Sibling comments have no clue what they're talking about. I actually engineer these things.
The short answer is cost and heat. What you're describing is a linear constant current driver, which are the gold standard for flicker-free operation. Drawback is a much more complex circuit and a transistor burning off a ton of waste heat.
A constant current driver requires sensing the current across the LED, which involves sensitive analog circuitry. A PWM drive requires essentially nothing more than a FET tied directly to your microcontroller.
There are switch-mode constant current drivers, best of both worlds. Essentially a buck-boost converter in current mode instead of voltage mode. These are slightly more expensive, so they don't appear in consumer lighting products.
All that aside, the reality is that if your PWM frequency is high enough, it doesn't matter. Above several KHz it's imperceptible. The reason that this still isn't done universally is that it's a third of a cent more expensive to use a controller that can switch above 1KHz. All hail the glorious race to the capitalistic bottom.
At work I just finished up a constant current driver circuit. At home I'm building a custom lighting system with bespoke driver circuits. Despite having a CC driver I can pull off the shelf, I still chose 10KHz PWM. It's easier and more efficient, and neither you or I could tell the difference in the quality of the output light.
I used to wonder WTF was so hard about LED lighting. One R, one G, one B, a shunting resistor and... and then I took a glance under the hood. Oh. Yeah, much dicier than heating up some tungsten and frosting the inside of the bulb.
(Yeah, inventing cheap blue LEDs was some serious heavy-lifting in itself, but that hurdle has been jumped.)
Dimming. And even without that, the "continuous DC" is usually provided by switch-mode power supplies that themselves have a PWM ripple. How much depends on the quality of the supply.
Since AC is pulsating you need to store some energy to get continuous DC, usually in a smoothing capacitor. And that capacitor is relatively big and when durable, then not cheap. And it requires some further complications (like avoid inrush current).
Sibling comments have no clue what they're talking about. I actually engineer these things.
The short answer is cost and heat. What you're describing is a linear constant current driver, which are the gold standard for flicker-free operation. Drawback is a much more complex circuit and a transistor burning off a ton of waste heat.
A constant current driver requires sensing the current across the LED, which involves sensitive analog circuitry. A PWM drive requires essentially nothing more than a FET tied directly to your microcontroller.
There are switch-mode constant current drivers, best of both worlds. Essentially a buck-boost converter in current mode instead of voltage mode. These are slightly more expensive, so they don't appear in consumer lighting products.
All that aside, the reality is that if your PWM frequency is high enough, it doesn't matter. Above several KHz it's imperceptible. The reason that this still isn't done universally is that it's a third of a cent more expensive to use a controller that can switch above 1KHz. All hail the glorious race to the capitalistic bottom.
At work I just finished up a constant current driver circuit. At home I'm building a custom lighting system with bespoke driver circuits. Despite having a CC driver I can pull off the shelf, I still chose 10KHz PWM. It's easier and more efficient, and neither you or I could tell the difference in the quality of the output light.
Another optical engineer here.
I used to wonder WTF was so hard about LED lighting. One R, one G, one B, a shunting resistor and... and then I took a glance under the hood. Oh. Yeah, much dicier than heating up some tungsten and frosting the inside of the bulb.
(Yeah, inventing cheap blue LEDs was some serious heavy-lifting in itself, but that hurdle has been jumped.)
What about the IKEA light drivers? They claim to be flicker free.
Dimming. And even without that, the "continuous DC" is usually provided by switch-mode power supplies that themselves have a PWM ripple. How much depends on the quality of the supply.
Since AC is pulsating you need to store some energy to get continuous DC, usually in a smoothing capacitor. And that capacitor is relatively big and when durable, then not cheap. And it requires some further complications (like avoid inrush current).
Cost.