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Home > Technologies > Power
> Build Your Own Transfomer

Build Your Own Transfomer

Mar 10, 2015
Eric Christensen
Electronic
Design

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7

When the right off-the-shelf part just can’t be found, you may want to
consider designing a custom transformer to fit your specific
requirements.

Thousands of ready-made off-the-shelf transformers out in the market are
available to be designed into electronic or electrical circuits. However, when
the right part just can’t be found, you may want to consider designing a custom
transformer to fit your specific requirements.

Download
this article in .PDF format
This
file type includes high resolution graphics and schematics when
applicable.

You need to determine a few things before you can start to design a
transformer, though. At the minimum, these include the input voltage(s) and
frequency, and the output voltage(s) and current(s). There may very well be
other parameters to consider, both physical and electrical, such as available
space for mounting, mounting style, isolation requirements, leakage currents,
etc. Environmental conditions may also be a consideration.

Note: At this step, it’s important to point out that all of the
decisions made concerning the design will be verified through calculations that
can only be made after many “rules of thumbs’” and “educated guesses” are used
to get us to that point.

Selecting the Core

The first step is to determine the type of core for the design. You should
consult with a core manufacturer to obtain the specific characteristics and
power-handling capabilities for each type and size of core. However, a general
starting point is:

• When less than 400 Hz, a silicon steel lamination is generally used.

• At 400 to 2000 Hz, consider a tape-wound or nickel-alloy core.

• Above 2000 Hz, look at ferrite.

Remember this is only a guideline; it’s not uncommon to go outside of these
ranges (e.g., audio transformers can use silicon steel laminations and operate
from 20 to 20,000 Hz). There are many other core types, and many sizes, shapes,
and material grades within the cores listed above. The exact core chosen may
depend on board spacing, location, mounting style, or any of a number of
physical and electrical parameters that only you can decide.

Related

White
Paper: Current-Sense Transformer Application Design Guidelines

Program
Quickly Calculates Transformer Design Parameters

Mind
The Gap And Improve Your Low-Power Flyback Transformer Design

Most core types will also need a winding bobbin to fit the core that you
choose, and possibly assist in the mounting of the finished product. Make sure a
sufficient bobbin style and material is available, and that you have all of the
mechanical measurements to determine winding details later on in the design.
Certain cores don’t require a bobbin, but we’ll save those for another
discussion.

After selecting a core and bobbin, you need to calculate the correct number
of primary turns needed using formula 1 or 2 (see “Basic Design Formulas,”
below). There are several variables to consider when using these formulas;
you will need to consult the core manufacturer’s data for answers to
specifications such as flux density and stacking factor.

Basic Design Formulas

1) N(p) = (V x 108) / (4.44 B A f K) sine wave

2) N(p) = (V x 108) / (4 B A f K) square wave

3) N(s) = V(s) / V(p) x N(p)

4) I(p) = (VA(s) + losses) / V(p)

5) I(s) = P(out) / V(out)

6) Open circuit voltage (Voc) = N(s)/N(p) x V(p)

7) Loaded voltage (Vld) = Voc-[IR(sec) + (IR(pri) x (N(s)/N(p))]

8) Temp rise (T(C)) = (losses/(0.008 x surface area))

N(p) = Primary turns

N(s) = Secondary turns

B = Flux density in gauss

A = Core area in centimeters squared

f = Frequency

K = Stacking factor

L = Inductance

A(l) = Inductance per turn squared for a given core

Windings and Wire

The primary winding current and wire size needs to be determined. The primary
current will be equal to the total output power plus transformer power losses,
divided by the primary voltage. For power losses, I start at a 10% increase in
the input power, assuming a 90% efficient transformer. For example, a
transformer with a 12-V, 2-A output at 120 V input would be:

12 V x 2 A = 24 VA; 24 VA x 1.10 (110%) = 26.4 VA needed in the primary
winding;
26.4 VA/120 V = 0.22 A in the primary
winding

The next step will be a subject for debate and adjustment depending on the
transformer characteristics: I generally start at approximately 500 circular
mills (cm) per amp to choose the starting wire gauge. This number may be smaller
for small transformers, and larger for large power transformers; that decision
is again up to the designer. Using the example above, 0.22 A x 500cm/A = 110cm;
I would start with a 29 gauge wire (127.7cm) for the primary.

