Pure Sine Wave Inverters - PWM techniques

DC to AC inverters have long been available since the availability of high wattage bipolar transistors. Advances in technology and techniques have seen inverter sizes shrink and efficiencies increase, as manufacturers move to MOSFETs and small, high frequency transformers. These days, inverters are relied upon more than ever, allowing people to take them camping, and to run mains-powered appliances from batteries where mains power is not available.

There are generally 4 types of inverters.

Square Wave (Transformer)

Perhaps of the earliest and most simplistic type, these were the forerunners to modern day inverters. These use a large transformer, driven by a 12v push-pull circuit at 50Hz. The transformer steps up the voltage (using a 1:20 ratio), and the output is fed to the load directly with minimal filtering. Many compupter UPS units operate like this, and some are synchronised to the mains voltage, so that when they switch over, there is less stress on the load. The switchover time can be <20ms.

Although it uses a low frequency transformer, harmonics from the square wave do make it to the output. Filtering before the load can help to reduce the HF components. Much of the harmonics are lost as heat in the transformer, which only operates optimally at 50Hz.

UPS type inverters are not designed for continuous load, and can only operate for up to 15 minutes before thermal overload. Some of the heat is generated in the switching transistors, however the bulk of it eminates from the heavy transformer. Dedicated inverters will have ventilation fans, which disperse the heat generated in the transformer.

Efficiency of these inverters is around 70%, and this can only be achieved with the inverter at least 50% loaded. An unloaded inverter may consume around 50W when idle.

Modified Sine Wave (MSW) / modified square wave inverters

Currently, these types of inverter are the most prevalent, and cheapest available.

These inverters operate in two stages. The first stage consists of a high frequency (HF) PWM push-pull DC-DC converter, which steps up 12v to approximately 330v DC, at a frequency of 50 to 150kHz. A small, compact high efficiency transformer is used. The second stage uses a H bridge (consisting of MOSFETs) to produce 230v AC MSW from the 330v DC bus.

These inverters are usually marketed as 'Modified Sine', but are far from it. The output is actually still very similar to a square wave, except that there is 'dead time' at each zero crossing - that is, the voltage goes positive for a while, then drops to zero for a brief moment, before going negative. Doing so helps to approximate a sine wave a little better than a true square wave.

Because of the small, efficient HF transformer, and simple switching of the high voltage side, these inverters are very efficient (for certain load types), and all recently manufactured inverters have at least 90% efficiency. Idle current draw is also low, typically less than 1 amp (for a 1000W unit) when the inverter's output is unloaded.

Pure Sine Wave (Low frequency transformer)

These are the 'traditional' pure sine wave inverters. Similar in design to the 'traditional' square wave inverters, the primary of the large iron core transformer is fed by a 12v PWM push-pull stage, which generates a sine wave. This is stepped up to 230v AC, and the secondary is fed into the load.

The resulting sine wave usually has up to 4% distortion. The output RMS voltage is on par with the mains. Like its predecessor, these inverters need good cooling, otherwise they will overheat rapidly. This is expecially important when operating near, or at their rated continous capacity.

Voltage regulation is below par, as the load must be driven through a large transformer. Reactive loads are more problematic as they interact with the transformer in complex ways. However, these pure sine inverters are still much better than modified sine wave inverters when used to power inductive or motor loads.

Pure Sine Wave (HF transformer with PWM output)

Like the modified sine wave inverter, this type uses two stages. The first stage is the same, generating high voltage DC via a PWM stage and small HF transformer. The high voltage side uses a high frequency PWM H-bridge to produce a pure sine wave. Simple low pass filtering easily removes all the HF PWM switching components from the output, leaving a clean, sine wave.

Voltage regulation is very good, because the PWM output stage directly senses the output waveform, and can very quickly alter the PWM duty cycle to cater for changing loads.

Efficiency is on par with modified sine wave inverters. The output MOSFETS are either switched fully on or off via the PWM drive, so very little heat is created.

The added complexity of the PWM high voltage stage increases the cost over modified sine wave inverters, but is still much cheaper than low frequency large-transformer pure sine units.

Inverter load compatibility

Many electronic appliances have a switchmode power supply, containing a diode rectified capacitor smoothed input stage. These power supplies only work with the peak voltage of the supply waveform, and work quite well with any inverter.

Resistive loads (such as incandescent lamps, toasters and heaters) also do not care about waveform purity. As long as the RMS voltage is sufficient, it will produce at its rated heat/light output. Any type of inverter would also be suitable here.

Inductive / reactive loads are a different matter. Loads such as motors (including washing machines, vacuum cleaners, fans) will function perfectly from a pure sine wave inverter. They may work OK or not at all from a MSW inverter. Often, motors run hotter and may buzz from a modified square wave. The higher dV/dT from the square waves can react negatively with inductive loads, often causing the output H-bridge transistors to heat up. This drastically reduces the efficiency of a MSW inverter.

Newer switchmode power supplies that incorporate active power factor correction circuitry also do not work well from a modified sine wave power source. These supplies normally attempt to draw power linearly from a sine wave by means of a boost regulator, so it is not drawing current only on the waveform peak (as is the case with diode/capacitor rectified units). Given a modified sine/square wave, active PFC power supplies will buzz loudly, generate more heat, cause more stress in the PFC components, and cause the output MOSFETS in the inverter to heat up. This is because the power supply is attempting to draw current over the full crest of the sine wave, but the modified sinewave inverter only supplies the voltage at a flat peak.

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