Switching vs. Linear LED drivers

[pelengineer]

i really have no idea about this so please help me. this is actually a research for school and i can't find references from the internet. :cryyy:

 

[Kansas Outcast]

Im still a little new to laser technology but from what i've read previously, linear drivers are more stable... if I'm wrong someone here with more knowlege will correct me,

 

[HIMNL9]

It's a bit more complex ..... way of working, capacity, the ability for switching to work also as boost (where the linears are just buck), efficence, and other .....

What exactly do you need to know, as research argument ? ..... principles ? ..... designs ? ..... circuits ? .....

 

[Kansas Outcast]

Whats the difference between boost and buck... I've been a tad lost on that, I'm guessing the boost drives boost up the voltage to the required voltage and the buck drives don't? Idk if you would know what would happen if you joined a buck and boost in parallel(sp), (other than there would be more mw/ma being put out)

 

[Meatball]

Linear drivers are your no nonsense basic voltage and current controllers. A few basic components can make a very powerful one.

Switching drivers are much more complicated. They require many more components. The end result is a driver with up to 95% efficiency, but switching devices often end up introducing more "noise" into the output. So if you go cheap with a switching device, it might not be as "quiet".

 

[HIMNL9]

Whats the difference between boost and buck... I've been a tad lost on that, I'm guessing the boost drives boost up the voltage to the required voltage and the buck drives don't? Idk if you would know what would happen if you joined a buck and boost in parallel(sp), (other than there would be more mw/ma being put out)

Well, as very basical difference, in a "buck" (or "step-down", if you prefer) regulator, the output voltage can be ONLY lower than the input voltage (more exactly, the output voltage can be, at maximum, the input voltage MINUS the complexive dropout voltage required from the regulator for work) ..... a "boost" (or "step-up") regulator can only have a voltage that is higher than the input voltage, or at minimum the input voltage PLUS some millivolts, it depend from the IC used ..... a "buck-boost" (or also "sepic" sometimes) regulator can work with an output voltage that can be indifferently higher or lower than the input voltage.

A linear regulator can be only a "buck" regulator, cause it only can reduce the voltage (and also, it can never give more current than the input one), where instead a switching regulator, depending from the IC architecture and specifics, can be a buck, a boost, or a sepic regulator type, and also give you more current than the input one (in specific conditions, ofcourse) ..... also, they have usually a better efficence than linear regulators, cause in a linear unit, ALL the unused power is turned in heat, where instead in a switching unit, is the circuit that regulate these parameters.

For the counterpart, a linear unit is much more easy to build, few components, no inductors, no special capacitors, no high frequencies and routing problems .....

 

[Kansas Outcast]

Thanks alot! That answered my question exactly. +rep for the great explanation

 

[pelengineer]

i just need some comparison between the two. Like, which is better for use in what types of LED circuits or LED applications. And maybe a circuit design for each type of driver.

and if you know of any website where i can find information?

 

[Bionic-Badger]

i just need some comparison between the two. Like, which is better for use in what types of LED circuits or LED applications. And maybe a circuit design for each type of driver.

and if you know of any website where i can find information?

Read up some of the application notes for LED powering chips on Texas Instruments' or Maxim' sites or other such companies.

 

[Illuminum3415]

Linear drivers sets a voltage limit on the output, what does not go through the output gets converted to heat. The upside is minimum parts count, and hardly any ripple. The downside is a higher foward voltage drop between input and output, step-down only, and as previously noted: heat.

Switching drivers uses some sort of a switch, inductor, diode combination to get the correct output by ways of storing incoming energy [usually in the sense of magnetic flux] then exporting that energy based on the set output. Some switchers are preset, others can be configured through the feedback pin.
Advantage: converts just about any reasonable [positive or negative] voltage to any reasonable [positive or negative] voltage needed. Output is stable and remains relatively unaffected by varying input voltage as long as it is within the thresholds to which the switcher is configured. Some DC-DC switchers can be configured to bidirectional so the inputs and outputs may be switched, as with a portable battery with a USB output can be recharged through the same plug as the output.
Diadvantage: May require more than a handful of parts, sensitive to electrical noise, sometimes produces electrical noise, may be cost prohibitive in some applications.

"Boost" and "Buck" is a matter of semantics. "Boost" = "step-up" and "Buck" = "step-down." When referenced from a voltage perspective Boost increases Vout to be higher than Vin while Iout is almost always less than Iin. Buck, then in terms decreases Vout below Vin but can supply high Iout currents that are roughly equal or less than Iin. Variances in chip technology allows Buck chips to be more efficient than boost chips.

Ways of telling which driver is which is relatively easy if you know the basic rules of boost versus buck. With a boost, the switch is connected after the inductor and before the diode. The switch [usually in the form of a FET, may be internal or external] is controlled by the driver. On startup, switch is closed with the other end of the inductor connected to Vin, which shorts the inductor to ground. As the inductor charges its current is increased. When the switch opens, the charged inductor now has the potential to bias the diode, which dumps its charge across it, which the load sees. When the charge is depleted, diode opens, switch closed and the process starts over.

Buck is a little hard to visualize the diode is connected anode to ground and cathode between the switch and the inductor. The other end of the switch goes to Vin and the other end of the inductor goes to the load, which shares the same ground as the diode. When the switch closes, power flows through the inductor to the load directly while steadily charging up magnetic flux. To a point the charge increase leads to reversebiasing the diode. At this point, the switch closes. The charge coming out of the inductor foward biases the diode, which moves the charge back into the inductor. The cycle continues until inductor current drops below the forward bias value of the diode, the diode turns off and the process repeats.

Its linear if its got a huge heatsink, no inductor, maybe a diode
Its switching if its got at least one diode, inductor and FET

 

[LarryDFW]

For Red laser diodes the working voltage is ~ 3 VDC
For 445nm Blue laser diodes the working voltage is ~ 4.5 VDC
For 405nm BluRay laser diodes the working voltage is ~ 5.5 VDC

A #18650 lithium ion battery is 4.2 VDC fully charged.

A Red would need a LM1117 step down regulator to deliver 3 VDC.

The two others would need a boost driver (TI 63000) to boost the voltage to 4.5-5.5 VDC.

LarryDFW