You now need to determine the number of turns that will be required for each
secondary winding. The first step is to use formula 3 (N(s) = V(s) / V(p) x
N(p)) to determine the turns for a perfect transformer. This number then needs
to be increased to account for the losses in the coils. As a rule of thumb
again, I start with a 10% increase in the number of turns, assuming a 90%
efficient transformer: N(s) x 1.10 = N Turns. This percentage will vary
depending on the characteristics of your design. Use the same method to
determine the secondary wire gauge(s) that you used for the primary.

Now you need to see if the windings fit into your winding area and determine
the actual losses of the coils. Start by determining the winding width (ww) of
the bobbin, the winding height (wh) of the bobbin, and the mean length of turns
(mlt) from the mechanical drawing (see the figure).

With this information, you can now calculate how many turns of wire can fit
in each layer of winding for each different winding. After that, determine how
many layers of each wire gauge used it will take to wind the calculated number
of turns for each given winding. Finally, calculate the total height of all of
the windings wound concentrically. Remember to include the insulation used
between the windings in figuring your total winding height.

Verification

The next step is to verify your design. Will it fit in the allowable winding
height with enough “play” to allow for error? Keep in mind that the windings
will not be perfectly layered, so you must allow for some bowing of the copper
wire as it bends around each corner of the bobbin. I use a maximum of 85% fill
(or build) when comparing the actual winding height to the allowable bobbin
winding height:

(Actual winding height/Allowable winding height) × 100 = < 85%

A build of around 75% is more desirable, and will make the transformer easier
to manufacture in the long run.

Next, you need to calculate the resistance of each winding, and the loaded
voltage losses in the winding to determine loaded output voltages. First, the
resistance of each winding is calculated by multiplying the mean length of turn
by the number of turns. This gives the total length of the wire conductor.
Whatever unit of measure you are using, convert this length to thousands of feet
(K ft); for example, a length of 400 in./12000 = 0.03 K ft. Now from a magnet
wire chart, you can get resistance of each gauge of wire in thousands of feet.
For our example, 29 gauge has a resistance of 81.21 Ω/K ft, so a 400-in. wire
would have a resistance of 0.03 K ft × 81.21 Ω /K ft = 2.44 Ω.

Once you have the resistance of a winding, you simply calculate the voltage
drop across that winding by multiplying the resistance by the current in the
winding. In this primary it would be 2.44 Ω x 0.22 A = 0.540 V. Calculate the
voltage drop across each winding in the same manner.

The next step is to calculate the open-circuit voltage of each secondary
using formula 6 and the loaded voltage using formula 7 of (see “Basic Design
Formulas”). I put these together because I usually do both calculations on
each secondary winding before moving on to multiple windings if I have them.

The key thing to note here is that the primary voltage drop is reflected into
the secondary loaded voltage by the ratio of the turns. After that, it’s added
to the secondary voltage drop. The sum of the two is then subtracted from the
open circuit voltage of the given secondary. This gives you the loaded output
voltage of that secondary (I realize the formula could be mathematically
simplified, but this format allows me easier use with a calculator; again, my
opinion).

If the voltage is not what you want, divide the desired output voltage by the
calculated output for an error ratio. Use this ratio multiplied by the number of
turns on the given secondary and recalculate everything starting at the number
of layers of winding.

