Active Triac Triggering Circuit for EMI Reduction
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Solution Overview
Problem
Thermostats face challenges in minimizing electromagnetic interference (EMI) emissions to comply with governmental regulations, particularly when using power stealing methods that require high power for consistent and reliable triac triggering, which existing solutions like current transformers often fail to achieve.
Innovation Solution
A power supply unit incorporating a triac and MOSFET combination with an active trigger circuit that stores energy and supplies it near zero-crossing, using a MOSFET gate driving circuit to shape gate signals and reduce EMI, and a bypass circuit that adapts to different power requirements through AC-DC converters or RC networks to comply with FCC regulations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If power stealing is used to provide high power for consistent and reliable triac triggering, then the reliability of triac triggering is improved, but electromagnetic interference emissions increase
Solution Approach 1:
The patent applies preliminary action by storing energy in advance in a capacitor during periods when line power is sufficient, so that this stored energy can be used to trigger the triac near zero-crossing points when line power is insufficient. This allows the system to maintain reliable triggering without drawing high instantaneous power from the line that would cause EMI.
Solution Approach 2:
The patent introduces an intermediary energy storage capacitor between the power stealing circuit and the triac trigger circuit. This capacitor acts as a buffer that decouples the high-power switching operations from the triac triggering, allowing energy to be transferred smoothly without generating EMI on the power line.
2Object-generated harmful factors
If triac triggering is delayed to reduce EMI emissions, then electromagnetic interference emissions are reduced, but the consistency and reliability of triggering performance deteriorates
Solution Approach 1:
The system performs preliminary energy storage during normal operation so that when zero-crossing triggering is needed for low EMI, sufficient energy is already available in the capacitor to ensure reliable and consistent triac activation without delay.
3Use of energy by moving object
If conventional power stealing methods are used, then power can be provided to the thermostat, but they fail to supply sufficient high power for active triac triggering near zero-crossing
Solution Approach 1:
The patent implements preliminary energy accumulation by continuously charging a capacitor from the power stealing circuit during periods when line voltage is adequate. This stored energy is then available to deliver the high instantaneous power needed for active triac triggering near zero-crossing points, overcoming the limitation of conventional power stealing methods.
Solution Approach 2:
The system uses periodic power stealing synchronized with the AC line cycle to recharge the energy storage capacitor, ensuring that sufficient energy is accumulated periodically to support high-power triac triggering events while maintaining overall power efficiency.
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 reduces EMI conducted emissions, ensuring compliance with FCC regulations by controlling triac transitions and providing sufficient power for reliable thermostat operation across various energy consumption levels.
Implementation Method 1
an energy storage module having an input connected to an output of the bypass circuit and an output connected to a second terminal
Implementation Method 2
a MOSFET power steal module having an input connected to an output of the triac and an output connected to the second terminal
Data Source
AI summary
A power supply unit for use with thermostats or other like devices requiring power. A power supply unit may be designed to keep electromagnetic interference emissions at a minimum, particularly at a level that does not violate governmental regulations. A unit may be designed so that there is enough power for a triggering a switch at about a cross over point of a waveform of input power to the unit. Power for triggering may come from a storage source rather than line power to reduce emissions on the power line. Power for the storage source may be provided with power stealing. Power stealing may require switching transistors which can generate emissions. Gate signals to the transistors may be especially shaped to keep emissions from transistor switching at a minimum.