The Basics of AC/DC Conversion

© 2016 ROHM Co.,Ltd.

The Basics of AC/DC Conversion

P. 1 © 2016 ROHM Co.,Ltd.

1. AC/DC Conversion Basics

2. DC/DC Conversion (Regulated) System after Smoothing

3. Design Procedure for AC/DC Conversion Circuits (Overview)

4. Issues and Considerations in AC/DC Conversion Circuit Design

The Basics of AC/DC Conversion: AGENDA



• Transformer System

• Switching System

• Transformer vs Switching

AC/DC Conversion Basics


Why is AC/DC conversion necessary?

Electricity is transmitted as AC.

Electronic circuits basically run on DC at a low voltage.

Why is power transmitted as AC?

High voltage/low current transmission minimizes transmission loss.

Voltage transformation is easy to accomplish, at a low cost.


AC/DC conversion is required for the moment!


Full-Wave Rectification

0

0

0

Input voltage

Rectified, without a capacitor

After rectification & smoothing,

with a capacitor

Half-Wave Rectification

0

0

0

Ripple voltage

LORD

LORD



AC/DC Conversion Basics : Transformer System


0

Transformer

100VAC

Diode Bridge Rectifier

Capacitor

VDC

Voltage

Regulator


Transformer

100VAC


Capacitor

VDC

Voltage

Regulator

AC/DC Conversion Basics : Transformer System



AC/DC Conversion Basics : Switching System

0

High-Frequency Transformer

100VAC


Capacitor

（＋）

（－）

Rectifier Diode

Capacitor

Control Circuit

Switching Element

VDC



Principles of Switching DC/DC Conversion (PWM)

75%

25%

50%

50%

75%

75%

50%

25%

25%

Voltage

Time (Duty Cycle %)

Voltage Averaging




AC/DC Conversion Basics : Switching System


100VAC


Capacitor

（＋）

（－）

Rectifier Diode

Capacitor

Control Circuit

Switching Element

VDC

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AC/DC Conversion Basics : Transformer vs Switching

Transformer

100VAC


Capacitor

VDC

Voltage

Regulator


100VAC


Capacitor

（＋）

（－）

Rectifier Diode

Capacitor

Control Circuit

Switching Element

VDC

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Transformer System

Transformer system

Relatively simple circuitry

Low noise (with a linear regulator provided for output)

Low cost

Bulky volume and significant weight

Substantial heat dissipation

Low efficiency

Switching System

Complex circuitry

Many high-voltage tolerant components

Presence of switching noise

Smaller size and lighter weight

Low heat dissipation

High efficiency



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Comparison of Wall Adapters

Left : A charger for portable devices

Input: 100 VAC

Output: 4.5V/600mA (2.7W)

Right : A charger for mobile phones Input: 100 VAC

Output: 5.4V/700mA (3.78W)



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DC/DC Conversion System after Smoothing


Control Circuit

Voltage

Regulator

（＋）

（－）

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① Linear Regulator

② Flyback

③ Forward

④ Buck (Step-Down)（Non-Islated）


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Linear Regulator

IN OUT

• Allowable input is up to about 80V (depending on the spec)

• Simple design/few components

• Little noise (no switching noise)

• Inexpensive

• Capable of stepping down only

• Large input/output difference means poor efficiency

• Heat sink may be required

• Actual maximum allowable loss is about 2W

VIN MAX

VOUT

Power loss

Available Input Range

Available Power

VDROPOUT

GND


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Flyback

• Simple configuration with a minimum number of components

• Wide input voltage range

• Suitable for small power switching power supplies

• Large ripple current of output capacitors

• In applications not requiring a great deal of output precision, the output can be set using a transformer winding ratio, and the Flyback system can be used as an unregulated power supply

• Self-exciting ringing choke converter (RCC), and separate-excitation (PWM)

•When the MOSFET turns on, a current flows to the primary winding on the transformer, producing a build-up of energy. In this case, the diode remains off.

•When the MOSFET turns off, the stored energy is output from the secondary winding in the transformer through the diode.


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Flyback System Operation (Continuous Mode)

Vgs

Vp

Vds

Ip

Is

VIN

VIN+VOR

Vs Vf+Vout

・VIN Ns

Np

VOR

・(Vf+Vout) Np

Ns VOR=

・Ipk Np

Ns

Ipk

ton toff

Vgs

VIN

Vds

Ip

Vp Is Np

Ns Vs

Vout Vf

Lp

VIN

Lp


•When the MOSFET turns on, a current flows to the primary winding on the transformer, producing a build-up of energy. In this case, the diode remains off.

•When the MOSFET turns off, the stored energy is output from the secondary winding in the transformer through the diode.

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• Supports small-power to large-power applications (1.5 KW)

• Control is stable

• Transformer utilization efficiency may not be very high

• In applications not requiring a great deal of output precision, the output voltage can be set with a transformer winding ratio, and the system can be used as a non-stable output power supply unit

•When the MOSFET turns on, the diode D1 turns on and supplies a current to the load through the inductor. In this case, the D2 remains off.

•When the MOSFET turns off, the energy stored in the inductor is supplied to the load through the diode D2. In this case, the D1 remains off.

D1

D2

Forward


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D1

D2

VIN

Vp

Np Ns

Vgs Vds

Ip

Lp L

IL Vgs

Vp

Vds

Ip

IL

VIN

VR

Ipk

ton toff

VIN+VR

・IL+Im Np

Ns Ipk= ・ton

VIN

Lp Im=

Iout

-Vout

L

Im

VIN


Forward System Operation

•When the MOSFET turns on, the diode D1 turns on and supplies a current to the load through the inductor. In this case, the D2 remains off.

