Power Tool Bulk Capacitor Pre-Charge for In-Rush Current Control
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Solution Overview
Problem
Existing power tool devices face challenges with high peak in-rush currents during startup and parasitic voltage issues when idle, requiring larger and more expensive electrical components to manage these conditions.
Innovation Solution
Implementing a capacitance control system with a pre-charge circuit and discharge circuit to manage bulk capacitor charging, using a pre-charge switch and discharge switches to limit in-rush currents and isolate capacitors from battery terminals when not in use, along with a charge pump circuit for controlled charging and discharging.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If bulk capacitors are connected directly to battery pack terminals during startup, then the capacitors charge quickly, but high peak in-rush currents occur requiring larger electrical components
Solution Approach 1:
A pre-charge circuit with current limiting resistance is activated before the main power switch to charge the bulk capacitors to a predetermined voltage level. This preliminary charging action prevents high in-rush current when the main switch closes, as the capacitors are already partially charged.
Solution Approach 2:
A current limiting resistance is introduced as an intermediary element between the battery pack terminals and the bulk capacitors during the charging phase. This resistance limits the in-rush current while allowing the capacitors to charge to the required voltage level.
2Ease of operation
If bulk capacitors remain connected to battery pack terminals when idle, then the system is ready for immediate operation, but parasitic voltage accumulates creating safety hazards
Solution Approach 1:
The bulk capacitors are electrically disconnected from the battery pack terminals when the power tool device is idle or not in use. This extraction of the capacitors from the powered state eliminates the accumulation of parasitic voltage on the battery pack terminals, removing the safety hazard while maintaining operational capability through the stored charge in the capacitors.
3Reliability
If high-rated electrical components are used to handle peak currents, then component reliability is improved, but device cost and size increase
Solution Approach 1:
By pre-charging the bulk capacitors before main power connection, the instantaneous power demand on the main switch and other electrical components is dramatically reduced. This allows the use of lower-rated, smaller, and less expensive components while maintaining system reliability, as the components only need to handle normal operating currents rather than peak in-rush currents.
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
Reduces the need for high-rated components by limiting in-rush currents and quickly discharging parasitic voltage, allowing the use of smaller, less expensive electrical components while ensuring safe operation.
Implementation Method 1
controlling the charge stored in the bulk capacitors
Implementation Method 2
limiting a charge current to the at least one bulk capacitor using a first resistance connected in series
Implementation Method 3
reducing a resistance in series with the at least one bulk capacitor
Data Source
AI summary
A power tool device including a housing, first and second battery pack terminals, at least one bulk capacitor, a pre-charge circuit, and a discharge circuit. The pre-charge circuit includes at least one resistance connected in series with the at least one bulk capacitor, and a pre-charge switch connected in series with the at least one resistance. The pre-charge switch is configured to selectively provide a conductive path to charge the at least one bulk capacitor. The discharge circuit includes a first switch and a second switch connected in series with the at least one bulk capacitor. The first switch and the second switch are configured to be turned on after the at least one bulk capacitor is charged to a DC bus voltage.