PFC AC/DC Converter Soft Start Under Distorted Input Voltage
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Solution Overview
Problem
Existing AC/DC converters in vehicle-mounted chargers face challenges with large precharge circuits that hinder size reduction and are prone to relay failures, and existing thyristor-based solutions struggle with false firing and chattering due to high distortion in commercial power supplies.
Innovation Solution
An AC/DC converter with a PFC circuit using thyristors and switching elements, controlled by phase synchronization circuits and comparators, performs a soft start by adjusting pulse widths based on phase angles to inhibit inrush current, even with distorted input voltages.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a precharge circuit with relay and resistance is used to inhibit inrush current, then inrush current is inhibited, but the charger size increases and relay reliability decreases
Solution Approach 1:
The invention extracts the inrush current inhibition function from the traditional precharge circuit with relay and resistance, and implements it through control of the thyristor's conduction angle. This eliminates the need for separate precharge components, reducing charger size while maintaining reliability.
Solution Approach 2:
The thyristor serves multiple functions: it acts as both the main power switching element and the inrush current protection device. By controlling its conduction angle during startup, it simultaneously performs power conversion and inrush current limitation, eliminating the need for dedicated precharge circuitry.
2Measurement precision
If zero-crossing detection is used to control thyristor firing timing, then firing timing is synchronized, but detection errors occur with high distortion power supplies causing false firing
Solution Approach 1:
The invention introduces a phase synchronization circuit as an intermediary between the distorted power supply and the thyristor control system. This circuit generates a clean reference signal synchronized with the power supply frequency, which then drives the thyristor gating circuit, eliminating direct dependence on zero-crossing detection of the distorted waveform.
Solution Approach 2:
The invention replaces the mechanical/electrical zero-crossing detection method with an electronic phase synchronization approach. Instead of detecting the actual zero-crossing point of the distorted waveform, the system uses a phase-locked reference signal to determine firing timing, which is much more reliable under distorted conditions.
3Reliability
If thyristor firing is inhibited during frequency fluctuation periods, then false firing is prevented, but chattering occurs and productivity decreases
Solution Approach 1:
The phase synchronization circuit continuously monitors the power supply frequency and automatically adjusts the thyristor firing angle in real-time. This feedback mechanism ensures that firing occurs at the correct phase angle regardless of frequency fluctuations, eliminating the need to inhibit firing during frequency variations and preventing chattering while maintaining productivity.
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 inhibits inrush current and ensures precise control, enabling a compact and efficient AC/DC converter that operates reliably with high distortion factors in commercial power supplies.
Implementation Method 1
the timing of firing of the first thyristor and the second thyristor is adjusted by changing a pulse width at a time when each of the first thyristor and the second thyristor is turned ON based on a phase angle detected by a phase synchronization circuit
Implementation Method 2
controls turning ON and OFF the first thyristor and the second thyristor in accordance with alternately repeated positive and negative half-cycles of an alternating-current voltage and controls turning ON and OFF the at least one switching element to convert the alternating-current voltage into a predetermined direct-current voltage
Implementation Method 3
a reactor; a first thyristor; a second thyristor; at least one switching element that includes a diode; and a capacitor that is arranged between a pair of pieces of direct-current output wiring on an output side relative to the reactor, the first thyristor, the second thyristor, and the at least one switching element
Implementation Method 4
a capacitor that is arranged between a pair of pieces of direct-current output wiring on an output side relative to the reactor, the first thyristor, the second thyristor, and the at least one switching element
Data Source
AI summary
An AC/DC converter including a PFC circuit. The PFC circuit includes: a reactor; a first thyristor and a second thyristor; at least one switching element; and a capacitor. When an input of an alternating-current voltage is started, with a function of the switching element being made inactive, a controller executes a soft start by adjusting a pulse width when each of the thyristors is turned ON by changing a timing when each of the thyristors is turned ON based on a phase angle.