by LELAND TESCHLER, Executive Editor
Surprise: A look inside five LED bulbs designed to replace 60-W incandescents reveals design regimes ranging from dead simple to startlingly sophisticated.
The average consumer might think that when it comes to light bulbs, one is about the same as another. This view might have been accurate back when every light socket contained an incandescent lamp. It is certainly not true for the LED bulbs designed as incandescent replacements.
We came to this conclusion after tearing down five LED bulbs marketed as equivalents for 60-W incandescent bulbs. The five bulbs we chose all got high marks from Consumer Reports Magazine. But that’s where the commonality stopped. When we got inside, we found vastly different approaches in construction techniques, thermal management and electronics design.
We start with a bulb called the E27 A19 LED from Home EVER Inc. in Las Vegas. The mechanics of the bulb and its electronics are dead simple. The two-sided circuit board seems to have been reflow soldered. Two wires connect the board to a metal plate holding 30 LEDs. Two more wires go to the light socket conductors. All four of these wires look as though they were hand-soldered.
The bulb is built around a 2-in.-high heat sink that weighs 2 oz and looks to be a metal casting. The base of the lamp contains a plastic housing that holds the ac/dc converter. The electrical connections to the lamp socket are at one end of the housing. The other end attaches to the heat sink with two small screws.
Additional attachments to the heat sink are a frosted polycarbonate bulb that encloses the LEDs and a 2-in.-diameter metal plate containing the LEDs. The plastic bulb apparently snap-fits into the heat sink while the LED plate attaches with three screws. There’s a couple spots of compound for thermal conduction applied between the LED plate and the heat sink.
The ac/dc converter design is straightforward. The only non-SMD components are two big capacitors, a surge resistor on the input and a transformer. Connections from the board to the screw-base and to the LED board are through discrete wires, but the connection to the bulb foot contact was done by machine. The electrical connection to the metal screw threads, though, is simply a length of bare wire squeezed between the plastic housing and the inside surface of the screw threads.
The electronics on the ac/dc converter are bare bones. The diode bridge on the input is four discrete diodes. There is a single IC on the board. It is a buck topology supply designed to provide a constant current and is made by Bright Power Semiconductor (BPS) in China. The chip, dubbed BP2812, incorporates a 600-V MOSFET. The spec sheet lists the chip operating current at 200 µA.
The “typical application circuit” listed on the BP2812 spec sheet comes extremely close to the actual circuit we found on the LED’s circuit board. Seven resistors go into simple networks that handle the Vcc voltage, sensing the buck inductor’s peak current, and regulating the input voltage to the IC. Five capacitors handle chores of ac line filtering, an ac by-pass for the Vcc pin and line-sense pins, and the buck topology. An in-line fuse cuts the power to the whole circuit in the event of too-high current draw.
Judging by graphics on the BPS web site, it looks as though BPS may have assembled the board itself. There are images there of example boards for a few other LED applications that look remarkably similar to this one.
It should be noted that the effect of temperature on LED operation doesn’t seem to be factored into the ac/dc converter. LEDs put out less light as their temperature rises. That’s generally not a problem for small temperature changes. The eye’s sensitivity to light is logarithmic, and the eye is not particularly sensitive to small changes in luminosity. It’s not unusual to see a 10% drop in LED lumen output as junction temperature rises from room temperature to 150° C.
But LED current can also be reduced at higher temperatures as a way of lessening the need for heat sinking. That said, there is no temperature sensing that we could see in the Home EVER bulb’s ac/dc converter. And there is no circuitry for dimming.
But all in all, the LED bulb probably works well in uses that don’t need a dimmable light.
Osram Sylvania’s 60-W equivalent LED bulb is notable in that it has a relatively small, two-piece heat sink. One piece is a 1-in.-high pentagon-shaped tower that doubles as a backing for six LED boards, five oriented in a pentagon shape with the sixth sitting atop the pentagon tower. The other is a 0.75-in.-long cylindrical cast heat sink that apparently snap-fits to the upper part of the plastic dome housing the LEDs. The cylindrical cast heat sink and tower together weigh 1.3 oz.
The base of the unit is a one-piece plastic housing that holds the ac/dc converter circuit board. Two wires connect it to the pentagon-shaped tower holding 18 LEDs, three on each face. The connections between the boards appear to have been reflow soldered. But the discrete wires between the circuit board and the LED assembly appear to have been hand soldered. Similarly, connections to the bulb base are discrete wires with one squeezed between the metal screw threads, the other a machine assembled to the bulb foot.
