Read the basic structure and working principle of LLC circuit in one article

Recently, LLC topology has been favored by the majority of power supply design engineers for its high efficiency and high power density. However, this soft-switching topology requires MOSFETs more than any previous hard-switching topology. Especially in the case of power on, dynamic load, overload, short circuit, etc. CoolMOS with its fast recovery body diode, low Qg and Coss can fully meet these requirements and greatly improve the reliability of the power system.

Summary

Recently, LLC topology has been favored by the majority of power supply design engineers for its high efficiency and high power density. However, this soft-switching topology requires MOSFETs more than any previous hard-switching topology. Especially in the case of power on, dynamic load, overload, short circuit, etc. CoolMOS with its fast recovery body diode, low Qg and Coss can fully meet these requirements and greatly improve the reliability of the power system.

For a long time, improving the power density, efficiency, and system reliability of the power supply system has been a major issue facing R&D personnel. 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 effect of increasing the frequency on the hard-switching topology is not very obvious, and the hard-switching topology has reached its design bottleneck. .

At this time, soft-switching topologies, such as LLC topologies, are sought after by the majority of design engineers for their unique characteristics. But this kind of topology puts forward new requirements for power devices.

Features of LLC circuit

The following characteristics of LLC topology make it widely used in various switching power supplies:

1. The LLC converter can achieve zero voltage switching in a wide load range.

2. The output can be adjusted when the input voltage and load change in a wide range, while the switching frequency changes relatively little.

3. Using frequency control, the duty cycle of the upper and lower tubes is 50%.

4. To reduce the voltage stress of the secondary synchronous rectification MOSFET, a lower voltage MOSFET can be used to reduce costs.

5. No output Inductor is required, which can further reduce system cost.

6. The use of lower voltage synchronous rectifier MOSFETs can further improve efficiency.

Basic structure and working principle of LLC circuit

Figure 1 and Figure 2 show the typical circuit and operating waveforms of the LLC resonant converter, respectively. As shown in Figure 1, the LLC converter includes two power MOSFETs (Q1 and Q2) with a duty cycle of 0.5; the resonant capacitor Cr, the center-tapped transformer Tr with the same number of secondary turns, the equivalent inductance Lr, and the magnetizing inductance Lm , Full-wave rectifier diodes D1 and D2 and output capacitor Co.

Read the basic structure and working principle of LLC circuit in one article
Figure 1 Typical circuit of LLC resonant converter

Read the basic structure and working principle of LLC circuit in one article

Fig. 2 Working waveform of LLC resonant converter

And LLC has two resonant frequencies, Cr and 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 will change. When the load increases, the switching frequency of the MOSFET decreases, and when the load decreases, the switching frequency increases.

Read the basic structure and working principle of LLC circuit in one article

1. Working sequence of LLC resonant converter

The steady-state working principle of the LLC converter is as follows.

1)〔t1,t2〕

Q1 turns off, Q2 turns on, the inductor Lr and Cr resonate, the secondary D1 turns off, D2 turns on, and the diode D1 is about twice the output voltage. At this time, the energy is converted from Cr, Lr to the secondary. Until Q2 turns off.

2)〔t2,t3〕

Q1 and Q2 are turned off at the same time. At this time, it is in the dead time. At this time, the inductor Lr, Lm current charges the output capacitor of Q2 and discharges the output capacitor of Q1 until the voltage of the output capacitor of Q2 is equal to Vin.

The secondary D1 and D2 are turned off Vd1=Vd2=0, and the phase ends when Q1 is turned on.

3)〔t3,t4〕

Q1 turns on and Q2 turns off. D1 is turned on, D2 is turned off, at this time Vd2=2Vout

Cr and Lr resonate at fr1. At this time, the current of Ls returns to Vin through Q1, until the current of Lr reaches the end of the zero-order phase.

4)〔t4,t5〕

Q1 turns on, Q2 turns off, D1 turns on, D2 turns off, Vd2=2Vout

Cr and Lr resonate at fr1, and the current of Lr flows back to the power ground through Q1 in the reverse direction.The energy is transferred from the input to the secondary until Q1 turns off and the phase ends

5)[t5t6)

Q1 and Q2 are turned off at the same time, D1 and D2 are turned off. The primary current I (Lr+Lm) charges the Coss of Q1 and discharges Coss2 until the Coss voltage of Q2 is zero. At this time, the Q2 diode begins to conduct. The phase ends when Q2 is turned on.

6)〔t6,t7〕

Q1 turns off, Q2 turns on, D1 turns off, D2 turns on, Cr and Ls resonate at frequency fr1, and Lr current returns to ground via Q2. The phase ends when the Lr current is zero.

The Links:   1DI400A-120 LB043WQ1-TD05

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