Buck Regulator vs. Linear Voltage Regulator

Discussion in 'General Electronics Chat' started by Fobio Design, Jun 6, 2016.

  1. Fobio Design

    Thread Starter New Member

    May 11, 2016
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    Hello-

    I am researching step-down regulators and was wondering if someone can explain some of the differences between a buck-regulator and linear regulator. Also, if you can please provide some links of buck-regulator schematics so that I may implement one into my circuit. I have attached the schematic of my circuit below. Thanks!
     
  2. dl324

    Distinguished Member

    Mar 30, 2015
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  3. ISB123

    Well-Known Member

    May 21, 2014
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    Add 10uF electrolytic cap at output of LM317.

    Linear regulators are very inefficient because the difference between input and output voltage is converted into heat. They have efficiency of around 30-40% which means they require big heat sinks which are expensive, take up a lot of PCB space and have negative impact on lifetime of El. Caps which are sensitive to temperature. Good side of linear regulators is that they are simple,cheap and very low noise components which makes them ideal for use in Audio equipment.

    Switching regulators relay on storing energy in inductor which then gets smoothed out by a capacitor to get a flat DC voltage.They have efficiency in range of 80%-90% which makes the ideal for use in high power PSUs and battery powered devices.
    Work principle goes like this:
    1. PWM signal is getting feed to the base/gate of transistor/fet. PWM is varied to keep the voltage steady.
    Ton is increased to prevent the voltage dip in case current hungry load is connected and the other way around if low power load is connected.
    2. Constantly switching transistor on/off induces noise in the circuit which is bad, so good PCB layout and filtering is crucial.
    Switching the regulator on/off send current trough inductor which stores it and charges the capacitor at same time, when transistor is switched off the current flows from the inductor and capacitor to the load and back trough diode.

    A simple SMPSU can be made with NE555.
     
  4. Lestraveled

    Well-Known Member

    May 19, 2014
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    Sometimes it is better to use a simple linear regulator, and sometimes a switcher is better. A switcher gives you high efficiency. What do you need for your application? That is the driving factor.

    I suggest you build a TI, "simple switcher" circuit and a LM317 version and experience the difference. One good experiment is worth more than 100 opinions.
     
  5. BobTPH

    Active Member

    Jun 5, 2013
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    Actually, the efficiency of a linear regulator is Vout / Vin.

    Bob
     
  6. ronv

    AAC Fanatic!

    Nov 12, 2008
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    @Fobio Design
    Is your crystal circuit correct?
    Edit:
    The linear regulator also needs more voltage difference between input and output to regulate. I see you are running motors on a small 9 volt battery. It's voltage may drop when the motors start causing the 5 volts to drop.
     
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  7. ISB123

    Well-Known Member

    May 21, 2014
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    True but in most cases they have average efficiency in circuits of around 30%-40%.
     
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  8. Lestraveled

    Well-Known Member

    May 19, 2014
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    @Fobio Design

    Wrong. The efficiency of a linear regulator is the ratio of the of the output voltage to the input voltage. There is no "rule of thumb" about it. You have to be careful about how much heat a linear regulator dissipates so this is one of the first calculations you have to make. (Vin - Vout) * I = Watts. You have to get rid of that heat, otherwise the regulator will not last long.
     
  9. ISB123

    Well-Known Member

    May 21, 2014
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    That is correct on regulators which have set voltage . On adjustable one you have to calculate for worst case scenario and if you average the results with your best case scenario you get efficiency of around 30% or so.
    Crystal is not properly connected it goes in parallel with the pins.
     
    Last edited: Jun 7, 2016
  10. MrAl

    Well-Known Member

    Jun 17, 2014
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    Hello there,

    Linear regulators can be more efficient than buck converters. it depends *highly* on the application.

    Linear regulators regulate using a continuous control to the pass transistor which causes it to work in the linear mode, while buck circuits use a pulsed control. which cause the pass transistor to work in the switch mode.

    Switch mode can be more efficient because the transistor is either on or off. Power is P=I*V so with zero current OR zero voltage there is no power in the transistor, whereas in the linear regulator there is always at least some power in the transistor which gets dissipated as heat and therefore is wasted energy.

    In spite of this general view, a more detailed view shows that a linear regulator, especially a low dropout type, can be more efficient. This happens when the input/output differential voltage is low.
    For example, with 5.5v in and 5v out, the linear regulator is 91 percent efficient. That is not that easy to get with a buck regulator as most typical bucks are about 80 percent efficient. So using a typical buck there would cause a loss of efficiency of 10 percent compared to the linear.
    Buck circuits can get as high as 90 percent, but it takes some effort usually. High quality parts and careful design will get you there, but a run of the mill buck like the Simple Switcher line wont cut it.
    Maybe someone makes a ready made buck that has higher efficiency these days. The main item needed is a low sat transistor or low Rds MOSFET.
     
  11. Techno Tronix

    Member

    Jan 10, 2015
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