I would not describe COB LEDs as "high power LEDs', because they are in reality arrays of low power LEDs in a single package. Having many, maybe many 10s, of LEDs spread over the package also spreads out the heat, so it's not so intense in any single spot, and the package includes a metal, usually aluminium, heatsink. So the individual leds don't get as hot as a "true" high power led, and their forward voltage does not change as much.
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