Electrostatic and Electromagnetic Shields in a Transformer

Article by Mike Horgan, Applications Engineer at Butler Winding

As high frequency switching power supplies, operating in the hundreds of kHz, have replaced 60Hz linear power supplies high frequency electrical noise on power lines has become more of a problem. In noise sensitive applications such as medical measuring equipment, audio gear, and telecommunication systems, an electrostatic shield can be used in the transformer to reduce high frequency electrical noise.

Fig. 1: An Electrostatic foil shield with attached
ground wire & primary winding.

An electrostatic shield can be used in a transformer to reduce the common mode noise and transients. The shield consists of one winding placed between the primary and secondaries. Typically it is a one turn insulated copper foil with foil thicknesses around 0.005” and with a foil width equal to the bobbin width, see picture 1. One end is connected to ground via the green wire and the other end of the winding left as an open circuit. Similarly a single layer winding of relatively fine wire like #36 can be used as an electrostatic shield with one end of the winding connected to ground and the other end left as an open circuit. The use of an electrostatic shield for medical transformer applications is increasing.

An electrostatic shield suppresses noise by using its capacitive coupling. A transformer winding is very inductive and presents a high resistance to any high frequency noise inhibiting current flow. By installing an electrostatic shield a capacitive network is introduced allowing this high frequency noise current to flow to ground. A typical electrostatic shield gives about 60dB of common mode noise attenuation over the frequency range of 100Hz to 1MHz. Adding a second shield between the secondary and the core may increase this. Additionally a separate common mode inductor can be used to provide more noise attenuation.

Fig. 2: An Electromagnetic enclosure (or can)
along with an assembled ferrite pot
Core Transformer.

 
While an electrostatic shield is used to attenuate high frequency conducted electrical noise an electromagnetic shield can attenuate radiated noise. An electromagnetic shield is typically a metal enclosure that envelopes the component or circuits being shielded. Picture 2 shows an electromagnetic shield for a ferrite pot core transformer. In addition to enclosing a transformer in an electromagnetic shield selecting a self shielding transformer core shape like the pot core as opposed to an EE core shape or a ferrite rod will help attenuate radiated electrical noise.

The Challenge of Reverse Engineering an Obsolete Military Power Transformer" Article by Mike Horgan

Unfortunately the wound
toroidal inductor broke into
several pieces during disassembly.
Read about our challenge

Our Engineering & Quality Manager, Mike Horgan, recently wrote an article on a reverse engineering project we completed.  The article is titled, "Reverse Engineering an Obsolete Military Power Transformer".  This project was challenging since there was incomplete and outdated documentation for us to work from including a lack of detail concerning the current.  Tobyhanna Army Depot worked to find all of the old documentation (possibly 30 years old) to help us custom design and recreate this transformer assembly.  Many necessary details were left out - making this quite the challenge.

Here is an excerpt from the article Mike wrote:

"...Butler winding recently completed a reverse engineering effort for the Tobyhanna Army Depot on what we initially thought was a multi-tapped power transformer in an encapsulated box with connectors. We were given a non-functioning assembly of an EMP Electronics Inc. part (see photo 1) and some system level drawings but nothing specific about the assembly. When we disassembled the unit we realized this was more than just one transformer."   Read more...

Practical Considerations of Common Mode Inductors

Common mode inductors, more generally known as chokes, are used to filter unwanted electrical noise usually caused by the switching action of switch mode power supplies.  Specifically it filters out common mode noise or noise that is common to both the positive and negative outputs of a dc power supply, for example.  The main idea behind a common mode inductor is that you put two symmetrical windings on a magnetic core, typically a ferrite toroid core, and mount it a plastic header, see figure 1. The windings are connected so the current flow will be in equal but in opposite directions.  This keeps the core from saturating since it will have two equal but opposite magnetic fields and the full inductance of the core will help attenuate the switching noise common to both lines, see chokes/common mode inductors.

The most common core shape used in a common mode inductor is the toroid. It has two big advantages compared to other shapes. First, if you think of it as a winding on one half of a toroid opposing a winding on another half of a toroid you have two very symmetrical shapes which helps prevent core saturation. Secondly, a toroid has no mating surface compared to other core shapes so it will have more inductance and is usually 35% higher in inductance compared to a similar sized two piece core shape. The two windings on the toroid need to be as symmetrical as possible since leakage inductance will be created by the current flow. Leakage inductance can start to saturate the core which will cause the common mode inductance to decrease. An unequal number of turns on the two windings or an unsymmetrical winding shape can increase leakage inductance resulting in reduced inductance leading to reduced filtering of the noise. For more details see the theory of common mode inductors.

