GB2310554A

GB2310554A - Generation of pulse-width modulating waveforms with dead-bands for three-phase, power switching circuit configurations

Info

Publication number: GB2310554A
Application number: GB9603587A
Authority: GB
Inventors: John Edward Fletcher; Barry Wayne Williams; John Hiley
Original assignee: PWM Drives Ltd
Current assignee: PWM Drives Ltd
Priority date: 1996-02-20
Filing date: 1996-02-20
Publication date: 1997-08-27
Anticipated expiration: 2016-02-20
Also published as: GB2310554B; GB9603587D0

Abstract

A pulse-width modulator for a three-phase inverter/converter circuit comprises a counter, 1, a ring counter, 2, and look-up table, 3, to produce suitably phase displaced, per-unit, flat-topped waveforms, SA, SB and SC. The waveforms are scaled by the modulation index, M, using multipliers 4, 5 and 6 to yield signals S1, S2 and S3. The shifting stage, 11-14, is configured to add or subtract the term 1-M from the signals S1, S2 and S3. The addition or subtraction is dependent on the current sextant of the fundamental cycle identified by the ring counter. The outputs from the arithmetic stage, D1, D2 and D3 control the pulse-width of the carrier waveform, C1, and through the drive electronics, 10, the inverter phase leg duty ratios. The addition or subtraction of the 1-M term from the modulating waveforms effectively shifts the flat-tops of the scaled waveforms S1, S2 and S3. The duty ratios, at the shifted flat-tops, are either 1 (leg clamped to positive rail) or -1 (leg clamped to zero volt rail). This produces the desired dead-bands in the waveforms D1, D2 and D3. The pulse-width modulator may be applied to a variety of power switching circuit configurations including inverters, converters, matrix converters, active VAr compensators and cycloconverters.

Description

Generation of pulse-width modulating waveforms with dead-bands for three-phase inverter and converter circuits Technical Fleld This invention relates to the generation of pulse-width modulating waveforms, and is concerned more particularly, but not exclusively, with a method to introduce deadbands in the waveforms.

Background Art Inverter circuits employing high frequency switching devices are used in conjunction with the pulse-width modulating principle to provide high quality variable frequency and variable voltage outputs. Additionally, the pulse-width modulating principle is employed in many other three-phase, power switching circuit configurations such as converters, matrix converters, active VAr compensators and cycloconverters. The waveforms used to modulate the pulse-width of the high frequency carrier, so as to produce fundamental frequency output voltages, are typically continuous and of a sinusoidal nature such that every switching device in the inverter be switched continuously, at the carrier frequency, over a fundamental cycle. The modulating waveforms are typically, but not exclusively, generated by clocking sequentially through a look-up table which holds a pre-stored, per-unit waveform. The waveform is then scaled by the required modulation index, M, and applied to the pulse-width modulators.

Discontinuous waveforms may also be used providing that the resultant output voltages are sinusoidal. The discontinuous waveforms have dead-bands where the phase connection point of an inverter phase leg is clamped to one of the direct current input rails and thus switching, at the carrier frequency, in the inverter phase leg is suppressed for a period of the fundamental cycle. The shape of the discontinuous waveform changes with the modulation index and, as such, variable modulation indices cannot be generated by applying a scaling factor to a pre-stored, per-unit waveform. The advantage of using discontinuous waveforms is a reduction in the average switching frequency of the switching devices over a fundamental cycle. This lowers the overall device losses by reducing the switching loss component.

Dlsclosure of Invention It is an object of the invention to provide a means of generating the pulse-width modulating waveforms, with dead-bands, from a pre-stored, per-unit waveform using only minimal additional circuitry to that of the continuous case.

According to the invention there is a pulse-width modulator, the circuit comprising of storage means to store a per-unit, 60 flat-topped, continuous waveform, a scaling and counting means to generate suitably displaced, continuous, pulse-width modulating waveforms, a shifting means to shift the modulating waveforms by a constant magnitude, controlled by the modulation index, such that the high frequency switching in the leg currently in a flat-topped section is suppressed and a modulating means to modulate the pulse-width of the high frequency carrier in accordance with the modulating waveforms.

Brief Descnption of Drawmgs In order that the invention be more fully understood, reference will now be made to the accompanying drawings, in which: Figure 1 is a circuit diagram of the prior art to generate continuous modulating waveforms.

Figure 2 shows per-unit modulating waveforms indicating the manner of operation of the circuit shown in Figure 1 using prior art.

Figure 3 show explanatory waveforms indicating the form of scaled modulating waveforms.

Figure 4 is a circuit diagram of a possible embodiment of the invention.

Figures 5 and 6 show explanatory waveforms indicating the manner of operation of the circuit shown in Figure 4.

Figure 7 shows another possible embodiment of the invention with reduced logic requirement.

