Bulk Capacitor Pre-Charge Circuit for Power Tool In-Rush Control

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Solution Overview

Problem

Existing power tool devices face challenges with high peak in-rush currents during startup due to uncontrolled charging of bulk capacitors, and there is a risk of parasitic voltage on battery terminals when idle, which can be hazardous.

Innovation Solution

A capacitance control system is implemented with a pre-charge circuit and discharge circuit to manage the charging of bulk capacitors, including a resistance in series with the capacitors to limit the in-rush current and a discharge circuit to quickly discharge parasitic voltage, using a charge pump circuit and discharge gate driver for controlled switching.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Speed

If bulk capacitors are directly connected to battery pack terminals during startup, then the capacitors charge quickly, but high peak in-rush currents occur that require higher-rated, larger electrical components

Engineering Contradiction:

Improvecharging speedVSAvoidelectrical component rating

Core Design Contradiction:

SpeedVSDevice complexity

Solution Approach 1:

The pre-charge circuit is activated before the main power connection to gradually charge the bulk capacitors to a predetermined voltage level. This preliminary charging action prevents high peak in-rush currents when the main power connection is made, allowing the use of lower-rated, smaller electrical components in the main power path.

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The pre-charge circuit acts as an intermediary between the battery pack terminals and the bulk capacitors during the startup phase. It provides a controlled charging path that limits current flow, mediating the energy transfer to prevent harmful in-rush currents while still enabling capacitor charging.

Inventive Principle:

Principle #24Intermediary (Mediator)

2Duration of action of stationary object

If bulk capacitors remain connected to battery pack terminals when idle, then the capacitors maintain charge readiness, but parasitic voltage on battery terminals creates safety hazards

Engineering Contradiction:

Improvecharge readinessVSAvoidparasitic voltage hazard

Core Design Contradiction:

Duration of action of stationary objectVSObject-affected harmful factors

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 battery connection eliminates the source of parasitic voltage on the battery terminals, removing the safety hazard while the capacitors retain their charge for quick startup readiness.

Inventive Principle:

Principle #2Taking out (Extraction)

Solution Approach 2:

The connection state of the bulk capacitors to the battery pack terminals is dynamically changed based on the operational state of the power tool device. During operation, the capacitors are connected for power supply; during idle periods, they are disconnected to eliminate parasitic voltage hazards. This dynamic reconfiguration allows the system to adapt to different operational requirements.

Inventive Principle:

Principle #15Dynamics

3Productivity

If uncontrolled charging current is allowed during startup, then the system responds quickly, but electrical components must be higher-rated and larger

Engineering Contradiction:

Improvesystem response speedVSAvoidelectrical component size

Core Design Contradiction:

ProductivityVSWeight of stationary object

Solution Approach 1:

The pre-charge circuit performs preliminary charging of the bulk capacitors before main power connection, enabling the system to respond quickly without requiring high peak current capability from the main electrical components. This maintains productivity while allowing the use of smaller, lighter components.

Inventive Principle:

Principle #10Preliminary action

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 system effectively limits peak in-rush currents and safely discharges parasitic voltage, allowing the use of smaller electrical components and ensuring user safety by isolating capacitors from battery terminals when not in use.

Implementation Method 1

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

Methodology Applied

Scientific EffectElectrical Resistance: Electrical Resistance

Implementation Method 2

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

Methodology Applied

Scientific EffectJoule Heating: Joule Heating

Data Source

PatentUS20260005541A1Controlling bulk capacitance charge in a power tool device

Publication Date: 2026.01.01 MILWAUKEE ELECTRIC TOOL CORP

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.