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ELEC3205 - Power Electronics I: Devices and Circuits 2

Announcement 3rd Tutorial this Wednesday/Thursday,

R261, 9AM Mid-semester exam on 9th Sept. (Tue)

at 9:30AM in EE351

Mid-semester exam: mostly calculationsfrom LT1 to LT5 including TUT1-3

Consultation: 12:30-13:30 on 5/9,14:00-16:00 on 8/9

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Power Electronics I Partial lecture slides prepared with content

from Mohan et al. Power Electronics: Converters, Applications,and Design. John Wiley & Sons, Inc. 2nd Ed., 1995.

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UoS Quick-scan

Power Converter

LT3-4 LT5-6

LT7

LT8

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Outline Power switching concept

Power semiconductor devices asswitches

Operation principle of buck converter Driving the transistor

Losses mechanism

Gate drive

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Outcome After this lecture, you will be able to

Identify the properties of power diode andMOSFET

Analyse a buck converter in different

operation modes

Apply circuit theory to analyse other

converters Describe the operation of gate drive for

MOSFET

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Electrical Energy Conversion by

Switching Electrical energy can be generated,

transmitted and converted for the load

Process electrical energy by powersemiconductor devices & storage elements.

Enable efficiency enhancement, size andweight reduction of electrical equipment.

Based on switching on and off the power

source by power semiconductors. Applications: power supplies for computers,

communication equipment, machine drives,

lighting, automobile and many applications.

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Power Semiconductor Diode Voltage overshoot & reverse-recovery

transition

Source: Mohan et al. 1995, pp. 535Switching

Loss

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Power Semiconductor Diode Power diodes used in

switching mode powersupply

Fast recovery/

SchottkyLine frequency

Example: MUR820

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Power Semiconductor DiodeDiode/Rectifier

MBR10451N4004Example

-Standard, UF,US

Type

PTC on IR &

NTC on VT

PTC on IR &

NTC on VT

Temp Coeff.

~100A~2ALeakage currentIR

Up to 250V> 1kVBreakdownvoltage VT

Negligible35ns - 2usReverserecovery trr

0.3V0.6VVf (Threshold)

Symbol

SchottkyBarrier Diode

Single P-Njunction

SBD favours

low-voltage-

high-currentapplications!

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Solving Reverse-Recovery Related

Problem Modified power converter

Critical / boundary mode of operation of theinductors

Input current is continuous

IEEE Transactions of Industrial Electronics, Vol. 50, No. 4, pp. 767-776August 2003

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Power Semiconductor MOSFET N-Channel MOSFET

Voltage-controlled device plus body diode Threshold voltage VGS(th) On-state resistance R(on) ( T, 1/V

DSS

)

Operating temperature

Source: IRSource: Mohan et al. 1995, pp. 25

Example: IRF540

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Power Semiconductor MOSFET

Relationship between VGS and ID

VGS = 15V

VGS = 4.5V

ID = 20A

VGS = ?

Source: IRF540

International Rectifier

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Power Semiconductor MOSFET

Example: In a power converter, the

maximum continuous current throughthe MOSFET is 5A and the drain-to-source voltage when off is 75V. If the

power dissipation due to MOSFETsconduction loss should be less than 3W.

Please calculate the required on-stateresistance and select a suitableMOSFET for this application.

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Buck (Step-down) Converter

S. Ang 2005, pp. 17-27

Circuit diagram

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Buck Converter

Key SwitchingWaveforms

Continuous

Conduction

Mode (CCM)

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Buck Converter

CCM voltage conversion ratio

Critical inductance

Output voltage ripple

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Buck Converter

Example: A buck converter has an inputvoltage of 12V. The switching frequency is50kHz. The load requires an average voltageof 5V with a maximum ripple voltage of 20mV.

The maximum ripple current of the outputinductor is 0.2A. Determine: (a) the duty cycle,(b) the output inductance, (c) the output

capacitance, and (d) the output inductance ifthe switching frequency is increased to100kHz.

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Discontinuous Conduction Mode

Voltage conversion ratio

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Discontinuous Conduction Mode

Example: A buck converter has the

following circuit parameters: Vs=24V,L=150uH, R=25R, C=1000uF, f=10kHz,D=0.4. Find the following:

(a) Show that the inductor is indiscontinuous mode

(b) Determine the output voltage

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Conduction Loss

Loss due to ohmic resistance of device

Example: A DCM buck converteroperates at 25kHz and duty

cycle at 25% with input voltage

equals 12V. The output voltage

is 5V and inductance is 10uH.

Calculate the conduction loss

of the MOSFET if R(on) is 0.1R.

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Switching Loss

Loss due to switching of current and

voltage

Source: Mohan et al. 1995, pp. 21-23How to reduce it ?

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Gate Drive for MOSFET

Totem-pole

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Gate Drive for MOSFET

Need of floating gate drive

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Gate Drive for Buck Converter

Floating gate drive

Source: IR2111


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Gate Drive for Buck Converter

Floating gate drive

Source: IR2111


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Suggested Readings

Ang and Oliva, 2005. Chapters 2.1-2.3.

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SMPS

Linear regulator

Size and Volume Reduction

Output power = 380W

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Applications of SMPS

Lighting

Lighting

AC Adaptor

Telecom

Car adaptor

Computer

and MorePower Amp

Motor Drives

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