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Taking a dim view of power factor | Cypress Semiconductor

Taking a dim view of power factor

We all treasure those "light-bulb moments', eh?  Let's hope that our heads get more efficient in future, when LED lighting enters the affordable mainstream.  I recently found out about Cypress's entry into this arena (check out http://www.cypress.com/?id=2495).  It's exciting, and should help to drive down the cost of LED lighting.

We have several reference designs; some incorporate power factor correction (PFC) and some are compatible with dimming systems but currently we don't have a design with both.  Someone mentioned that another supplier had gone to great lengths (and cost, and board area) to do this, so why didn't we?  PFC is not mandatory for devices that consume this level of power.  But wouldn't it be more "good citizen" to include PFC in our dimmable designs, to take advantage of the cleaner input current waveform, despite the costs?  Well, incorporating PFC into a dimmable LED light bulb is less valuable than you'd think.

It's true that poor load power factor causes significant extra wasted energy in the infrastructure that connects you to your electricity supplier.  I studied this in brain-hurting detail when I looked at how to use the new PSoC3 and PSoC5 devices in smart electricity meter applications.  Clarifying the misinformation about this out there on the web is worth an entire long article on its own!  But, to stay on-topic for today's blog, it's important to know that a major contributor to the "dirtyness" of the current drawn by many homes and offices is... the traditional light dimmer.  People often ask for dimmability in a modern LED bulb, and consider that the ultimate LED bulb would be both dimmable and PFC'd.  Let me tell you why I think that could be a waste of time and effort.

It's simple.  Whatever load you attach to a conventional triac-based dimmer (the vast majority of dimmers out there), the average current is controlled by switching off the current flow after a certain percentage of the AC cycle has occurred.  The resulting "spiky" current has a power factor that degrades dramatically as the dimming effect kicks in.  It's only an empirical observation, backed up by some informal canvassing, but a great many modern homes, particularly in the US, seem to have lighting installations comprising a multitude of very bright bulbs that are consequently run "very dim" by turning the dimmers most of the way down.

As a result, the otherwise perfect power factor of the ubiquitous filament bulb is competely squandered by the phase-angle dimming technique.  And it shouldn't be much of a surprise that replacing the filament bulbs with PFC'd LED bulbs will make no difference to the power factor.  You'll still be causing the power utility's transformers and cabling to get hotter than would be expected from the actual billable wattage you're consuming (which will be reduced by using the LED lamps, obviously).  The ideal situation would be to rip out all these triac-based lighting dimmers and replace them with a much more intelligent solution to lighting level management (ideally using PFC'd bulbs).  If you're going to keep these dimmers, then for sure you need a dimmable light bulb design, but you definitely don't need a PFC'd design. 

So, not much about filters this week, then.  Just a cautionary tale about not getting over-excited about solutions to non-problems - a message I could sometimes do with contemplating myself in the filter world...  Happy illuminating! 

Best - Kendall

Comments

Amol's picture

Dear candell,
can we use th cypress for dimming purpose, it will save th power that we r using regularly.
can we develop the th module which can drive the triac, and will definately save th power.

--
Amol k.

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