YOUR GUIDE TO DC-AC POWER INVERTERS

Power inverter

is a device that converts electrical energy from dc to ac form using electronic circuits. Its typical application is to convert a battery voltage into conventional household AC voltage allowing you to use electronic devices when grid is not available. Inverters are used in a wide variety of applications from small car adapters to large converters in solar and wind powered systems. Unlike generators they do not have any moving parts.

There are three basic types of dc-ac converters: square wave, modified sinewave, and pure sine wave (see the diagram below). The square wave is the simplest and the least expensive type, but nowadays it is practically not used commercially because of low quality of power. The modified sine wave topologies (which are actually modified square waves) supply square waves with some dead spots between positive and negative half-cycles. They are suitable for many electronic loads, although their THD is about 25%. Inverter waveforms

Priced in the range of $.05-$0.10 per watt, modified sinewave models are the most popular low-cost inverters on the consumer market today, particularly among car inverters. If the description of a model does not state that it is a pure sinewave type, then most likely it is a modified one. Note that output voltage waveform in conventional modified sinewave DC-AC circuits has only two levels: zero or peak voltage. By adding another voltage level, the THD can be reduced typically from 25% to 6.5%. Periodically connecting the output to a specific voltage level with proper timing can produce a multiple-level waveform which is closer to sinusoidal than conventional modified sinewave.
A

pure sine wave

inverter produces voltage with low total harmonic distortion (normally below 3%). It is the most expensive type, which is used when there is a need for clean sinusoidal output for some sensitive devices such as medical equipment, laser printers, stereos, etc.

There is a number of topologies used in the power inverter circuits. Many commercially manufactured models use the same basic multi-stage concept: first a switching pre-regulator steps up a voltage from an input source to a DC voltage corresponding to the peak value of the desired AC voltage. The output stage then generates an AC. This stage usually uses an SMPS with a full-bridge or half-bridge configuration. If a half-bridge is used, the DC-link voltage should be at least twice the peak of the generated output voltage. Input to output galvanic isolation is provided by either a high-frequency transformer in the SMPS switching pre-regulator, or by a large low-frequency output transformer. If a low-frequency transformer is used, the sinusoidal voltage is generated on its primary side and transformed to the secondary side. Cheap square wave circuits suitable primarily for hobbyists projects may also use a push-pull converter with a step-up transformer. The output voltage level can be controlled either in square-wave mode or in pulse width-modulated (PWM) mode. Sine wave circuits use PWM mode, in which the output voltage and frequency are controlled by varying the duty cycle of the high frequency pulses. Chopped signal then passes through a lowpass LC-filter to supply a clean sinusoidal output.

Power inverters for cars

can often be plugged directly into a cigarette lighter. Note, however, that the cigarette lighters are protected by a fuse rated typically between 15 and 20 A. If you need to feed an electrical load that consumes more then 12V×(fuse amperage)×0.95 volt-amps, where 0.95 is a typical efficiency of an auto inverter, your unit has to be connected directly to the car battery. That's why the models above 200 VA usually don't even provide a plug for cigarette lighters.

Here you will find a theory for designing power inverters, electronic schematics for hobbyists and other online resources.

FIND DC-AC INVERTER BY SPEC

THEORY, DESIGN TEXTBOOKS, REVIEWS

SCHEMATICS
FOR HOBBYISTS

Inverters- theory of operation and analysis (a power electronics textbook for students)

DC-AC isolated inverter with quasi-square wave output- a reference design

Design and analysis of PWM inverters with low harmonic distortion

Switch mode DC-AC converters for ac motor drives and UPS

Sinewave inverter design tutorial

Power electronics books

2kVA 12VDC to 120VAC sinewave inverter for power backup- schematics, PCB

12V to 110V MOSFET converter with square wave output- schematic, circuit explanation

Inverter Circuits (reversed engineered from commercial products)

12V to 110VAC inverter for cars

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