POWER ELECTRONICS DC-DC CONVERTERS (CHOPPERS) PART 1 SITI ZARINA BINTI MD NAZIRI TMT 404 ADVANCED ENGINEERING 4

Schedule Class/Week Topic Reading Assessment Lecture 1 DC-DC Converters (Choppers) (Part 1) Daniel W. Hart (Chapter 6) Quiz Lecture 2 (Part 2) (Chapter 7) Lecture 3 DC-AC Converters (Inverters) (Chapter 8) - Lecture 4 Presentation (?) A schedule design for optional periods of time/objectives.

Content Content Introduction Bulk Converter Boost Converter Buck-Boost Converter Questions/Discussions (provided in separate sheet) Introductory notes. SOURCES: “Power Electronics”, Daniel W. Hart, Mc-Graw Hill, “Power Electronics & Drives”, Lecture Notes, Dr. Zainal Salam, UTM

DC-DC CONVERTER (CHOPPERS) DEFINITION: Converting the unregulated DC input to a controlled DC output with a desired voltage level. General block diagram: APPLICATIONS: Switched-mode power supply (SMPS), DC motor control, battery chargers

Linear Regulator Transistor is operated in linear (active) mode. Output voltage: The transistor can be conveniently modeled by an equivalent variable resistor, as shown. Power loss is high at high current due to:

Switching Regulator Power loss is zero (for ideal switch): when switch is open, no current flow in it, when switch is closed no voltage drop across it. Since power is a product of voltage and current, no losses occurs in the switch. Power is 100% transferred from source to load. Switching regulator is the basis of all DC-DC converters

Buck Converter Circuit Switch closed, diode OFF Switch open, diode ON

Bulk Converter Waveforms closed open Inductor voltage Inductor current Capacitor current

Bulk Converter Switch closed, diode off

Bulk Converter Switch open, diode on

Bulk Converter Steady-state Operation Steady-state operation requires that iL at the end of switching cycle is the same at the beginning of next cycle. The change of iL over one period is zero, i.e:

Bulk Converter Average, Max & Min Inductor Current

Bulk Converter

Bulk Converter Output Voltage Ripple

Bulk Converter (Design) Calculate D to obtain required output voltage. Select a particular switching frequency: preferably >20KHz for negligible acoustic noise higher fs results in smaller L, but higher device losses. Thus lowering efficiency and larger heat sink. Also C is reduced. Possible devices: MOSFET, IGBT and BJT. Low power MOSFET can reach MHz range.

Bulk Converter (Design) Determine Lmin. Increase Lmin by about 10 times to ensure full continuous mode. Calculate C for ripple factor requirement. Capacitor ratings: must withstand peak output voltage must carry required RMS current. Note RMS current for triangular w/f is Ip/3, where Ipis the peak capacitor current given by ΔiL/2 Wire size consideration: Normally rated in RMS. But iL is known as peak. RMS value for iL is given as:

Bulk Converter Equivalent Series Resistance (ESR)

Boost (Step-up) Converter Circuit Switch closed, diode OFF Switch open, diode ON

Boost Converter Waveforms Inductor voltage Diode current Inductor current Capacitor current

Boost Converter Switch closed, diode OFF

Boost Converter Switch open, diode ON

Boost Converter Steady-state Operation Boost converter produces output voltage that is greater or equal to the input voltage. Alternative explanation: when switch is closed, diode is reversed. Thus output is isolated. The input supplies energy to inductor. When switch is opened, the output stage receives energy from the input as well as from the inductor. Hence output is large. Output voltage is maintained constant by virtue of large C.

Boost Converter Output Voltage Ripple

Boost Converter Average, Max & Min Inductor Current

Boost Converter For continuous operation,

Buck-Boost Converter Circuit Switch closed, diode OFF Switch open, diode ON

Buck-Boost Converter Waveforms Inductor current Inductor voltage Diode Capacitor current

Bulk-Boost Converter Switch closed, diode OFF

Bulk-Boost Converter Switch open, diode ON

Bulk-Boost Converter Steady-State Operation

Bulk-Boost Converter Output Voltage Output of a buck-boost converter either be higher or lower than the source voltage. If D>0.5, output is higher If D<0.5, output is lower Output voltage is always negative Note that output is never directly connected to load. Energy is stored in inductor when switch is closed and transferred to load when switch is opened.

Bulk-Boost Converter Inductor current Inductor voltage Diode current Capacitor current

Bulk-Boost Converter Average Inductor Current Assuming no power loss in the converter, power absorbed by the load must equal power supplied the by source, i.e. But average source current is related to average inductor current as: Substituting for Vo:

Bulk-Boost Converter Max & Min Inductor Current For continuous current,

Bulk-Boost Converter Output Voltage Ripple