Typically common mode inductors handle currents ranging from tens of mA to about 20A. This means wire gauges from #38 to #12 are commonly used. Currents much above 20A are usually not wound on a toroid since the core can break due to winding stress associated with heavy gauge wire. Other core shapes and unique windings are usually used above 20A. Also multiple inductors in parallel could be used. High permeability ferrite materials are very susceptible to winding stress and inductances can be down to 50% of the expected inductance due to the winding stress on the core. Usually heating the wound cores slowly relaxes the stress and brings the inductance back up. I emphasize that the heating should be ramped up and down slowly since ferrite is a ceramic and can crack due to rapid temperature changes. 

One of the more surprising aspects of toroidal common mode inductors, or toroids in general, is that they are wound automatically, not by hand on toroidal winding machines.

written by Mike Horgan
Engineering Manager
at Butler Winding

Clamp on current transformers using permanent magnets

A clamp on current transformer, or a current clamp, is a common piece of test equipment used to measure current flow in a primary cable without making physical contact with the cable. A toroidal shaped current transformer is generally regarded as the best choice for a core shape, but a frequent dilemma is how to connect the transformer without disconnecting the cable. A spring loaded current transformer with a cut toroid shape is often employed to solve this problem. Another approach is to use two permanent magnets to clamp the split toroid back together.

We recently manufactured some split core current transformers using this idea. First an epoxy coated silicon iron tape wound core is cut in half, permanent magnets are glued to the core mating surfaces along with an alignment bracket, and a plastic hinge is taped on the toroid halves, see figure

1. Two secondary windings are wound onto the core, see figure

2. The windings are connected in series so the current from the two windings adds together. A final inductance test is performed at 100mV/1kHz to ensure the windings are connected correctly. Inductance measures about 15.5mH on properly connected parts and 9.9mH on parts with the connections reversed. A test limit of 12.5mH minimum is used. More insulation is added to help isolate the primary and secondary windings and a power cord is also assembled, see figure

3. These current transformers were coated bright yellow to make them easier to locate, see figure

4. The inside diameter of this current transformer is greater than 4.5” so current flow in very large cables can be measured.

Written by Mike Horgan
Engineering Manager
Butler Winding

Testing Transformers used in Switch Mode Power Supplies

Most electrical equipment is powered by a switch mode power supply. Switching the power at high frequency, 25kHz to 250kHz typically, reduces the size and cost of the power supply. One of the key components in a switching power supply is the transformer. The transformer consists of a ferrite core set, two or more coils wound on a coil former or bobbin, lead wires or printed circuit pins, and a clip or some other means of holding everything together. Butler Winding manufactures custom high frequency transformers and tests 100% of them to make sure they meet all design criteria.

Almost all our transformers are tested on a Voltech AT3600 transformer tester which is calibrated yearly. A four wire Kelvin connections is made to make certain that the voltage and current measurements are made as close as possible to the device under test commonly referred to as the DUT, see figure 1. The first test performed on the DUT is either continuity or resistance. Continuity makes sure the DUT is connected correctly and test fixturing or test leads are correct. Resistance testing additionally measures winding resistance which ensures the correct wire gauge has been used.

Figure 1: A Voltech AT3600 automated transformer tester with a 200W, 50kHz ferrite
EE core transformer under test connected with Kelvin flying test leads.

The second test on the DUT is series inductance. This makes sure the correct type of ferrite material has been used and the correct number of turns has been wound on the bobbin. The test conditions this measurement is made at must be carefully selected. The voltage must be chosen so that the resulting magnetic field density within the ferrite core is between 5 – 10 Gauss. The test frequency should be relatively low, like 1kHz – 10kHz, so you are well below the self resonance frequency of the DUT.
Based on their application, some transformers have an intentional air-gap in the ferrite core. It is common to have an air-gap on the center leg of a ferrite E core set to ensure the core does not saturate if it is operated in a unidirectional application like a switching flyback transformer. If the DUT has an air-gap then leakage inductance is commonly measured. This makes sure the gap is the correct size, 0.001” to 0.040” typically, and that the windings are positioned correctly.

Turns ratio testing is performed on the DUT by applying a signal to one winding and measuring the transformed signals on all the other windings. Polarity is also tested to make sure the windings were wound is the correct direction; this is commonly referred to as the transformer dot convention.

Since isolation is often a transformer requirement, HIPOT testing either AC or DC is performed on the DUT. Voltages range from 100V to 5kV. The high voltage is applied across winding to winding or winding to core and the resultant leakage current is measured. A maximum current limit is set commonly 250uA.

