Rectifier Charge Switch Using FET to Limit Inrush Current
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Solution Overview
Problem
Motor driving devices with rectifiers experience wear and arc discharge issues due to frequent start and stop cycles, particularly at the contact switch, leading to inrush current problems and contact bounce.
Innovation Solution
A rectifier design incorporating a bridge circuit, smoothing capacitor, pre-charge resistor, charge switch, and controller that controls the charge switch to manage voltage thresholds and reduce inrush current, thereby minimizing contact wear by optimizing the charging process and reducing unnecessary power loss.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a switch is used to open and close the current path between the AC/DC converter and the smoothing capacitor for initial charging, then the capacitor can be charged before motor startup, but the switch contact experiences bounce and arc discharge causing wear during frequent start and stop cycles
Solution Approach 1:
The patent replaces the mechanical switch contact with a solid-state field effect transistor (FET) as the charging switch. This substitution eliminates mechanical bounce and arc discharge issues inherent in contact-based switches, thereby extending the operational lifespan while maintaining reliable capacitor charging functionality during frequent start-stop cycles.
Solution Approach 2:
The patent introduces a control mechanism that dynamically changes the resistance value of the charging path by controlling the FET's on-resistance based on the voltage difference between the AC/DC converter output and the smoothing capacitor. This parameter change allows for optimized charging current control, preventing excessive inrush current while ensuring reliable charging, thereby protecting the solid-state switch from stress.
2Productivity
If the switch is closed to short-circuit the charge resistor during initial charging, then the charging process completes efficiently, but frequent closing and opening causes contact wear and arc discharge
Solution Approach 1:
The patent replaces the mechanical switch that causes arc discharge and contact wear with a solid-state FET-based charging switch. This solid-state implementation eliminates the harmful arc discharge phenomenon while maintaining the ability to quickly close the charging path and short-circuit the charge resistor, thereby preserving high charging speed without generating contact wear.
Solution Approach 2:
The patent introduces a control circuit as an intermediary between the power conversion circuit and the charging switch. This control circuit monitors the voltage difference and intelligently controls the FET's switching timing and duration, optimizing the charging process to achieve fast charging while minimizing stress on the solid-state switch, thereby preventing degradation from frequent operation.
3Reliability
If a pre-charge resistor is used to reduce inrush current, then the capacitor charges safely, but the resistor causes power loss and heating during frequent start and stop cycles
Solution Approach 1:
The patent transitions from a static pre-charge resistor to a dynamic charging switch (FET) whose resistance can be dynamically adjusted. The control circuit modulates the FET's on-resistance based on real-time voltage difference measurements, allowing the charging path impedance to adapt during the charging process. This dynamic control maintains safe inrush current limitation while minimizing resistive power losses, especially important during frequent start-stop cycles where energy efficiency is critical.
Solution Approach 2:
The patent changes the resistance parameter of the charging path from a fixed pre-charge resistor value to a dynamically controllable FET on-resistance. By adjusting the FET's gate voltage, the control circuit optimizes the charging resistance to balance between limiting inrush current and minimizing power dissipation. This parameter change reduces energy loss as heat in the charging path while maintaining reliable inrush current control during frequent operations.
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 contact wear and arc discharge, enabling the rectifier and motor driving device to endure frequent start and stop cycles without significant degradation, thus enhancing reliability and longevity.
Implementation Method 1
a pre-charge resistor that is provided between the bridge circuit and the smoothing capacitor and reduces an inrush current
Implementation Method 2
a charge switch that is provided in parallel with the pre-charge resistor... the switch is closed (turned on) to short-circuit the ends of the charge resistor
Data Source
AI summary
A rectifier according to an aspect of the present disclosure includes: a bridge circuit that derives a positive voltage component and a negative voltage component from an AC power supply voltage; a smoothing capacitor that smooths an output voltage from the bridge circuit; a pre-charge resistor that is provided between the bridge circuit and the smoothing capacitor, and reduces an inrush current; a charge switch that is provided in parallel with the pre-charge resistor; and a controller that controls the charge switch between an open state and a close state, wherein when application of the AC power supply voltage to the bridge circuit is started, the controller closes the charge switch if a voltage applied between ends of the pre-charge resistor is determined to become equal to or lower than a predetermined resistor threshold voltage.