A cycling and flow of knowledge and ideas.

Friday, 4 April 2014

Transformer, Electronic Transformer and Switch Mode Power Supply differences.

I have been looking at writing a post on electronic transformers due to a conversation I had with a friend.  I was explaining why I was changing my low voltage electronic transformer to a different version because I wanted to change from the original 20w 12v halogen light to an energy saving 5w LED which had roughly the same light output but a quarter the power consumption.  During the conversation it became apparent that my friend did not know what an electronic transformer was.  He incorrectly assumed that it was a switch mode power supply (SMPS). My friend is not a technophobe and was clearly unaware that there was a difference.  So in the following post I will describe what is the difference between a transformer, an electronic transformer and a switch mode power supply, specifically in regard to the 12v domestic options designed for home use.


What is a transformer and how does it work?

What is an electronic transformer?

Why does an electronic transformer create a higher frequency?

So, what is a Switch Mode Power Supply (SMPS) and how is it different from an electronic transformer?


What is a transformer and how does it work?

First I will describe the simplest device which is the transformer. I think it is important to understand the basics and what makes a transformer "transform." As its name suggests it is used to transform a voltage from one to another, such as: 240 v to 12 v or 120 v to 240 v.  The process is achieved by mediating the energy by way of a magnetic flux.  This sounds quite complicated but it is actually quite simple. 

"Any electric current," will cause a magnetic field as it travels along a conductor.  Note that it is the flow of current and not the voltage that generates this magnetic field. The strength of the magnetic field is proportional to the current flowing i.e. the greater the current the greater the strength of the magnetic field.
  •  In most day to day cases this field is so weak that it is generally ignored.  
It is possible to concentrate the magnetic field by simply winding the conductor in a loop and the weak flux is added together to make a stronger flux. For any consecutive winding, the current will be travelling in the same direction and so generates the field identically to its neighbouring loop. The more the loops the stronger the concentration of magnetic field. This concept is used as the basis for magnetic devices such as electromagnets, inductors, motors and transformers.  I think it's important to understand this point as we will come back to this concept several times throughout this post.

  • These summed up magnetic fields can be considered as a single entity called a Magnetic Flux. The magnetic flux is effectively stored energy. Without the appreciation that the magnetic flux is stored energy, magnetic devices just seem like magical and mysterious devices working by je ne se quoi.

  • Remember in physics, energy is neither created nor destroyed. This line will appear quite a lot in this post.

When current flows, the magnetic flux builds up and invisibly wafts around the coil.  When the current generating the flux stops the magnetic field will start to collapse.  The process of this collapse releases the stored energy and causes a current to flow in the coil in the same direction as the original flow.

  • The ability to generate current from a magnetic field is called, "induction."

  • As the flux is neither contained nor constrained, it will also try to induce a current in anything else that is conductive, as well as the original coil.

  • The building up of the flux takes its energy from the current flow through the coil. This storing of energy, as a magnetic flux, effectively resists the current flow, as though it is reluctant to allow the current to change. Equally, as the current wanes and the field starts to collapse, the energy is released back into the windings creating more current again as though their is a reluctance for a change to the current flow. This affect is known as, "reluctance."

    • Though an important consideration for the choice of transformer, it is a property more critical for SMPS.  I will describe this further later in the section  What is a Switch Mode Power Supply section.

To help concentrate this magnetic field, and stop it wafting around, a ferrite core or laminated soft iron core can be used to increase magnetic efficacy. This is due to a property of the core called its, "magnetic permeability." The choice of the core shape and the material it's made from is very dependent on many factors.  These factors include cost and designers are paid serious money to get this design requirement right, but it is a subject way beyond the limit of this post.

  • The second black and white video, embedded in this post, is an excellent explanation of some of the considerations relating to the core design in transformers.  The video is long, at roughly 27 minutes, but is one of the best I have found on Youtube. Don't be put off by the antiquated music, presentation or graphics as this video is bang on the money!

