High Frequency Transformer for Multi-Output SMPS Applications

1. High Frequency Transformer for
Multi-Output SMPS Applications
By,
Kaushik K Naik
naikkaushik93@gmail.com
Contact for detailed report, ppt file

2. Introduction
2
• High-frequency transformers operate using the same basic principles as
standard transformers.
• The primary difference is that, as their name implies, they operate at much
higher frequencies — while most line voltage transformers operate at 50 or
60 Hz, high-frequency transformers use frequencies from 20 KHz to over
1MHz.
• Operating at a higher frequency has many benefits:
• As the frequency of operation increases, the size of the transformer
decreases.
• Since the transformer is smaller, less copper wire is needed, thus
reducing the losses and helping to make the transformer more
efficient.
• But minimizing the issues such as skin and proximity effects are a serious
concern when designing high frequency transformers.

3. Introduction
3
• Skin effects are caused by the tendency of high frequency currents
to flow on the surface of conductors.
• This effect can be overcome by using multiple strands of wire
instead of single thick wire.

4. Introduction
4

5. Introduction
5
• Proximity effects are also known as eddy current losses are caused
by the magnetic fields from adjacent conductors either in adjacent
windings, but mainly due to field present in the core window, which
causes current to flow in unintended patterns or in eddy currents.
• Ferrite cores are used for its high permeability and its low
electrical conductivity. These two benefits allow ferrite to
prevent eddy currents.

6. Flyback Topology
6
• Flyback provides isolation through the
use of a transformer that acts as the
storage inductor.
• The transformer provides isolation,
facilitates multiple outputs, and allows
for voltage adjustment by varying the
turns ratio.
• This topology is widely used for low
power applications, and can also be
designed for higher output voltages,
but, it should not be used for anything
above 10 amps.
• With simple circuitry and no need for a
separate inductor, Flyback is one of the
most common topologies.

7. Designing High-Frequency Transformers
7
• Two factors decides the
mechanical size of transformers,
• the amount of power that
needs to be transferred
• the operating frequency.
• Transformers are basically
classified as core type and shell
type.
core type shell type

8. Flyback Converter - Example
8
Output power requirements:

9. Flyback Converter - Example
9
Inputs for design:
1. Minimum Input Voltage (𝑉𝑖𝑛𝑚𝑖𝑛) = 20V
2. Maximum Input Voltage (𝑉𝑖𝑛𝑚𝑎𝑥) = 30V
3. Maximum Output Power(𝑃𝑂𝑚𝑎𝑥) = 22W
4. Transformer Efficiency (𝜂)= 90%
5. Switching Frequency (𝐹𝑆)= 100kHz
6. Maximum Duty Cycle (𝐷 𝑚𝑎𝑥)= 0.45
7. Core cross-sectional Area(𝐴 𝐶)= 97 sq.mm
8. Core Saturation Flux Density (𝐵𝑠𝑎𝑡)= 0.15T
9. Diode forward voltage drop(𝑉𝐹) = 1V

10. Core selection for high frequency transformer
10Design of Multi Output SMPS Combining Forward and Flyback Topology for Industrial Application
The selection of core is an important step while designing a transformer.
For a square wave voltage, the magnetic flux variation is as below:
The e.m.f equation,
The same e.m.f equation holds good for transformer secondary side also.

11. Core selection for high frequency transformer
11Design of Multi Output SMPS Combining Forward and Flyback Topology for Industrial Application
The total area occupied by the windings is a fraction of area available in core for winding
Substituting
The core should be
selected such that the its
area product is greater
than the calculated
value.

12. Core selection for high frequency transformer
12Design of Multi Output SMPS Combining Forward and Flyback Topology for Industrial Application
Core Selection Based on Area Product Method:
For Pprimary = 25W, Ku = 0.025, F = 100kHz, Bm = 0.15T and J = 3A/mm2
Thus ETD 34/17/11 type of core is selected since it has area product greater than
11,111 mm4.

13. Flyback Converter - Example
13
Converter Design Calculations
Sl.No SMPS Outputs Vo(n) VF(n) Ns(n) KL(n) Isec(n)rms
Area
(mm2)
SWG
SWG-area
mm2
SWG
SWG-
area
mm2
No. of
Strands
1 Primary winding 20 6.18 2.10 0.42 21 0.519 30 0.078 5
2 Output 1 12 1.00 4.91 0.55 1.60 0.53 20 0.657 30 0.078 7
3 Output 2 5 1.00 3.27 0.45 2.88 0.96 18 1.167 30 0.078 12
Required Used

14. Flyback Converter - Example
14
Converter Design Calculations

15. High-Frequency Transformer Hardware
15
Core Type = ETD34x17x11
Number of Pins = 14
Operating frequency = 100kHz
Primary Inductance = 16uH

16. High-Frequency Transformer Hardware
16
Sl.
No.
Layer Start Finish Description
1 A - - 1 Layer of Insulation Tape
2 B 7 1
6 Turns, 5 strand of 30SWG
wire
3 C - - 2 layer of Insulation Tape
4 D 14 12
5 Turns,7 strands of 30SWG
wire
5 E - - 2 layer of Insulation Tape
6 F 10 8
3 Turns, 12 strands of 30SWG
wire
7 G - - 2 layer of Insulation Tape
8 H - - Insert the core
9 I - -
Use cable tie to tighten the
core
Transformer winding procedure:

17. Interleaved primary and secondary winding
17
• Sandwiched type windings
(where the secondary winding is
sandwiched between two halves
of the primary) are used to
reduce leakage inductance of
the windings.
• Sandwiching increases the
insulation requirement between
the windings

18. 18