Amsdell's Small Wonder [ White Paper ]

July 9, 1997
By Professor Fu Ning Wu

Inverters performing a DC to AC inversion are widely used in various industries, such as uninterruptible power supplies, communication and solar power systems, and any back up systems driven by AC power.

There are two major kinds of inverter:
The simplest kind inverts DC into AC by using push-pull circuitry, driving the two primary windings of a transformer alternatively, to provide the secondary winding with a two-phase AC output. Basically, the output is a square waveform. Some modification is necessary to control the width of the square wave so as to obtain an effective voltage (VRMS) equal to that of the sine wave and peak voltage. Such inverters are simple but very heavy because of the bulky, low-frequency transformer used. Also, such inverters are not suitable for use in critical equipment which is sensitive to high-frequency interference since the rise or fall time of a square wave is very fast and difficult to control because it contains very high-frequency components. Therefore, the circuits are mostly used in small UPSs. The same circuit is also used to produce sine waves by switching the primary winding at high-frequency instead of with a wide square pulse, and passing the resultant signal through an L.C. filter. When used in this way, the efficiency is low because the circuit operates at high-frequency but uses an iron transformer.

The other kind of inverter performs DC to AC inversion in a full bridge circuit having four switches. Switching the two diagonal switches on and off alternately produces a bi-directional output pulse between the two middle terminals of the bridge.

The second kind of inverter does not have a large transformer, and the output waveform may be produced as either a sine or stepped waveform, by switching the full-bridge transistors at high frequency.

The shortcoming of this second type of inverter is the difficulty of driving the upper two switches with a floating voltage. Usually, there are two basic methods: an optic-coupler and a transformer. The optic-coupler needs a floating DC source, which is complex and expensive. The transformer has difficulty accommodating a sine wave because of its limited bandwidth.

Another difficulty with this circuit is its problems in protecting against overload and short-circuit conditions. There are four conditions that can cause a short circuit when combined: two diagonals can cause a load short, and two verticals can cause a switching error. Because short-circuit currents rise so quickly, they are rarely detected, or protected against, in time.

Amsdell has designed a new patent-pending inverter that eliminates these problems. The new Amsdell inverter is discussed in simplified form below:
The Amsdell full-bridge switches use SCRs (Silicon Control Rectifiers) instead of MOSFETs or other transistors. A main switch is placed between the bridge and ground, instead of grounding the full-bridge directly. The advantage of this scheme is that SCRs are inexpensive, very powerful, and small in size. In addition, SCRs can be easily switched on and remain on for so long as the current flow across them is of a certain value (IH), which is very small.

A power MOSFET is used as the main switch to control the full-bridge by turning it on and off. It also provides overload and short-circuit protection as well as waveform modulation. Because the load current and any short-circuit current must go through this main switch, it is easy to sample the current from a resistor which is located between the switch and ground. An inductance is placed in series, between the bridge and switch, so as to increase the rise and fall times of the load current, giving the circuit sufficient time to react when a short-circuit occurs.

The output voltage can be modulated to produce any waveform by turning on and off the main switch instead of the four full-bridge switches. This makes the control more simple, more reliable, and highly efficient when compared with a traditional inverter.

Professor Fu Ning Wu is the inventor of the minute inverter and heads the Power Electronics team at Amsdell Inc.

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