In which an important lesson is learned: The presence of a large transformer-like component does not necessarily mean an LED bulb is isolated.
I was so charmed at the clever heat-dissipating pc board design in the new Philips SlimStyle LED bulb that I breezed right over the electronic circuit used in the driver design. My first look showed a large magnetic component with multiple connector pins and I quickly assumed that it was an isolation transformer, which would have made this the first new LED bulb design I’ve seen recently that was isolated. (Because of their savings of space and components, non-isolated designs have quickly taken over the replacement bulb design wins. See, Isolated or non-isolated LED drivers for light bulbs?)
Fortunately for me, an astute reader pointed out that this is not always the case, and you can’t judge a transformer by its yellow wrapping. By looking at the photos, the reader pointed out, you can tell that the magnetic component has insufficient creepage distance to satisfy safety requirements for a non-isolated design.
(Creepage distance is the distance between the primary and secondary windings of a transformer. For example, in a typical stand-alone switch-mode power supply, a rule of thumb is to allow an 8-mm creepage distance between primary and secondary circuits.)
So if this is not an isolation transformer, what is it, and what kind of a driver topology does the bulb use?
Both flyback and non-isolated buck boost designs use a primary side-controller. They both require a custom magnetic, but rather than being a transformer, with a primary and secondary winding used for voltage transformation, it’s a coupled inductor: There are still two windings, but one is used for inductance, and one for bias.
Taking a closer look at the backside of the pc board, we can see that the output of the power FET Q1 goes directly to D8, the output of which goes to the yellow wire that powers the LED string, pointing to a non-isolated buck-boost design.
Some further information about the LED board: The voltage applied to the 26 LEDs measures just over 78V, indicating the LEDs are in a series string (26 LEDs x 3V/LED = 78V).
One more thing: In the photo below you can see the tiny component next to the LED at the top of the photo: This is a resistive temperature sensor that throttles back the current to the LEDs in case of overheating – a nice feature that adds to the bulb’s reliability.
Just to reiterate from my initial review: The Philips SlimStyle is an impressive and clever bulb design.