Three-Phase LLC Resonant Power Supply Current Balancing
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Solution Overview
Problem
Existing serial resonant switching power supplies face challenges in balancing currents across parallel circuits, leading to increased circuit size and output voltage fluctuations, particularly under light loads, due to the use of Si-based MOSFETs and the need for large leakage inductances and balance reactors.
Innovation Solution
A power supply device with three-phase LLC resonant DC-DC converters, where the operation phases are displaced by 120 degrees, using SiC-based MISFETs and separate cores for transformers with split bobbins to achieve current balancing without increasing circuit scale, and incorporating current detection and control circuits to manage resonance and balance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If Si-based MOSFETs are used as switching elements in serial resonant switching power supply, then the device can operate with lower cost and simpler structure, but the upper limit of input voltage is restricted to about 400V and the circuit cannot cope with higher frequencies
Solution Approach 1:
The power supply circuit is divided into multiple parallel serial resonant circuits (three-phase configuration), where each phase operates independently with its own switching elements. This segmentation allows each MOSFET to handle a portion of the total voltage and current, enabling the use of lower-voltage-rated MOSFETs while achieving higher overall input voltage capability through parallel operation.
2Power
If parallel serial resonant circuits are operated with the same phase to increase output power, then the output power is increased, but the ripple currents in outputs add up to a high ripple current
Solution Approach 1:
The parallel serial resonant circuits are configured with phase displacement (120 degrees for three-phase), creating asymmetric operation timing among the parallel circuits. This phase asymmetry causes the ripple currents from different phases to partially cancel each other out, significantly reducing the total ripple current in the output while maintaining high output power capability.
3Stability of the object's composition
If balance reactors are wound around the same core to achieve current balancing in parallel circuits, then current balance is achieved, but the circuit scale increases and it becomes difficult to detect and safely monitor currents
Solution Approach 1:
Separate balance reactors are provided for each parallel circuit phase, with each reactor having its own independent core. This segmentation allows for individual current detection and monitoring in each phase, simplifying the detection and control circuits while maintaining current balance. The separate cores enable independent adjustment of leakage inductances for each phase.
Solution Approach 2:
Current detection circuits are provided for each phase to detect the currents passing through the balance reactors. These detection circuits provide feedback signals that are used by control circuits to adjust the operation of switching elements, thereby maintaining current balance among parallel circuits through active control rather than relying solely on passive reactor design.
4Reliability
If a special shaped transformer is used to isolate primary and secondary sides, then isolation is achieved, but the transformer cannot be made compact
Solution Approach 1:
The isolation function is distributed to multiple separate transformers, each serving a specific phase or function. By segmenting the isolation task across multiple smaller transformers rather than using one large special-shaped transformer, the overall configuration becomes more compact while maintaining the required isolation between primary and secondary sides.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration allows for balanced currents across parallel resonant circuits without unnecessary circuit expansion, reduces transformer size, and maintains stable output voltage by distributing resonance conditions and using high-frequency SiC-based components.
Implementation Method 1
serial resonant circuits which are connected to output nodes of the switching circuits respectively
Implementation Method 2
first transformers, second transformers, and resonance capacitors
Implementation Method 3
SiC-based MISFETs (metal-insulator-semiconductor field-effect transistors)
Implementation Method 4
The first transformers respectively include separate cores, and respectively have the primary and secondary windings isolated from each other with leakage inductances present between the primary and secondary windings
Data Source
Figure 1
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AI summary
A power supply device (1) has resonance-type DC-DC converters (10-*) (*=1, 2, 3) of three phases connected in parallel, said converters respectively having operation phases shifted from each other by 120°. The converters (10-*) include switching circuits (11-*), series resonant circuits (12-*), and rectifying smoothing circuits (13-*), respectively. The series resonant circuits (12-*) include transformers (T*1 and T*2) and resonance capacitors (C*1 and C*2), respectively. Each of primary winding wires (T*1a) of the transformers (T*1), each of primary winding wires (T*2a) of the transformers (T*2), and respective resonance capacitors (C*1 and C*2) are connected in series. Each of secondary winding wires (T+2b) of the transformers (T*2) is connected to each of the rectifying smoothing circuits (13-*). The transformers (T*1) are provided with different cores, respectively, the primary winding wires (T*1a) and the secondary winding wires (T*1b) are insulated from each other by means of dividing bobbins, and the secondary winding wires (T*1b) of respective phases are connected in parallel.