The action of generating a magnetic field and then collapsing it, is used as the basis of the transformation.  When the field collapses it does not really care what it starts to induce the current in and so it can be used to generate a current in a totally different coil to the originating coil.  In fact the process of generating a current is due to the changing flux and happens both at the time that it is created and built and again at the time it is collapsing.  The energy within the magnetic flux is the same but the number of turns in the first and second coils may be different and so it affects the coils differently. I will explain the significance of this an a bit.

The coil which has the current driven through it, is called the primary and the other coil which has the current induced by the changing field is called the secondary.  As it happens, the ratio of turns from the primary to the secondary coil is the inverse ratio to the induced current from the primary to the secondary.  As the power in the flux is the same before and after (remember, energy can neither be created nor destroyed) the voltage must be proportional to the primary and secondary turns ratio. This makes the maths so much simpler.

It is slightly more complicated than described but essentially it is good enough for the calculation and design of the majority of circuits using transformers. For a nice visual description please see the following video.

A transformer is considered a passive device as it does not have anything that can be said to actively change either the voltage or current.  The current has to be supplied by some external source.  In the case of a domestic low voltage lighting transformer that source is the mains.  The mains is supplied by magical means and arrives at your home as 240v at 50 Hz (120v at 60 Hz in North America).  Essentially this is connected via a fuse, or breaker, to the transformer.  The properties described above cause a magnetic flux to be generated in the primary and that energy in the flux is used to induce a lower voltage but higher current in the secondary.  So the turns ratio of the primary to the secondary is 20 to 1.  Thus the voltage goes from 240v to 12v in the same ratio.

  • One excellent advantage of using a transformer is that the primary and secondary coils can be kept physically separate and this is termed, "isolation." Important in so many safety considerations especially where there is a real chance of coming into contact with the electrics or where such an event would be particularly dangerous, such as in a swimming pool or in a bathroom.

  • Another feature is that the energy transfer is moderated by the amount of flux that can be held in the transformer's core. (Remember, energy can neither be created nor destroyed) The amount of energy in an iron core is not infinite and there is a limit.  The point this limit is reached is called, "magnetic saturation."

    • A transformer with an air core (i.e. nothing) cannot saturate but, then again, it is unlikely that the energy will be recovered as the air cannot contain nor constrain the magnetic flux as a well designed core can. Consequently air cores are not as efficient at power transformation but they do have desirable properties for other applications.

  • One disadvantage is that if the energy is not consumed by the secondary side load, then it will end up being driven back into the primary side. As energy can neither be created nor destroyed, it must go somewhere. This unused energy is the root cause of an effect called power factor.  

    • It is also the reason why trailing edge dimmers (the most common type) cannot be used if a transformer is fitted. The switching of the dimmer can cause the transformer to force back the energy through the dimmer, damaging the dimmer. 
  •  But it is only a disadvantage when it is not desired.  The switch mode supply uses this affect to great effect but I will describe that later.

Actually the domestic mains source is not magic but actually a series of distribution transformers which boost the voltage from the power station and then drop the voltage before it gets to you from the local substation.  The process of transmission is a true wonder to behold and the source of everything we consider to make us an advanced civilisation.  As you can imagine the energy required is massive and so the flux needed to transform the voltage and current is also enormous.  To get the scale of energy transfer requires some awesomely large transformers making up the power grid infrastructure.  (Remember, energy can neither be created not destroyed). Considerations such as transmission losses govern the voltage at each stage of the journey from the power station to your door. Again sadly, this is beyond the limit of this post.

Here is a videos of the operation of transformers which explain the concept very well.

What is an electronic transformer?

