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How to Design and Calculate High Frequency Transformer?

Author: Apogeeweb
Date: 5 Mar 2021
 6973
high frequency transformer design

Introduction

A transformer is a passive electrical device that transfers electrical energy from one electrical circuit to another, or multiple circuits. Its transmission current is AC. Transformer is commonly used to increase or decrease the supply. As one of the types, high-frequency transformers use frequencies from 20 KHz to over 1MHz. This paper tells the design process of high-frequency transformers (HFTs), that is, how to calculate high frequency transformer?

How to Make High Frequency Transformer?

Catalog

Introduction

Ⅰ Transformer Core

1.1 Magnetic Core Material

1.2 Core Structure

1.3 Core Parameters

1.4 Coil Parameters

1.5 Coil Turns

1.6 Assembly Structure

1.7 Temperature Rise Check

Ⅱ Types of High Frequency Transformer

2.1 Transformer Classifications

2.2 Design Rules

Ⅲ Transformer Core Selection Cares

Ⅳ Main Transformer Parameters

Ⅴ How to Calculate High Frequency Transformer?

5.1 Design Principles and Methods of Transformers

5.2 AP Method Analysis

5.3 Parameters of Power Supply

5.4 Transformer Turns Calculation


Ⅰ Transformer Core

In real transformers, the two coils are wound onto the same iron core. The transformer core provides a magnetic path to channel flux. The use of highly permeable material (which describes the material's ability to carry flux), as well as better core construction techniques, helps provide a desirable, low reluctance flux path and confine lines of flux to the core. The following introduces some important aspect of the transformer core.

1.1 Magnetic Core Material

Which material is best for high frequency transformer core? Soft ferrite is widely used in switching power supply due to its own characteristics. Its advantages are high resistivity, low AC eddy current loss, low price, and easy processing into various shapes. It also has disadvantages, including low working magnetic flux density, low permeability, large magnetostriction, and relatively sensitive to temperature changes. Choosing suitable materials can fully meet the design requirements of high-frequency transformers, and they have ideal performance and price advantage.

1.2 Core Structure

Transformer core as a main part, the factors to be considered when selecting the magnetic core structure are: reducing magnetic leakage and leakage inductance, increasing the heat dissipation spacing of the coil, which is beneficial to shielding, easy coil winding, and convenient assembly and wiring. Magnetic leakage and leakage inductance are directly related to the core structure. If the magnetic core does not require an air gap, a closed ring-shaped or square-shaped magnetic core is better.

1.3 Core Parameters

In the design of the magnetic core parameters, special attention should be paid to the magnetic flux density on working not only limited by the magnetization curve, but also by the loss, and the working mode of power transmission. When the magnetic flux changes in one direction: ΔB=Bs-Br, which is not merely limited by the saturation magnetic flux density, but also mainly by the loss, (the loss causes a temperature rise to affect the magnetic flux density). Working magnetic flux density Bm=0.6~0.7ΔB.
Opening the air gap can reduce Br to increase the magnetic flux density change value ΔB. After then, the excitation current increases, but the magnetic core volume can be reduced. For magnetic flux work in two-way: ΔB=2Bm. In this case, it is also necessary to pay attention to the fact that the volt-second area of the positive and negative changes of the excitation is not equal due to various reasons, and the DC bias problem occurs. Therefore, a small air gap can be added to the magnetic core, or a DC blocking capacitor can be added in the circuit design.

1.4 Coil Parameters

Coil parameters include the number of turns, wire section (diameter), wire form, winding arrangement and insulation arrangement.
The wire diameter is determined by the current density of the winding. Usually J is 2.5~4A/mm2. The choice of wire diameter should consider the skin effect. If necessary, make adjustments after checking the temperature rise of the transformer.

1.5 Coil Turns

Generally used winding arrangement: The primary winding is close to the magnetic core, and the secondary winding feedback winding is gradually arranged outward. Two winding arrangements are recommended as following:
1) If the voltage of the primary winding is high, and the secondary winding voltage is low, the secondary winding can be used close to the magnetic core, and next is the feedback winding, and the primary winding is in the outermost, which is beneficial to the primary winding to the magnetic core. Insulation arrangement.
2) To increase the coupling between the primary and secondary windings, half of the primary windings can be close to the core, then the feedback winding and secondary windings, and another half primary windings in the outermost layer, which will reduce leakage inductance helpfully.

