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January 2007

~ PFC's Affect on Electronics

~ Waveform of the Month

~ Notice in the waveforms below how the multiple zero crossing causes a severe di/dt change in current and is very high in amplitude.

A PFC (Power Factor Correction) event causes two problems for high speed electronics: Over Voltage and Multiple Zero-crossings. The Over Voltage caused by the event, breaks down internal components, in particular the FCC and CE/UL filters that have capacitors and MOVs. These components start to breakdown and leak current to ground. Not only does the filtering become ineffective, but the ground current causes the system to reset and act erratic.

While over voltage to electronics is easily seen, how the multiple zero-crossing affect the system is less obvious. To see this, let’s understand how the power supply works.

How AC/DC power supplies work:

Transformer + Rectifier

The varying DC output is suitable for lamps, heaters and standard motors. It is not suitable for electronic circuits unless they include a smoothing capacitor.

In order to be more suitable for electronics, add the smoothing capacitor:

Transformer + Rectifier + Smoothing

The smooth DC output has a small ripple. It is suitable for most electronic circuits. The DC voltage can be further rectified and regulated by AC/DC converters.

Now that we understand that the DC bus is directly affected by the AC, what happens if there is a sag on the AC? How does that affect the DC?  Well, from the above diagram it is easy to see that if the AC goes down, so will the DC. But the DC has a “buffer” in the “Smoothing Capacitor.” This cap helps ride-through momentary AC sags. As the cap discharges, when the AC comes back, there will be a requirement to fill the current to the load as well as a requirement to provide current to the Cap. And basic physics tell us that the cap will get the current before the load will and in some cases it oscillates back and forth (extending the sag event).

Now, for multiple zero-crossings, this causes an internal problem for the power supply. It is neither positive nor negative and so there is an extended "Zero” state in the rectifier that causes a huge current draw and a DC voltage dip. This can cause electronic problems which can cause the system to lock-up. How does this occur? The Power supply to the diodes gets “hung up" at the zero-crossing and causes a null state where the voltage goes to zero and the current increases exponentially. This rise in current cause the power supply capacitor to deplete and the DC voltage to sag. As the AC voltage comes back, the load is drawing current as is the capacitor (to recharge) and the I2R losses come into play. The larger the current the more heat (squared) goes to the load (e.g. laser diodes). Heat eventually kills the diodes if the reverse-biasing does not get them first! This is why the diodes fail prematurely. This is a major issue in pre-press imaging equipment and gradient amplifiers as well as many other medical imaging systems.

Does this affect home electronics and computers? No. the Power supplies typically in these devices are large enough to handle the event and the electronic system is relatively low speed compared to the advanced imaging equipment. Light bulbs are the biggest issue as the cheap ones will overheat and blow prematurely. Plasma and some LCD televisions may also be affected but that would depend on the make model and if it was on during the PFC event (which is usually at 5:30AM). No study has been conducted on home electronics, but the fact also remains that businesses see more PFC events than houses because the loads in houses are still mostly resistive (and it does not pay for the Utility to add capacitors to residential lines randomly). Utilities will add PFC capacitors in residential areas; it is just not as common.

How do you prevent it? Unplug the device during the known times of the event or try to filter out the 60Hz. waveform. This would require a double conversion (on-line) UPS. Some VRDUs (Voltage Regulator Distribution Units) can accomplish this, but they are mainly UPS without the batteries. Tap switch voltage regulators will not filter this event because it is too low in frequency and too quick for the tap switcher to react.