An electronic transformer is exactly the same as a normal transformer but rather than using the frequency supplied by the distribution company of say 50 Hz the device generates its own frequency which is usually in the range of 20 to 100 KHz.  The electronic transformer uses the same transformer concept of mediating the conversion via magnetic flux and transforms the voltage based on the same turns ratio.  The generation of this high frequency is achieved electronically hence the name, "Electronic Transformer." As the input voltage is the same, the output voltage is dependent on the turns ratio of the transformer.  Rather than mediating the flux at the mains frequency it mediates substantially more frequently using its own built in oscillator.  The output is at the oscillator frequency of say 50 KHz but as the mains is supplied at 50 Hz it would look like a modulated output of 50 KHz with the shape of the 100 Hz as an envelope to the 50 KHz fundamental frequency.

  • This is not strictly true as the frequency of the oscillator is dependent on the load, but it is close enough. The faster it oscillates the more power it will deliver but the voltage is fundamentally defined by the turns ratio.

Why does an electronic transformer create a higher frequency?

Remember, the energy in the transformer is in the magnetic flux and it only creates a current when it is changing. For an ordinary mains transformer supplied by a standard mains cycle of 50 Hz that means that the transfer of energy happens 100 times a second (twice per cycle, for each positive and negative change in current).

  • If your frequency is fixed then to get more power you need more flux and consequently a larger transformer.  This is why distribution transformers, such as in your local substation, are very large.

The alternative way is to transfer the energy more frequently.  So rather than 100 times a second (at 50 Hz) you may use 100000 times a second (at a frequency of 50 KHz).  As you are transferring energy more frequently per second of time, you don't need such a physically large transformer for the same power handling.  This is the core concept of the electronic transformer and the reason for its relatively small form factor when compared to an equivalent passive transformer.  In fact, most of the space and weight of the electronic transformer is the supporting electronics both generating the high frequency and electrical noise filtering circuitry rather than the actual output transformer used to transform the voltage and current.

One advantage of a smaller transformer is that the energy stored in it, as flux, is small and so the unused energy, potentially fed back into the mains, is also small.  This coupled with some filtering and suppression allows the electronic transformer to have a small power factor. In the region of 95 ~ 99 %.

I will describe the operation and workings of an electronic transformer in a following post.

So, what is a Switch Mode Power Supply (SMPS) and how is it different from an electronic transformer?

This is an interesting question as the electronic transformer uses switching to generate the high frequencies used in the transform process.  A switch mode supply can also use transistors to switch but it tends to do this for the purposes of regulation rather than just as a supply to a transformer. The term regulation means to keep the voltage (or sometimes current) constant irrespective of the current (or voltage) drawn by the load. The transformer and the electronic transformer use the energy in the magnetic flux to transform the voltage and current from the primary to the secondary.  In the electronic transformer, the value of the transformed voltage and current is defined by the turns ratio while the power available is defined by the frequency.

The switch mode does something much cleverer.  The switch mode is able to vary the voltage (and current) independently of the input voltage and current.  A feature which is essential for successful regulation.  This is achieved by using another property of the magnetically stored energy as flux within a wound component.

In the earlier explanation of transformers I stated that a disadvantage was that if used with a trailing edge dimmer the collapsing magnetic flux can back feed and blow up the dimmer.  This is because the flux energy has nowhere to go and it can't just vanish. Remember energy can neither be created nor destroyed.  The trailing edge dimmer works by going open circuit partway through the mains cycle but the energy stored in the transformer must go somewhere.  The result is that the voltage climbs in the transformer sufficiently to back-drive the dimmer, thus damaging the dimmer. But this property is what allows an SMPS to vary its output voltage. Though not strictly examples of a switch mode power supply, here are some examples of where this property to ramp up the voltage due to a collapsing field is used throughout the world, closer to you than you may think.

  • This exact property is used to start an old fashioned fluorescent light.  The energy is built up through the ballast inductor and the starter circuit trips open.  The field collapses in the ballast inductor and causes the voltage to rise sharply, as the ballast tries to conduct the trapped flux energy. This sudden voltage rise is used to generate a spark in the fluorescent tube, known as a strike. That is the initial flash that allows the fluorescent to work.

