Cookbook for Do-It-Yourself Transformer Design - Digikey

more than you expect

Cookbook for do-it-yourself transformer design

Content

02


Introduction

04

Transformer Design

06

Step-by-step to flyback converter design

07

1st step: Definition of the turns ratio and the duty cycle

10

2nd step: Definition of inductance

11

3rd step: Selection of the core

12

4th step: Calculating primary turns

13

5th step: Defining the wire cross section

15

Transformer Construction

17

Step-by-step-construction

18

Glossary

19

Speedy Design Service

20

Product Overview

22


23 03

Introduction The Speedy Design Kits are made for engineers to wind

SPEEDY DESIGN SERVICE is the world´s fastest sample

first transformer samples to test and optimize their Switch

service for customized transformers. The service offers

Mode Power Supply (SMPS). The material in the Design

the unique possibility to get samples designed to your

Kits is standard material. Thus there will be no material

requirements and delivered when you need them –

shortage in mass production. The material is suitable for

guaranteed! Order our SPEEDY DESIGN SERVICE when

the power ranges:

requesting samples and samples will be shipped within the selected time.

• 5-15 W (Low Power Kit) Order Code: 750 102 • 15-30 W (Medium Power Kit) Order Code: 750 101 • 5-30 W (All inclusive Design Rack) Only available on request This “Cookbook” in hand shows you examples how to design and wind a transformer. For engineers which want to concentrate on there circuit and not design their own transformer we also offer our SPEEDY DESIGN SERVICE. For the SPEEDY DESIGN SERVICE please see page 20.

04

PLEASE NOTE: Althought great care has been taken to provide accurate and current information, neither the authors nor the publisher, nor anyone else associated with this publication, shall be liable for any loss, damage, or liability directly or indirectly caused or alleyed to be caused by this book. All appropriate material is only valid for low power applications. For applications with 60 VDC / 48 VAC or more, please refer to relating safety regulations.


Content of the Speedy Design Kits

Speedy Design Kit Low Power 5-15 W

Speedy Design Kit Medium Power 15-30 W

Speedy Design Rack (only available on request)

Bobbins

ER11, ER14.5, EFD15, EE13

EFD20, EE16, EE20, EE25

ER11, ER14.5, EFD15, EFD20, EE13, EE16, EE20, EE25

Wires (ø)

0.1 mm (AWG38) 0.15 mm (AWG34) 0.2 mm (AWG32) 0.28 mm (AWG29) 0.3 mm (AWG28)

0.1 mm (AWG38) 0.3 mm (AWG28) 0.35 mm (AWG27) 0.4 mm (AWG26) 0.5 mm (AWG24)

0.1 mm (AWG38) 0.15 mm (AWG34) 0.2 mm (AWG32) 0.28 mm (AWG29) 0.3 mm (AWG28) 0.35 mm (AWG27) 0.4 mm (AWG26) 0.5 mm (AWG24)

Wrapper tape

Suitable for all bobbins in Kit

Suitable for all bobbins in Kit

Suitable for all bobbins in Rack

Cores

ER11, ER14.5 (different airgaps) EFD15 (different airgaps) EE13 (no gap)

EFD20 (different airgaps) EE16, EE20, EE25 (no gap)

ER11, ER14.5 (different airgaps) EFD15, EFD20 (different airgaps) EE16, EE20, EE25 (no gap)

Gapping Material

Mylar 0.05 mm, 0.1 mm, 0.15 mm, 0.19 mm

Mylar 0.05 mm, 0.1 mm, 0.15 mm, 0.19 mm

Mylar 0.05 mm, 0.1 mm, 0.15 mm, 0.19 mm

Extras

-

-

Design Guide „Abc of transformers“ and Software for Flyback Design „WE-FLEX-DESIGNER“

Order Code

750 102

750 101

on request

Tab. 1: Contents of the Speedy Design Kits

05

Transformer Design The following example gives you an idea how to design a transformer for a flyback converter.

Compile specifications

Fig. 1 is an overview on how to proceed. As you see from this flow chart transformer design is a highly iterative process.

Define duty cycle (max.) and turns ration

Further transformer designs for forward converters and push pull converters are integrated in Würth Elektronik´s Application and Design Guide “Abc of Transformers”.

Calculate inductance

Decide on core

Define the number of turns and calculate core losses no Core loss ok? yes Define wire thickness and calculate copper losses

Order Code: English version 749 002

German version 749 001

French version 744 044

Fig. 1: Flow chart for the approach in designing a flyback transformer 06

Copper losses ok? yes Construct a model and measure in the circuit

no


Step-by-step to flyback converter design Fig. 2 shows the basic schematics of a flyback converter.

Fig. 3 shows the current and voltage profile on the

The switch S1 is a controlled switch, e.g. a MOSFET.

primary and secondary sides of the transformer.

To understand the basic function of the flyback converter the switching processes are described as follows:

Fig. 2: Circuit diagram of a flyback converter

Fig. 3: Current and voltage profiles at the transformer of a flyback converter

1. Switch closed:

Two flyback converter operating modes are distinguished

The closed switch applies the input voltage on the

depending on the current profile.

transformer´s primary. As a result of the inductance a current rises linearly on the primary side. The polarity

1. Continuous mode:

of the transformer is that the diode blocks the current

In continuous mode (trapezoid operation or continuous

on the secondary side. The energy fed is stored in

conduction mode CCM) energy is still stored at the end of

the gap.

the switching cycle. The linear decline in current does not return to zero.

2. Switch open: With the switch open the current is interrupted on primary

2. Discontinuous mode:

side. The inductance of the transformer tries to maintain

In discontinuous mode (triangular operation or discontinu-

the flow of energy, so that the polarity of the secondary

ous conduction mode DCM) the current on secondary side

side changes. The diode becomes conducting and a linear

will be zero at the end of the cycle. There are current gaps

declining current flows on the secondary side.

in which no current flows, neither on the primary nor on the secondary side.

07

Prior to design the following parameters must be known:

Especially the safety requirements such as dielectric withstand voltage, creepage and clearance distances should be considered in the design phase, as a

• Input voltage range • Output voltage • Output power or output current • Switching frequency • Operating mode • Maximum duty cycle of the IC • Safety requirements • Ambient temperature

transformer requires a larger package if these requirements are considered. Special care should be taken for Off-line applications.

An idea about the clearance and creepage distances and the dielectric withstand voltages are given in Tab. 2 and 3. The values therein are based on IEC60950. Attention: Supplied Copper Wire is not able to withstand high voltage applications. Please take care about common practice for safety in transformers.

Operating voltage RMS-voltage or DC 50 100 125 150 200 250 300 400 600 800 1000

Creepage distance Polution degree 2 [mm] CTI>600 0.6 0.7 0.8 0.8 1.0 1.3 1.6 2.0 3.2 4.0 5.0

Basic insulation 400