Circuit driver / Booster explanation?

I think you'll get better response in the "Electronics" subforum .... Thread moved (and post approved)

It would be possible that the pulse frequency/width could be set so that it would be impossible to sense with the naked eye, but it is unlikely. The only way to be sure about a light when reviewing it would be to use an oscilloscope to measure either the voltage going to the emitter or in conjunction with a photocell, to see if there was any fluctuation present.

Nox said:

Hello all. Long time lurker, first time poster and all that.

I am looking at reviews of a light which are unanimous in their belief that the light has no PWM flicker.

Yet when I check the manufacturer information on the light it says this:

"Circuit Driver: PFM/PWM Synchronous booster intelligent conversion circuit."

Does this PFM/PWM booster not have anything to do with flickering of a light? I sort of know what a booster is, and I did a quick Google on the difference between PFM and PWM, but I'd rather understand from someone who knows what they're talking about.

Thanks.

Click to expand...

I will take a stab at it. I'm familiar with switching power supplies and I'm pretty sure this terminology of "PFM, PWM, and synchronous" is referring to the process that this particular boost IC works. In short this sounds like a pretty good driver and is not related to PWM to the LED. I'll explain in more detail.

An efficient LED driver with constant current regulation requires a type of driver that is either a boost type if the battery voltage is below the LED's Vfwd, or a buck type if the battery voltage is above the Vfwd of the LED. Boost or Buck drivers are in essence switching power supplies. The only way in electronics to convert one voltage to another is via turning an inductor on and off in a controlled fashion via the switching controller IC. A diode, a mosfet transitor switch, and capacitor are also used in this configuration. The capacitor smooths the output to DC such that this output is regulated going to its load (ie: the LED). This is non PWM to the LED.

Now, there are a variety of methods that different switching IC controllers utilize to turn this inductor on and off. Most will use a fixed switching frequency and adjust the duty cycle (PWM) of the mosfet transitor to adjust between light load or heavy load. The capacitor holds the output to DC. "The PFM/PWM synchronous booster" is terminology used with this particular boost controller IC. They are using a combination of pulse width modulation and pulse frequency modulation. The "synchronous" means that they have replaced the diode with another mosfet to improve efficiency. Because mosfets have a lot lower voltage drop than do diodes.

Sorry this is probably too much information. Basically, every decent switching type of dc to dc power conversion circuit that drives an LED has some type of PWM going on. That particular PWM though is part of the controller IC and is not connected to the LED. A less expensive method of controlling an LED is to turn it on and off rapidly so that the eyes don't detect the off time and the output then is averaged based on the duty cycle. That would be PWM direct to the LED and is not my favorite way to regulate the brightness.

What a great explanation, I am very ignorant of electronics in general and have wondered the same thing.

Good question and great answer!

We are fortunate at CPF to have such a wide range of knowledgable people who are prepared to chime in.

hiuintahs said:

I will take a stab at it. I'm familiar with switching power supplies and I'm pretty sure this terminology of "PFM, PWM, and synchronous" is referring to the process that this particular boost IC works. In short this sounds like a pretty good driver and is not related to PWM to the LED. I'll explain in more detail.

An efficient LED driver with constant current regulation requires a type of driver that is either a boost type if the battery voltage is below the LED's Vfwd, or a buck type if the battery voltage is above the Vfwd of the LED. Boost or Buck drivers are in essence switching power supplies. The only way in electronics to convert one voltage to another is via turning an inductor on and off in a controlled fashion via the switching controller IC. A diode, a mosfet transitor switch, and capacitor are also used in this configuration. The capacitor smooths the output to DC such that this output is regulated going to its load (ie: the LED). This is non PWM to the LED.

Now, there are a variety of methods that different switching IC controllers utilize to turn this inductor on and off. Most will use a fixed switching frequency and adjust the duty cycle (PWM) of the mosfet transitor to adjust between light load or heavy load. The capacitor holds the output to DC. "The PFM/PWM synchronous booster" is terminology used with this particular boost controller IC. They are using a combination of pulse width modulation and pulse frequency modulation. The "synchronous" means that they have replaced the diode with another mosfet to improve efficiency. Because mosfets have a lot lower voltage drop than do diodes.

Sorry this is probably too much information. Basically, every decent switching type of power conversion circuit that drives an LED has some type of PWM going on. That PWM never sees the LED though and is only part of the conversion to a DC level that is appropriate for the LED.

Click to expand...

Okay, your explanation makes a lot of sense, but then why do some lights exhibit PWM to the LED? Are they dropping the inductor/capacitor altogether? Or are they slowing down the frequency to the point that the pulses actually become visible?

Timothybil said:

Okay, your explanation makes a lot of sense, but then why do some lights exhibit PWM to the LED? Are they dropping the inductor/capacitor altogether? Or are they slowing down the frequency to the point that the pulses actually become visible?

Click to expand...

I'm guess that if its a current regulated switching type of driver that exhibits what might be PWM, it could just be a noisy output. Perhaps not a big enough capacitor on the output, improper feedback stabilization, etc. I once had a problem with 2 different Fenix LD12, gen 2's that I personally feel Fenix had a mistake in their assembly or manufacturing process. They denied it but when I put a scope on the LED, it was just flat out noisy. Many others were also unhappy and were questioning if it was PWM. Let's just say the end result was kind of similar as a strictly PWM driven LED.