How Does the Three-Phase Five-Leg Integrated Inductor Address Pain Points in High-Power Inverter Circuits?
Three-phase inverter circuits are widely used in medium-to-high-power power supplies. When discrete components are adopted for inverter inductors, issues such as large volume and heavy weight arise; in contrast, conventional magnetic integration schemes and wiring methods lead to high ripple. To tackle these problems, Delta has proposed a new type of three-phase five-leg coupled integrated inverter inductor, which integrates three inductors onto a pair of parallel strip-shaped yokes.
1. Introduction
Power density and efficiency have always been among the key indicators for evaluating the performance of switching power supplies. Magnetic components account for a large proportion of the volume and loss of the entire switching power supply, usually ranging from 25% to 40%. Therefore, reducing the volume and weight of magnetic components and lowering their loss is a crucial goal pursued in the development of switching power supplies.
The use of magnetic integration technology to reduce the volume, weight, and loss of magnetic components has long been a research focus in the industry. For high-power or high-current power supply products, extensive technical research and exploration have been conducted in the past targeting interleaved parallel BUCK or BOOST circuits, and various inductor integration schemes have been proposed, achieving certain results [1-6].
However, in terms of integration effects, most of these integrations are coupling or decoupling integration schemes for two or more interleaved parallel magnetic components. Research on three-phase integrated inductors in both academia and industry has mainly focused on phase-separated magnetic components and three-phase fully coupled or three-phase decoupled integrated magnetic components [7-11], with less research on the comprehensive performance of three-phase magnetic components—such as considering volume, weight, loss, and the impact on high-frequency ripple current simultaneously.
This paper focuses on researching how to balance the reduction of volume, weight, and loss of three-phase inductors, as well as the high-frequency ripple current flowing through their windings. A three-phase five-leg coupled integrated inverter inductor [12] is proposed, which can effectively meet the above technical requirements.
In the following sections, the operating characteristics of the three-phase inverter circuit and its requirements for inductor performance are first analyzed in detail. Then, the three-phase coupled integrated inductor scheme is theoretically analyzed and studied from the perspective of magnetism. Finally, an 11kW three-phase inverter prototype is designed, and the experimental test results are almost completely consistent with the theoretical analysis. This coupled integration scheme can also be further promoted and applied to three-phase PFC circuits, etc.
2. Operating Characteristics of the Three-Phase Inverter Circuit
As shown in Figure 1, it is a topological diagram of a commonly used three-phase inverter circuit, which is mainly applied to inverter circuits such as high-power solar photovoltaic power supplies. It can be connected to the grid or supply power independently to AC electrical equipment.
From left to right, the circuit consists of a DC-side large capacitor, a three-phase inverter bridge, three-phase inverter inductors, a three-phase star-connected AC-side capacitor, and a three-phase power grid or three-phase AC electrical equipment.
The three-phase bridge arms of the three-phase inverter circuit are electrically connected to the three-phase inductors respectively. Through the switching action of the upper and lower switching tubes of the three-phase bridge arms, a three-phase power frequency current with 120° interleaving is formed on the AC side of the output end of the three-phase inverter circuit; however, the three-phase high-frequency ripple current has no fixed 120° phase difference.
In the design of the three-phase inverter circuit, when three-phase separated inductors or decoupled integrated inductors are used, the volume and weight are large; when three-phase fully coupled inductors are used, the ripple current is large.
To reduce the volume and weight of the inverter inductor and reduce the high-frequency current ripple at the output end of the power supply (i.e., the AC side), a three-phase five-leg coupled integrated inverter inductor scheme is proposed. This scheme features a structure where adjacent winding legs are positively coupled and spaced winding legs are negatively coupled. It can effectively reduce the high-frequency current ripple on the AC side while maintaining the small volume and light weight of the inverter inductor, ensuring better comprehensive electrical performance.
Figure 1 Three-Phase Inverter Circuit
To maintain the goal of reducing the volume and weight of magnetic components and also reduce the high-frequency current ripple output by the inverter circuit, a built-in three-phase five-leg coupled integrated inductor scheme as shown in Figure 2 is adopted. That is, from left to right, three winding legs are arranged at intervals: a non-winding leg 1 is set between winding leg A and B, and a non-winding leg 2 is set between winding leg B and C, forming five magnetic legs in the sequence of A-1-B-2-C.