    • The ballast inductor has a dual role. The main role of the ballast inductor and the reason it is called a, "ballast," is to limit the current flowing through the fluorescent tube once the tube has started to conduct.  As mentioned earlier in the transformer section, the inductor has a property called reluctance which hinders and moderates the current flow though it. As the reluctance is caused by the generation and release of flux energy it does not consume power in itself. Without this ballasting of current the fluorescent tube, directly connected to the mains, would probably explode catastrophically.
  • The same technique is used in a petrol engined car.  The battery is only capable of generating 12v so the high voltage used for the spark plug is generated using a switching technique.  The magnetic flux is built up in the ignition coil which is a transformer.  The transformer has a turn ratio of roughly 1 turn on the primary for every 100 turns of the secondary.  This should only result in a secondary voltage of 1200v if used in a standard transformer configuration.  The voltage need to generate a spark is roughly 20 000v nearly 20 times greater.

    Once the flux has built up in the transformer a contact opens and causes an open circuit.  The magnetic flux collapses releasing the energy back into the transformer windings but it cannot conduct as it's open circuit.  The result is the voltage rises to try and bridge the gap on the primary side so it can shed the energy.  On the primary side, this voltage would rise to nearly 240 volts.  As it's a transformer, if the voltage is 240v on the primary it would be roughly 24 000v on the secondary and thus sufficiently high to generate a spark (through the distributor) at the correct spark plug. So the energy is eventually shed as a spark on the secondary side.

Similarly, the switch mode power supply uses this change in voltage by storing the energy magnetically and releasing it to allow the desired voltage to be reached. By changing the time the switch is open or shut changes the voltage output.  This subtle changing of the switching times is called pulse width modulation.  It is using the energy stored in an inductor or transformer to vary the output voltage. As the energy is stored and release in a wound component there is little energy loss in the voltage conversion.  Switch mode supplies require much more sophisticated electronics to monitor the output voltage and precisely vary the switching frequency accordingly, to accurately change the voltage.

Unlike the comparatively crude method used in a car or the old fashioned fluorescent light, the switch in a SMPS is not an electromechanical device but a precisely controlled solid state component such as a power transistor. The result is the same, that the inductive component has energy built up as flux and then it is disconnected and isolated forcing the collapsing flux to raise the voltage to shed the energy.  When this method is used with a transformer, the turns ratio governs the main drop and the switching allows for the subtle regulation. This is the method found in most computer or laptop Power Supply Units ( PSU).

An SMPS does not need to even use a transformer as this property is common to all inductive components allowing a cheaper inductor to be used. A transformer is still used where isolation from the primary driving circuit is required such as for electrical safety regulations.

A raise in voltage is termed as a "boost" or "step up".  The opposite of a, "boost" is called a, "buck" or "step down." Buck converters are usually found inside laptops or incorporated into the computer's motherboard.  Effectively the hard work is done by the PSU and the onboard SMPS provides the plethora of different voltages required by processors, memory and peripherals.

A buck, or a drop in voltage, is achieved in a slightly different way.  Usually an SMPS will use a different property which is the property of the inductor to try and maintain a constant current. As briefly touched on in my explanation of transformers this property is called reluctance.  It is where the inductor appears to want to maintain a constant current flow.

An electronic transformer may vary the frequency but only for the increase in power consumption as the output is governed by the turns ratio and input voltage. Unlike the switch mode power supply the wave form is also symmetrical.


The power transformer is a passive component and converts the voltage dependent on the turns ratio.  For more power you need a larger transformer.  The Electronic transformer similarly converts voltage dependent on the turns ratio but if you want more power you have the option to either increase the switching frequency or get a larger transformer. A switch mode power supply changes both the switching frequency and the duty cycle of the switching pulse to achieve the desired voltage and power but at a whole level of complication greater than either the electronic transformer or the power transformer.

For further reading and references: please see my resource page

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