Solid-State AC Switch Zero-Cross EMI Reduction
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Solid-state AC switches generate electromagnetic interference (EMI) due to non-zero current during turn-on, particularly caused by the Miller Effect and voltage steps across transistors, which existing technologies attempt to mitigate by synchronizing turn-on with AC waveform zero-crossings but still result in undesirable current flow.
Innovation Solution
A solid-state AC power switch circuit with a zero-cross detector and controller generating control signals for a specific turn-on sequence, combined with bias resistors in parallel with each transistor to discharge stored energy and extend zero-crossing windows, preventing voltage steps and EMI by ensuring transistors are turned ON during zero-voltage conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-generated harmful factors
If transistors are turned ON during AC waveform zero-crossings to prevent EMI, then electromagnetic interference is reduced, but capacitance associated with each transistor (Miller Effect) still results in storage and subsequent undesirable flow of current during turn-on
Solution Approach 1:
The patent applies preliminary action by turning on the first transistor Q1 before turning on the second transistor Q2 during the zero-crossing transition. This sequencing ensures that when Q2 turns on, the voltage across it is already near zero, preventing voltage steps and EMI. The controller detects the zero-crossing point and initiates the turn-on sequence at the optimal moment, before the full voltage transition occurs.
Solution Approach 2:
The patent segments the turn-on process into two distinct stages: first turning on Q1, then subsequently turning on Q2. This segmentation allows each transistor to be controlled independently with appropriate timing, ensuring that the capacitive effects of one transistor do not cause EMI when the other transistor turns on. The staggered turn-on sequence divides the single problematic event into two controlled events.
2Device complexity
If transistors are turned ON simultaneously during zero-crossings to simplify control, then device operation is simplified, but voltage steps across transistors contribute to EMI generation
Solution Approach 1:
The controller performs preliminary action by detecting the zero-crossing point of the AC waveform and initiating the transistor turn-on sequence at the precise moment when the voltage transitions through zero. This timing ensures that both transistors turn on when the voltage across them is minimal, preventing voltage steps and EMI while maintaining relatively simple control logic based on zero-crossing detection.
3Reliability
If Miller Effect capacitance is present in transistors to enable normal operation, then transistor functionality is maintained, but stored energy in capacitance causes undesirable current flow during turn-on
Solution Approach 1:
The patent applies preliminary action by sequencing the turn-on of transistors Q1 and Q2 so that Q1 is turned on first, establishing a conductive path before Q2 turns on. This sequencing ensures that the Miller Effect capacitance in Q2 discharges through the already-conductive Q1 rather than causing EMI-generating current spikes. The functionality of both transistors is maintained while their capacitive effects are managed through proper timing.
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 by ensuring transistors are turned ON during zero-crossing windows, preventing voltage steps and current flow during turn-on, thereby minimizing electromagnetic interference in the AC source.
Implementation Method 1
a detection means comprising a zero-cross detector circuit arranged to monitor the AC input to determine zero-crossings of the monitored AC input at which the AC input equals zero
Implementation Method 2
capacitance associated with each transistor (i.e., the Miller Effect) results in the storage and subsequent undesirable flow of current during turn-on
Data Source
Figure 1
Figure 2
Figure 3A~3D
AI summary
A solid-state alternating current (AC) switch provides for the sequential turn-on of the associated solid-state switches (14a, 14b, 14c) to reduce the generation of electromagnetic interference (EMI). The solid-state AC switch includes at least first and second solid-state switches (Q1, Q2, Q3, Q4, Q5, Q6) connected in series between an AC input (A, B, C) and an AC load (20). A zero-cross detector circuit (18) monitors the AC input to determine zero-crossings associated with the monitored AC input. A controller (16) turns on the first solid-state switch and the second solid-state switch according to a turn-on sequence in which the first transistor is turned ON during a detected zero-crossing window associated with the first transistor and the second transistor is subsequently turned ON during a detected zero-crossing associated with the second transistor.