Friday Quiz: Transformers and Inductance has an interesting set of questions about the important components in nearly every power supply. Try to answer them and check how many of your answers you get correct. It could show that you have some misconceptions on transformers. You most propably get learning experience from correct answers.
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Tomi Engdahl says:
The Top 3 Reasons for Transformer Failures
https://www.electronicdesign.com/power/top-3-reasons-transformer-failures?NL=ED-003&Issue=ED-003_20180822_ED-003_789&sfvc4enews=42&cl=article_1_b&utm_rid=CPG05000002750211&utm_campaign=19411&utm_medium=email&elq2=6d46f91a84cf4744afdd2ac8c79ca2d9
The culprits are all sins of omission: a lack of craftsmanship, high-quality materials, and good design.
Tomi Engdahl says:
Build Your Own Transformer
https://www.electronicdesign.com/power/build-your-own-transformer
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.
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.
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.
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
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.
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).
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.
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.
Tomi Engdahl says:
Build Your Own Transformer
https://www.electronicdesign.com/power-management/article/21800646/build-your-own-transformer?PK=UM_Classics05120&utm_rid=CPG05000002750211&utm_campaign=31628&utm_medium=email&elq2=0de882419b384085bdb63685fedfb499&oly_enc_id=7211D2691390C9R
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.
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.
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.
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.
assuming a 90% efficient transformer. For example, a transformer with a 12-V, 2-A output at 120 V input
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.
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
Now you need to see if the windings fit into your winding area and determine the actual losses of the coils.
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:
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.
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.
The next step is to calculate the open-circuit voltage of each secondary
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
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.
Tomi Engdahl says:
Autotransformer followup – Inductance
https://www.youtube.com/watch?v=LAOJRjrkwWE
Tomi Engdahl says:
A transformer designed to be suitable for application can be used. It possibly needs to be big and expensive.
Transformers are passive components that are designed to work in alternating current. They will not pass DC through, but will not be damaged by some amount of DC fed to them (how much depends on transformer design).
“if the amplifier output is in direct current (signal >=0) you risk overloading the core and causing it to melt.”
If large amount of DX is fed to transformer, it can saturate the transformer core. DC will not melt the core. If you feed too much DC, it will not melt iron or ferrite core, but the current if it is too much can overheat the coils.
When transformer core is saturated, the transformer works almost like there were no core in there (very low impedance for any AC signal).