LED bulbs then and now: Teardown of the EcoSmart 60-W equivalent LED bulb

LED bulb assembly practices circa 2015 included epoxy potting material used as a structural element for the bulb screw threads and base contact, beefy metal heat sinks (top), and handsoldered wire connections to the LED plate.

Compared to the LED bulbs of only a fewyears ago, modern-day versions are simplerand assembled via more automated methods.

If you tore down an LED bulb manufactured a few years ago you’d likely find evidence of hand soldering and kludgy design practices. We found both in evidence when we examined LED bulbs back in 2015. We took bulbs from several manufacturers that all received the highest rankings from Consumer Reports. Several of them used epoxy potting material apparently to both add stability to the screw threads and to help manage thermal dissipation.

A number of these mass-produced bulbs also displayed evidence of hand soldering. The most typical location was in making a connection between the LED plate and the circuit board holding the bulb electronics, but some bulbs contained other instances of solder globs that looked as though they had been done by hand.

Back then, it was also common to see bulbs carrying sizable heat sinks. Many of the bulbs we looked at had metal heat-spreading components weighing in at a few ounces. And the circuitry driving the LEDs tended to be comprised of at least a dozen discrete components placed around the LED driver IC.

Things have changed quite a bit in four years. We recently procured a new batch of 60-W equivalent LED bulbs to see the advances that have ensued since 2015. Like the last batch, these, too, were selected because they all got high ratings from Consumer Reports.

Consider the EcoSmart A19 LED bulb, which is assembled in China but comes from the Lighting Science Group in Florida. This 9.5-W bulb illustrates how simple LED bulb electronics can be so long as the bulb doesn’t need to be dimmed. Cut away the translucent plastic cover and you’ll find a dozen LEDs sitting on the standard metal plate. The plate attaches to the bulb’s plastic housing via two Philips screws and to the PCB electronics via two connectors.

The connectors are worth a comment. They are simple and apparently inexpensive. They make a connection when metal posts on the PCB are pushed through them. They also seem to be designed for one-time use, fine for an LED bulb.

Simple though they are, these connectors are in contrast to the typical LED plate connection scheme we found in Chinese-made bulbs four years ago -- the typical means of electrical connection was via soldered wires.

Also interesting is the method used for connecting the electronics to the bulb base contact and the metal screw threads. A press-fit connector makes contact with a metal post extending up from the base contact. The metal screw thread connection is via a metal strap that puts a spring-load against it for contact. The metal screw threads and base post are one assembly that presses on to the plastic housing that supports the translucent bulb and the PCB.

The simple configuration of the bulb base contrasts with what we typically found on LED bulbs in 2015. It wasn’t uncommon to find the metal screw threads supported mainly by epoxy potting material rather than by the plastic housing. In some bulbs the metal threads didn’t even touch the plastic housing. The epoxy did all the work. Similarly, the base contact on some bulbs was separate from the metal screw threads. And electrical connections to the base and to the metal screw threads tended to be via discrete wires.

The electronics for LED bulbs that don’t allow dimming can be simple. In the case of the EcoSmart bulb, a single IC (BP9916D) from Bright Power Semiconductor in China handles LED driving chores. It is a buck constant-current device and contains a 500-V power MOSFET for handling LED current.

The circuit we found on the bulb PCB is basically the same as the reference circuit on the BP9916D data sheet. Three capacitors, one inductor, a diode bridge, one resistor, one discrete diode, and a fuse for safety are the basic circuit components. There are an additional three resistors that seem to be there as part of a test circuit. The main resistor is a sense resistor used to convert the buck converter from a voltage-output to a current-output device. The discrete diode seems to be there to head off any ringing from the switched inductor/capacitor.

A point to note about the electronics is that the only heat sink is the metal LED plate itself. The PCB fits in the plastic housing via plastic slots. There is a metalized area on the inside of the housing, but it doesn’t touch the PCB – it’s probably there for EMI shielding.

Bright Power doesn’t provide any details about the internal circuits of the LED driver chip. One explanation it does provide is that the chip operates in critical conduction mode, meaning that the current in the inductor goes to zero before the next switching cycle initiates. Circuits that operate this way generally include a means of sensing when the inductor current hits zero, and this sensing usually takes place through a small sense winding used to show when voltage on the winding has dropped to near zero. However, a point to note about the Bright Power chip is that it doesn’t need a sense winding. The data sheet mentions something about a patent-pending MOSFET driving technique, which may have something to do with its ability to operate in critical conduction mode.

All in all, the EcoSmart bulb is an interesting example of how LED bulb assembly techniques have advanced over the past few years.