Power LED with 6 to 20 volts keeping LED at full brightness

I am trying to power a LED 20ma 3 volt keeping LED at same brightness at 20 volt and at 6 volt dc. I tried a 4.6 volt zener across from + to - and a resistor ahead of zener on the positive side the putting another resistor in positive line to drop 4.6 volts down as not to burn out LED. I never got that far as when you place the zener across from positive to negative it gets hot at 20 volts I either have a bad zener or there are limits to how much voltage one can apply. The zener is rated 1/2 watt I do not know if this would be enough for driving a LED. I tried just a 900 ohm resistor to power the LED at 20 volts works good but as voltage drops to 6 you loose brightness. Voltage regulators usually have fixed input and variable output, but I want fixed output and variable input?

Use an LT3092 https://www.google.com/url?sa=t&rct...soF7flwEufoUdYA&bvm=bv.73612305,d.cWc&cad=rja or LM334 https://www.google.com/url?sa=t&rct...922LIPb81jMVoaZUQ7KuCVQ&bvm=bv.73612305,d.cWc

Set the current to 20 mA.

Or you could use a LM317L with a 62 ohm resistor between the output pin and the adjust pin. Input to pin 3, resistor from pin 1 to pin 2 and the output connected from pin 1 to ground.

The current regulators look good. LM317, T3092, ect.
20V X 20mA = 400mWatts. Some of the power is in the LED but most of the power will be in the regulator. With the small packages the heat will be near maximum.

Keep it simple !
----> https://www.digikey.com/product-detail/en/NSIC2020BT3G/NSIC2020BT3GOSCT-ND/3694474