Capacitive Dropper Power Supply EMI Reduction via Synchronized Switching
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Solution Overview
Problem
Capacitive dropper power supplies face challenges in reducing electromagnetic interference (EMI) and maintaining stability during AC mains power interruptions, with high-frequency switching leading to increased EMI and inefficient energy preservation.
Innovation Solution
Synchronizing the openings and closings of the shunt switch with zero crossings of AC current to limit switching frequency to twice the line frequency, and incorporating disconnect circuitry to reduce current draw from the output filter capacitor during AC mains failure.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If high-frequency switching is used to control the shunt switch element, then voltage ripple magnitude is reduced and smaller capacitor value is sufficient, but electromagnetic interference (EMI) increases
Solution Approach 1:
The patent applies periodic action by synchronizing the shunt switch element to operate at the line frequency (50/60 Hz) rather than using high-frequency switching. The switch is controlled to conduct during specific portions of the AC cycle, creating periodic action that matches the input frequency. This approach reduces EMI by avoiding high-frequency switching while still achieving voltage regulation through cyclic operation aligned with the AC waveform.
2Reliability
If the shunt switch element is actuated frequently to maintain output voltage, then voltage regulation is improved, but electromagnetic interference (EMI) increases
Solution Approach 1:
The patent implements periodic action by controlling the shunt switch element to operate in synchronization with the AC line frequency. The switch is actuated only during specific intervals of the AC cycle when voltage regulation is needed, rather than using continuous or high-frequency switching. This periodic operation at line frequency maintains adequate voltage regulation while minimizing EMI by avoiding high-frequency switching transients.
3Loss of energy
If disconnect circuitry is added to reduce current draw during AC mains failure, then energy preservation is improved, but device complexity increases
Solution Approach 1:
The patent applies self-service by using the existing control circuitry that monitors output voltage to automatically detect AC mains failures and activate the disconnect function. The same control system that regulates voltage during normal operation also detects when input power is lost and subsequently opens the shunt switch element to prevent discharge current. This eliminates the need for separate disconnect circuitry, achieving energy preservation without increasing device complexity.
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
This approach significantly reduces EMI and enhances the power supply's robustness and energy preservation by minimizing abrupt current and voltage transients, while also simplifying circuit implementation and reducing unnecessary current drain.
Implementation Method 1
employ a capacitive element, operative as a current limiting element, placed in series with the incoming AC voltage where its effective impedance at the AC input's frequency serves to reduce, by virtue of the current flowing through that impedance, the voltage presented to an immediately following rectifier element
Implementation Method 2
The resultant rectified DC current may be filtered by a filter capacitor
Implementation Method 3
The resultant rectified DC current may be filtered by a filter capacitor, and a variety of feedback and control means have been employed to realize a regulated DC voltage at said filter capacitor
Data Source
AI summary
In one aspect, the present invention reduces electromagnetic interference (EMI) caused by a capacitive dropper power supply by synchronizing the openings and closings of a shunt switched used for regulation control of the DC output voltage generated by the power supply, to zero crossings of AC current from the current-limiting resistor disposed in series at the AC input of the power supply. In one or more other embodiments, the capacitive dropper power supply includes disconnect circuitry that senses a loss of the input AC voltage source and in response wholly or partly disconnects internal regulation control circuitry from the supply's output filter capacitor to reduce the current drawn from the filter capacitor, thereby reducing the decay rate of the DC output voltage from the filter capacitor. The contemplated power supply may also be implemented in a Bipolar, BiCMOS or CMOS process, for realization in a compact integrated circuit device.