Chapter 3 DC to DC Converters Outline 3.1 Basic DC to DC converters 3.1.1 Buck converter (Step- down converter) 3.1.2 Boost converter (Step-up converter) 3.2 Composite DC/DC converters and connection of multiple DC/DC converters 3.2.1 A current-reversible chopper 3.2.2 Bridge chopper (H-bridge DC/DC converter) 3.2.3 Multi-phase multi-channel DC/DC converters

3.1 Basic DC to DC converters 3.1.1Buck converter SPDT switch changes dc component Switch output voltage waveform Duty cycle D: 0 ≤ D ≤ 1 complement D: D´ = 1 - D

Dc component of switch output voltage

Insertion of low- pass filter to remove switching harmonics and pass only dc component

Basic operation principle of buck converter

Thought process in analyzing basic DC/DC converters 1) Basic operation principle (qualitative analysis) –How does current flows during different switching states –How is energy transferred during different switching states 2) Verification of small ripple approximation 3) Derivation of inductor voltage waveform during different switching states 4) Quantitative analysis according to inductor volt-second balance or capacitor charge balance

Actual output voltage waveform of buck converter

Buck converter analysis: inductor current waveform

Inductor voltage and current subinterval 1: switch in position 1

Inductor voltage and current subinterval 2: switch in position 2

Inductor voltage and current waveforms

Determination of inductor current ripple magnitude

Inductor current waveform during start-up transient

The principle of inductor volt- second balance: Derivation Inductor defining relation: Integrate over one complete switching period: In periodic steady state, the net changes in inductor current is zero: Hence, the total area(or volt-seconds)under the inductor voltage waveform is zero whenever the converter operates in steady state. An equivalent form: The average inductor voltage is zero in steady state.

Inductor volt-second balance:Buck converter example Integral of voltage waveform is area of rectangles: average voltage is Equate to zero and solve for V:

3.1.2Boost converter Boost converter example

Boost converter analysis

Subinterval 1: switch in position 1

Subinterval 2: switch in position 2

Inductor voltage and capacitor current waveforms

Inductor volt- second balance

Conversion ratio M(D) of the boost converter

Determination of inductor current dc component

Continuous- Conduction- Mode (CCM) and Discontinuous Conduction-Mode (DCM) of boost

3.2 Composite DC/DC converters and connection of multiple DC/DC converters 3.2.1 A current reversible chopper

3.2.2Bridge chopper (H-bridge chopper)

3.2.3Multi-phase multi-channel DC/DC converter