•When the MOSFET turns off, the energy stored in the inductor is supplied to the load through the diode D2. In this case, the D1 remains off.

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Buck（Non-Isolated）

• Use for step-down conversion

• For non-insulation, small-power applications

• Same operation as forward system (MOSFET operates in the same manner as diode D1 in forward system)

• Because control by MOSFET exclusively determines output voltage, output feedback is mandatory

•When the MOSFET turns on, a current flows to the load through the inductor, and energy accumulates in the inductor as well. In this case the diode remains off.

•When the MOSFET turns off, the energy stored in the inductor is supplied to the load through diode D2. During this operation the MOSFET remains off.

D1

D2

Forward System


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Vgs VL

Vout

VIN

Ip IL

L

Vgs

VL

Vds

Ip

VIN-Vout Vout

ton toff

VIN

IL

Iout

VIN-Vout

L

-Vout

L

Buck System Operation (Continuous Mode)

•When the MOSFET turns on, a current flows to the load through the inductor, and energy accumulates in the inductor as well. In this case the diode remains off.

•When the MOSFET turns off, the energy stored in the inductor is supplied to the load through diode D2. During this operation the MOSFET remains off.


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I. Firming Up Required Specifications

II. Selecting a Power Supply Control IC

III. Design and Peripheral Components Selection

IV. Prototyping & Evaluation

V. Mass Production Design, Evaluation and Shipment Inspection

Design Procedure (Overview)

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I. Firming Up Specifications a. Input/output: input voltage range, output voltage/accuracy

b. Load: current, with or without transient (including sleep/wakeup)

c. Standby power, efficiency

d. Temperature: Max/Min, cooling

e. Size: foot print, height (form factor)

f. Required protection: low voltage, over-voltage, over-heating

g. Environmental/application conditions: automotive, aerospace/communication, RF

h. Cost

II.Selecting a Power Supply Control IC a. System: Transformer, Switching

b. System: Step-up, Step-down, Buck-Boost, Inverting

c. System: Linear, Flyback, Forward

d. Insulation/Non-insulation


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III.Design and Peripheral Components Selection

a. Major transformation components：Transformers, bridges, diodes, and capacitors

b. Components required for the IC

c. Calculation and optimization of constants

d. Transformer design: Size, inductance, number of turns, structural design (wire diameter and layer construction) * Refer to next slide.

IV.Prototyping and Evaluation

a. Using an evaluation board/tools

b. Board prototyping and evaluation of operations and performance under assumed conditions

c. Debugging & optimization

d. Check for compliance with required specifications & Trade-off

e. Mass production design, evaluation and shipment inspection

V. Mass Production Design, Evaluation and Shipment Inspection


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NP1 NS1 ND NS2 NＰ2

2mm

Barrier tape

4mm

Insulation tape 3T

Tolerance P-S ： AC3.0KVrms １MIN. 2mA or AC3.6KVrms 1S 2mA Beginning of winding: Fixed with barrier tape PS-CORE ： AC1.5KVrms １MIN. 2mA or AC1.8KVrms 1S 2mA Beginning of winding: Fixed with barrier tape IR ： P-S，PS-CORE 100MΩ MIN. at DC 500V Winding direction: Uniformed

NS1

1

2

3

12

NP1

NP2

4

5

ND

6 7

10

11

NS2 9

8

Example of Transformer Design


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Discrete Configuration vs. Power Supply IC With an in-depth knowledge of the technology, the discrete configuration

may also be an option

The power supply IC, integrating myriad features including the protection function, can provide an improved ease of use

In terms of size reduction and improved reliability, the IC may be a step ahead

Efficiency Efficiency is the ratio of output power to input voltage

Efficiency depends on the system and parts used

Efficiency is a critical factor to accommodate regulations and certification

Efficiency is basically traded off with size

AC/DC efficiency, compared to DC/DC efficiency, is a bit more difficult to handle and leaves more room to improve

Issues and Considerations

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Downsizing – Number of Parts and Their Sizes Adopting the switching system contributes downsizing The control IC integrates protection and other functions, reducing the parts

count and foot print Transformers and other discrete parts are also undergoing downsizing

Protection Functions Essential in terms of safety is protection against input over/under voltage

and output over loading A discrete configuration requires a large number of parts to implement

protection functions The control IC, integrating most required protection functions, provides

significant benefits

Certifications and Regulations Relate to efficiency, standby power consumption, safety, and noise Vary from country to country Examples of AC adapters: PSE (Japan), CE (EU), UL (USA), CSA (Canada),

EN (EU) Examine the requirements in advance and take a methodical approach

Issues and Considerations

P. 31 © 2016 ROHM Co.,Ltd.

1. AC/DC conversion basics • Transformer system

• Switching system

• Transformer vs switching

2. DC/DC conversion system after smoothing out • Linear regulator

• Flyback

• Forward

• Diode rectification

2. Design Procedure for AC/DC Conversion Circuits (Overview) • Firming up the Specifications

• Selecting a Power Supply Control IC

• Design and Peripheral Parts Selection

• Prototyping and Evaluation

• Mass Production Design, Evaluation, and Shipment Inspection

3. Issues and Considerations in AC/DC

Conversion Circuit Design • Discrete Configuration vs. Power Supply IC

• Efficiency

• Downsizing – Number of Parts and Their Sizes

• Protection Functions

• Certifications and Regulations

The Basics of AC/DC Conversion: Summary

P. 32 © 2016 ROHM Co.,Ltd.

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© 2016 ROHM Co.,Ltd.

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The Basics of AC/DC Conversion