Figure 2: EFD15 size test fixture to which facilitates fast transformer testing. The operator inserts the transformer, clamps it down, and pushes one button to test it. A green/red light switch indicates the pass/fail results.

Test fixturing is used for rapid 100% testing of all transformers. Both thru-hole and surface mount transformers are tested. See figure 2 for our recently tooled EFD15 surface mount test fixture. All test data is saved under its part number and manufacturing date. Special tests beyond what was discussed here can be made. Contact Butler winding to discuss further.

Written by Mike Horgan
Engineering and Quality Manager at Butler Winding

Can a Pulse Transformer Solve World Hunger?

Small electric pulses, processed by a
pulse transformer, stimulate growth

One of our customers sells a system to stimulate plant growth.  The stimulation is accomplished by passing small electric pulses through the ground.  These pulses stimulate the plant roots to grow faster and a larger.  This more robust root network allows the plant to grow faster.  It is more complicated than just electrically stimulating the roots; maximum performance is when the pulses of electricity vary over a wide range of frequencies – processed by a pulse transformer.

The problem our customer brought to us was that the waveform was not the correct shape, and the electrical efficiency of the system was poor.  Since this is a product that is in the market, the customer’s requirement included a limitation on the physical size.  It could not increase because the customer committed to keeping the same outer dimensions.

The restrictions on physical size, tight waveform criteria, and the wide range of operating frequencies made us rethink the entire design.  The first step was to change to a high efficiency ferrite core.  This did two things; it allowed the system to operate at a high level of electrical efficiency and the physical size was actually reduced.  Yes, sometimes the best things come in small packages.  The windings were carefully calculated to ensure that the core did not saturate.  Core saturation would cause poor efficiencies and also variable and unpredictable performance.

The end result was a pulse transformer with a ferrite core that does a superb job in a smaller package.  This made the customer, and lots of plants, very happy.

Written by
Denny Wist
President of ButlerWinding

Ideal Transformer – Fact or Dream?

We all have dreams.  You may dream of the perfect vacation, winning the lottery or just having a wonderful day in the sunshine.  My dream is a lot more farfetched.  I dream of the ideal power transformer.

Like all dreams, my dream is about as likely to occur as finding the “ideal” man or woman.  A lovely concept but never actually experienced by anyone.

The ideal transformer is a physics concept that refers to perfect magnetic coupling where there is 100% efficiency in the transfer of power between the primary and the secondary windings.   The physics equation for an ideal transformer is;

The ratio voltage of the primary (Vp) to the voltage of the secondary (Vs) is the same ratio as the number of turns on the primary (Np) to the number of turns on the secondary (Ns).

A transformer that takes the primary voltage and increases it is called a “step up” transformer because the secondary voltage is higher than the primary because it has more turns on the secondary.  Likewise, a transformer that has fewer turns on the secondary (compared to the primary) is called a “step down” transformer because the voltage is reduced.

So if you want the voltage to step down by exactly 50% then you need 50% of the windings on the secondary compared to that of the primary.  If only it were this simple.  Apparently God has a keen sense of humor and decided that if things were this easy then all of us would pass physics and that would mean that there would be rampant unemployment in the ranks of teachers and teaching assistants!

There are two factors that interrupt the perfection of an ideal transformer; imperfect coupling and core or power losses.  In order to achieve perfect coupling, all of the magnetic flux produced by the primary must be transferred and captured by the transformer core.  One problem that quickly arises is simply a matter of geography.  The shape of the windings and the core would have to be infinitely close in order to have all of the lines of flux completely cut through the core.  While the transformer and core manufacturers struggle mightily to come up with shapes that maximize the coupling, it is impossible to get all of the flux transferred into the core.  Stray flux lines radiate in all directions thus reducing the efficiency of energy transfer from the primary to the core.

Even if we could get perfect flux transfer from the primary to the core, the core has issues itself.  The purpose of the core is for it to be a conduit of flux to the secondary.  The core absorbs flux form the primary and then by producing a magnetic field for the secondary, creates a voltage in the secondary.  Over the years better and better core materials have been developed to reduce power losses but we still are far from perfection.  Even tiny imperfections know as grain boundaries in the crystals that make up the core slow down the flux and resist its movement.  This resistance becomes heat.  If you want a real life experience of this, Just touch any transformer under power and you will see that they range from warm to very hot (depending on the efficiencies).

I will continue to dream of the ideal transformer while the rest of the world dreams of more achievable goals like world peace or the perfect enchilada.

Written by Denny Wist

President of Butler Winding