I've looked into them. Very inefficient.
They are made to look good not for an efficient light source.
The filaments are connected in series and there are about 25 LEDs in each filament.
Most these "60W eqv" filament LED bulbs are in the same class of 7-10 Watts.
Typically 75 lm/W
For example:
5 Watt, 375 Lumens, Filament LED Bulb
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This is a 60W Equivalent Soft White by Westinghouse, it draws 7.5 Watts.
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FROM THE US PATENT OFFICE
The LED light bulb includes, in its base
#10 hermetically sealing the opening of a glass cover
#12, a control board (not shown) that converts commercial electric power into electric power for driving the LEDs.
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The bulb base 10 is e.g. but not limited to an E26/E27/B22 connector. If being applied in a small LED light bulb, an E12 connector used in a small nightlight can be adopted. If being applied in a large illumination lamp, an E40 connector can be adopted. According to one embodiment of the present invention, an E27 connector is adopted for illustration and shall not be a limitation to the scope of the present invention.
The insulation part 20 is disposed on the bulb base 10, formed as e.g. but not limit to a hollow barrel-shaped structure, and formed with an accommodation space 21, the bottom thereof is formed with a thread 22 for being screw-fitted in the bulb base 10, two sides thereof are respectively formed with a fasten post 23, the top end of the fasten post 23 is formed with a fasten hole 24. Wherein, the insulation part 20 is made of a plastic material. In addition, a slide groove 25 is respectively formed between the two fasten posts 23 of the insulation part 20. Moreover, the insulation part 20 can be served to insulate the power module 30 thereby complying with relevant safety regulations.
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FOR FURTHER READING
Older article (2015) but still valid
http://www.ledinside.com/knowledge/2015/2/the_next_generation_of_led_filament_bulbs