Breakthrough fixed voltage output limit LLC innovation LED drive circuit design

In a highly competitive market environment, circuit efficiency and cost are the designers of LED systems, and the focus is on the selection of drivers. In order to meet market expectations, it is a trend to replace the three-level architecture with a two-stage architecture with an isolated converter combined with LED constant current control, and a high-efficiency LLC converter is the first choice for this type of design.

Medium-power LED power supplies are used in large-scale construction lighting, street lighting, advertising billboards and greenhouse lighting, with output power ranging from 100 to 300W. Traditionally, the circuit topology of such LED driving power supplies mostly adopts a three-stage architecture, which is a boost power factor calibration (PFC) circuit, an isolated DC converter and a buck converter (Buck Converter).

However, in the highly competitive market environment, LED system designers pay more and more attention to the circuit efficiency and cost of the driver. Therefore, the two-stage architecture of the isolated converter combined with the LED constant current control can meet the market expectation, and the high efficiency LLC The converter is preferred.

Breaking the LED three-level drive concept, the two-level architecture has obvious advantages

In medium power LED applications, the benefits of a two-stage architecture save circuit space and cost and improve overall converter efficiency compared to a three-stage architecture. The choice of LLC converter for the second-stage architecture can reduce the switching loss. Compared with the flyback converter (FlybackConverter), it can reduce the leakage inductance loss and reduce the voltage rating of the primary and secondary power components, so it is expected to be used. LED driver stage, as shown in Figure 1.

Schematic diagram of the two-stage architecture of the power LED application in Figure 1.

However, for wide voltage output applications, power supply designers lack design experience and still use flyback converters to cover the mid-power range, which cannot further improve the performance of the driver circuit. This article will be led by the working principle of LLC to understand the design concept of a wide range of output voltage LLC, the main purpose is to grasp the change of the gain curve, that is, to control the LED current.

The contribution of the resonant converter to circuit efficiency is to assist the power switch to switch at zero voltage, reducing switching losses. By controlling the on-time of the switch, the output energy is adjusted, and the charge originally stored on the stray capacitance of the MOSFET is taken away before the switching commutation time, and the energy is transferred to the output end, and the resonant frequency is designed in conjunction with the resonant tank. However, the premise is that the stored energy of the transformer's magnetizing inductance or leakage inductance is enough to take away the charge in the stray capacitance, as shown in Figure 2. If the switching on-time of the resonant converter is symmetrical, the energy storage and release time of the transformer are the same, and the voltage and current stress of the secondary power component can be averaged; this type of resonant converter adopts frequency conversion control, and the common series resonance ( SRC), LLC and LCC converters.

Figure 2 resonant circuit to achieve switching zero voltage switching

The LLC converter can regulate the output voltage through charge and discharge of the magnetizing inductor under light load conditions, reducing the frequency range from light load to full load, while the SRC must operate at very high frequencies to maintain the light load output. The application of large output voltage fluctuations makes it difficult to find operating points.

The LCC has a larger voltage swing of the resonant capacitor by the parallel connection of the equivalent capacitance of the transformer, and has a wider voltage operating range than other resonant converters. In the past, LCCs were often used in gas discharge lamps to easily achieve high voltage ignition (Ignite) voltages and zero voltage switching under steady state operation. However, for converter efficiency, an increase in the primary switching current will result in greater conduction losses, leaving the LCC architecture still more than the evaluation phase. For LED applications, most customers require a wide range of voltage outputs to cover more lighting applications, and the requirements for dimming are more stringent, especially in the Lightquality section. For example, at very low output power, the LED application still does not allow the driver to enter the Burst mode because it causes flicker.

The only simple thing is that LEDs are different from information products in terms of hold-up time. Therefore, the PFC output voltage is determined within the allowable PF value and voltage stress range. The optimized design of a single input voltage can be considered for the resonant tank. .

To simplify the discussion, the author considers the LED load variation range as a different DC resistance value, as shown in Figure 3. An approximate equivalent model is obtained by simplifying the secondary DC resistance to a simple AC resistance Rac. Where Rac can be expressed as Equation 1.

Figure 3 LLC resonant AC equivalent circuit

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