THE BASICS. Most standard appliances are designed to accept only AC (alternating current) voltages because that's how electricity is supplied from the grid. In order to run an electronic device from a DC (direct current) source you obviously need to transform it into AC. A device that converts electricity from DC form to AC form using electronic circuits is known in power industry as inverter. Note that the same term is used in digital electronics for a circuit that switches the logic level of a signal. To avoid confusion, the device we are talking about is a power inverter. Its typical application is to convert a battery voltage into conventional residential AC.

Inverters are used in a wide variety of applications from small car adapters to large grid-tie systems that can supply electricity to an entire home.

THE TYPES. There are three basic types of dc-ac converters depending on their AC output waveform: square wave, modified sinewave, and pure sine wave (see the diagram below). The square wave is the simplest and cheapest type, but nowadays it is practically not used commercially because of low power quality (THD≈45%). The modified sine wave topologies (which are actually modified squares) provide rectangular pulses with some dead spots between positive and negative half-cycles. They are suitable for most electronic loads, although their THD is almost 24%.

The models that employ such a technique are priced in the range of $.05-$0.10 per watt. They are the most popular low-cost inverters on the consumer market today, particularly among car inverters. The reason most electronic devices do not care about the supplied waveform is they have an internal SMPS, which rectifies input voltage anyway. Nevertheless, some equipment require cleaner power. Unfortunately, the type of AC output is rarely mentioned in consumer-grade products. If you see a device whose description does not state that it is a pure sinusoidal type, then most likely it is a square wave or a modified one. We saw that output waveform in conventional modified sinewave DC-AC circuits has only three levels: zero or peak voltage of both polarities. By adding two more voltage levels, a designer can reduce THD from 24% to typically 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 one.

A true sinewave inverter produces output with the lowest total harmonic distortion (normally below 3%). It is the most expensive type of AC source, which is used when there is a need for a sinusoidal output for certain devices, such as medical equipment, laser printers, stereos, etc. This typs is also used in grid-connected applications.

There is a number of topologies utilized in the power inverter circuits. Cheap circuits suitable primarily for hobbyists projects may have just a push-pull converter with a step-up transformer. If such a converter uses an external excitation without current-mode control, its transformer may suffer from flux imbalance that can lead to power transistors failure. The described circuit is an example of a single-stage design. Most commercially manufactured models use a multi-stage concept. With such a technique, first a switching pre-regulator raises a voltage from the input source to a regulated DC level corresponding to the peak value of the desired sinusoidal voltage. The output stage then generates the AC. This stage usually uses a full-bridge (see diagram to the right) or half-bridge configuration. Note that with a half-bridge, the DC-link voltage should be more than twice the peak of the generated output. Input to output galvanic isolation is provided by either a high-frequency transformer in the switching pre-regulator, or by a large low-frequency (LF) output transformer. If an LF transformer is used, the sinusoid is generated on its primary side and transformed to the secondary side. There are also transformerless inverters, which are gaining popularity in solar systems.
In the designs with square-wave operation, output level has to be controled on the DC side. Sine wave circuits operate in pulse width-modulated (PWM) mode, in which the generated voltage and frequency are regulated by varying the duty cycle of the high frequency pulses. The "chopped" voltage then passes through a low-pass LC-filter to supply a clean sinusoidal output. Although such approach is more expensive, it is usually employed in grid-tie devices, which require high quality of power. By the way, in the past, the inversion was accomplished using alternators driven by DC motors. Nowadays, an inverter does not have any moving parts and unlike a generator, does not burn fuel and does not emit toxic fumes.

POWER INVERTERS FOR CARS. Automotive inverters often come with a jack that can be plugged into a cigarette lighter. Note, however, that the cigarette lighters are protected by a fuse rated typically between 10 and 20 A. This is usually enough to run your laptop or other portable electronics. In general, the maximum power you can draw from a cigarette lighter receptacle is 12V×(fuse amperage)×η volt-amps, where η is efficiency of the inverter (typically η=0.95-0.98). If you need to feed electronics that consumes more than that, your device 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 DC outlet. Instead they include jumper cables that can be hooked up to battery terminals. Note that commercial inverters are usually rated in volt-amps (VA). The real power (watts) they can supply will depend on the power factor of your load: Watts=VA×PF, where PF is always
Next Post »