Understanding & Using Solar DC-AC Inverters – Part 1

DC-AC inverters are electronic devices used to produce .mains voltage. AC power

from low voltage DC energy (from a battery or solar panel). This makes them

very suitable for when you need to use AC power tools or appliances but the

usual AC mains power is not available. Examples include operating appliances in

caravans and mobile homes, and also running audio, video and computing

equipment in remote areas.

Most inverters do their job by performing two main functions: first they convert

the incoming DC into AC, and then they step up the resulting AC to mains voltage

level using a transformer. And the goal of the designer is to have the inverter

perform these functions as efficiently as possible . so that as much as possible

of the energy drawn from the battery or solar panel is converted into mains

voltage AC, and as little as possible is wasted as heat.

Modern inverters use a basic circuit scheme like that shown in Fig.1. As you can

see the DC from the battery is converted into AC very simply, by using a pair of

power MOSFETs (Q1 and Q2) acting as very efficient electronic switches. The

positive 13.8V DC from the battery is connected to the centre-tap of the

transformer primary, while each MOSFET is connected between one end of the

primary and earth (battery negative). So by switching on Q1, the battery current

can be made to flow through the .top. half of the primary and to earth via Q1.

Conversely by switching on Q2 instead, the current is made to flow the opposite

way through the .lower. half of the primary and to earth.

Therefore by switching the two MOSFETs on alternately, the current is made to

flow first in one half of the primary and then in the other, producing an

alternating magnetic flux in the transformer.s core. As a result a corresponding

AC voltage is induced in the transformer.s secondary winding, and as the

secondary has about 24 times the number of turns in the primary, the induced

AC voltage is much higher: around 650V peak to peak.

By the way if you.re wondering why MOSFETs are used as the electronic switches,

to convert the DC into AC, it.s because they make the most efficient high-current

switches. When they.re .off. they are are virtually an open circuit, yet when

they.re .on. they are very close to a short circuit (only a few milliohms). So very

little power is wasted as heat.

(In DC-AC inverters designed to deliver high power, there are actually quite a few

MOSFETs connected to each side of the transformer primary, to share the heavy

current. However because they.re essentially connected in parallel, you can still

think of them as behaving very much like the single transistors shown in Fig.1.

They just behave like very high-power MOSFETs, able to switch many tens of

amps...)

Note that because the switching MOSFETs are simply being turned on and off,

this type of inverter does not produce AC of the same .pure sinewave. type as the

AC power mains. The output waveform is essentially alternating rectangular

pulses, as you can see from Fig.2. However the width of the pulses and the

spacing between them is chosen so that the ratio between the RMS value of the

output waveform and its peak-to-peak value is actually quite similar to that of a

pure sinewave. The resulting waveform is usually called a .modified sinewave.,

and as the RMS voltage is close to 230V many AC tools and appliances are able

to operate from such a waveform without problems.

It.s true, though, that this kind of waveform is not close enough to a sinewave

for some appliances. That.s mainly because the rectangular pulses contain not

just the fundamental mains frequency, but quite a lot of its harmonics as well.

So if the inverter is operating at the Australia/New Zealand mains frequency of

50Hz, the output will also contain components at 100Hz, 150Hz, 200Hz, 250Hz

and so on.

These harmonics can disturb the operation of some appliances.

It.s because of this shortcoming that manufacturers have come up with a more

complex type of inverter, which does deliver a pure sinewave output. More about

these later.