How to choose the topology of the LED driver power supply
The high reliability of LEDs (lifetime over 50,000 hours), high efficiency ("120 lumens/watt" and near-instantaneous responsiveness make it an attractive source of light. Compared to the response time of an incandescent bulb of 200mS, the LED will illuminate in just 5nS response time. Therefore, they are currently widely used in brake lights in the automotive industry.
Drive LED
Driving LEDs is not without challenges. The adjustable brightness requires a constant current to drive the LED and it must be kept constant regardless of the input voltage. This is more challenging than simply connecting an incandescent bulb to a battery to power it.
The LED has a forward VI characteristic similar to a diode. Below the LED turn-on threshold (the white LED's turn-on voltage threshold is approximately 3.5V), the current through the LED is very small. Above this threshold, the current will increase exponentially in the form of a forward voltage. This allows the LED to be shaped as a voltage source with a series resistor with a warning that the model is only valid at a single operating DC current. If the DC current in the LED changes, the resistance of the model should also change to reflect the new operating current. At large forward currents, power dissipation in the LEDs can cause the device to heat up, which will change the forward voltage drop and dynamic impedance. It is important to fully consider the thermal environment when determining the LED impedance.
When driving LEDs through a buck regulator, the LEDs often conduct the AC ripple current and DC current of the inductor based on the selected output filter arrangement. This not only increases the RMS amplitude of the current in the LED, but also increases its power consumption. This increases the junction temperature and has a significant impact on the life of the LED. If we set a 70% light output limit as the life of the LED, the LED's lifetime will extend from 15,000 hours at 74 degrees Celsius to 40,000 hours at 63 degrees Celsius. The power loss of the LED is determined by multiplying the LED resistance by the square of the RMS current plus the average current multiplied by the forward voltage drop. Since the junction temperature can be determined by the average power consumption, even a large ripple current has little effect on power consumption. For example, in a buck converter, the peak-to-peak ripple current equal to the DC output current (Ipk-pk = Iout) increases by no more than 10% of the total power loss. If you exceed the above loss level, you need to reduce the AC ripple current from the power supply to keep the junction temperature and operating life constant. A very useful rule of thumb is that for every 10 degrees Celsius reduction in junction temperature, semiconductor lifetime will be doubled. In fact, most designs tend to have lower ripple currents due to the suppression of the inductor. In addition, the peak current in the LED should not exceed the maximum safe operating current rating specified by the manufacturer.
Topology Selection Analysis of LED Driver Power Supply
In LED lighting applications with AC-DC power supplies, the power conversion building blocks include discrete components such as diodes, switches (FETs), inductors, and capacitors and resistors to perform their functions, while pulse width modulation (PWM) regulators are used to control Power conversion. The isolated AC-DC power conversion with a transformer usually included in the circuit includes topologies such as flyback, forward and half bridge. See Figure 3, where the flyback topology is the standard choice for medium and low power applications with power less than 30 W. The half-bridge structure is best suited to provide higher energy efficiency/power density. For transformers in isolated structures, the size of the transformer is related to the switching frequency, and most isolated LED drivers basically use "electronic" transformers.
Figure 1: LLC Half-Bridge Resonant Topology
In the LED lighting application using DC-DC power supply, the LED driving methods that can be used are resistance type, linear regulator and switching regulator. The basic application diagram is shown in Figure 4. In the resistive drive mode, the current sense resistor connected in series with the LED can be used to control the forward current of the LED. This drive mode is easy to design, low in cost, and has no electromagnetic compatibility (EMC) problem. The disadvantage is that it depends on the voltage and needs to be filtered. (binning) LED with low energy efficiency. Linear regulators are also easy to design and have no EMC problems. They also support current regulation and fold back, and provide an external current set point. The power dissipation problem is insufficient, and the input voltage is always higher than the forward voltage. Voltage, and energy efficiency is not high. The switching regulator continuously controls the opening and closing of the switch (FET) through the PWM control module to control the flow of current.
Figure 2: Common DC-DC LED Drivers
Switching regulators have higher energy efficiency, are independent of voltage, and can control brightness. Insufficient is relatively high cost, high complexity, and electromagnetic interference (EMI) problems. Common topologies for LED DC-DC switching regulators include buck, boost, buck-boost, or single-ended primary inductor converter (SEPIC). Where the lowest input voltage under all operating conditions is greater than the maximum voltage of the LED string, a step-down structure is used, such as driving 6 series LEDs with 24 Vdc; in contrast, when the maximum input voltage is less than the minimum output voltage under all operating conditions The boost structure, such as driving 6 LEDs in series with 12 Vdc; and the input voltage and output voltage range overlap can be buck-boost or SEPIC structure, such as driving 12 LEDs in series with 12 Vdc or 12 Vac However, the cost and energy efficiency of this structure is the least desirable.
The way of directly driving LEDs with AC power has also achieved certain development in recent years. See Figure 5 for the application diagram. In this configuration, the LED strings are arranged in opposite directions, operating in a half cycle, and the LEDs are turned on when the line voltage is greater than the forward voltage. This structure has its advantages, such as avoiding power loss caused by AC-DC conversion. However, in this configuration, the LED is switched at a low frequency, so the human eye may be aware of flicker. In addition, LED protection must be added to this design to protect it from line surges or transients.
Figure 3: Schematic diagram of direct use of AC driven LEDs
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