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5 Volt Switching (Buck) Regulator


I decided to build this project after experimenting with a cheap buck regulator purchased online from China which worked OK if run at low currents but got rather hot if asked to deliver more than 1 amp. Fitting a heatsink to the regulator IC may have helped, but that was impractical due to it being surface mounted. Some of the other components seemed a bit underrated too. The circuit described here uses an LM2576T-5 fixed 5 volt regulator and closely follows the design found in the data sheet, so this is really just showing a method of assembly. I use the regulator to step down 12 volts produced from my solar panel, but of course it has many other uses

       


The trusty old 78xx series linear regulators (eg. LM7805) waste quite a lot of energy as heat and are increasingly being replaced by the more modern (and efficient) switching regulators, also known as buck regulators. A switching regulator is basically a high powered square wave oscillator that dumps energy into a coil. When its drive waveform transitions from low to high, a magnetic field builds up around the coil and when the transition goes from high back to low it stops putting energy into the coil and the magnetic field collapses. This collapsing field (or stored energy) causes a voltage to appear across the coil of the opposite polarity. When this happens, a reverse biased diode connected to the input side of the coil starts to conduct, connecting this now negative side of the coil to ground, which allows the positive side to continue supplying the load even though the regulator is off





Because the switching device (transistor) within the regulator is producing a square wave, it is essentially either fully on or fully off and if it's fully on then it's a short circuit. In theory a pure short circuit cannot have a voltage across it, and if there's no voltage across it then there can be no power dissipated (Power = Volts x Amps so 0 Volts x any amount of Amps = 0 Watts). In the real world though, nothing is perfect and there will always be some losses due to small amounts of resistance with small amounts of voltage across that will produce small amounts of power in the form of small amounts of heat. But with the combination of reduced heat and the regulator being off for some of the time, high efficiency can be achieved. To produce the wanted voltage at the output, the mark to space ratio of the square wave is varied which in turn varies the 'on' and 'off' time of the energy delivered to the coil, and thus the energy produced by the coil when its magnetic field collapses. The resulting pulses are smoothed by the output capacitor and an average voltage is produced. By using feedback resistors within the IC, the correct voltage level can be maintained regardless of the output load (variable voltage versions are also available that use external feedback resistors to set the desired output)




Matrix board layout (viewed from above)


I usually use veroboard as my preferred method of construction, but for this project, I chose a 50x70mm matrix (or prototype) board instead. Veroboard tracks simply cannot handle the current and will act like low value resistors with a small voltage drop across them. The main criteria for the layout is to ensure that the components are located as close to the IC as possible to reduce instability and to provide the maximum amount of current transfer. High current with minimum voltage drop is achieved by using the thick copper conductors stripped out of some twin and earth mains cable




Matrix board layout (viewed from below)

Running from a 12 volt car battery with a 2.5Ω dummy load connected to the output, the input current from the battery is just over 1 amp when the output current into the load is 2 amps. A linear regulator running under the same conditions would be drawing 2 amps from the car battery to supply 2 amps into the load! With a simple aluminium heatsink (100mm x 100mm x 2.5mm) bent at 90, currents of up to 2.7 amps have been achieved without it complaining, but personally I wouldn't want to push the circuit much harder than this even though the spec says it is capable. It's always good practice to underrun things and keep within the limits and so I would rate this circuit as 2 amps continuous

Design Notes

The Coil:

The coil is made from a ferrite core salvaged from a scrap PCB. I don't know how accurate the value is but I have commercially made 100uH chokes that have 50 turns wound on a Yellow core, so that's what I used, except mine is a much larger core with fairly thick 0.8mm enamelled copper wire. I thought this would surely deliver the current... and it can! Coils within switching regulators sometimes produce an audible whistle, as this one sometimes does depending on the load. It has nothing to do with the load current as the whistle can occur at any current. It's more to do with the type of load. I've attempted to cure this effect by using ferrite filters, trying different coils and even coating the coil in epoxy resin but nothing seems to work. For the moment I will live with it as it's not too bad and doesn't seem to affect the operation of the circuit in any way

The Capacitor:

While I was experimenting with this circuit it became apparent that the type of 1000uf capacitor used at the output was quite important, not so much for its smoothing ability but for its ability to stay cool. The first one I tried got quite hot which I was not happy with and so I looked for an alternative with a higher ripple current (low ESR). The one I used in the final design was the Panasonic EEUFR1V102 (FR series) which has an excellent ripple current rating of 2.6 amps. The capacitors used are rated at 35 volts

The Diode:

A Schottky diode is used here as it has a much lower voltage drop than a standard diode and therefore less power loss. The data sheet shows a 1N5822 which is difficult to mount onto matrix board due to having legs that are too thick to go through the holes! So instead I used an MBRF1645 which has a higher current rating and legs that actually fit