Mumbai University > Electronics Engineering > Sem7 > Power Electronics 2

Marks: 10M

Year: 2012

1. A Buck-Boost converter is a type of switched mode power supply that combines the principles of the Buck Converter and the Boost converter in a single circuit.

2. The Buck converter produces a DC output in a range from 0V to just less than the input voltage. The boost converter will produce an output voltage ranging from the same voltage as the input, to a level much higher than the input.

3. By combining these two regulator designs it is possible to have a regulator circuit that can cope with a wide range of input voltages both higher and lower than that needed by the circuit. Fortunately both buck and boost converters use very similar components; they just need to be re-arranged, depending on the level of the input voltage.

1. In Fig. 3.3.1 the common components of the buck and boost circuits are combined. A control unit is added, which senses the level of input voltage, then selects the appropriate circuit action. (Note that in the examples in this section the transistors are shown as MOSFETs, commonly used in high frequency power converters, and the diodes shown as Schottky types. These diodes have a low forward junction voltage when conducting, and are able to switch at high speeds).

2. Use of buck boost converter

   a) +Vin, -Vout: This configuration of a buck-boost converter circuit uses the same number of components as the simple buck or boost converters. However this buck-boost regulator or DC-DC converter produces a negative output for a positive input. While this may be required or can be accommodated for a limited number of applications, it is not normally the most convenient format.

• When the switch in closed, current builds up through the inductor. When the switch is opened the inductor supplies current through the diode to the load.

• Obviously the polarities (including the diode) within the buck-boost converter can be reversed to provide a positive output voltage from a negative input voltage.

  b) $+Vin, +Vout:$ The second buck-boost converter circuit allows both input and output to be the same polarity. However to achieve this, more components are required. The circuit for this buck boost converter is shown below.

In this circuit, both switches act together, i.e. both are closed or open. When the switches are open, the inductor current builds. At a suitable point, the switches are opened. The inductor then supplies current to the load through a path incorporating both diodes, D1 and D2.

1. State-space models for the buck boost dc-dc converter

   • Consider the simplified circuitry of the buck-boost converter of Fig. 19.1

The differential equations governing the dynamics of

The state vector = $[I_L,V_0]^{T}---(^T $denotes the transpose of vector of matrices is):

$L_i\frac{di_L}{dt}=V_{DC} \\ C_0\frac{dv_0}{dt}=-\frac{V_0}{R_0}$ for 0 ≤ t ≤ $\delta_1T\delta(t)$=1, $Q_1$ is on $D_1$ is off

$L_i\frac{di_L}{dt}=-v_0$ for $ \delta_1 T ≤ t ≤ T \delta T \delta(t)$=0, $Q_1$ is on $D_1$ is off