Here you go! How the LLC Resonant Half-Bridge Power Converter Works

This section describes a typical isolated LLC resonant half-bridge converter - its operation, circuit modeling simplification, and the relationship between the input and output voltages, called the voltage gain function. This voltage gain function forms the basis of the designs in this topic.

I. converter principle This circuit is very similar to the circuit in Figure 1b. For convenience, Figure 1b is reproduced as Figure 1b with the concatenated elements interchanged for side-by-side comparison with Figure 1a. The converter configuration in Figure 1a has three main parts:

, power switches Q1 and Q2, typically MOSFET, configured to form a square wave generator. The generator generates a unipolar square wave voltage Vsq by driving switches Q1 and Q2, each with a 50% duty cycle. A small dead time is required between successive transitions to prevent the possibility of cross-conduction and to allow time for ZVS to be implemented.

, resonant circuit, also known as resonant network, consists of resonant capacitor Cr and two inductors - series resonant inductor Lr and transformer excitation inductance Lm. The transformer turns ratio is n. The resonant network circulates current and, therefore, energy circulates and transfers to the load through the transformer. The primary winding of the transformer receives the bipolar square wave voltage Vso. This voltage is transferred to the secondary side and the transformer provides both electrical isolation and turns ratio to provide the desired voltage level to the output. In Figure 1b, the load R'L includes the load RL of Figure 3a and the losses from the transformer and output rectifier.

. On the secondary side of the converter, two diodes form a full-wave rectifier that converts the AC input to a DC output and powers the load RL. Output capacitor smooth and rectified voltage and current. The rectifier network can be implemented as a full wave bridge or center tap configuration with capacitive output filter . Rectifiers can also be implemented with MOSFETs to form synchronous rectification to reduce conduction losses, especially beneficial for low voltage and high current applications.

2. Working process

1, resonant frequency in SRC

Fundamentally speaking, the resonant network of SRC presents minimum impedance to sinusoidal current at the resonant frequency, regardless of the frequency of square wave voltage applied to the input terminal . This is sometimes referred to as the selective characteristic of the resonant circuit . Away from resonance, the circuit exhibits a higher impedance level. Then, the amount of current or associated energy to be circulated and delivered to the load depends primarily on the value of the resonant circuit impedance at that frequency for a given load impedance. As the frequency of the square wave generator changes, so does the impedance of the resonant circuit to control what portion of the energy is delivered to the load.

An SRC has only one resonance, the series resonance frequency, expressed as

The circuit frequency fc0 at peak resonance is always equal to its f0. Therefore, the SRC needs a wide frequency change to accommodate input and output changes.

2, fc0, f0 and fp

in LLC circuits However, LLC circuits are different. With the addition of the second inductor (Lm), the frequency of the LLC circuit at peak resonance (fc0) becomes a function of the load, moving within the range fp ≤ fc0 ≤ f0 as the load changes.f0 is still described by equation (1), and the pole frequency is described by equation (1)

With no load, fc0 = fp. As the load increases, fc0 moves towards f0. When the load is short-circuited, fc0 = f0. Therefore, LLC impedance adjustment follows a series of curves fp ≤ fc0 ≤ f0, unlike in SRC, where one curve defines fc0 = f0. This helps reduce the frequency range required for LLC resonant converters, but complicates circuit analysis .

As is evident from Figure 1b, f0 described by equation (1) is always true regardless of load, but fp described by equation (2) is true only at no load. As will be shown later, LLC converters are designed to operate around f0 in most cases. For this reason and others yet to be explained, f0 is a critical factor in converter operation and design.

3 Operating at f0, below and above f0

LLC resonant converter The operation is characterized by the switching frequency (denoted as fsw) as a function of the series resonant frequency (f0). Figure 2 illustrates typical waveforms for an LLC resonant converter with switching frequencies at, below, or above the series resonant frequency. Graph showing Q1 gate (Vg_Q1), Q2 gate (Vg_Q2), switching node voltage (Vsq), resonant circuit current (Ir), magnetizing current (Im), and secondary-side diode current (Is) from top to bottom . Note that the primary current is the sum of the field current and the secondary current referenced to the primary; however, since the field current flows only on the primary side, it does not contribute to the power transferred from the primary side source to the secondary side load.

Figure 2 The working mode of the LLC resonant converter

A, works at the resonant frequency (Figure 2a)

In this mode, the switching frequency is the same as the series resonant frequency.When switch Q1 is turned off, the resonant current drops to the value of the magnetizing current and no more power is delivered to the secondary side. This circuit achieves primary side ZVS and obtains soft commutation of secondary side rectifier diode by delaying the on-time of switch Q2. The design conditions for realizing ZVS are discussed later. However, it is clear that operation at series resonance produces only one operating point. To cover input and output variations, the switching frequency must be adjusted away from resonance.

B, operating below the resonant frequency (Fig. 2b)

Here, the resonant current has dropped to the value of the magnetizing current before the end of the drive pulse width, causing the power transfer to cease even if the magnetizing current continues. Operation below the series resonant frequency still achieves primary ZVS and obtains soft commutation of the secondary-side rectifier diode. The secondary side diode is in discontinuous current mode and requires more circulating current in the resonant circuit to provide the same amount of energy to the load. This extra current results in higher conduction losses on the primary and secondary sides. However, one characteristic that should be noted is that the primary ZVS may be lost if the switching frequency becomes too low. This will lead to high switching losses and several related problems. This will be explained further later.

C, operating above the resonant frequency (Figure 2c)

In this mode, the primary side presents a small circulating current in the resonant circuit. This reduces conduction losses because the current in the resonant circuit is in continuous current mode, resulting in less RMS current for the same load. The rectifier diodes are not soft commutated and have reverse recovery losses, but operation above the resonant frequency can still achieve primary ZVS. Under light load conditions, operation above the resonant frequency may result in a significant increase in frequency.

The previous discussion shows that converters can be designed by using fsw ≥ f0 or fsw ≤ f0, or by changing fsw on either side around f0. Further discussion will show that the best operation is around the series resonant frequency, where the advantage of the LLC converter is maximized. This will be the design goal...

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