A True Sine Wave ” Inverter
Design Example
Raymond Mack
Working on this section and the primary side of the switchmode power supply are no less danger-
ous than poking a screwdriver into an AC socket. If you do not understand what you are doing,
you could end up on your back, or at least with some fireworks occurring in your hands. Graphic
analogies aside, you must take the right precautions when probing the switching power supply.
One time after setting up the instrumentation on a bench and making sure I had all of the equip-
ment I needed, I started my AC/DC converter. It was operating fine, until I went to view the
drain voltage of the main power switch. Within a millisecond I had vaporized the ground clip
of the oscilloscope voltage probe with a large flash and pop. After a few expletives, I realized
I had neglected to insert an earth ground isolator onto the AC power plug of the oscilloscope.
Happily, the oscilloscope survived, having been designed for accidents or idiots such as myself.
I had learned two lifelong lessons: make sure your life insurance is up to date and always keep
several ground isolator plugs in your tool box. Today, my oscilloscope is always isolated from
ground, no matter what type of circuit I am working on.
Several other habits I have developed over the years: don t work barefooted, know where the
earth ground points are on the rest of your workbench, and know where the workbench s circuit
breaker is located. It is not so much a matter of being afraid, but more a matter of having a
healthy respect for the power you are working with. It is no different than operating your back-
yard BBQ grill with its hot surfaces and its propane bomb (sorry…tank) just underneath it.
This chapter is an adjunct to Chapters 5 and 6. This topic is how to interface a switching power
supply to the various AC power grids in the world. The AC/DC rectification stage has three or
four major functions: Creating a high voltage DC voltage for the input to the switching power
supply, filtering the noise from the input to the power supply from the AC mains, filtering the
noise created by the switching power supply to the AC mains, protecting the power supply from
adverse transients from the AC mains, and perhaps performing power factor correction of the
AC input current. This is a lot to do for a small, often minimized, section of an AC/DC power
The design of this section is important to the reliable operation of your final product and
whether the safety and emissions regulating agencies will let you sell your product around the
world. It can be the most frustrating portion of the design cycle. You will set your product on

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