For the three windings, their reference access terminals are set as a1, b1, c1 respectively. When the same reference current is applied, the magnetic flux directions formed on the three winding legs are upward on legs A and C, and downward on leg B. In other words, legs AB and BC are positively coupled, while leg AC is negatively coupled.
Figure 2 Schematic Diagram of the Built-in Three-Phase Five-Leg Coupled Integrated Inductor
3. Operating Characteristics of the New Three-Phase Five-Leg Coupled Integrated Inductor
3.1 Analysis of Power Frequency Voltage and High-Frequency Ripple Current
Figure 3(a) shows the variation of the duty cycle of the switching tubes of the three-phase inverter circuit within a power frequency cycle. When the ordinate value is > 0, it indicates that the upper tubes (S1, S2, S3) are turned on and represents the duty cycle value; when the value is < 0, it represents that the lower tubes (S4 (complementary to S1), S5 (complementary to S2), S6 (complementary to S3)) are turned on. The red curve represents Phase A, the green curve represents Phase B, and the blue curve represents Phase C.
Any point in the power frequency cycle can be selected for research. For example, take t = 0.0104 ~ 0.010425s (i.e., a high-frequency switching cycle). The duty cycle of the switching tubes is shown in Figure 3(b). It can be seen from the figure that at the same moment, the duty cycles of the switching tubes on each arm of the three-phase bridge arm are different and have no 120° phase difference. Therefore, free magnetic legs 1 and 2 as shown in Figure 2 are required.
(a) Duty cycle of switching tubes within the power frequency cycle
(b) Duty cycle of switching tubes at t = 0.0104 ~ 0.010425s
(c) Phase relationship diagram of output power frequency current
Figure 3 Operating Characteristics of the Three-Phase Inverter Circuit
As shown in Figure 3(c), within a power frequency cycle, the output-side power frequency AC current of the circuit can be divided into 6 intervals, namely a1, a2, a3, a4, a5, a6.
According to the aforementioned setting of the connection mode between the coupled integrated inverter inductor, the circuit bridge arm, and the output end, from the perspective of power frequency current:
(1) In interval a1, AB, BC, and AC are actually negatively coupled;
(2) In interval a2, AB is actually negatively coupled, while BC and AC are positively coupled;
(3) In interval a3, AB and AC are actually positively coupled, while BC is negatively coupled;
(4) In interval a4, AB, BC, and AC are actually negatively coupled;
(5) In interval a5, AB is actually negatively coupled, while BC and AC are positively coupled;
(6) In interval a6, AB and AC are actually positively coupled, while BC is negatively coupled.
3.2 Analysis of Maximum High-Frequency Ripple Current of Each Phase
Combining the wiring methods shown in Figure 1 and Figure 2, based on the reference current flow direction and the 6 intervals in Section 3.1, the maximum high-frequency ripple current of a specified phase can be calculated using Equations 1 ~ 3.
Where L11, L22, and L33 are the self-inductances of Phases A, B, and C respectively; M12, M23, and M13 are the mutual inductances between AB, BC, and AC respectively. The self-inductances are all positive, and the mutual inductances are positive or negative depending on the 6 intervals: if the actual current and the reference current are in the same direction or both in the opposite direction, the mutual inductance is positive; otherwise, it is negative.
di1/dt, di2/dt, and di3/dt are the ripple current change rates of Phases A, B, and C at the calculated time point respectively. VAL, VBL, and VCL represent the voltages applied to the inductor windings of each phase.
By solving Equations 1, 2, and 3 together, Equations 4, 5, and 6 are obtained, which can be used to calculate the current change rate of any branch of A, B, and C (e.g., di2/dt). By comparing different coupling methods of the coupled integrated inductor, it is found that when the wiring method shown in Figure 2 is adopted (i.e., the reference directions of the magnetic flux of the winding legs on both sides are opposite to that of the middle winding leg), the change rate is the smallest, meaning the high-frequency ripple current is the smallest.
