AC Power Control Device Using MOSFETs for EMI Reduction
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Solution Overview
Problem
Conventional thyristor-based power control devices experience issues with electromagnetic interference (EMI) and fail to effectively control power in reactive loads due to abrupt voltage and current transitions, leading to inefficient power delivery and interference with other electronic equipment.
Innovation Solution
A power-control device utilizing MOSFETs with synchronized rectification and PWM control signals to manage energy transfer during both import and export phases of the alternating voltage waveform, allowing for bidirectional energy transfer and reducing harmonic content through linear switching characteristics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If thyristor-based conduction-angle modulation is used for power control, then the device simplicity and high efficiency are improved, but abrupt voltage and current transitions occur causing electromagnetic interference (EMI)
Solution Approach 1:
The patent divides the power control function into two separate circuits: an energy-import circuit for delivering power to the load and an energy-export circuit for receiving power from the load. Each circuit uses its own switching device (MOSFETs), allowing independent control of power delivery and reception. This segmentation enables smooth transitions in each circuit without the abrupt switching characteristic of thyristors, thereby reducing EMI while maintaining control simplicity.
Solution Approach 2:
The patent employs MOSFETs with PWM control instead of fixed conduction-angle thyristors. The MOSFETs can be dynamically switched on and off with precise timing control through PWM signals, enabling smooth transitions and variable duty cycles. This dynamic control allows the system to adapt switching timing to minimize EMI while maintaining high efficiency, unlike the static conduction-angle approach of thyristors.
2Productivity
If thyristor-based lagging conduction-angle control is used, then power control is achieved, but the abrupt transition of voltage and current creates detrimental EMI effects
Solution Approach 1:
The patent inverts the traditional unidirectional power flow approach by implementing bidirectional power flow capability. The energy-export circuit allows power to flow from the load back to the source, enabling the system to handle both power delivery and power reception. This inversion of the conventional model allows for smoother transitions during power transfer and eliminates the need for lagging conduction angles that cause abrupt transitions and EMI in thyristor-based systems.
Solution Approach 2:
The patent incorporates feedback control through PWM signaling that monitors and adjusts the switching of MOSFETs based on actual power flow conditions. This feedback mechanism allows the system to dynamically adjust switching timing and duty cycles to maintain optimal power control while minimizing abrupt transitions and EMI, providing superior control compared to the open-loop conduction-angle modulation of thyristors.
3Object-generated harmful factors
If choke circuit is added to reduce EMI, then EMI effects are reduced, but device complexity and power loss increase
Solution Approach 1:
The patent replaces the passive mechanical choke inductor used for EMI filtering with active electronic switching control using MOSFETs and PWM. Instead of relying on the inductive reactance of a choke to slow current rise, the system uses precisely controlled electronic switching to achieve smooth current transitions. This substitution eliminates the need for bulky choke inductors and associated power losses while maintaining EMI reduction through controlled switching waveforms.
4Ease of operation
If thyristor remains on until current falls to zero, then simple control is achieved, but conduction angle must always be lagging causing abrupt transitions
Solution Approach 1:
The patent uses MOSFETs with PWM control that can be dynamically switched at any point in the AC cycle, unlike thyristors that must wait for current zero-crossing. The MOSFETs can be turned on and off precisely when needed based on PWM signals, enabling leading, lagging, or unity power factor operation without abrupt transitions. This dynamic control maintains simplicity while eliminating the fundamental limitation of thyristor-based lagging conduction control.
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 achieves efficient and controlled power delivery to resistive, inductive, and capacitive loads, minimizing EMI and improving efficiency by ensuring smooth transitions and reducing power dissipation, thereby enhancing the overall performance and reliability of the power control system.
Implementation Method 1
The first linear-switching device is capable of transferring electrical energy from a utility-generated sine wave to a load
Implementation Method 2
The second linear-switching device is capable of transferring electrical energy from the load to a neutral terminal
Implementation Method 3
A first pulse width modulation (PWM) control signal controls the first linear-switching device during a first portion of the alternating voltage waveform
Data Source
AI summary
A power-control device comprises an energy-import portion and an energy-export portion. The power-control device may additionally include a general processing and power supply circuit providing linear control of the power-control device's production of power to the load. The energy-import portion is coupled between a VLINE terminal and a load terminal, and is capable of importing energy to the load terminal during a first portion and a third portion of an alternating voltage VAC waveform. The energy-export portion is coupled between the load terminal and a NEU terminal, and is capable of exporting energy from the load terminal during a second portion and a fourth portion of the alternating voltage VAC waveform. The first, second, third and fourth portions of the alternating voltage VAC waveform are equal to a period of the alternating voltage VAC waveform and respectively are consecutive during the period of the alternating voltage VAC waveform. The power-control device provides variable power control to the load terminal in response to a variable on/off time of a PWM control signal.