Dual-mode chokes teach old inverters new tricks

April 01, 2019 //By Michael Freitag
Dual-mode chokes teach old inverters new tricks
Important changes in the way electricity is generated and used—such as increasing reliance on energy from renewable sources, the change to efficient variable-speed drives in industrial and domestic appliances, and adoption of hybrid or battery-electric vehicles—are increasing demands for electronic inverters that can be controlled to provide ac power at a desired voltage and frequency.

Taking renewable energy as an example, utility companies’ strategies are moving toward distributed power generation, with micro generators feeding into the grid at multiple points in the network. There’s also interest in small non-grid-tied generators for deployment on consumer or farming and light commercial/industrial sites. Such applications demand compact and low-cost electronic power conditioning. This would enable conversion of the harmonic-rich and unstable output of a wind turbine, or the changing dc output of an array of photovoltaic panels, first into high-voltage, capacitor-stabilized dc that’s then input to inverters to generate a consistent ac waveform at a frequency suitable for feeding into the grid.

Similarly, in hybrid/EV or motor drives, where constantly adjusting the inverter’s output frequency using logic or software commands is key to controlling the motor speed, compact dimensions, low weight, and affordability are critical to ensure market growth.


Operating principles and noise sources

Fig. 1: A simple single-phase full-bridge inverter.

An inverter like the bridge inverter shown in figure 1 commutates current through the load by turning on and off the upper and lower power switches in alternate legs in sequence. The power switches may be insulated gate bipolar transistors (IGBTs) or super-junction MOSFETs, or—in high-end applications such as premium electric vehicles or where ultimate energy efficiency is required—wide-bandgap devices such as silicon-carbide (SiC) MOSFETs. Each gate is controlled in sequence relative to all others using a pulse-width-modulated (PWM) signal.

If the power switches are IGBTs, the frequency of the PWM signals applied to each gate is typically about 20 kHz. MOSFETs can operate at much higher frequencies up to several hundred kilohertz. In either case, rapid switching produces abrupt changes in voltage across the transistors, causing oscillations containing high-frequency noise at harmonics of the switching frequency.

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