VSD and Power Electronics--Special topics, new developments

Soft-switching

One of the possible designs for achieving this goal is the resonant link inverter, which is shown for a single-phase case . The front-end is identical to a normal 'hard switched' inverter, except for the series inductor and shunt capacitor between the DC bus and the inverter stage.

The circuit is controlled as follows:

• Assume that the capacitor is momentarily discharged from the previous cycle of inverter operation.

• All inverter switches are turned ON at zero voltage, applying zero volts to the load and shorting out the capacitor. The inductor current then ramps up through the switches.

• When the inductor current reaches an appropriate level, one switch in each leg is opened (at zero voltage) to apply voltage to the load. The capacitor voltage then rings up to a value exceeding the supply while the inductor current decreases. The oscillation continues with capacitor voltage now decreasing.

• When the capacitor voltage decreases to zero, the anti-parallel diodes clamp the capacitor voltage from going negative, in effect placing a short-circuit across the capacitor and momentarily discharging it.

• The process is repeated.

++++ The topology of a single-phase resonant link 'soft' inverter.

The supply across the inverter legs has the form of a series of pulses with the same waveform as the capacitor voltage . The inverter legs must switch at one of the voltage zeros if the resonance is to be continued and low switching losses are to be achieved.

The present design of PWM voltage source inverters involves a constant DC link voltage supply and semiconductor switching devices, with anti-parallel diodes, feeding an inductive load. When device switching occurs, the other anti-parallel diode in the same leg conducts and assures that full voltage is across the switching device. This gives it the so-called clamped inductive load switching waveform. This leads to the simultaneous large voltages and currents that give rise to high switching losses in an inverter.

A new inverter topology, which is under investigation, gives either zero voltage or zero current during switching, to reduce switching power loss to a very low level. This new technique is called soft switching and should allow future semiconductor devices to be switched at much higher frequencies, thereby giving better waveform control.

++++ Characteristic of clamped inductive load switching waveform.

++++ Resonant link inverter waveform at inverter leg.

This circuit does not have the ability to give pulses with continuous variation in pulse width as with the conventional inverter. The output voltage must be controlled by discrete pulse modulation rather than pulse width modulation. However, the resonance is at such a high frequency (50-100 kHz) that this does not limit the smoothness of the output current because the load inductance is very effective at filtering such a high frequency.

There are several other types of soft-switching circuits under investigation at present.

Some of them don’t maintain continuous resonance but are controlled to resonate at a desired moment of switching, which allows continuous pulse width modulation. This type of design, called a quasi-resonant link inverter, is related to the force-commutation circuits, except that operation is at much higher frequencies and is used with gate controlled devices which can inherently be turned off. Other designs allow zero-current rather than zero-voltage turn off.

One of the advantages of this type of switching is that it results in lower levels of RFI because of the slower rates of rise of voltage and current.

Most of the potential designs share common problems.

• Resonance inherently causes higher voltages than that of the supply, which places higher stresses on the power switches and the load. This can be overcome with the addition of other switches and energy storage elements to absorb excess energy.

• They require more complex control systems because the instant of switching has to be varied with the load to maintain resonance. This control must be implemented at about 20 times the switching frequency of a conventional inverter. This requires fast controller hardware and software, such as a digital signal processor.

The matrix converter

All AC converter designs discussed in this guide so far contain energy storage elements, such as inductance and capacitance, as well as the semiconductor power switches. The energy storage components result in extra losses, are bulky, and contribute to unreliability. The matrix converter attempts to eliminate these storage devices.

++++ Matrix converter connection circuit.

The concept is very simple, consisting of a matrix of switches joining each of the 3 input lines to an output line.

The output voltage waveform is made up of sections of the input. It has been demonstrated that the circuit can operate in all four quadrants with an input line current of any desired power factor.

++++ The input and output voltage waveforms of the matrix converter.

The main difficulty with the circuit is the requirement that the switches must be able to conduct and block in both directions. Although it’s possible to fabricate devices using two power switches and two diodes, this is not an economical solution. There have been attempts to produce a single chip 'universal switch' over the last three years with no commercial success to date.

++++ Four-quadrant semiconductor switch.