Summary
Recently, the LLC topology has been favored by power supply design engineers for its high efficiency and high power density. However, this soft switching topology requires more MOSFETs than any previous hard switching topology. Especially in the case of power start, dynamic load, overload, short circuit, etc. With its fast recovery body diodes, low Qg and Coss, CoolMOS can fully meet these requirements and greatly improve the reliability of the power system.
For a long time, improving the power density, efficiency and reliability of power systems has always been a major issue for researchers. Increasing the switching frequency of the power supply is one of the methods, but the increase of the frequency will affect the switching loss of the power device, so that the lifting frequency is not very effective for the hard switching topology, and the hard switching topology has reached its design bottleneck. . At this time, soft switching topologies, such as LLC topology, are sought after by design engineers for their unique characteristics. But... This topology puts new demands on power devices.
2. Features of the LLC circuit
The following features of the LLC topology make it widely used in a variety of switching power supplies:
1. LLC converters enable zero voltage switching over a wide load range.
2. The output can be adjusted with a wide range of input voltage and load changes, while the switching frequency changes relatively small.
3. Using frequency control, the duty ratio of the upper and lower tubes is 50%.
4. Reduce the voltage stress of the secondary synchronous rectifier MOSFET, you can use a lower voltage MOSFET to reduce costs.
5. No need for output inductors, which can further reduce system cost.
6. The use of lower voltage synchronous rectifier MOSFETs can further improve efficiency.
3. The basic structure and working principle of the LLC circuit
Typical circuit and operating waveforms of the LLC resonant converter are shown in Figures 1 and 2, respectively. The LLC converter shown in Figure 1 consists of two power MOSFETs (Q1 and Q2) with a duty cycle of 0.5; a resonant capacitor Cr, a center-tap transformer Tr with the same number of turns, an equivalent inductance Lr, and excitation. Inductor Lm, full-wave rectifier diodes D1 and D2 and output capacitor Co.
Figure 1 Typical circuit of a LLC resonant converter
Figure 2 The operating waveform of the LLC resonant converter
The LLC has two resonant frequencies, Cr, Lr determine the resonant frequency fr1; and Lm, Lr, Cr determine the resonant frequency fr2.
When the load of the system changes, the operating frequency of the system changes. When the load increases, the switching frequency of the MOSFET decreases. When the load decreases, the switching frequency increases.
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