Temp Calculations

After calculating the turns, you need to know the calculated temperature
rise. There are two main causes of temperature rise in a transformer: core power
losses and winding power losses. To determine the core power losses, refer to
manufacturer’s datasheets and the flux density used in your design. Winding
power losses are easily calculated by multiplying the voltage drop across the
winding again by the current in the winding (I2R). The sum of the
power losses in the windings, primary(s), and secondary(s) is multiplied by 1.33
(I’ll be honest, I don’t fully know where this factor came from, but it has to
do with heat transfer and concentric windings, and it works) to give the total
effective heating losses of the windings. Add the core losses to the winding
losses for total power dissipation.

To determine how well the transformer dissipates power losses, we need to
calculate the surface area of the completed device. This simply requires looking
at all of the surfaces that will be exposed to air and adding them up in inches
squared. Now use formula 8 from the design formulas to calculate the estimated
temperature rise of the transformer.

Download
this article in .PDF format
This
file type includes high resolution graphics and schematics when
applicable.

Once again, what constitutes an acceptable temperature rise depends on the
application and the designer.  I always use 50ºC as my maximum rise
allowed. Keep in mind that forced air cooling or heat sinks may be used in the
end product, which could push that number higher.

Now that you’ve made all of your decisions on the design aspects, based on
“rules of thumb” and “educated guesses,” you should be able to confirm the
choices made using the formulas and calculations discussed in the article. If
things anywhere along the way don’t work, you have to go back, make the
appropriate changes, and start over. This includes the temperature-rise
calculation.

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Discuss this Article 7

jeffl

on Mar 16, 2015

You haven't included anything about determining the correct amount of air gap
in the core to allow, if you don't have one for a power transformer you may get
saturation and unacceptable losses, for a signal transformer this will manifest
itself as unacceptable distortion at certain frequency and power levels. I think
you're trying to vastly oversimplify a field of design that it takes most people
an entire career to master.

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engineer_bill

on Mar 16, 2015

But if you include an air-gap, inductance will drop ... requiring more turns
to restore a reasonable "excitation current" in the primary. It's generally
better to just use more core material to avoid saturation. The only transformers
that normally use air gaps are those that have substantial DC current in primary
or secondary ... so they're generally used only in switch-mode power supplies or
single-ended signal (audio) transformers. All else being equal, they'll have
poorer low-frequency response. Also, any transformer that operates near core
saturation (as do most 50/60 Hz power transformers are to make them cheap) will
also radiate a substantial external field ... making them troublesome neighbors
for nearby equipment. Designing transformers is truly "a world of trade-offs."
Bill Whitlock, Technology Manager, Jensen Transformers, Inc.

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steph

on Mar 17, 2015

I have a line isolation TRF (<500W with @16cm2 core) and wanna modify it
to fuse rods of small diameter (max. 10mm2). Do you know how to workout the
current to fuse metals and at what voltage?

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palefish

on Mar 16, 2015

I'd love to meet you guys for lunch once a week, to kick this subject
around.

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to post comments

Dennis
F

on Mar 16, 2015

" I think you're trying to vastly oversimplify a field of design that it
takes most people an entire career to master."

Profoundly true, and it takes some courage to write in one article a
transformer design guide. A few additional hand-waves:

1. To optimize transfer power, full core utilization is the goal. The core
should be run at both its power-loss and saturation limits (with design
margins). Then the smallest volume core can be used. Transfer power applies to
coupled inductors, but the same basic utilization criteria apply to
transformers.

2. To optimize power transfer, the winding and core power losses should be
about the same. The goal is to maximize power transfer through the transductor
(transformer or coupled inductor) by adjusting the design for winding to core
power loss such that output power / input power is maximized. This happens
neither at equal core and winding loss nor at minimum loss but in-between the
two.

Core and winding losses can be traded off by adjusting turns and wire size.
However, circuit design constraints also affect both. Wire size is fairly easy
to choose from a wire table and circuit current requirements. The optimal turns
is the geometric mean between the max-power-loss minimum and the saturation
maximum.

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andrewt

on Mar 17, 2015

the pdf available for download seems to be corrupted.
The start of each
page has omissions.
It stops after the table on page 4.

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to post comments

nicka

on Mar 17, 2015

Andrew: copy and paste into word doc works great.

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