Please explain difference between buck, buck boost, and PWM

[LLCoolBeans]

Can someone please explain to a layman what the difference is between buck, buck-boost, and PWM drivers?

In what applications would one use one of these over the others? I've read here that buck is more efficient than buck-boost, but buck-boost is necessary for using 17670 and 18650 Li-Ion cells. Do I have that right?

Also, are they all constant current and what is so great about constant current anyway?

Please forgive my ignorance, I'm trying to rid myself of it.

 

[Marduke]

This might answer some of your questions:
http://www.candlepowerforums.com/vb/showpost.php?p=2632695&postcount=9

The Welcome Mat should answer more.

 

[Kiessling]

buck = step-down
The converter reduces the voltage when the power source has a higher voltage than Vin. When the batteries drain and their voltage drops below Vin of the LED, the driver goes direct-drive and the light gradually dims as the batteries deplete.
You select the current at which to drive the LED.

Boost = step-up
The driver boosts the voltage of the battery which is less than Vin of the LED. When the battery dies, the driver draws more current to compensate for the lack of voltage to keep in regulation. This can be uncool for serial batteries and it is less efficient than buck, but it requires less batteries.

Buck/Boost
Is a converter that can do both, meaning it steps down when the batteries have more voltage than the Vin of the LED and when they deplete and the voltage sags, the converter steps up. Generally, this will be used for lights that use either one CR123 or one Li-Ion cell as the Li-Ion is just above the Vin of the white LED and will fall out of regulation soon AND as the CR123 needs a boost anyway, being under the Vin.

Current regulation
... is so important because it controls the current to the LED, thus the brightness. It is the only means to do that, as the Vin of the LED will shift over time and thus the drive current will change, it will grow and can kill your LED. And current regulation is of course important if you want constant brightness throughout the life of your battery. Otherwise the light will dim over the lifetime of the cell, which means it won't have its initial brightness for most of its lifetime.
Current regulation is a must for almost any good light, to ensure the output you want and to ensure the life of the LED.

PWM
... is "dimming by flickering" the LED in certain intervals. The less on-time, the dimmer it seems for the eye. This is in fact playing tricks on the human perception. PWM dimming works with ot without current regulation, both are indipendent concepts. Comparing the two is too much for my post, bust you might wanna search the good threads about this topic. Well worth it, you can leanr a lot.

bernie

 

[mudman cj]

I'll give it a shot.

Buck is a circuit that reduces (bucks) the input (battery) voltage down to drive a load (LED) which requires less than the battery voltage in order to run at the desired current in this case.

Boost increases the input (battery) voltage to a level required to drive a load (LED) at the desired current level.

Buck/Boost is necessary when you may want to switch battery configurations that would fall both below and above the voltage needed to drive the load. An example would be using either a primary CR123a cell with a nominal 3V output or a rechargeable RCR123 cell with a voltage beginning as high as 4V or more under some load conditions to drive an LED that needs 3.4V to operate at the desired current. If the desired current were 350mA, then the value of 3.4V would equal the forward voltage for the LED (freebie factoid), at least the way most LEDs are rated.

PWM uses a switching circuit to rapidly turn the current on and off. When the switch is on, the circuit flows whatever current it is configured to run. This could be by virtue of a constant current regulator, or it could just be direct drive. Direct drive PWM circuits are used as dimmers, so when the output is at 100% the LED is essentially in direct drive and thus you must be careful not to overdrive the LED. Direct drive also includes the use of resistors to reduce current flow in cases that battery voltage is too much greater than the forward voltage of the LED and therefore too much current would flow.

Constant current is great because it can be very efficient, it can provide a predictable and safe output level given a variety of battery types and/or combinations, and it continues to do so until the batteries are depleted. In contrast, direct drive causes the current to drop appreciably as the battery voltage decreases during discharge. The use of resistors in direct drive can have a large impact on efficiency. PWM loses efficiency as the switching frequency increases and as the percentage of the cycle spent on decreases. Low frequency PWM can bother some people (myself included) in certain situations, so there has been a trend towards higher frequencies in newer lights.

