Zero-Crossing Solid State Relay for Low AC Line Emissions
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Solution Overview
Problem
Solid state relays cause transient disturbances and radio frequency (RF) emissions on AC supply lines due to semiconductor-based devices, leading to interference with other equipment, necessitating the use of expensive filters to meet emission standards, which increase equipment size and cost.
Innovation Solution
The implementation of a solid state relay with zero-crossing sensing and power MOSFETs in series opposition configuration, which minimizes conducted emissions by synchronizing the switching with the zero-crossing point of the AC waveform and using high power field effect transistors to stabilize current flow, reducing the need for external filters.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If external filters are installed to minimize conducted emissions, then emission interference is reduced, but equipment size and cost increase
Solution Approach 1:
The harmful transient disturbances are extracted and eliminated at their source by using zero-crossing sensing to detect when the AC voltage passes through zero, triggering the MOSFETs to switch at this precise moment when current is minimal, thereby removing the need for external emission filters
Solution Approach 2:
The zero-crossing detector performs preliminary detection of the AC waveform phase before triggering the switching action, ensuring that the solid state relay activates only when the voltage is at or near zero, which prevents the generation of transient disturbances in the first place
2Reliability
If semiconductor-based devices are used for switching, then switching performance and reliability are improved, but transient disturbances and RF emissions are generated
Solution Approach 1:
The switching parameters are changed by synchronizing the MOSFET turn-on moment with the zero-crossing point of the AC waveform, transforming the switching action from an arbitrary time point to a precisely controlled moment when voltage and current are at their minimum, thereby eliminating transient disturbances while maintaining semiconductor reliability
Solution Approach 2:
The solid state relay uses periodic zero-crossing detection of the AC waveform to trigger switching actions at regular intervals synchronized with the mains frequency, ensuring that switching always occurs at the optimal point in each AC cycle where emissions are minimized
Data Source
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AI summary
According to one embodiment, a solid state relay with ultra low emissions is disclosed. The solid state relay includes one or more inputs for receiving a control signal, an input circuit for processing the received control signal. An output circuit responsive to the control signal to close the solid state relay so that power may be delivered from a power source to an electrical load. The input circuit can include zero-crossing functionality configured such that the solid state relay does not turn on until an AC source signal crosses from a negative value to a positive value or from a positive value to a negative value. The zero crossing functionality may comprise opto-couplers with zero crossing functionality. The input circuit may further comprise a low emission driver portion that is to reduce emissions during the portion after the initial start up.