the resistor divider is fixed at 6.2V using a 250mW Zener diode.
When the device is at 70°C or lower, the 1.25V supplied by the reference voltage limits the DIM input and the LEDs supply 550 mA. Once the temperature exceeds the 70°C threshold, the resistor divider output drops below 1.25V. The DIM input then follows a resistive divider profile, which reduces the LED drive current as the temperature continues to rise.
Simulation can be used to estimate the operation of the circuit. Simulation results for this example show that the DIM voltage stabilizes at 1.25V up to the temperature threshold and then decreases exponentially until it reaches a minimum output of 0.3V when the temperature reaches 120°C (Figure 3).
Figure 3 Simulation results of simulated dimming performed by a buck LED driver. Source: MPS
One disadvantage of this system is how the NTC resistance changes with temperature, according to the Steinhart-Hart formula (calculated by Equation 1):
Formula for calculating NTC resistance
The Steinhart-Hart equation shows that the relationship between temperature and NTC resistance value is nonlinear, so the resistor divider also has a nonlinear relationship with temperature. Therefore, the current reduction due to temperature is also non-linear. This drop can be estimated using Equation 2:
Equation for calculating the current drop due to temperature
Nonetheless, this circuit provides a small and simple solution to reduce the LED drive current at high temperatures, thereby increasing the life expectancy of these components.
Result verification
To test circuit performance, a system was built to simulate a real-world use case (Figure 4). A 3Ω resistor replaces the LED, which heats up by applying a voltage difference across the poles. The NTC of choice is then attached to the resistor with thermal paste to ensure the most accurate resistor/temperature detection. Finally, the NTC is connected to the designed circuit. By varying the temperature of the resistor (sweeping the power supplied to it), a DIM voltage curve was obtained.
Figure 4 The test setup was created to simulate simulated dimming of a real-world use case (such as a car light), and the test system was built to simulate a real-world use case. Source: MPS
The test is performed in a temperature range of 25°C to 145°C. Figure 5 shows that the expected circuit performance is achieved. When the temperature is below 74°C (close to the estimated 70°C threshold), the output voltage (VDIM) of the circuit remains stable at 1.25V. Above this temperature, the voltage drops to 0.25V at 145°C.
Figure 5 Test results showing dimming voltage as a function of temperature Source: MPS
Figure 6 shows that the obtained drive current is set to 100% when the LED temperature is below 74°C. Once the temperature exceeds this value, the drive current is reduced and the LED is dimmed to reduce heat dissipation and counteract the effects of temperature rise. This test, as well as the test shown in Figure 5, confirmed the intended functionality of the design. By successfully limiting the output current at high temperatures, circuit components can be protected from thermal damage.
Figure 6 Test results showing drive current versus temperature Source: MPS
This article demonstrates how the circuit’s implementation can control the drive current of an LED by using a simple sensing circuit and the pre-existing dimming function in most LED drivers. This solution provides automotive lighting system manufacturers with a stable, cost-effective option that can significantly increase the life expectancy of components in circuits while taking up very little board space. The circuit presented in this paper can be applied to many existing lighting systems with relative ease and a cheap bill of materials.
Xavier Ribas and Tomas Hudson are application engineers at Monolithic Power Systems (MPS)
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