At its most basic, a light bulb is a commodity item. It should turn on instantly and emit a (hopefully pleasing) white light. Any internal part that adds both space and cost is a liability.
So it’s no surprise that light bulb manufacturers should speedily move to non-isolated light bulb designs, where 120V ac line power enters at the light bulb base and is rectified and fed directly into the power management portion of the light bulb circuit with no isolating transformer required.
Neither the Underwriters Labs (UL) nor the Energy Star program specifies either isolated or non-isolated LED drivers for LED replacement bulbs. However, if a non-isolated bulb has its LED components mounted on a heat sink for thermal dissipation, and the heat sink is exposed metal, then the heat sink must be insulated from the ac input voltage for safety reasons.
This is kind of a Catch-22: electrical insulation generally decreases thermal conductivity, so the heat sink is less effectiveness because it’s not mated directly to the heat-generating LEDs. Worse, since the heat sink is not electrically grounded it can radiate EMI. So, non-isolated LED bulbs save on space and cost, but require careful thermal management and EMI control.
I asked Andy Smith, senior product manager for LED lighting at Power Integrations, what the advantages of an isolated LED bulb design might be. His reply:
“Traditional isolated benefits are:
1. Isolation makes it somewhat more straightforward to meet safety requirements.
2. The transformer also allows you to step up or step up/down the output voltage almost any way you want. While buck and buck-boost topologies give you good performance the output voltage they can support is more dependent on the input voltage range than it would be for an isolated driver.
3. The switch in a non-isolated buck converter sees the full load current which means the switching MOSFET needs to be large for high current outputs and tends to be a problem above about 20 W output. (However, 20W is as big as most LED bulbs get today). This is not the case with the transformer-enabled current-scaling of an isolated driver.
You can get around 2. and 3. in a non-isolated bulb design by using a tapped-buck topology which will still require a smaller wound component than the isolated design, so this is not a clear advantage for isolated drivers in LED lighting apps. Also LED [driver] output voltages are relatively high (30-60V being common) with correspondingly low output current which makes them ideal candidates for non-isolated buck and buck boost converters.
In summary if you can isolate the bulb effectively and meet agency requirements [including EMI], you should look really hard at using a non-isolated approach.”
Power Integrations recently introduced the LYTSwitch-0 series, which are off-line switcher ICs for cost-sensitive, non-isolated, non-dimming LED lighting applications. The 8-pin parts sell for $0.29 in high volume.
Another IC driver design was also recently introduced by iWatt, one of the earliest LED driver IC companies. iWatt is moving to give designers a choice in non-isolated or isolated designs with its iW3606/08 (8W/15W) drivers, which can support either isolated or non-isolated designs while also supporting dimming and high PFC (power factor correction). The parts start at $0.46 in low volume.