Soft switching technology overcomes drive noise

IMO Precision Controls Limited launched its new Jaguar VXM and VXSM inverters at the recently held Drives and Controls 2000 Exhibition to considerable critical acclaim.

IMO Precision Controls Limited launched its new Jaguar VXM and VXSM inverters at the recently held Drives and Controls 2000 Exhibition to considerable critical acclaim.

Ranging from 0.2 to 400kW, the new drives are interchangeable with earlier models and incorporate many advanced features and functionality.

All have a newly designed IPM (Intelligent Power Module) with advanced 'soft switching' technology and carry IMO's unique 5-year warranty.

They are also designed to couple with an all-new 'combination' power filter which was launched at the same time.

Stuart Kemp, IMO's Technical Manager highlights why the new VXM and VXSM products incorporate 'soft switching': "Although it has been an extremely successful product, the Jaguar VX range of inverters, like just about every other general purpose inverter on the market today, could generate quite large amounts of electro-magnetic noise over a wide frequency spectrum due to the technology used in its power circuitry.

These high frequency emissions can easily cause disturbances to other sensitive equipment installed in the process plant if correct installation precedures are not followed closely." To overcome this problem, IMO's next generation Jaguar VXM and VXSM series has been carefully designed using soft switching techniques for both power devices and switch mode power supply devices with the result that potentially troublesome emissions are greatly reduced.

It is a well known fact that the rate of voltage increase (dv/dt) across today's super fast switching devices - such as IGBT's, will excite stray capacitances and inductances normally dormant in low frequency circuits, motors and cables.

The result is high frequency harmonic currents propogating through these elements, which can then couple easily into troublesome ground loops or onto other 'victim' circuits.

Theoretically, if the power dv/dt's are reduced, then the overall emissions from the drive will also reduce, but care has to be taken that switching losses (in the form of heat) are carefully controlled due to longer transition states within the semi-conductors.

Failure to design correctly on this critical issue would lead to lower drive efficiency and much larger heatsinks.

In an IGBT for example, increasing resistance of the gate will lower dv/dt.

In the Gate resistance, and hence gate current can be controlled to keep dv/dt smaller.

At the same time, current transitions in the previously unwanted stray inductance - Ls inside the IGBT, are detected to suppress the gate resistance and voltage at on-off time points, to keep power losses small.

The next generation Jaguar VXM series inverters have undergone a major redesign of the switch mode power supply, so as to incorporate the latest developments in soft switching techniques.

Increasing gate resistance to lower dv/dt is OK for main circuit IGBT's operated at carrier frequencies of around 16kHz or less, but when we take a typical IGBT or MOSFET device used in the power supply circuit operating at carrier frequencies of between 100-200kHz, dv/dt cannot easily be adjusted by the previous methods as this would make high frequency switching difficult and losses high, making the pulse transformers and heatsinks large.

In the new design in the latest IMO drives, a DC-DC converter system using resonance phenomena is deployed which has lower emissions than the standard SMPS hard switching techniques still used by other manufacturers.

A timing chart would show that after resetting the pulse transformer (Mode III), a resonance occurs between the primary inductance of the pulse transformer and the resonance capacitor (Mode IV).

The device voltage drops according to a dv/dt determined by the resonance phenomena.

If the MOSFET (or IGBT) device is triggered into the ON state at the lowest part of the resonant voltage curve, the dv/dt can be made much smaller than by traditional switching methods.

In theory, no switching power loss is produced at zero voltage switching.

Because the resonant capacitor acts as a snubber, the dv/dt can be reduced as the device turns OFF.

Furthermore, the switching power loss becomes theoretically zero because the device current commutates to the snubber at the same time.

Emissions from this type of switching are generally significantly below emissions from conventional power switching, particularly in the area of 30 to 50 MHz, where interference with sensitive process equipment is increasingly likely to occur.

Stuart Kemp then added: "The new 'combination' power filter is specifically designed for use with our new range of drives and will ensure compliance with existing EMC requirements and the new European harmonic legislation EN61000-3-2 which comes into effect on the 1st January 2001.

This legislation requires that all electrical equipment connected to the public mains drawing current of less than 16 amps must be fully EN61000-3-2 compliant and is part of an EMC directive for CE marking of goods from 1st January 2001." The new filter contains all of the EMC circuits as expected and a DC reactor for harmonic suppression.

IMO believes at present, this approach is unique.

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