1.6 Assembly Structure

The assembly structure of a high-frequency power transformer is divided into two types: horizontal and vertical. If using plane magnetic cores, chip magnetic cores and thin film magnetic cores, they all adopt a horizontal assembly structure.

1.7 Temperature Rise Check

The temperature rise check can be carried out by calculation and sample testing. The experimental temperature rise is lower than the allowable temperature rise by more than 15 degrees, increasing the current density and reducing the wire section appropriately. If it exceeds the allowable temperature rise, appropriately reduce the current density and increase the wire section. For example, increase the heat dissipation area of the magnetic core and wire diameter.

transformer symbol

Transformer Symbol

Ⅱ Types of High Frequency Transformer

2.1 Transformer Classifications

Power transformers are divided into three categories according to the topology:
(1) Flyback transformer
(2) Forward transformer
(3) Push-pull transformer (in full-bridge/half-bridge)
The suitable topological structure of the magnetic core structure is shown in the table on the following:

Core Structure

Transformer Circuit Type

Flyback Type

Forward Type

Push-pull Type

E cores

+

+

0

Planar E Cores

-

+

0

EFD Cores

-

+

+

ETD Cores

0

+

+

ER Cores

0

+

+

U Cores

+

0

0

RM Cores

0

+

0

EP Cores

-

+

0

P Cores

-

+

0

Ring Cores

-

+

+


Remarks: "+"=Appropriate   "0"=Normal   "-"=None

2.2 Design Rules

1) If the DC filter inductor, and the inductor core only works in one quadrant, the inductors belonging to this type include Boost inductors, Buck inductors, Buck/boost inductors, forward and push-pull transformer filtering inductors, and single-ended transformers.
2) The magnetic core of the forward transformer only works in one quadrant, so the transformer needs to be magnetically reset.
3) The magnetic core of the push-pull transformer is bidirectional alternating magnetization. Converters belonging to this category include push-pull converters, half-bridge and full-bridge converters, and AC filter inductors.

 

Ⅲ Transformer Core Selection Cares

1) Soft ferrite is widely used in switching power supply due to its low price, good adaptability and high frequency performance.
2) Soft ferrites are common in two series: manganese-zinc ferrite and nickel-zinc ferrite. The components of manganese-zinc ferrite are Fe2O3, MnCO3, and ZnO. It is mainly used in various filters below 1MHz, inductors, transformers, etc., with a wide range of applications. The components of nickel-zinc ferrite are Fe2O3, NiO, ZnO, etc., which are mainly used for various induction windings above 1MHz, anti-interference magnetic beads, and sharing antenna matching devices.
3) Manganese-zinc ferrite cores are the most widely used in switching power supplies. Depending on their use, the choice of materials is also different. The cores used in the power input filter part are mostly high-permeability, and their material grades are mostly R4K~R10K, that is, ferrite cores with a relative permeability of 4000~10000. For main transformers and output filters, most of them have high saturation magnetic flux density, and their Bs is about 0.5T (ie 5000GS).

 

Ⅳ Main Transformer Parameters

a.Transformer Topology

With a higher saturation magnetic flux density Bs and a lower residual magnetic flux density Br,  Bs has a certain impact on the transformer and winding results. Theoretically, if Bs is high, the number of winding turns will decrease, and the copper loss will also decrease. In practical applications, there are many circuit forms of switching power supply high-frequency converters. For transformers, their working forms can be divided into two categories:

  • Bipolar

The circuit is half-bridge, full-bridge, push-pull, etc. The positive and negative half-cycle excitation currents in the transformer primary winding are identical in magnitude and opposite in direction. Therefore, the magnetic flux changes in the transformer core also move symmetrically up and down. Maximum change range of B is △B=2Bm, and the DC component in the core basically cancels out.

  • Unipolar

The circuit is single-ended forward, single-ended flyback, etc. The primary winding of the transformer adds a unidirectional square wave pulse voltage in one cycle (single-ended flyback is the case). The transformer core is unidirectionally excited, and the magnetic flux density varies from the maximum value Bm to the residual magnetic flux density Br. At this time, △B=Bm-Br. If Br is reduced and the saturation magnetic flux density Bs is increased, △B can be increased. It can reduce the number of turns and the copper loss.

