LLC Resonant Converter: Structure and ZVS Operation

The Anatomy of an LLC Converter

At first glance, an LLC converter looks similar to other isolated converters. It has a primary-side switching stage, a transformer for isolation, and a secondary-side rectification stage. The magic, however, lies in the "resonant tank" placed between the switches and the transformer.

Let's start by looking at the fundamental building blocks.

As Figure 1 shows, the converter has four main parts:

1. Power Switches: This is an inverter stage, typically a half-bridge (two switches) or a full-bridge (four switches), that converts the DC input voltage into a high-frequency square wave. Your familiarity with Buck and Boost converters means you're already comfortable with this concept of a switching leg.
2. Resonant Tank: This is the core of the converter. It takes the square wave from the switches and, through resonance, filters it into a sinusoidal current. This tank is composed of three elements: a series resonant inductor ($L_r$), a series resonant capacitor ($C_r$), and the transformer's magnetizing inductor ($L_m$). The name "LLC" comes from these three components ($L_r$, $L_m$, $C_r$).
3. Transformer: Provides galvanic isolation and steps the voltage up or down.
4. Diode Rectifier: Converts the AC from the transformer's secondary back into DC for the output. A key advantage of the LLC topology is that it does not require a separate, bulky output inductor like those found in Buck, Boost, or PSFB converters.

To get a more detailed look at these components, let's turn to a great introductory article.

As the reading highlights, half-bridge configurations are common for lower power (under 1kW), while full-bridge is preferred for higher power. This is a practical rule of thumb you can use when evaluating schematics.

The Heart of the Matter: A Multi-Talented Resonant Tank

The genius of the LLC converter lies in how its resonant tank behaves under different load conditions. This is what allows it to maintain high efficiency from full load all the way down to no load.

The tank effectively has two different resonant modes depending on the load:

1. At Heavy Loads: The output load, when reflected across the transformer to the primary side, has a very low impedance. This low impedance essentially "shorts out" the magnetizing inductance ($L_m$), preventing it from participating in the resonance. In this state, the tank behaves like a simple Series Resonant Converter (SRC), with resonance determined only by the series inductor ($L_r$) and series capacitor ($C_r$).

2. At Light Loads: As the load decreases, its reflected impedance increases. The magnetizing inductance ($L_m$) is no longer shorted out and begins to participate in the resonance. The tank's behavior now also incorporates the characteristics of a Parallel Resonant Converter (PRC), where resonance is determined by $L_r+L_m$ and $C_r$.

This ability to transition between two modes is what gives the LLC converter its remarkable flexibility and wide operating range.

The following reading explains this dual-mode behavior and introduces the concept of two resonant frequencies, which we will explore further in the next lesson.

How the Tank Delivers Wide-Range ZVS

Now we can connect the tank's operation to achieving ZVS. In a PSFB, we saw that achieving ZVS at light load was difficult because there wasn't enough energy in the primary current to discharge the switch capacitances. The solution was to increase the magnetizing current, but this came with the penalty of higher circulating losses.

In an LLC converter, the magnetizing current isn't a patch; it's a fundamental feature. The magnetizing current flowing through $L_m$ is what enables ZVS, and because it is largely independent of the load, ZVS can be achieved even with no load connected.

Let's walk through how this happens. The process is perfectly illustrated in the following video.

Let's summarize the key steps for achieving ZVS on switch Q2 (as shown in the first video clip):

1. Q1 turns off: The dead time begins. Both switches are off.
2. Magnetizing Current Finds a New Path: The current that was flowing through the magnetizing inductor ($L_m$) cannot stop instantly. It is redirected to charge the parasitic output capacitance ($C_{oss}$) of Q1 and discharge the parasitic output capacitance of Q2.
3. Vds of Q2 falls to zero: The magnetizing current successfully discharges Q2's capacitance, and its drain-source voltage ($V_{ds}$) drops to zero.
4. Body Diode Conducts: Once $V_{ds}$ reaches zero, the current continues to flow, but now through Q2's intrinsic body diode. This clamps the voltage at (or near) zero.
5. Q2 turns on with ZVS: The gate signal for Q2 is applied while its body diode is conducting. Since the voltage across the switch is already zero, it turns on with virtually no switching loss.

The exact same process occurs in reverse to achieve ZVS for Q1. Because this mechanism relies on the magnetizing current—which is always present—and not the load current, it works across the entire load range. As noted in the thesis "Chapter 4 LLC Resonant Converter" (resource LINK), "The ZVS is achieved with magnetizing current, which is not related to load current, so ZVS could be realized even with zero-load." This is the LLC's single biggest advantage over the PSFB.

Conclusion

In this lesson, we've dissected the LLC resonant converter and uncovered the secret to its high-efficiency operation. By moving beyond the trapezoidal currents of hard-switched converters and the limitations of the PSFB, the LLC uses a carefully designed resonant tank to create smooth, sinusoidal currents and ensure soft switching across a vast operating range.

Key Takeaways:

• An LLC converter consists of four main blocks: power switches, an LLC resonant tank ($L_r$, $L_m$, $C_r$), a transformer, and a rectifier. It notably lacks a separate output inductor.
• The resonant tank's unique structure allows it to behave like a series resonant converter at heavy loads and a parallel resonant converter at light loads.
• This dual-mode capability, combined with the inherent magnetizing current ($I_m$), is the mechanism that enables ZVS across the entire load range, from full load down to no load. This overcomes the major drawbacks of the PSFB topology.

In our next lesson, we will dive deeper into the behavior of the resonant tank. We'll explore how designers use frequency modulation to control the converter's output voltage by manipulating its gain characteristics, and we'll introduce the critical concepts of resonant frequency and quality factor (Q).