3.3 Position Setting of Phases A, B, and C
Through further analysis of Equations 1 ~ 3, it can be seen that the currents of Phases A, B, and C are independent of their specific positions on the winding legs. For example, as shown in Figure 2, the order from left to right can be Phase A, B, C; or Phase A, C, B; or Phase B, A, C; or Phase B, C, A, etc.
4. Scheme Application
Based on the above-proposed new three-phase coupled integrated inverter inductor structure, an 11kW three-phase solar photovoltaic inverter prototype is designed. The input voltage of the inverter Vin = 580Vdc, the output voltage Vac-rms = 210Vac, and the switching frequency fs = 40kHz.
The inductor adopts the built-in three-phase five-leg coupled integrated structure shown in Figure 2. The five magnetic legs (including winding legs and non-winding additional legs) and the two yokes are all made of the same alloy powder core material. The cross-sectional area of the winding leg Ae1 = 490mm², the cross-sectional area of the additional leg Ae2 = 308mm², the cross-sectional area of the yoke Ae3 = 450mm², and the number of turns of the three windings is the same.
The inductance values are: L11 = 704uH, L22 = 879uH, L33 = 705uH, M12 = 187uH, M23 = 194uH, M13 = -55uH.
The maximum ripple current of each phase occurs when its output voltage is zero. For example, to calculate the maximum high-frequency ripple current of Phase B, at this time Vgb = 0 (corresponding to the junction of intervals a2 and a3). The Vga = 257.5V and Vgc = -257.5V can be calculated through the timing relationship of each phase in Figure 3(c). As shown in Figure 1(b), the voltages applied to the three-phase windings are VAL = +32.5V, VBL = +290V, and VCL = -32.5V.
Figure 4 Simulation Analysis
The 3D simulation structure of the inductor is shown in Figure 4. To understand that the newly proposed coupled integrated inductor scheme can effectively reduce high-frequency ripple and achieve a more uniform magnetic field distribution in the iron core, the differences in magnetic flux density values under different phase inductor positions and couplings are compared, as shown in Table 1.
Table 1 Magnetic Flux Density Distribution Under Different Reference Coupling Modes (Unit: T)
It can be seen from Table 1 that the proposed scheme is insensitive to the specific position setting of Phases A, B, and C, and has strong adaptability. As shown in the yellow background area, the maximum magnetic flux density distribution on the entire iron core is also more uniform.
Furthermore, the maximum ripple current of Phase B can be calculated using Equation 5 as 8.76A. Compared with the traditional built-in three-phase five-leg coupling and wiring method where the reference magnetic flux directions of the three winding legs are the same (with a ripple current of 10.01A), this value is reduced by 12.5%.
The 3D diagram is shown in Figure 5(a), and the test waveforms are shown in Figures 5(b) and 5(c). The maximum ripple current in Figure 5(b) is 9.4A, and the maximum ripple current in Figure 5(c) is 8.2A; the error between the theoretical analysis and the test is approximately 6%, which meets the requirements of engineering applications.
(a) 3D Diagram of the Three-Phase Five-Leg Inductor
(b) Phase B Current with the Same Reference Magnetic Flux Direction of the Three Winding Legs (Comparison Scheme)
(c) Phase B Current with Opposite Reference Magnetic Flux Directions of Adjacent Winding Legs (Proposed Scheme)
Figure 5 Three-Phase Five-Leg Coupled Integrated Inductor and Test Waveforms
5. Conclusion
Considering the power supply requirements for small volume, light weight, low cost, and small ripple, this paper proposes a new type of three-phase five-leg coupled integrated inductor. The theoretical analysis, simulation, and practical verification on the 11kW three-phase inverter prototype are consistent.
1. The three-phase windings of the proposed coupled integration scheme do not require specific position arrangement on the winding legs; only the adjacent legs need to be set for reference positive coupling (and the spaced legs for reference negative coupling).
2. Compared with the traditional integration scheme, the proposed three-phase coupled integrated inverter inductor scheme reduces the output high-frequency ripple current by 12.5%.
3. The proposed three-phase coupled integrated inverter inductor scheme has a simple structure and is easy to