For reasons that are not completely clear, the designers of the Osram bulb chose to pot the ac/dc converter board. The relatively small heat sink in this board, compared to other designs we’ve seen, might indicate the potting is meant to improve thermal dissipation, though potting material doesn’t completely fill the empty space between the electronic components and the external shell. The potting did, however, complicate the process of deciphering the circuit.
The main board for the Osram LED bulb is two sided. It contains two ICs, one a diode bridge for the ac input, the other an SSL21082AT driver IC from NXP Semiconductors. Features implemented on the NXP chip include dimming, over-temperature protection and LED over-temperature control, output short protection, and a restart mode in the event of a brown out. This IC has an integrated internal HV switch and work as a boundary conduction mode (BCM) buck converter.
BCM is a quasi-resonant technique used to enhance energy efficiency. The fundamental idea of BCM is that the inductor current starts from zero in each switching period. When the power transistor of the boost converter is turned on for a fixed time, the peak inductor current is proportional to the input voltage. The current waveform is triangular; so the average value in each switching period is proportional to the input voltage.
Energy stores in the inductor while the switch is on. The inductor current is zero when the MOSFET is on. The amplitude of the current build-up in the inductor is proportional to the voltage drop over the inductor and the time that the MOSFET switch is on. When the MOSFET is switched off, the energy in the inductor releases toward the output. The LED current depends on the peak current through the inductor and on the dimmer angle. A new cycle starts once the inductor current is zero.
The 3M LED has a distinctive look thanks to the 2-in.-high white cylindrical column visible under its semitransparent plastic dome. The column is just a metal heat sink; it apparently has nothing to do with the dispersion of light.
The LEDs sit on a flex circuit board attached to another 2-in.-high heat sink that also forms a support for the base of the bulb. A plastic sleeve goes on the bottom of the heat sink to hold the metal screw threads and support the foot contact at the bottom of the base. The heat sink plus column together weigh in at 2.4 oz.
The flex circuit board holding the LEDs also holds the ac/dc driver circuit. It is a CL8800 from Microchip Technology. The reference design consists of the CL8800, six resistors and a bridge rectifier (a Fairchild device). Two to four additional components are optional for various levels of transient protection. Microchip’s reference design is quite close to what we found in the 3M bulb.
The driver circuit divides a string of 25 LEDs into two sets of five, one set of four, and one set of six. We’re not sure why 3M divided the number of string LEDs this way. Their orientation, however, is interesting. They sit on a ledge formed by the heat sink and are oriented straight up. The transparent carbonate globe fits onto the same ledge, so the LED light output is actually up into the edge of the plastic globe itself, rather than shining through the globe from the inside of the shell.
The LED driver circuitry is quite simple and laid out on the flex circuit without any potting compound to get in the way. According to the Microchip data sheet, six linear current regulators sink current at each tap and are sequentially turned on and off in a manner tracking the input sine wave voltage. The chip minimizes the voltage across each regulator when conducting, providing high efficiency.
The output current at each tap is individually set by a resistor. An RC network, consisting of a resistor and three capacitors in parallel, on the input of the bridge rectifier provides phase dimming. Two other components handle transient protection on the connection to the ac line. In all, there are 13 discrete components on the flex circuit that make up the transient protection, phase dimming, and set the currents in the LED strings.
Feit Electric Co.
The bulb from Feit Electric had the oddest orientation for LEDs of any we examined. The 1 7⁄8-in.-diameter plate onto which the 36 LEDs mount is partly hidden in the assembled bulb by a circular plastic piece with a 1-in.-diameter hole in the middle. This piece mounts over top the LED plate. So, a look at the assembled bulb provides a view of the plastic piece and just five LEDs visible on the center of the plate below the hole in its middle.
We are at a loss as to why Feit installed the plastic piece over top most of its LEDs. The piece blocks most of the light they emit. (We have no way of quantifying the amount of light getting through the plastic. But informal tests here indicate little of it penetrates.) So the vast majority of the emitted lumens come from the five LEDs in the center of the plate.
The rest of the bulb’s mechanical design is less mystifying. The LED plate mounts to the top of a hefty 3.8-oz cast metal heat sink with three screws. The heat sink serves as the main body of the bulb. The ac/dc converter circuitry fits in a plastic cylinder that slides into the base of the heat sink and attaches to it with two screws.
