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.
