Technische Universität München

DC/DC Converter Fundamentals

Power Electronics

Prof. Hans-Georg HerzogTechnische Universität München

Elektrische Energiewandlungstechnik

1. Overview on DC/DC Converter

2. One-Quadrant Converter

• Buck Converter

• Boost Converter

• Buck-Boost Converter

• Cuk Converter

3. Two-Quadrant Converter

4. Multi-Phase DC/DC Converter

Outline

2

Overview on DC/DC Converter

Fields of Application

• Switched-Mode Power Supplies (≤ 300W)

– Supply of µC

– PC Power Supply

• Automotive (some kW)

– Coupling of Multi-Voltage On-Board Supply Networks

– Connection of Energy Storage Devices, Thermo-Electric

Generators, Solar Panels, ...

• Controlled DC-Drives (several 10 kW)

3

Fields of Application in Vehicles

4

Buck Converter

06.07.2

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DC/DC Converter Fundamentals (Prof. Herzog) 5

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• Unidirectional Coupling of Two On-Board Networks

• Connecting Components with Lower Voltage Level to

a Higher Voltage On-Board Network

Fields of Application:

One-Quadrant Converter

Buck Converter – Principle Circuit

Network A

(e.g. HV On-

Board Network)

Network B

(e.g. LV On-

Board Network)

Buck

Converter

Source: [1]

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Power

Buck Converter – Switching States

Source: [1]

Source: [1]

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Assumption:

VO = const.

Buck Converter – Switching States

Source: [1]

Source: [1]

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Assumption:

VO = const.

Buck Converter – Switching States

Source: [1]

Steady-State: Area A = Area B

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Assumption:

VO = const.

Buck Converter – Output Voltage

Source: [1]

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Buck Converter – Simulation Results

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Source: [1]

Boost Converter

U

I

• Unidirectional Coupling of Two On-Board Networks

• Connecting Components with Higher Voltage Level

to a Lower Voltage On-Board Network

Fields of Application:

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One-Quadrant Converter

Network A

(e.g. HV On-

Board Network)

Network B

(e.g. LV On-

Board Network)

Boost

Converter

Power

Boost Converter – Principle Circuit

Source: [1]

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Boost Converter – Switching States

Source: [1]

Source: [1]

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Boost Converter – Switching States

Source: [1]

Source: [1]

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Boost Converter – Switching States

Source: [1]

Steady-State:

16

Boost Converter – Output Voltage

Source: [1]

Source: [1]

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Boost Converter – Simulation Results

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Source: [1]

Buck-Boost Converter

• Voltage Inversion

• Connecting Components to a Lower/Higher

Voltage On-Board Network

Fields of Application:

U

I

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One-Quadrant Converter

Network A

(e.g. HV On-

Board Network)

Component B

(e.g. Negative

Voltage)

Buck-Boost

Converter

Power

Buck-Boost Converter – Principle Circuit

Source: [1]

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Buck-Boost Converter – Switching States

Source: [1]

Source: [1]

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Buck-Boost Converter – Switching States

Source: [1]

Source: [1]

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Buck-Boost Converter – Switching States

Source: [1]

Steady-State:

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Buck-Boost Converter – Output Voltage

Source: [1]

Source: [1]

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Buck-Boost Converter – Simulation Results

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Source: [1]

Potentialtrennung ?

Cuk Converter – Principle Circuit

Source: [1]

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Network A

(e.g. HV On-

Board Network)

Component B

(e.g. Negative

Voltage)

Cuk

Converter

Power

Cuk Converter – Switching States

Assumption:

vC1 = const

→ C1 big enough

VC1 = Vd + VO

Diode D conducting

• iL1 und iL2 flow through D

• iL1 charges C1

• iL2 delivers Output Current

→ iL1 and iL2 decrease

Source: [1]

Source: [1]

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Cuk Converter – Switching States

Switch T conducting

• iL1 and iL2 flow through T

• C1 delivers Energy to Output and L2

• Energy in L1 rises

→ iL1 und iL2 increase

Source: [1]

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Source: [1]

Assumption:

vC1 = const

→ C1 big enough

VC1 = Vd + VO

Cuk Converter – Output Voltage

Source: [1]

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Assumption:

vC1 = const

→ C1 big enough

VC1 = Vd + VO

Two-Quadrant Converters

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• Bidirectional Coupling of Two On-Board Networks

• Connecting Components with Lower Voltage

Level to a Higher Voltage On-Board Network

• Current Inversion

• Step Up-Step Down Converter

Fields of Application:

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Two-Quadrant Converters – Principle Circuit

Source: [2]

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Step Down Mode

Network A

(e.g. HV Supply)

Network B

(e.g. LV Supply)

Power

Step Down/

Step Up

Converter

Two-Quadrant Converters – Principle Circuit

Source: [2]

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Step Up Mode

Network A

(e.g. HV Supply)

Network B

(e.g. LV Supply)

Power

Step Down/

Step Up

Converter

Step Down/Step Up Converter – Simulation

33

Source: [2]

Step Down/Step Up Converter – Simulation

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Source: [2]

Step Down/Step Up Conv. – Requirements

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Limited at High Power because of

• Slow Switching of Large Semiconductor Devices

• Large Smoothing Inductances (due to High Current)

• High Ripple Current Stress in Smoothing Capacitor

Cost-Intensive Passive Components

→ „Silicon instead of Passives“

→ Multi-phase DC/DC Converter

Half-Bridge – Multi-Phase Approach

Uout

Uin

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Multi-Phase Approach

Ripple-Current Superposition of Individual Phases

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Multi-Phase Approach – Pros & Cons

Advantages:

+ Less Current per Phase

+ Higher Modulation Frequency

+ Higher Effective Modulation

Frequency by Phase-Shift in

PWM Triggering

→ Compact and Cheap Set-Up

+ Modular Design possible

Disadvantages:

– Risk of Ring Currents

– Asymmetrical Phase Currents

Balancing Alternatives:

Series Resistors

Central Control

Master-Slave Approaches

Magnetically Coupled Coils

Fuzzy Logic

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DC/DC Converter – Losses

Uout

Uin

Switching Losses:

Ohmic Losses:

On-State Power Losses Transistor:

On-State Power Losses Diode:

Gate-Triggering:

Reverse Recovery Diode:

Total:

Example: 2Q Converter

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References

[1] N. Mohan, T. Undeland, W. Robbins,

„Power Electronics – Converters, Applications, and Design“,

3rd Edition, Wiley, 2003

[2] D. Schröder,

„Leistungselektronische Schaltungen – Funktion, Auslegung und

Anwendung“,

2. Auflage, Springer, 2008

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