 

b. Low Power Loss at High Frequencies
The power loss of ferrite not only affects the output efficiency of the power supply, but also causes the core heating, waveform distortion and other undesirable consequences. 
The heating problem of the transformer is extremely common in practical applications. It is mainly caused by the copper loss and core loss. If Bm is selected too low when designing the transformer, and more winding turns will cause the winding to heat up, and at the same time transfer heat to the magnetic core. Conversely, if the core is the main heating body, it will also cause the winding to heat up.
When selecting ferrite materials, the power loss is required to have a negative temperature coefficient relationship. If the core loss is the main body of heat, the temperature of the transformer will rise, which will cause the core loss to increase further, eventually burn out the power tube, transformer and other components. Therefore, when developing power ferrites at home and abroad, it is necessary to solve the problem of the negative temperature coefficient of the magnetic material itself. This is also a significant feature of the magnetic material for power supply.

 

c. Permeability
How much is the appropriate permeability? This should be determined according to the switching frequency of the actual circuit. Generally, materials with a relative permeability of 2000 have an applicable frequency below 300kHz, and sometimes it can be higher, less than 500kHz. For materials higher than this value, a lower magnetic permeability should be selected, generally around 1300.

 

d. Higher Curie Temperature
The Curie temperature is the temperature at which the magnetic material loses its magnetic properties, general above 200℃. However, the actual working temperature of the transformer should not be higher than 80℃. This is because when the temperature is above 100℃, its saturation magnetic flux density Bs has dropped to 70% of that at room temperature. Therefore, an excessively high operating temperature will cause the saturation flux density of the magnetic core to drop more severely. Furthermore, when it is higher than 100°C, its power consumption has a positive temperature coefficient, which will lead to a vicious circle. For the R2KB2 material, the temperature corresponding to its allowable power consumption has reached 110°C, and the Curie temperature is as high as 240°C, which meets the requirements for high-temperature use.

 

Ⅴ How to Calculate High Frequency Transformer?

5.1 Design Principles and Methods of Transformers

There are two principle methods for designing transformers: area product AP method. AP is the product of the core cross-sectional area Ae and the coil effective window area Aw.

AP method

PT-power of transformer
Ae- effective cross-sectional area
Aw- core window area
Ko-core window utilization factor, typical value is 0.4.
Kf-form factor, square wave is 4, and sine wave is 4.44.
Bw-the working magnetic intensity of the magnetic core
Fs-switch operating frequency
Kj-current density coefficient, take 395A/cm2
X-core structure coefficient

5.2 AP Method Analysis

According to the design method of power transformer, the general steps of designing transformer with area product AP method:
1. Select the core material and calculate the apparent power of the transformer.
2. Determine the core cross-sectional size AP, and then select the core size according to it.
3. Calculate the inductance and number of turns of the primary and secondary sides.
4. Calculate the length of the air gap.
5. Find the wire diameter according to the current density and the effective value current of the primary and secondary sides.
6. Determine whether the copper loss and iron loss meet the requirements (allowable loss and temperature rise).

5.3 Parameters of Power Supply

Input voltage: 175-264VAC
Output voltage: 21V
Output power: 3A
The frequency is set at 60KHz, and the duty cycle is initially set at 0.45. 
Using a flyback topology, choose the core material and determine the apparent power PT of the transformer.
Consider the cost, choose PC40 material here:
Check the PC40 data and get Bs=0.39T, Br=0.06T

Bmax

Bm= ΔBmax*0.6=0.198T, round it to 0.2T
In order to prevent the magnetic core from being saturated momentarily, reserve a certain margin and take Bm= ΔBmax*0.6=0.198T, take 0.2T.
Transformer apparent power PT, for the flyback transformer:

Transformer apparent power

Calculate AP:

AP

Where:
J is the current density, usually taking 395A/cm2.
Ku is the effective use coefficient of the copper window, which is determined according to the safety requirements and the number of output channels, generally 0.2 to 0.4. Take 0.4 here to adapt to the sudden load current. The power supply is designed in critical mode, and the critical current I0B=0.8×I0=2.4A

