Design of three-phase interleaved LLC resonant converter

Since its introduction, LLC series resonant converter (SRC) has become a very common topology due to its special performance, especially its efficiency and power density are far superior to other DC-DC converter topologies. However, since it does not contain an inductor output filter and only requires a capacitor filter at the output stage, it will inevitably generate high ripple current in the output capacitor. Therefore, LLC-SRC can be used as an ideal application for high voltage and low current, such as PDP continuous power supply.

Of course, it is also suitable for medium voltage and medium current applications, such as LCD power supplies, but it is necessary to use many very low ESR capacitors in parallel at the output stage to reduce the output ripple voltage and current stress of the output capacitor. Since the high ripple current of the output capacitor may cause the output capacitor to deteriorate and reduce the service life of the DC-DC converter, the recently developed two-phase interleaved LLC DC-DC converter is expected to significantly reduce the output ripple current in the output capacitor.

Theoretically, the output ripple current of two-phase interleaved operation is about 1/5 of that of a conventional converter. However, this is not enough for very high current applications such as power converters for electric vehicles, battery chargers or server power supplies.

This paper proposes a novel three-phase interleaved LLC resonant DC-DC converter design. The converter contains three ordinary LLC resonant DC-DC converters, each of which operates with a phase difference of π/3. Therefore, the ripple current of the output capacitor is significantly reduced and the service life of the converter is extended. In order to ensure the effectiveness of the proposed converter, this paper uses a 1kW (12V/84A) DC-DC converter prototype for experimentation and presents the test results, which prove the effectiveness of the scheme under low voltage and high current output conditions.

The circuit architecture proposed in this article: the circuit diagram of the three-phase interleaved LLC-SRC and the circuit diagram of the equivalent single-phase operation are shown in Figure 1, and the theoretical waveform is shown in Figure 2. The composition of the two resonant circuits depends on the load state: one is composed of Lr, Lm, and Cr under no load, and the other is composed of Lr and Cr under a large load. Therefore, it is necessary to analyze the two different resonant frequencies according to the following formulas:

Figure 1: Three-phase interleaved LLC-SRC.

The quality factor (Qs) is derived from the following formula:

(3)

Here, n=N1/N2, Zr1 is the characteristic impedance when fs=fr1, and Ro=Vo/Io. If the switching frequency is lower than the first resonant frequency fr1, the secondary rectifier can perform soft commutation, so the reverse recovery loss can be ignored. Under low voltage and high current application conditions, the ripple current ΔIc of the output capacitor will be very high. We assume that Imax - Imin = ΔIc. Then, the ratio of the ripple current can be determined by the following formula:

(4)

When the switching frequency fs = fr1, the output current Io is derived from the following formula:

(5)

Even if the capacitor current stress must be suppressed to ensure a longer service life, the LLC-SRC capacitor ripple current is bound to be high because its output filter only contains capacitors. However, if the interleaved control technique is applied, the LLC-SRC output ripple current can be significantly reduced. When the switching frequency fs is the same as the first resonant frequency fr1, the non-power supply period, t2~t4 in Figure 2 can be ignored.

Figure 2: Theoretical waveforms for single-phase operation.

Under the condition of fs = fr1, the ripple ratio from single-phase to six-phase LLC-SRC interleaved operation is calculated. The results show that the ripple current in three-phase interleaved operation is about 1/11 of that in single-phase operation.

Experimental results and conclusions: To verify the effectiveness of the three-phase interleaved LLC resonant converter, we conducted an experiment using a 1kW three-phase interleaved LLC resonant converter with an input voltage of 400V and an output of 12V/84A. The control scheme we built for the three-phase interleaved operation is shown in Figure 3, and the resonant parameters are shown in Table 1. Figure 4 shows the waveform of the resonant current and the ripple current of the capacitor under full load conditions. The phase difference between different phases is 60°, and the ripple current ΔIc is measured to be 20.4A and %ΔIc is 24.3%.

Figure 3. Three-phase interleaved control scheme.

Table 1. Resonance parameters.

Figure 4. Resonant current and ripple current of capacitor.

Even though the obtained ripple current ratio is different from the calculated result due to the non-powered period and the imbalance in the resonant current, it is verified that the ripple current of the output capacitor can be significantly reduced through interleaving. Because the DC gain characteristics must be different for each converter's load condition, current imbalance is generated between phases. Therefore, further research is needed on load sharing methods using phase management functions.

This paper proposes a multi-phase interleaved LLC-SCR and its control strategy. Because the output ripple current can be significantly reduced through interleaved operation, it is particularly suitable for low voltage and high current applications, such as server power systems, while traditional LLC-SRC is usually only suitable for high voltage and low current applications. By reducing the current stress, a smaller capacitor can be used and the service life of the power supply can be extended.