Use low loss LED drivers to improve the "green" process of the power system

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Use low loss LED drivers to improve the "green" process of the power system

February 14 , 2023 0

Today there are many different solutions for powering high-brightness leds. Since most systems are powered by batteries, energy efficiency is the key to extending battery life and system operating time. Improving battery efficiency also helps speed up the "green" process of the system. Extending the time between charges could add hundreds of hours to a battery's useful life for the same number of charges. That means far fewer batteries are sent to landfills or hazardous waste sites for destruction.

Drivers for low power lighting typically use a simple linear voltage regulator configured in constant-current mode (Figure 1a). Linear voltage regulator has the advantages of simple design. However, its main disadvantage is high power consumption, because the excess voltage is dissipated through the galvanometer resistance and the heat of the regulating tube. Such heat loss also severely impedes the "green" process of the system. The greater the heat loss, the greater the demand for cooling (fans or large metal radiators), the greater the energy consumption, the greater the space and weight, and the increased material cost and manufacturing time.

An alternative solution is to use a switch mode tuning architecture, such as the buck regulator (Figure 1b). This type of regulator usually requires a feedback voltage of 0.8V to 1.3V to regulate the current flowing through the LED. The current measuring circuit used to establish this voltage is usually a small resistor in series with the LED. The voltage at both ends of the resistor acts as a feedback voltage to maintain constant current power supply for the LED. This architecture reduces the loss of the regulator itself, but the power dissipation of the galvanometer resistance makes the system loss still exist.

Figure 1a. A simple linear voltage regulator architecture has high power consumption due to the adjusting tube and current setting resistors. This circuit has the advantage of being simple and without any EMI, but it is only suitable for low-voltage applications and there is some heating.

Figure 1b. Basic switching mode regulation scheme. Power consumption mainly comes from the energy loss of galvanometer resistance. The solution is highly efficient and can be reconfigured to boost pressure. However, the circuit is relatively complex and produces EMI

In order to reduce the power consumption of the galvanometer resistance, a low loss current detection circuit should be used, such as a combination of resistance/op amp to provide the feedback voltage required by the switching converter. A dedicated precision galvanometer amplifier, such as the MAX9938T, can be used to produce a detection gain of 25V/V for the voltage at both ends of the galvanometer resistor. This scheme can reduce the loss of the feedback circuit to tens of milliwatts.

In the circuit shown in Figure 2, the boost converter uses a MAX9938T galvanometer amplifier and is powered by two NiMH batteries in series using a MAX8815A boost converter. The MAX8815A operates at a maximum switching frequency of 2MHz with an efficiency of up to 97%. High switching frequency minimizes the size of external components; Internal compensation reduces the number of external components and is suitable for cost - and space-sensitive products. The converter can produce an output voltage of 3.3V to 5V when powered by two NiMH batteries.

Figure 2. Derived from Figure 1b, a galvanometer amplifier such as MAX9938T is used to reduce the power consumption of the galvanometer resistor to tens of milliwatts. Compared to the circuit in Figure 1, which has a loss of several hundred milliwatts or more, the power consumption of this circuit is much reduced

The MAX9938T galvanometer amplifier controls the current flowing into the LED. The amplifier incorporates a gain setting resistor at its input, setting the gain at 25V/V. In addition, the amplifier has a high accuracy class, with a VOS of less than 500µV (maximum) and a gain error of less than ±0.5% (maximum). Since the feedback voltage of MAX8815A is 1.265V, the LED current generated by the 100mΩ galvanometer resistor is (1.265V/25)/0.1Ω ≈ 0.5A.

The MAX9938T requires an external input common-mode filter consisting of two sets of 10Ω/100nF to remove the common-mode voltage at the input due to the high frequency ripple at the MAX8815A output. The 200nF capacitor at the MAX9938T output reduces the bandwidth of the amplifier, thereby preventing oscillations.

The design idea provides a solution with very few components, which effectively prolongs the battery life by minimizing the power consumption of the regulator and control loop.