A buck converter delivers a lower DC voltage than the supplied voltage.
Circuit Diagram Edit
Input output voltage relationship Edit
For continous conduction mode. Remember energy is conserved! Energy coming into inductor leaves the inductor each cycle.
This is best expressed in using
and conservation of energy
Boundary between dis and cts Conduction Edit
Discontinuous conduction is more likely the lower the duty cycle, as the inductor has less time to store energy and more time dispensing it.
find when the minimum current is zero.
from the inductor equations
the load current for which we will enter into discontinuous mode.
Note: maximum current supplied when D=0.5, so the maximum boundary current is when D=0.5
It is useful to find the minimum conditions for the inductor and frequency to avoid discts conduction mode
Start anaylsis similar method as to ccm.
But now we need to find an expression for
Output Voltage Ripple Edit
If our output voltage is constant the output capacitors average current is zero.
Hence and So capacitor essentially smooths the output current wave form, filling in the supply gaps when the inductor is out of energy.
Ok so we know
We then have essentially the area under capacitor current shown in figure above(triangle).
during Toff the voltage across the inductor is Vo.
is the cut off frequency of the LC filter, hence it is very desirable for
In practice, the voltage drop across the equivalent series resistance (ESR) of the capacitor may contribute significantly to the output voltage ripple. This drop has been neglected.
Practical Considerations and Ratings Edit
Transformer Considerations Edit
Switch Ratings Edit
Two Switch Flyback Converter Edit