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Now, let's have a more practical look at these blocks. At first we choose a DC-DC converter to learn the basics, and then we can study more complicated DC-AC inverters and AC-AC cycloconverters.

Switching DC-DC converters use a power electronic switch (either a BJT,a MOSFET, a Thyristor or an IGBT in cut off and saturated states) to switch the direct current input voltage through to the output at a variable and adjustable duty cycle. Later this resulted square wave signal can be converted to DC again with the use of suitable L-C filters. The controller block, varies the duty cycle so that the average voltage of the rectangular voltage pulses could be equal to designed output. Surely, a commercial power supply should be equipped with additional properties, galvanic isolation, short circuit protection, over voltage and overload protection,shutdown circuit and so on, but let us begin with a basic converter now. A Buck converter :

Buck Converter

Buck converter is a non-isolated only step-down DC-DC converter which normally used in low input voltage on-board applications. Fig2 illustrates a basic Buck converter. The MOSFET X1 serves as a switch. MOSFETS are voltage controlled current sources. Their current/voltage equation is:

For N-Channel Mosfet :

In Cut off mode :

\( V_{GS} \leq  V_T\;\;\;\;  \rightarrow\;\; I_D = 0  \)

In Linear mode : 

\( V_{GS} > V_T , V_{DS} \leq V_{GS} - V_T \)