Best Modes for Carrying Out the Invention Referring to the circuit of Figure 1 the circuit comprises a counter, 1, which in conjunction with a ring counter, 2, and look-up table, 3, produces suitably phase displaced, per-unit, flat-topped waveforms, SA, SB and SC, Figure 2, when clocked at a constant rate, CLK. The look-up table is pre-stored with the sextant defined by

where o,t = fundamental angle SA = value of pre-stored sextant at xOt The phase displaced, per-unit waveforms are scaled by the modulation index, M, using multipliers 4, 5 and 6 to yield the phase leg modulation depths, S 1, S2 and S3 as shown in Figure 3 for M=0.8.

Using prior art, S1, S2 and S3 are applied directly to the pulse-width modulators, 7, 8 and 9, which control the pulse-width of the high frequency carrier waveform, C l, and hence the duty ratios of the inverter phase legs. In this description, the duty ratio over a carrier period is defined as the difference between the on-times of the top and bottom switching devices in an inverter phase leg divided by the carrier period (between -1 and +1).

Figure 4 shows a possible embodiment of the circuit which generates dead-band pulse-width modulated signals. The circuit retains similar, if not identical, logic blocks as the prior art circuit in Figure 1, but has an additional shifting (or arithmetic) stage.

The shifting stage, consisting of 11-14, is configured to add or subtract the term l-M from the signals S1, S2 and S3. The addition or subtraction is dependent on the current sextant of the fundamental cycle identified by the ring counter. Figure 5 shows the effective shift added to each of the phase modulating waveforms for M=0.8. The outputs from the arithmetic stage, D1, D2 and D3 control the pulse-width of the carrier waveform C1 and through the drive electronics, 10, the inverter phase leg duty ratios.

The addition and subtraction of the l-M term from the modulating waveforms effectively shifts the flat-tops of the scaled waveforms in Figure 3. The duty ratios, at the shifted flat-tops, are either 1 (leg clamped to the positive rail) or -1 (leg clamped to OV rail), Figure 6. This produces the desired dead-bands in the waveforms D 1, D2 and D3 as shown. It is the combination of the pre-stored, flat-topped sextant and the constant amplitude, variable sign shift that reduces the required logic to generate the dead-band waveforms.

The logic requirements of the circuits in Figure 1 and Figure 4 may be reduced by multiplexing phase information into a single calculation block instead of providing a separate block for each phase. The multiplexed system for dead-band generation is shown in Figure 7 where 13 multiplexes phase information to a single calculation block and 14 demultiplexes the phase information to be applied to the pulse-width modulators.

It is possible to use other pre-stored continuous waveforms in the generation of deadband modulating functions. These functions will not have 60 flat-topped segments as in the described case. The shift required to give dead-band regions will not be a of constant magnitude l-M and will be dependent on the form of the pre-stored waveshape.

This complicates the logic required to generate the prescribed shift, 11. However, this complication allows the possibility of incorporating a shift in the position of the deadbands relative to the fundamental angle, zoot. This attribute is useful for load power factors below unity (leading or lagging) or delta connected loads where the maximum reduction in switching losses in the commutating devices can be obtained by optimally positioning the dead-bands with respect to the peaks of the sinusoidal line currents and hence inverter phase leg currents.

Industrial Applicability The described methods of introducing the dead-bands into the modulating waveforms reduces the logic requirements of circuit configurations that do not use a microprocessor based system. This allows dead-band waveforms to be utilised in lowcost pulse-width modualtors which cannot to tolerate the expense of a microprocessor.

Dead-band modulating waveforms are used to reduce the overall switching losses apparent in an inverter when compared with continuous wavefrorms under the same load conditions. This improves power stage reliability by reducing the average junction temperatures of the switching device and lowering snubber losses.

Claims

1. A pulse-width modulator, the circuit comprising of storage means to store a perunit, 60 flat-topped, continuous waveform, a scaling and counting means to generate suitably displaced, continuous, pulse-width modulating waveforms, a shifting means to shift the modulating waveforms by a constant magnitude, controlled by the modulation index, such that the high frequency switching in the leg currently in a flat-topped section is suppressed and a modulating means to modulate the high frequency carrier in accordance with the modulating waveforms.

2. A pulse-width modulating circuit according to claim 1 where phase information is multiplexed into a single calculation block.

3. A pulse-width modulating circuit according to claim 1 where the shifting means is dependent on the pre-stored waveform.

4. A pulse-width modulating circuit according to claims 1 and 3 where the deadbands are optimally positioned with respect to the inverter phase leg currents.

5. A pulse-width modulating circuit according to claims 1 and 3 where the position of the dead-bands may be adjusted with respect to the inverter phase leg currents.

6. A pulse-width modulating circuit according to any previous claim and applied to a three-phase power switching circuit configuration.

7. A pulse-width modulating circuit substantially as described herein with reference to Figures 1-7 of the accompanying drawing.