5.4 Transformer Turns Calculation

1) Minimum input voltage: Vimin=ViACmin*1.2=210V
2) Turns Ratio
n=[Vimin/(Vo+Vf)]*[Dmax/(1-Dmax)]
n=[210V/(21V+1V)]*[0.45/(1-0.45)]=7.8
3) Secondary Side Peak Current
^IsB=2*IoB/(1-Dmax)
^IsB=2*2.4A/(1-0.45)=8.72A
4) Secondary Side Inductance
Ls=(Vo+Vf)*(1-Dmax)*[1/(Fs*1000)]/^IsB*1000000
Ls=(21V+1V)*(1-0.45)*[1/(60KHz*1000)]/^8.72A*1000000=23.58uH
5) Primary Side Inductance
Lp=n*n*Ls
Lp=7.8*7.8*23.58uH=1434uH
6) Secondary Side Peak Current (continuous mode)
^IsB=Io/(1-Dmax)+(^IsB/2)
^IsB=3A/(1-0.45)+(8.72A/2)=9.81A
7) Primary Side Peak Current (continuous mode)
^Ipp=^Isp/n
^Ipp=9.81A/7.8=1.257A

transformer core

Primary Winding and Secondary Winding Turns
1) Primary Winding Turns
Np=Lp*^Ipp(^B*Ae)
Np=1434uH*1.257A/(0.2*84.8)=106.28T,round it to 106T
2) Secondary Winding Turns
Ns=Np/n
Ns=106T/7.8=13.58T,round it to Ns=14T
3) Feedback Turns
Nv=(Vcc+Vf)/[(Vo+Vf)/Ns]
Nv=(14.5V+1V)/[(21V+1V)/14T]=9.87T, round it to Nv=10T

In order to avoid saturation of the magnetic core, an appropriate air gap is added to the magnetic circuit, and the calculation is as follows:

air gap calculation

It may be necessary to correct the number of turns based on the edge effect of the air gap flux.

There are two methods for the wire diameter of the primary, secondary and auxiliary windings:
Bare wire area
Primary Winding diameter: effective current
Iprms=Po/^n/Vimin
Iprms=63W/0.8/210V=0.375A
Wire diameter (J current density is 4A/mm2)

Wire diameter

wire diameter calculation

Use two 0.18mm diameter wires and wind them together, or use AWG #28 single stranded wire.
Secondary winding diameter

secondary side diameter

Use 4 wires with a diameter of 0.25mm to be wound in parallel and calculate the current skin depth:

skin depth

The wire diameter of multiple strands must be less than or equal to dwH. For single wire winding, if the wire diameter exceeds the dwH, it is necessary to consider the use of multiple strands.

The calculation of copper loss Pcu and iron loss Pfe (transformer total loss Ploss)
a) Primary winding and secondary winding losses. Among them, MLT is the average turn length of the magnetic core.

primary side and secondary side winding losses

 

b) Calculate the allowable total loss Ploss and iron loss under the efficiency η.

c) Find the actual loss under the operation according to the core loss curve.
Iron loss per unit weight, it actually occurred

actual Ploss

The actual iron loss should be lower than the allowable value.

d) Calculate the loss per unit area Φ=Ploss/As. If the temperature rise caused by the Φ value is less than 25 degrees, the design is good.
Bw Calculation:

Bw calculation

The working magnetic flux density Bw should be met the design index requirements, Bw<Bs-Br, to avoid saturation of the magnetic core.

 

Frequently Asked Questions about High Frequency Transformer Design

1. What is high frequency transformer?
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. ... For any given power rating, the higher the frequency, the smaller the transformer can be.

 

2. What are the design aspects of high frequency transformer?
Design of HF transformers. High frequency transformers transfer electric power. The physical size is dependent on the power to be transfered as well as the operating frequency. The higher the frequency the smaller the physical size.

 

3. What is the use of high frequency transformer?
These transformers are designed to handle up to 15,000 volts safely and accurately, converting high voltage and current levels between coils by magnetic induction. High Voltage, High Frequency Transformers are relied on for applications ranging from power supplies to laser equipment and particle accelerators.

 

4. What is difference between high frequency and low frequency?
When we talk about sound, we talk in terms of high and low-frequency waves. ... This measurement of cycles per second is expressed in Hertz (Hz), with a higher Hz representing higher frequency sound. Low-frequency sounds are 500 Hz or lower while high-frequency waves are above 2000 Hz.

 

5. What is the frequency of transformer?
What is Transformer Frequency. The three common frequencies available are 50Hz, 60Hz and 400Hz. European power is typically 50Hz while North American power is usually 60hz. The 400 Hz is reserved for high-powered applications such as aerospace and some special-purpose computer power supplies and hand-held machine tools.

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