Single stage LLC resonant converters with inherent power factor correction are getting popularity in AC-DC converters due to its reduced size and weight. However, single stage topologies are usually less efficient in regulating the dc bus capacitor voltage pertaining to line and load transients. This paper proposes a multi-level flying capacitor based single stage AC-DC LLC topology to address the issue of voltage balancing of dc-bus capacitor and to reduce the voltage stress of the switching devices.
THREE LEVEL INVERTER
The proposed three-level inverter topology guarantees zero voltage switching, less circulating currents, reduced switching stress and losses. The converter uses bridgeless rectification scheme for better efficiency and the power factor is made nearly unity by operating the source-side inductor in discontinuous current conduction.
Variable switching frequency control is used to regulate the output voltage of the converter and pulse width modulation is used to control the dc-bus voltage. This dual control scheme is very effective to keep the dc-bus voltage nearly constant over a wide range of line and load variations. The proposed topology and control scheme have been validated by hardware results on a 250W resistive load.
- LLC resonant converters
- AC-DC converters
- soft switching
- DC bus
Figure 1. Proposed Three-Level Single Stage Llc Converter.
EXPECTED SIMULATION RESULTS:
Figure 2. Source Voltage And Current Waveform.
Figure 3. Input Current Waveform Through Input Inductor.
Figure 4. Flying Capacitor Voltage Waveform, Vc1.
Figure 5. Dc Bus Voltage Waveform, Vc1 C Vc2.
This paper has proposed a three-level flying capacitor based topology for AC/DC LLC resonant converters. The converter has a bridgeless topology which reduces the number of con- ducting devices. The controller uses a dual control scheme which varies duty ratio and frequency to regulate DC bus and output DC voltages respectively. The converter is designed to operate in discontinuous conduction mode to obtain a near unity power factor without having any active current control techniques.
Furthermore, the topology provides low voltage stress, ZVS for all the four switches, and reduces losses. For verification, a 250W, 230V to 48V AC-DC converter proto- type has been designed and implemented. The DC bus voltage is held constant at 750V with a peak overshoot of 3.3% even when the load is reduced by 30% from full load.
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