SSPC Pre-charge Circuit for Capacitive Load Inrush

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Solution Overview

Problem

High power direct current (DC) solid state power converters (SSPCs) face excessive peak inrush currents when turning on large capacitive loads, leading to stress on electrical components, potential hazards, and electromagnetic interference (EMI), necessitating higher fault current handling and increased power dissipation.

Innovation Solution

A pre-charge circuit with a MOSFET, inductor, and diode connected in parallel with the SSPC, controlled by a DSP-based gate driver using pulse width modulation (PWM) to gradually charge capacitive loads before turning on the SSPC, limiting inrush current and reducing power dissipation.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If a DC SSPC is turned on to a large capacitive load, then the load receives power, but excessive peak inrush current occurs causing stress on electrical components and potential hazards

Engineering Contradiction:

Improvecomponent operational lifeVSAvoidpeak inrush current

Core Design Contradiction:

ReliabilityVSObject-affected harmful factors

Solution Approach 1:

The pre-charge circuit is activated before the main SSPC to gradually charge the capacitive load, preparing the load in advance to avoid inrush current when the main switch closes

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

A dedicated pre-charge circuit with MOSFET, inductor, and diode serves as an intermediary component between the power source and the capacitive load, controlling the charging current to prevent excessive inrush

Inventive Principle:

Principle #24Intermediary (Mediator)

2Ease of operation

If the instantaneous trip level of the SSPC is set higher to bypass inrush current, then the SSPC can handle the peak current, but the SSPC must be designed to handle even higher fault current putting more stringent requirements on the solid state switching device

Engineering Contradiction:

Improvetrip level settingVSAvoidSSSD design requirements

Core Design Contradiction:

Ease of operationVSDevice complexity

Solution Approach 1:

The harmful inrush current is converted into a controlled pre-charge process through the dedicated pre-charge circuit, allowing the main SSPC to operate at its rated trip level without needing to be oversized for inrush conditions

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

3Object-affected harmful factors

If PWM drive or repeated tripping is used to gradually ramp up load side voltage, then inrush current is limited, but excessive switching loss occurs on the SSPC and full load current may rapidly discharge the capacitor

Engineering Contradiction:

Improveinrush currentVSAvoidswitching loss

Core Design Contradiction:

Object-affected harmful factorsVSLoss of energy

Solution Approach 1:

The power delivery function is segmented into two independent circuits: a pre-charge circuit for initial capacitor charging and a main SSPC for full power delivery, allowing each to operate in its optimal mode without interfering with the other

Inventive Principle:

Principle #1Segmentation

4Object-affected harmful factors

If current limiting is used to supply constant current to turn on capacitive load, then inrush current is controlled, but excessive power dissipation occurs on the solid state switching device driving up size, weight and cost

Engineering Contradiction:

Improveinrush currentVSAvoidSSPC size and weight

Core Design Contradiction:

Object-affected harmful factorsVSWeight of stationary object

Solution Approach 1:

The pre-charge circuit acts as an intermediary that handles the current limiting function externally, allowing the main SSPC to remain compact without requiring oversized current-limiting components

Inventive Principle:

Principle #24Intermediary (Mediator)

Solution Approach 2:

The current limiting function is extracted from the main SSPC and implemented in a separate pre-charge circuit, allowing the main SSPC to be optimized for its primary function with reduced size and weight

Inventive Principle:

Principle #2Taking out (Extraction)

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 minimizes inrush currents, reduces stress on SSPC components, enhances system reliability and safety, and mitigates EMI, while maintaining efficient operation and cost-effectiveness in high voltage DC electric power architectures.

Implementation Method 1

an inductor in series with the MOSFET and to a load side of the switching device

Methodology Applied

Scientific EffectInductance: Inductor

Implementation Method 2

a diode connected between the MOSFET and the inductor

Methodology Applied

Scientific EffectDiode rectification: Diode

Implementation Method 3

controlled by a DSP-based gate driver using pulse width modulation (PWM) to gradually charge capacitive loads

Methodology Applied

Scientific EffectPulse width modulation:

Data Source

PatentEP2398146B1High power DC SSPC with capability of soft turn-on to large capacitive loads

Publication Date: 2018.08.08 HONEYWELL INTERNATIONAL INC

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AI summary

Pre-charge circuitry (26) allows capacitive loads (24) connected to a solid state power controller (10) to be gradually charged up by a PWM, generated with a cycle by cycle current limit, switching a single MOSFET (28) in series with an inductor (30), before the SSPC (10) is turned on. The pre-charge circuitry (26) may require only three additional components, e.g., a MOSFET (28), an inductor (30) and a diode (34), along with a designated MOSFET gate driver (32).