Dual TMS320F2808 Based Polarity-Inverting TIG Welding Power Supply

Literature Overview

This 2013 study by Bai Hongwei, Zhang Yongting, and Li Jing, published in "Welding Technology," presents the design and implementation of a polarity-inverting TIG welding power supply based on dual TMS320F2808 digital signal processors (DSPs). The research was conducted at Henan Mechanical and Electrical Higher Vocational College and Henan Hoisting Machinery Co., Ltd. The work addresses a critical need in TIG welding of reactive metals, particularly aluminum and magnesium alloys, where AC TIG welding is essential for cathodic cleaning action.

Technical Background

Polarity-inverting TIG (AC TIG) welding alternates between electrode positive (EP) and electrode negative (EN) configurations. The EN half-cycle provides deeper penetration, while the EP half-cycle delivers cathodic cleaning action that removes the oxide film (Al₂O₃) from the aluminum surface. The balance between these two half-cycles, known as the balance ratio, determines the weld penetration profile and cleaning effectiveness.

Parameter	Specification	Function
DSP microcontroller	TMS320F2808	32-bit, 150 MHz, high-speed control
Output power	0–400 A	Range for aluminum welding
Frequency	50–200 Hz	Balancing cleaning and penetration
Balance ratio	30%–70%	EP/EN duty cycle adjustment
Control sampling rate	10 kHz	Precise current waveform shaping
Switching frequency	20 kHz	IGBT gate drive
DC bus voltage	540 V	IGBT inverter topology

Dual-DSP Control Architecture

The innovative aspect of this design is the use of two TMS320F2808 DSPs working in coordination. One DSP handles the main power conversion control (PWM generation for IGBT gates), while the second DSP manages the polarity inversion logic and welding parameter modulation. This architecture provides:

Independent current waveform control: Precise shaping of both EP and EN half-cycles
Dynamic balance adjustment: Real-time modification of balance ratio during welding
Arc sensing and regulation: Fast response to arc voltage variations
Soft-start capability: Gradual current ramp-up to prevent arc blow-off

Polarity Inversion Principle

The polarity inversion is achieved by controlling the gate signals of the full-bridge IGBT inverter. The DSP generates complementary PWM signals with a 50% duty cycle at the welding frequency (typically 50–100 Hz for aluminum). At each half-cycle boundary, the gate signals are inverted to reverse current direction.

The balance ratio is controlled by adjusting the PWM duty cycle of each half-cycle:

Higher EP percentage (60–70%) → stronger cleaning action, shallower penetration
Higher EN percentage (60–70%) → deeper penetration, reduced cleaning

Performance Characteristics

The developed power supply demonstrates the following performance metrics:

Performance Metric	Measured Value	Industry Standard
Current regulation accuracy	±1%	±3–5%
Frequency stability	±0.5 Hz	±1 Hz
Balance ratio accuracy	±2%	±5%
Arc starting reliability	>99%	>95%
Current rise time	<2 ms	<5 ms
Frequency switching response	<100 μs	<1 ms

Engineering Applications

This power supply technology is particularly valuable for:

Welding of 2xxx and 7xxx series aluminum alloys in aerospace structures
Magnesium alloy welding where oxide removal is critical
Welding of dissimilar aluminum alloys with different oxide thicknesses
Precision welding of thin aluminum sheets (0.5–2 mm) in automotive applications

The dual-DSP architecture enables advanced welding strategies such as:

Progressive balance ratio adjustment during multi-pass welding
Adaptive frequency control based on arc voltage feedback
Multi-frequency welding for enhanced oxide cleaning
Pulse-polarity combination for complex joint configurations

Key Reflections

The use of dual DSPs represents a significant advancement over single-processor designs, particularly in achieving precise current waveform control. The TMS320F2808's 150 MHz clock speed and hardware PWM modules enable sub-microsecond response times that are essential for stable arc operation during polarity transitions.

From an engineering practice perspective, the balance ratio adjustment capability allows welders to optimize the process for specific joint geometries and material conditions without hardware modifications. This flexibility is particularly valuable in production environments where multiple aluminum alloy grades may be welded on the same equipment.

The study demonstrates that digital control technology has fundamentally transformed TIG welding power supply design, enabling capabilities that were previously impossible with analog control systems. The dual-processor architecture provides the computational resources necessary for implementing complex welding algorithms while maintaining real-time control performance.