You don't have to use buck/boost with a single Li-ion cell such as a 17670 or 18650. The reason this is done is to assure that the LED is driven at the same current level throughout the battery discharge cycle. If the circuit were buck only, then once the battery voltage was below a certain point relative to the LED forward voltage the drive current would begin to decrease.

Yes, buck usually is more efficient than buck/boost, but only by a few percent.

 

[LLCoolBeans]

Thanks guys! That was extremely helpful.

 

[rushnrockt]

Alright, I didn't see that stated, but PWM does not have to apply to just the dimming. In fact, buck, boost, buck/boost and other topologies (such as SEPIC) are commonly done with PWM controllers. Hopefully that's not going to cause too much confusion

 

[LLCoolBeans]

Alright, I didn't see that stated, but PWM does not have to apply to just the dimming. In fact, buck, boost, buck/boost and other topologies (such as SEPIC) are commonly done with PWM controllers. Hopefully that's not going to cause too much confusion

I see, so buck, boost, and buck/boost are not necessarily separate from PWM? A buck driver can also be PWM, for example, do I understand you correctly?

What is SEPIC?

 

[Marduke]

Alright, I didn't see that stated, but PWM does not have to apply to just the dimming. In fact, buck, boost, buck/boost and other topologies (such as SEPIC) are commonly done with PWM controllers. Hopefully that's not going to cause too much confusion

Umm, you cannot boost a voltage with PWM, and you cannot lower it with PWM... You can lower the "average" voltage, but that will still burn out the LED before too long if the voltage peaks are significantly higher than the max Vf of the LED.

In the flashlight world, voltage buck/boost, regulation, and PWM dimming are generally mutually exclusive.

 

[Mr Happy]

I see, so buck, boost, and buck/boost are not necessarily separate from PWM? A buck driver can also be PWM, for example, do I understand you correctly?

It is better to say that PWM may be applied within inductive voltage converter circuits like buck, boost and buck/boost. The output voltage is regulated by varying the duty cycle of a PWM that is driving the input to the circuit.

What is SEPIC?

SEPIC is another kind of buck/boost circuit (it is short for Single Ended Primary Inductor Converter).

Umm, you cannot boost a voltage with PWM, and you cannot lower it with PWM... You can lower the "average" voltage, but that will still burn out the LED before too long if the voltage peaks are significantly higher than the max Vf of the LED.

In the flashlight world, voltage buck/boost, regulation, and PWM dimming are generally mutually exclusive.

You can indeed raise or lower voltages with PWM when the PWM is part of a buck or boost circuit. Many common forms of voltage converter circuit use some kind of switching technique such as PWM or PFM at their core.

So they are not mutually exclusive, they may be complementary. One may be part of another, or one may be used together with another.

 

[rushnrockt]

You can indeed raise or lower voltages with PWM when the PWM is part of a buck or boost circuit. Many common forms of voltage converter circuit use some kind of switching technique such as PWM or PFM at their core.

So they are not mutually exclusive, they may be complementary. One may be part of another, or one may be used together with another.

Thanks for responding and clarifying things after me, too quick/short responses do lead to confusion apparently

 

[Marduke]

You can indeed raise or lower voltages with PWM when the PWM is part of a buck or boost circuit. Many common forms of voltage converter circuit use some kind of switching technique such as PWM or PFM at their core.

So they are not mutually exclusive, they may be complementary. One may be part of another, or one may be used together with another.

Exactly, the boost/buck circuit raises or lowers the max voltage/current. The PWM can only operate to lower the "average" voltage/current from that level.

A PWM circuit by itself cannot raise voltage which does not already exist.

 

[rushnrockt]

Exactly, the boost/buck circuit raises or lowers the max voltage/current. The PWM can only operate to lower the "average" voltage/current from that level.

A PWM circuit by itself cannot raise voltage which does not already exist.

Exactly where did anyone say that PWM circuit by itself can raise voltage? I never stated that and MR. Happy's explanation is quite concise and clear.