I just did some simple testing. With an eagle eye X6 with FET only driver and XPL HI V2 3B, I simultaneously tested current and output for different PWM modes (30, 50, 70, 100%). To measure the current, I completed the circuit at the tailcap with a 0.0094 Ohm piece of wire while measuring the voltage across it. To measure the lumen output I used my ceiling bounce test, so the lumen numbers are approximate, but the relative values should be accurate.
Below I have listed lumen values to demonstrate the approximate efficiency of PWM dimming. The first column is the approximate PWM %, the second column is the current that I measured, the third column is the lumen output I measured, the fourth column is the lumen output that would be expected if the PWM dimming resulted in a linear drop in output (ratio of the currents), the fifth column is the approximate output one would expect if the current were constant, estimated using the curve for the XPL HI here . I’m just using this curve to estimate relative changes in output; in other words it is like I scaled this curve by a factor close to 0.8, which, for instance, could be close to the efficiency of the reflector/lens system.
PWM*Imeas.*lin.**__const. curr.
100%5.34A1315lum———_———-
30%1.37A397lum337lum_458lum
I recharged the 30Q cell and switched mode groups (using guppy rev. 2).
PWM*Imeas.*lin.**__const. curr.
100%5.53A1384lum———-———
70%3.54A932lum886lum_1040lum
50%2.48A685lum621lum__803lum
So the actual output from PWM dimming is greater than if the dimming resulted in a completely linear drop, but not as high as if the dimming was constant current; pretty much what I would expect.
Now to approximately compare the PWM efficiency to a linear regulator efficiency. To output 685 lumens with PWM dimming requires (2.48A)(3.35V)=8.31W. For the linear regulator, outputting 685 lumens only takes 2.1A since it is constant current, so (2.1A)(3.27V)=6.87W in the LED, plus (3.80V-3.27V)(2.1A)=1.11W burned off in linear regulator leads to 7.98W total. (this voltage difference is between the forward voltage at 5.5A and 2.1A. The voltage difference would in reality be even greater taking into account the voltage sag of the battery, which would result in approximately an extra 0.1V burned off in the regulator)
So in this case, the efficiency advantage that the linear regulator has as a result of using constant current is mostly negated by the power dissipated in the regulator itself.
Edit: well, I (and possibly my DMM) have been confused by measuring voltage and currents with PWM. I think the power estimate above for the PWM dimming is not right. If relevant measurements with PWM have been done, I would be interested to see them.