A buck-boost converter is used to convert a switched DC voltage of some value to another DC voltage of some fixed value.
They are primarily used in applications where you need some fixed DC output voltage even if your DC input voltage is changing.
For example, in electronics such as phones, tablets and lap top computers that run on a DC battery, the electronics inside need a fairly constant voltage to operate. If the battery were to start running low and if all the electronics ran directly off the battery voltage you would notice things such as the display dimming, or sound not being as loud…
A buck-boost converter will take that ever-lowering battery voltage and via varying the switching duty cycle and frequency will continue to output the same fixed voltage for use in these electronics.
The general make-up of a buck-boost converter is a PRI side DC source, a diode, an inductor, a capacitor and of course the switch use to “pulse” the current though the inductor. There’s more than one way to configure these elements but the function of the buck-boost is the same.
See schematic below.
Now when the switch is in the closed position current flows through it and down through the inductor returning to the battery. The diode is reverse biased in this case and so acts as an open. After some period of time and allowing a magnetic field of some value to build around the inductor, the switch is then opened.
Current flow through an inductor can never be changed instantly and so the magnetic field immediately begins collapsing around the inductor itself trying to keep current flowing in the same amount and direction as it was before the switched was opened.
This current flows down and then up charging the capacitor and in doing so a voltage is developed across the plates. The anode side of the diode sees more positive voltage than the cathode side and so is forward biased allowing current to flow through it back to the inductor completing the circuit path. Initially when the capacitor has no net charge across its plates it acts as a short and all the current flows through it and none through the load resistor. As it develops a voltage across it a current then flows through the load due to the in parallel voltage of the capacitor.
Once charged at some voltage the capacitor acts to “smooth” and keep that voltage. Just as an inductor cannot change its current instantly the capacitor voltage also cannot change instantly. Any variation in capacitor voltage can be minimized by using a sufficiently large capacitor. This variation is known as “ripple” as it ripples above and below some average DC value at the switching frequency.
You can vary the voltage across the capacitor, and hence across the load, by varying the duty cycle and frequency of the switching action. As the capacitor initially becomes charged and has an increasing voltage across it that voltage acts against the inductor voltage trying to reverse bias the diode but the inductor will induce whatever voltage it needs to in order to deplete itself of any stored magnetic energy.
If you are drawing less current from the capacitor than is given to the capacitor from the inductor the capacitor voltage will keep increasing during each switching period and the inductor voltage will keep increasing more so in order to forward bias the diode and deplete itself of magnetic energy.
This in turn keeps increasing the capacitor voltage which cannot decrease as fast as the increase due to the lesser need for current by the load from it. At some point though, the voltage becomes great enough that the current draw from it becomes sufficient so that any further increases in capacitor voltage are offset by decreases in that voltage from load current draw.
Again, if the capacitor is of a large enough value this back-and-forth action is barely noticeable, i.e. very small ripple voltage, and instead what you have is essentially a fixed DC voltage for use. If your input voltage drops as in the case of a dying battery, then the switching duty cycle can be increased a bit allowing more magnetic energy storage during ON times which is transferred during OFF times keeping the capacitor voltage at the desired value.
Here at CET, we know the choice of inductor for this type of converter is of great importance. It is almost always the case that size is a factor with smallest being best.
There are tradeoffs involved. What determines the inductance of an inductor is the core material (permeability), the magnetic area of the core, the magnetic length of the core and the number of turns wound around the core. You can achieve a higher inductance with a higher permeability material, but often high permeability materials have a lower maximum flux density value.
You can’t just pick any small inductor of a size you want and of an inductance value you desire and know it will work. Often this is not the case. Depending on the current through the inductor it may be ok, or it may not be ok.
If your current is of some value or higher for a given inductance your core will saturate, and your buck-boost converter will be rendered useless. Cores of different materials will have different maximum flux density values. Ferrite is maybe 0.48T while decent steel is as high as 1.8T.
Flux density in the core is directly related to the product of inductance and current and inversely related to the product of turns and core area.
You need to be certain your inductor is designed with certain tradeoffs in mind. If you need more inductance to store more magnetic energy in your buck-boost converter then you need to either lower your current, and/or increase your turns, and/or increase your core area….
This is shown in the following formula:
The current through your inductor is a function of applied volt-secs…. that is your DC voltage applied to your inductor for some period of time dictates the peak current reached in that period of time…. the time applied is the period of your switching frequency multiplied by your switching duty cycle.
All of this is a balancing act that we here at CET know very well. Let us help you with your buck-boost converter needs by designing your inductor to meet those needs and then manufacture it in any qty, at a price you will like.
