Power Conversion Device Neutral Point Control for THD Reduction
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Solution Overview
Problem
Conventional PFC circuits experience waveform distortion and increased Total Harmonic Distortion (THD) in the input current from three-phase AC power supplies, particularly near zero crossing points of AC voltage.
Innovation Solution
A power conversion device with a bridge circuit, high-frequency filter, reactor, smoothing circuit, snubber circuit, and switch circuit is configured to shape the input current waveform by forming low-impedance current passages through smoothing and snubber capacitors, even near zero crossing AC voltage, using switch elements with different duty ratios and Zero Current Switching (ZCS) and Zero Voltage Switching (ZVS) techniques.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If conventional PFC circuit operation is used, then switching loss is reduced through ZCS and ZVS, but input current waveform distortion increases and THD increases near zero crossing
Solution Approach 1:
The patent divides the conventional single smoothing capacitor into two series-connected smoothing capacitors (C1 and C2), and divides the single snubber capacitor into two series-connected snubber capacitors (Cs1 and Cs2). This segmentation allows independent control of voltage distribution and creates additional neutral points for selective switching, enabling the circuit to maintain ZCS/ZVS while shaping the input current waveform during zero-crossing periods.
Solution Approach 2:
The patent introduces a preliminary switching action controlled by switches S4 and S5 that operates before the main power switches during zero-crossing periods. By preemptively adjusting the circuit configuration through S4 and S5, the circuit prepares low-impedance current paths in advance, preventing waveform distortion before it occurs while maintaining the conditions for subsequent ZCS/ZVS operation.
2Loss of energy
If switch elements are turned off when current is not passed through reactor for ZCS, then switching loss is reduced, but current discontinuity causes waveform distortion near zero crossing
Solution Approach 1:
The patent introduces auxiliary switches S4 and S5 as intermediary elements that mediate between the discontinuous reactor current and the AC input. These intermediary switches provide alternative current paths through the segmented capacitor banks during zero-crossing periods, bridging the gap caused by reactor current discontinuity and preventing waveform distortion while allowing the main switches to operate with ZCS/ZVS.
3Loss of energy
If snubber capacitor is discharged when switch element is turned off for ZVS, then switching loss is reduced, but voltage variation affects input current waveform near zero crossing
Solution Approach 1:
The patent applies local quality control by independently managing the discharge characteristics of the two segmented snubber capacitors (Cs1 and Cs2) through separate switch control. During zero-crossing periods, the control circuit selectively discharges specific capacitor segments to minimize voltage variations at critical moments, thereby preventing waveform distortion in the input current while maintaining overall ZVS operation.
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
The solution effectively shapes the input current waveform, reduces THD, and minimizes switching losses by enabling ZCS and ZVS operations, improving power conversion efficiency.
Implementation Method 1
The smoothing circuit includes two smoothing capacitors connected in series between a positive line and a negative line, and the neutral point is connected between the two smoothing capacitors. The snubber circuit includes two snubber capacitors connected in series between the positive line and the negative line, and the neutral point is connected between the two snubber capacitors.
Implementation Method 2
In the PFC circuit, the switch element of the bridge circuit is turned off when the current is not passed through the reactor, thereby performing ZCS (Zero Current Switching). Therefore, a switching loss can be reduced.
Implementation Method 3
In the PFC circuit, the switch element of the bridge circuit is turned off while the snubber capacitor is discharged, thereby performing ZVS (Zero Voltage Switching). Therefore, a switching loss can be reduced.
Data Source
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AI summary
A power conversion device includes: a bridge circuit in which a switch element is provided in each of plural arms; a filter that is provided on an alternating current side of the bridge circuit, and connected to a neutral point; a reactor provided between the bridge circuit and the filter; a smoothing circuit provided on a direct current side of the bridge circuit; a snubber circuit provided between the bridge circuit and the smoothing circuit; and a switch circuit provided between the smoothing circuit and the snubber circuit. The smoothing circuit includes two smoothing capacitors connected in series between a positive line and a negative line, and the neutral point is connected between the two smoothing capacitors. The snubber circuit includes two snubber capacitors connected in series between the positive line and the negative line, and the neutral point is connected between the two snubber capacitors. The switch circuit includes a first switch provided on the positive line and a second switch provided on the negative line.