Mon 7 Jan 2008
In analog circuits, it is not uncommon to see noise at 60Hz and harmonics of 60Hz. While some portion of this noise, especially in sensitive equipment, can be attributed to pickup of local electromagnetic fields, there is also an issue of noise coupling in through DC power supplies. Even the best regulators still have a finite power supply rejection ratio and let in a little bit of noise from the power lines. The problem is compounded when power circuits are specifically designed filter 60Hz noise but are unable to cope with higher order harmonics.
Reactive/non-linear loads are the core of the problem when it comes to many 60Hz harmonic noise problems. The supply cables have finite impedance, therefore, any harmonics on the current load will also be reflected on the delivered voltage, and vice versa, resulting in some level of harmonic contamination for all of the devices sharing the particular power circuit. A simple example is figure above. When the capacitor bank is charged, the bridge rectifier diodes will only conduct for the brief period of time when their output voltage is slightly higher than the capacitor voltage resulting in current spikes as indicated. Since the current spikes are much narrower than the rectified voltage signal, they will contain higher frequency harmonics.
This is where power factor correction and better design can come in to save the day. The primary idea behind power factor correction is to remove any apparent reactance or non-linearities from the load and make it look like a plain resistance. Another key feature of power factor correction is maximizing the power delivered to a device as the real power, related to the in-phase components of the voltage and current supplied, is what typically does the work in the system. Although this second feature is very important over all, it is rarely important to neurology related electronics as they are typically based on sensitive amplifiers and not heavy electrical loads. (The obvious counter example would be an fMRI machine.)
In any case, here is a short (11 page) primer on power factor correction from Fairchild Semiconductor followed by the more substantial (208 page) power factor correction handbook from On Semiconductor.
( an-42047.pdf ) ( HBD853-D.PDF )
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