Girish R. Kamath

An electrical circuit based 3-D thermal model of a fan cooled 600μH, 80A inductor for a plasma cutting power supply

This paper applies a 3-D electrical circuit based thermal model technique described in [1] to analyze the thermal performance of a 600 muH, 80 A inductor for a plasma cutting application. The forced convective heat transfer terms in the model are derived using the empirical correlation method [2, 3]. This approach is cost-effective since it does not use an FEA tool for the analysis. Also, the circuit based nature of the model facilitates seamless coupling of the magnetic and thermal physics that affect the inductor in a standard circuit simulator environment. A comparative evaluation of the simulation results with experiment shows that the average coil temperature error is in the 10-12% range while the average layer temperature error is in the 3-16% range. This is lower than that seen in [1] where errors in the range 25-40% are recorded. However, the model has limitations due to the presence of individual winding layer temperature errors in the range 18-50%. Factors affecting the performance of the model are also discussed in the paper.

A passive coupled-inductor flying-capacitor lossless snubber circuit for plasma cutting power supply

Plasma cutting power supplies in the power range 20 kW and above commonly use the hard-switched buck converter topology. This topology is simple and efficient when compared with SCR based methods. In addition, circuit reliability is achieved by using current controlled PWM technique at a typical switching frequency of 15 kHz. It is however estimated that device switching losses make up about two-thirds of the total switch power loss. This significantly influences device cooling requirements and overall system efficiency. This paper presents a passive lossless snubber circuit that reduces the converter switching losses. It is based on a coupled inductor with flying-capacitor network and is an improvement over a turn-on coupled inductor snubber circuit proposed in H. Levy, et al. (1997) in that it reduces switch turn-off losses as well. The circuit operation is analyzed in the paper and verified by means of experiment, results of which are presented

A passive reduced rating output rectifier snubber for plasma cutting power supply

R-C snubber circuits are used to absorb the reverse recovery energy of a rectifier diode and limit the associated voltage spike across it as presented by MacMurray (1972). This circuit, while being simple and reliable is unsuitable for applications with power levels above 10 kW due to its high power dissipation. Existing alternatives efficiently recycle the diode reverse recovery energy into the load. However, these suffer from drawbacks such as excessive diode voltage stress or circuit complexity. This paper proposes a reduced rating diode snubber that limits the diode voltage stress over the entire load operating range while keeping the snubber power dissipation to a minimum. A simple circuit model is simulated to evaluate its operation over the entire power supply operating range. It is first experimentally verified with a proof-of-concept 7 kW prototype. Experimental results show a 60 % reduction in power dissipation in addition to a 30 % reduction in diode voltage overshoot when compared with the conventional R-C snubber under nominal operating conditions. Further experimentation with a 100 A, 150 V plasma cutting power supply shows that the output rectifier voltage stress is kept within reasonable limits over a wide load operating range verifying the robustness of the proposed circuit.

Simulation study of a simple flux saturation controller for high-frequency transformer link full-bridge DC-DC converters

High-Frequency Transformer Link Full-bridge DC-DC converter systems such as those used in plasma cutting applications are prone to transformer flux saturation. It can cause unit shutdown due to over-current protection or even catastrophic failure under extreme situations [1]. This is especially unacceptable in large plasma cutting applications where unexpected production stoppages can lead to severe economic loss to the customer. A simple flux control method that overcomes the disadvantages of current flux saturation control methods has been presented in [2]. Here, the transformer flux is controlled without affecting the dynamics of the main control loop. The proposed method is verified by simulating a 16 kW DC-DC full-bridge converter circuit model in ORCAD-PSPICE software. Results of this exercise show a 50% improvement in dynamic response, a 25% reduction in transformer size and weight, and improvements in system reliability and efficiency when compared with the conventional approach. It is seen that the proposed method can be retro-fitted on an existing power supply whether voltage or current controlled, with minimal change to its circuitry. In addition, it can also be extended to converter topologies like the push-pull as well.

Forced air cooled quasi-planar 60A, 600μH inductor for a plasma cutting power supply

This paper presents the design and experimental results of a forced air cooled 60A, 600 μH quasi-planar buck converter inductor for a plasma cutting application. The inductor consists of PCBs (Printed Circuit Boards) with air ducts between the adjacent boards and conventional powder iron E cores. The winding assembly is simple and enables direct cooling of the PCB layers. A multi-physics based approach that considers factors like winding loss, air flow and heat transfer is used for the component design. Results from a 3-D FEA (Finite Element Analysis) based model that analyzes these physics are compared with experimental data from a proof-of-concept inductor prototype. A reasonable correlation is observed with average temperature errors in the 10-20% range. Factors affecting the performance of the model are also discussed. A comparison with an equivalent conventional inductor shows a four-fold increase in cooling efficiency and a three-fold reduction in copper usage. Thus, this approach is seen to be cost competitive and feasible for this application.

Planar Tesla coil arc ignition transformer for a plasma cutting power supply

The arc igniter in a plasma cutting system is a High Voltage High Frequency (HVHF) Pulse Generator typically realized using a Tesla coil resonant circuit. The ignition voltage pulse is a 5-15kV pk., 2-3 MHz sinusoid lasting for not more than 5-10 cycles. It is used to ionize the gas inside the torch thereby producing plasma [1]. The Tesla coil resonant circuit is the preferred method since it is composed of rugged components like the Tesla coil transformer, spark gaps and capacitors. However, the conventional Tesla coil transformer is expensive since it requires a special custom bobbin and the windings are manually wound. This also leads to variations in unit to unit performance. In view of this, a planar technology based Tesla coil transformer as a cost effective alternative is proposed. With this approach, there is no need for a custom bobbin or manual winding. This simplifies the build, reduces cost and ensures consistency in unit performance. The design procedure consists of obtaining characteristics of the current transformer and designing the planar version to match those characteristics. Details of the design procedure are covered in the paper. FEA (Finite Element Analysis) based models of the current and planar Tesla coil versions are used for the electromagnetic analysis and validated with experimental data. Reasonable correlation is obtained with experimental data over a 1 kHz-3 MHz frequency range. This confirms the efficacy of the analysis and the feasibility of planar technology for this application.