EE682 – Group Project Design

Prof. Ali KeyhaniBoost DC/DC Converter Design

Lecture #1 Power Switch Design

Boost Converter Review1. Circuit topology

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Power Switch Design

Boost Converter Review2. Continuous conducting mode (CCM)

Current slope: VL/LWhere VL is inductor voltage

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Boost Converter Review3. Discontinuous conducting mode (DCM)

Inductor current is notcontinuous

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Design Tasks

1. Power switch design

2. Inductor design

3. Capacitor design

4. Drive circuit

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Design Specifications

1. Input voltage: 24 V

2. Output voltage: 48 V

3. Output power: 240 W

4. Inductor current ripple: 15 %

5. Capacitor voltage ripple: 0.1 %

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Power Switch Design1. Power BJTs, Power MOSFETs, and IGBTs

1. BJTs – greater capacity, low ON state loss2. MOSFETs – fast switching, voltage driven3. IGBTs – combined modules, powerful and expensive

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Power Switch DesignIllustrate the design procedures with a design example:

Design requirement:

A 240-watt DC/DC boost converter with Vin=24V and Vout=48V.

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Power Switch Design1. Current and voltage rating requirements

Design: Peak transistor current equals to

Iin=P/Vin=240W/24V=10A

Voltage rating requirements

VTmax=Vout+VF(diode)=48+0.7V=48.7V

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Power Switch Design2. Device selection based on the requirements

Design: Candidate I: BJT 2N6547

IC=15A>10A and VCE=400V>48V

Candidate II: power MOSFET HUFA75307D3ID=15A>10A and VDS=55V>48V

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Power Switch DesignDatesheet 2N6547

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Power Switch DesignDatesheet HUFA75307D3

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Power Switch Design3. Base/Gate drive requirements2N6547: For IC=15A, must have IB>=3A, not desirable

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Power Switch Design3. Base/Gate drive requirements (cont’d)HUFA75307D3 is voltage driven:Threshold (mininum ON) gate-source voltage VGSth =4V Maximum gate-source voltage VGSmax=20V

Can be driven by TTL (+5V) or CMOS logic +15V digital circuits

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Power Switch Design4. Transient performances2N6547

Rise time: tr=1.0µs Fall time: tf=1.5µs

HUFA75307D3 Rise time: tr=40ns Fall time: tf=45ns

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Power Switch Design5. Selection

Power MOSFET HUFA75307D36. Switching loss

( ) ( )

( ) W768.01010060

102012

1048

21

9

3

___

=×+

××

×=

+=+==

−

OFFONdds

OFFlossONlossloss

lossSW ttTIVWW

TTWP

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Power Switch Design7. ON state loss

1. ON state time

2. ON state loss

( )

( ) W684.81073.25075.015

102011

1

62

3

1)(2_

_

=×××

×

=

==

−

trITT

WP ONDSD

lossONlossON

s 73.25sec 48

247.0481020

1131 µ=

−+

×=

−+=out

inFout

VVVV

ft

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Power Switch Design8. Overall loss

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9.452W __ =+= lossONlossSWloss PPP < PD = 45 W

(see the datasheet of HUFA75307D3)

Power Switch DesignCalculation of junction to sink temperature difference

JCJClossJC RtZPtT θθ )()( %)50(=∆

C175 TC2.31W452.9C/W3.3 jmax °=<=×=×=∆∴ oolossjsjs PRT θ

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Power Switch DesignHUFA75307D3 ON-resistance and turn-on and turn-off time

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