Transistors｜Evaluation

Basic Operation of LLC Converters

2024.04.03

Points of this article

・LLC converters generally operate in region (2) where the gain is greater than 1 and MOSFET turn-on ZVS operation occurs.

・Circuit operation of an LLC converter in region (2) is divided into ten modes.

In succession to the previous ” Features of LLC converter operation, this is the third article explaining “Basic Operation of LLC Converters”.

■ Basic Configuration of an LLC Converter
■ Features of LLC converter operation
■ Basic Operation of LLC Converters
■ Importance of MOSFET Recovery Characteristics for Off-Resonance of LLC Converters

Basic Operation of LLC Converters

An LLC converter is generally used in region (2) in the previously presented Fig. 2, since in this region MOSFET turn-on ZVS operation occurs. Fig. 3 shows operation waveforms for region (2). The graph indicates that Q1 and Q2 drain current waveforms (I_{D_Q1}, I_{D_Q2}) are in ZVS operation during turn-on.

Fig. 3. LLC converter operation waveforms in region (2)

Circuit operation of an LLC converter in region (2) is classified into ten modes, as indicated at the bottom of Fig. 3. The operation in the different modes is explained in Fig. 4-1 to 4-10 below.

	Mode （1）・Q1 is on, Q2 is off. ・On the primary side, both a load current and a magnetizing current flow. ・On the secondary side, a current I_D1 flows through D1; at this time, a sinusoidal current flows due to the resonance of L_r and C_r. ・When I_D1 reaches 0 A, there is transition to the next mode.
	Mode （2）・Following Mode （1）, Q1 is on and Q2 is off. ・When the current flowing in D1 goes to 0 A, the primary-side load current also goes to 0 A. At this time a magnetizing current is flowing on the primary side, and this magnetizing current charges C_r. ・If this charging interval is long, the amount of electric charge is increased, and the output voltage rises.
	Mode （3）・Q1 turns off. ・A magnetizing current flows, and charging of the Q1 output capacitance C_{oss_Q1} begins. ・At the same time, discharge of the Q2 output capacitance C_{oss_Q2} begins.
	Mode （4）・After the completion of C_{oss_Q1} charging and C_{oss_Q2} discharging, a magnetizing current flows through the Q2 body diode. ・Due to the current in the body diode, V_{DS_Q2} becomes nearly 0 V. As a result, the MOSFET turn-on switching loss is also nearly zero.
	Mode （5）・Q2 turns on. ・Before turn-on, a magnetizing current is flowing in the body diode, and therefore ZVS operation occurs during turn-on. ・By being charged, C_r functions as a power supply. Consequently a load current flows on the primary side, and a load current flows on the secondary side via D2. ・A magnetizing current flows in the direction opposite a load current, but a negative voltage is applied to the transformer primary side, so that the magnetizing current is reduced, and finally is in the same direction as the load current.
	Mode （6）・Q1 is off, Q2 is on. ・On the primary side, both a load current and a magnetizing current flow. ・On the secondary side, a current I_D2 flows through D2; at this time, a sinusoidal current flows due to the resonance of L_r and C_r. ・When I_D2 reaches 0 A, there is transition to the next mode.
	Mode （7）・Following Mode （6）, Q1 is off and Q2 is on. ・When the current flowing in D2 goes to 0 A, the primary-side load current also goes to 0 A. At this time a magnetizing current is flowing on the primary side, and this magnetizing current charges C_r. ・If this charging interval is long, the amount of electric charge is increased, and the output voltage rises.
	Mode （8）・Q2 turns off. ・A magnetizing current flows, and charging of the Q2 output capacitance C_{oss_Q2} begins. At the same time, discharge of the Q1 output capacitance C_{oss_Q1} begins.
	Mode （9）・After the completion of C_{oss_Q2} charging and C_{oss_Q1} discharging, a magnetizing current flows through the Q1 body diode. ・Due to the current in the body diode, V_{DS_Q1} becomes nearly 0 V. As a result, the MOSFET turn-on switching loss is also nearly zero.
	Mode （10）・Q1 turns on. ・Similarly to Mode (5), a magnetizing current is flowing in the body diode before turn-on, and therefore ZVS operation occurs during turn-on. ・By being charged, C_r functions as a power supply. Consequently a load current flows on the primary side, and a load current flows on the secondary side via D1. ・A magnetizing current flows in the direction opposite a load current, but a negative voltage is applied to the transformer primary side, so that the magnetizing current is reduced, and finally is in the same direction as the load current.

Fig. 4-1 to 4-10: The ten operating modes of an LLC converter

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