The electronics is potted into the plastic cylinder that serves as its housing. The potting material is extensive, filling the cylinder. It also doubles as a structural element supporting the screw base of the bulb and the contact foot. The circuit board holding the electronics is two-sided and extends back nearly to the foot of the bulb base. The negative lead to the board is held to the metal screw threads by the potting material. Two wires run from the board to the LED board and seem to be hand soldered there. The board itself is reflow soldered.
The potting material obscured some of the details on the PCB, but on the board are two power MOSFETs, a diode bridge chip, five large caps, transformer, and at least 22 discrete components comprised of resistors, small caps and diodes. The input bridge rectifier seems to be protected with a fusistor.
The main chip is an SSL2103T LED driver from NXP Semiconductors. The SSL2103 is basically a flyback converter that operates in combination with a phase cut dimmer circuit directly from the rectified mains. It implements dimming through integrated circuitry that optimizes the dimming curve. Drive outputs are available for resistive bleeder switching.
Though potting material obscures some of the connection details, the circuit seems to be close to that of NXP reference designs for the chip. The mains voltage is rectified, buffered and filtered in the input section and connected to the primary winding of a transformer. The transferred energy is stored in a capacitor and filtered before driving the LED chain.
The circuit board also includes two power MOSFETs. One seems to be part of a dimming circuit that divides and filters the mains rectified voltage to provide an input for the generation of the dimming curve. A bleeder drive output from the NXP chip drives the MOSFET to switch bleeder resistors that are involved in a timer for the dimming function. The other MOSFET is the main switch for the flyback transformer.
There is also a buffer circuit consisting of two capacitors and an inductor. The circuit stores energy to ensure the converter can transfer power continuously to the LED chain despite any mains power fluctuations. It also filters ripple current generated by the converter to keep down any mains-conducted emissions.
Finally, another portion of the circuit consists of a capacitor, a rectifier diode, a peak-current-limiting resistor and a protection zener diode and is used to generate an external VCC supply for the IC.
Philips Lighting Co.
One noteworthy point about the Philips bulb pertains to heat sinking. The other bulbs we examined had metal heat sinks ranging in weight from 1.3 to 3.8 oz. The Philips bulb manages to handle thermal issues without any extra heat sinking. The only component that spreads heat is the 2.5-in.-diameter disk onto which the 26 LEDs mount, 13 to a side. Moreover, you might expect that designers would stagger the LEDs on the disk such that they wouldn’t mount directly opposite each other—this mounting arrangement would also help spread heat. But the LEDs on either side of the disk sit directly opposite each other. It appears that LED heat just wasn’t an issue in this design.
One of the reasons why is the presence of a negative temperature coefficient (NTC) thermistor on the LED board. But it proved to be impossible to trace out the temperature compensation network exactly because the driver PCB has three layers, one hidden. Further complicating the analysis of the circuit is the fact that two six-pin ICs seem to handle the ac-dc conversion and neither is marked with either a manufacturer logo or part number.
Because the main ICs can’t be identified, we can only hypothesize about how the LED driver works. The presence of a transformer, two large capacitors and an npn power transistor (from STMicroelectronics) on the PCB would seem to indicate that the converter has a flyback design. Our guess is that the temperature compensation network is in the biasing of the switch providing current to the LEDs from the flyback transformer. Two transistors seem to handle the LED current. In all, we counted 32 small discrete components made up of resistors, diodes and capacitors. Rounding out the board components were a bridge rectifier chip and three other power capacitors.
It turns out that the mechanical design of an LED bulb that contains no heat sink can be quite simple (and some might call it elegant). The Philips bulb is basically a plastic enclosure that encases the LED plate and driver PCB while also supporting the metal screw threads and contact foot.
The form factor differs from that of other bulbs because of the two-sided LED plate. The Philips bulb isn’t so much a bulb as a disk. Rather than encasing the LEDs in a transparent globe-like enclosure, the Philips device presents a flat profile with plastic encasing the two-sided LED plate. The enclosure seems to just snap together over top the LED plate and driver PCB.
And because the Philips bulb contains no heat sink, it is quite light weight. But its disk-like outline might look a little weird to consumers accustomed to screwing things that are shaped like spheres into light sockets. And it beams most of its light out on the two sides defined by the orientation of the LED plates. It relies on diffusion by the plastic enclosure for lighting in other directions.
Filed Under: Power Electronic Tips, Capacitors