As low-power LED bulbsbegin to displace traditional incandescent and halogen bulbs in residential and retail applications, there’s a growing need to design compact, cost-effective LED replacements that fit in standard bulb types.
The basic requirements are similar across all bulbs – every design needs to have a low component count and cost, a small PCB outline, high efficiency, high power factor (PF), and low total harmonic distortion (THD) – but the details vary depending on the specific bulb. In this blog, we look at the three most popular bulb formats and suggest ways to design for each.
20 to 50W spot and flood (MR11/16)
These are typically halogen lamps with ratings of 20, 30, or 50W. Space is limited. The LED replacement bulb can use one of two PCB footprints: round with a diameter of less than 30mm (with taller components within 5mm of the center connector), or vertical with measurements less than 30 x 20mm.
The lamps can use an input voltage of AC 12 or 24V, or can be plugged directly into an AC main supply of 120 or 230V. A transformer is typically used to convert the AC main voltage to the appropriate DC voltage. The DC-DC power stage is highly efficient (it can reach up to 90 percent), but the step-down transformer is not. Improving the transformer efficiency can be difficult in such a tight footprint.
We recommend using the FL7701 smart non-isolated PFC buck LED driver shown in Figure 1.

Figure 1. FL7701 smart non-isolated PFC buck LED driver

It has direct AC line input voltages, so the overall footprint is small, and it eliminates the electrolytic capacitors typically used for the input, output, and VCCvoltages. This extends the product life and increases the size and cost reductions.
Screw-in “Edison” socket (A19, E14/17, E26/27)
These lamps typically are incandescent bulbs and operate from an AC input voltage. Dimming is a common feature and isolated drivers are preferred for safety. Because the power rating is higher, the LED driver is likely to be bigger and that, again, places size constraints on the design.
A single-stage Power Factor Corrected (PFC) flyback topology eliminates the input electrolytic bulk capacitor, so it reduces the PCB size. Even more PCB space can be saved with a Primary Side Regulation (PSR) flyback topology.
The FL7730 includes PFC and dimming control. The FL7732 is a drop-in replacement for the FL7730, without the dimming feature. A sample application, using the FL7730 for a PSR flyback topology, is shown in Figure 2.

Figure 2. PSR flyback topology with FL7730

Higher-wattage spot and flood (PAR 16, 20, 30, 38)
This socket uses either a screw-in E26/27-type format or a two-pin GU10-type format. The larger lamp means there’s more space for the LED driver. The FL7730 single-stage PFC flyback topology described earlier can be a good choice for using single-stage flyback controllers with pulse width modulation (PWM) and Critical Conduction Mode (CCM) PFC. Note that the higher wattage can create a higher VDS peak across the MOSFET in a flyback topology, so the MOSFET needs a higher BVDss rating. Using a single-stage PFC controlled flyback is simpler than a two-stage design and delivers good efficiency. Figure 3 gives a basic schematic with the PFC controllers FL6961, FL7930B, or FL7930C:

Figure 3. CRM PFC Topology

Fairchild has innovative solutions for low-power LED bulbs. The options include AC-DC non-isolated PFC buck and single-stage PFC primary-side regulation offline topologies, so designers have compact, efficient choices when it comes to increasing performance and lowering cost in the most common bulb types.

This post is also available in: Chinese (Simplified)