IGBTs and Solar Inverters

IGBTs are excellent choices for use in solar inverters where voltage from a solar panel array on a residential or commercial building is converted from direct current to alternating current at a specific voltage output and frequency. IGBTs, like MOSFETS, use voltage rather than current as a means of control, with current applied across their gates being typically very low.

IGBTs are a type of bipolar junction transistor or BJT which have a semiconductor gate structure composed of metal oxide. BJTs have a higher current capability compared to MOSFETs. The speed at which an IGBT turns off is determined by how rapid the minority carrier recombines. The turn off time has an inverse relationship with the voltage drop or VCEON. This means that IGBTs with inherently rapid turn off times have a higher voltage drop and vice versa. Ultra fast IGBTs do switch off much faster than standard IGBTs, even if the IGBTs have the same dimensions and are basically manufactured the same way. The exact combination of speed and voltage drop is determined by adjusting the minority carrier recombination rate this in turn controls the turn off time.

Four switches are typically employed in a solar inverter. Two of the switches are high side IGBTs, while the other two are low side IGBTs. Solar inverters of this type produce a sinusoidal wave form and single phase alternating current. The frequency and voltage depend on the specific use required. In a household solar array system, the inverter will normally deliver voltage and frequency similar or the same as that provided by the mains electricity provider as it will be used to power household appliances that are designed to use mains power.

Inverters for installation in a residential capacity are usually linked to the power grid. These installations usually provide power to the grid when there is a surplus with tariff benefits depending on the location and provider. To enable this feature, the solar inverter is required to pulse width modulate the IGBTs above 20kHZ. The modulation frequency is normally around 50 to 60 Hz. This sort of pulse width modulation means that the two outut indicators can be maintained relatively small in size and they will also have the benefit of suppressing harmonics effectively.

This sort of solar inverter has switching speeds much higher than can be heard by the human ear so they remain basically noiseless.

To keep the power dissipation as low as possible, pulse width modulation is restricted to the two high side IGBTs while the low side IGBTs are commutated at 50 to 60kHz.