Switching Power Supply Zero-Cross Detection for Inrush Suppression

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Solution Overview

Problem

Existing switching power supplies face challenges in suppressing inrush currents and detecting zero crossing points of AC power supplies, especially when the load is small, which can lead to low-noise operation issues.

Innovation Solution

A switching power supply design that includes a rectifier unit, a smoothing capacitor, a switching unit, a first switch element, a second switch element, voltage detection circuits, and a control unit. The control unit detects zero crossing based on voltage detection circuit outputs and synchronizes the second switch element with the zero crossing point to suppress inrush currents.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If the zero crossing point is detected by detecting the voltage of a primary smoothing capacitor, then the semiconductor switch element can be brought into conduction at the zero crossing point to suppress the inrush current, but when the load of the switching power supply is small, the voltage of the primary smoothing capacitor does not drop to a value corresponding to the instantaneous value of the AC power supply, so the zero crossing point cannot be detected

Engineering Contradiction:

Improveinrush current suppressionVSAvoidzero crossing detection under small load

Core Design Contradiction:

ReliabilityVSAdaptability or versatility

Solution Approach 1:

The patent introduces a dedicated zero crossing detection circuit that directly monitors the AC power supply voltage, serving as an intermediary between the AC power supply and the control unit. This separate detection path does not depend on the smoothing capacitor voltage, enabling reliable zero crossing detection under all load conditions including small loads.

Inventive Principle:

Principle #24Intermediary (Mediator)

Solution Approach 2:

The patent separates the zero crossing detection function from the main power processing path by using a dedicated detection circuit. This segmentation allows the detection function to operate independently without being affected by the load-dependent behavior of the smoothing capacitor voltage.

Inventive Principle:

Principle #1Segmentation

2Ease of operation

If a relay is used as the switch element, then the switching power supply can be connected to the AC power supply, but it is difficult to precisely control the timing at which the contact becomes conductive

Engineering Contradiction:

Improveswitching connectionVSAvoidconduction timing control

Core Design Contradiction:

Ease of operationVSMeasurement precision

Solution Approach 1:

The patent replaces the mechanical relay with a semiconductor switch element (such as a triac or thyristor) that can be precisely controlled by electrical signals. This substitution enables accurate timing control of the conduction moment while maintaining the switching connection function, allowing the switch to be brought into conduction at the detected zero crossing point.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

3Productivity

If the switch element is brought into conduction away from the zero crossing point, then the switching power supply can be connected quickly, but an inrush current flows through the electrostatic capacitance according to the instantaneous value of the AC power supply voltage, causing element failure

Engineering Contradiction:

Improveconnection speedVSAvoidelement durability

Core Design Contradiction:

ProductivityVSReliability

Solution Approach 1:

The patent performs preliminary detection of the zero crossing point before actuating the switch element. By detecting the zero crossing point in advance and then triggering the switch element at that precise moment, the system ensures that connection occurs when the AC power supply voltage is at its minimum, preventing inrush current while maintaining quick connection response.

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 proposed solution effectively suppresses inrush currents and allows for accurate detection of zero crossing points regardless of the load, ensuring low-noise operation in switching power supplies.

Implementation Method 1

a rectifier unit to which a single-phase alternating-current (AC) power supply having a first phase and a second phase is connected and that is configured to subject the AC power supply to full-wave rectification

Methodology Applied

Scientific EffectFull-wave rectification: Diode

Implementation Method 2

a smoothing capacitor configured to smooth an output voltage of the rectifier unit

Methodology Applied

Scientific EffectCapacitance: Capacitance

Data Source

PatentUS20250038671A1Switching power supply

Publication Date: 2025.01.30 CANON KK

AI summary

A switching power supply includes a rectifier unit that subjects the AC power supply to rectification, a smoothing capacitor that smooths a rectified voltage, a switching unit disposed in parallel with the smoothing capacitor, a voltage detection circuit disposed between the AC power supply and the rectifier unit, a first switch element disposed between the AC power supply and the voltage detection circuit, a second switch element disposed between the rectifier unit and the smoothing capacitor, and a control unit. When the AC power supply, and the smoothing capacitor and the switching unit are connected, the second switch element is made conductive at a zero crossing point of the AC power supply.