Mumbai University > Electronics Engineering > Sem7 > Power Electronics 2

Marks: 10M

1. Basic buck converter or regulator

• The fundamental circuit for a step down converter or buck converter consists of an inductor, diode, capacitor, switch and error amplifier with switch control circuitry.

• The circuit for the buck regulator operates by varying the amount of time in which inductor receives energy from the source.

• In the basic block diagram the operation of the buck converter or buck regulator can be seen that the output voltage appearing across the load is sensed by the sense / error amplifier and an error voltage is generated that controls the switch.

• Typically the switch is controlled by a pulse width modulator, the switch remaining on of longer as more current is drawn by the load and the voltage tends to drop and often there is a fixed frequency oscillator to drive the switching.

  

2. Buck converter operation

• When the switch in the buck regulator is on, the voltage that appears across the inductor is Vin - Vout. Using the inductor equations, the c urrent in the inductor will rise at a rate of (Vin-Vout)/L. At this time the diode D is reverse biased and does not conduct.

  • When the switch opens, current must still flow as the inductor works to keep the same current flowing. As a result current still flows through the inductor and into the load. The diode, D then forms the return path with a current Idiode equal to Iout flowing through it.

  • With the switch open, the polarity of the voltage across the inductor has reversed and therefore the current through the inductor decreases with a slope equal to -Vout/L.

3. Current waveforms at different times during the overall cycle

1. State-Space Averaged Model for an Ideal Buck Converter

$\begin{bmatrix} \ \dot{x_1}\\ \ \dot{x_2} \\ \end{bmatrix} =\begin{bmatrix} \ 0 & -\frac{1}L \\ \ \frac{1}C & \ -\frac{1}{RC} \\ \end{bmatrix} \begin{bmatrix} \ {x_1}\\ \ {x_2} \\ \end{bmatrix} \begin{bmatrix} \ \frac{1}L \\ \ \ 0 \\ \end{bmatrix} [u_1]$

$0=L\dot{x_1}+{x_2} \ \ \ x_1=C\dot{x_2}+\frac{x_2}{R}$