With a gapped core almost 100% of the energy is stored in the air gap. When a transformer is used as a true transformer, the flux in the core does not change appreciably with load current, since the primary ampere•turns are cancelled by the secondary ampere•turns. As power increases, for a given frequency, the core size must increase to allow enough window area to fit windings heavy enough to handle the currents. Extra windings are not used to discharge leakage inductance. Leakage inductance is there because of imperfect magnetic coupling between the windings that are already there. You might split a winding for better coupling to reduce leakage inductance. A reset winding is used in a forward converter where there is no means of discharging the energy stored in the magnetizing inductance. Normally you don't fill a bobbin fully end-to-end because of the need for safety margins for "creepage and clearance." Triple insulated wire can be a boon, though an expensive one, because the insulation removes the need for the c & c allowance, letting you use the full bobbin. Typically it is used either for the primary or the secondary(s), but not often both. A copper foil external shorting band is normally used only on a gapped transformer, which means only (in general) for a flyback converter. I've designed 1 kW switchers. I'd hang my head in shame if they were as horrible as only 80% efficient. I've achieved much better than that overall in designs with active power factor correction (so the overall circuit is essentially a specialized boost converter followed by a half-bridge). It *is* difficult to get really high efficiency with very low output voltage due in large part to diode losses. Synchronous rectifiers can be a big help there. Ignoring the skin effect issues, wire size is based on RMS current, not peak or average, though of course they all go together, but the ratios depend on the waveforms. I don't think the wire shown as litz wire actually is, but it's hard to tell.. It looks like it is merely a large bundle of fine strands each of which is individually insulated. In true litz wire the design is much more complex and the strands are arranged to that all of them "spend the same time" at the available positions in the bundle. That is, a conductor will move from the centre of the bundle out to the periphery and then back toward the center. This isn't necessarily done for each and every individual conductor, though that is often the case for small litz wires, but for small bundles of conductors. Litz wire is expensive, nice to wind because it is very flexible, and hard to terminate because each strand is insulated. Copper foil for windings has some significant benefits, but like everything else in a switcher there are compromises that must be evaluated. I've used it in several designs. If you go to a contract manufacturer for transformers they may prefer using parallel strands of magnet wire because their equipment is set up to handle it well. A copper foil external shorting band is normally used only on a gapped transformer, which means only (in general) for a flyback converter.
Hi. Thanks for taking the time to watch all the videos and make clear and concise comments. Its difficult to go into detail in videos while keeping them clear and simple for beginners but also accurate for advanced people. My aim is that the video serves as a introduction for beginners, while more advanced people can correct any mistakes I make in the comments, so thanks again. However, I am confused about one of your points : "When a transformer is used as a true transformer, the flux in the core does not change appreciably with load current, since the primary ampere•turns are cancelled by the secondary ampere•turns. As power increases, for a given frequency, the core size must increase to allow enough window area to fit windings heavy enough to handle the currents."...This sounds like a transformer such as forward converter will never enter saturation because the flux is always canceled...is that correct?
@@ElectricMonkeyBrain You most definitely can saturate the core in true transformers. Acting essentially in parallel with the transformer is the primary as a simple inductor. When you apply voltage to the primary winding the current in it ramps up according to its inductance, independent of any emery being transferred to the secondary. If there is insufficient inductance the flux can build to the point where the core saturates. Keep in mind that if the input voltage to a forward converter were fixed and the output voltage were fixed, the duty cycle would be constant, regardless of load current, once the output inductor started operating in continuous current mode, ignoring losses (typically you'd set the output filter inductance to make the transition form discontinuous to continuous current at somewhere in the range of about 20% to 40% of the full rated output current of the supply). Of course losses do increase with increasing load current so the duty cycle must increase a bit to compensate. Typically what you would do is set the number of turns for the primary winding high enough so that the maximum volt-second product you would apply wouldn't ramp the magnetizing current up to the saturation point. When the converter is operating normally, the volt•second product will be essentially constant with the output inductor in CCM. However, things get messy if you have a wide input voltage range. Normally you still have constant volt-second product when the converter is regulating, but "large signal" perturbations can result in transient increase. You have to be careful. Converters that employ cycle-by-cycle peak current limiting can allow less conservative design with fewer primary turns (lower primary inductance). The current limiting ends any switching cycle that might saturate the core before saturation can happen, assuming, of course, that it is properly designed. Without such current limiting you might be forced to use the maximum possible ON time and the maximum possible instantaneous input voltage to determine how many turns you need for the primary so that you can be sure, not matter what happens, you won'd drive the core to saturation. (the current limiting can't distinguish between over current due to load current or due to the core approaching saturation, but that's OK) Generally you don't want any more turns in any winding than you really need since you have to fit the wire and consider the resistive losses. Sometimes you do need to adjust the number of primary turns to get an integer number of secondary turns because half turns for the secondary are not easy. This is more likely to occur if you are designing for a very low voltage output where the total winidng might be only 2 or 3 turns. Forward converters bring another saturation problem. A forward converter operates in only one quadrant of excitation of the core. You apply voltage to the primary and store some energy in its inductance. You want to get all of that energy out and drive the core back to essentially zero residual magnetism. If you don't do that, each successive cycle will drive the residual magnetism higher and higher until the core saturates. There is no inherent path to get that energy out - it has nowhere to go on the primary side and it can't go into the secondary circuit because of the diode. This is where the "reset winding" comes in. It allows for the energy stored in the inductance of the primary to be discharged, "resetting" the core. Typically the reset winding has the same number of turns as the primary and the maximum duty cycle is limited to 50%. That assures that there is sufficient time to discharged the stored energy in the worst-case condition. There are ways to allow higher duty cycle but care is required. Many control ICs don't allow easy precise control of maximum duty cycle - you can get nearly 50% easily or nearly 100% easily, but something like 70% might be difficult with any consistency unit-to-unit. Even with converters that operate in two flux quadrants (push-pull, bridge, half-bridge) you can get into trouble with staircase saturation if some persistent imbalance in volt-seconds between the two working quadrants arises. Switchers are horrible beasts! There are all sorts of opportunities for things to go wrong. I often describe switchers as something in which everything is in conflict with everything else.
Thanks a lot, that much more clear. I will pin this comment. I will also ask for your advice if I decide to design a SMPS in the future, if you dont mind of course. While I have your attention I would like to ask you a different question....do you know of any chips/micro-controllers suitable for controlling multiple switches? I have a little research project where I need to control the phase between two full bridges driving resonant transformers. I have done this using analogue chips with some limited success but it always contains some nonlinearity in frequency and pulse width control, further complications occur when I want to introduce pulse packets, like interruption or amplitude modulation. Then I moved to an Arduino which also worked, but the min pulse width is limited because of the internal computation which is going on. Know any chips suitable for this? thanks@@d614gakadoug9
Absolutely wrong: With a gapped core almost 100% of the energy is stored in the air gap. No, it depends on the magnetic path to gap ratio. Using a gap in a transformer core is a bad practice.
Hello, there is no way to message you, but I want to ask one question which prevents me from designing my own SMPS. It's the frequency. Do I just pick the frequency myself or the core needs to be compatible at my frequency or how would I choose it? Can you help me? Thanks in advance
What is described here is not a transformer at all, it is a multi-winding inductor. Inductors store magnetic energy. For transformers storing magnetic energy is calamity.
The tape shown is polyimide (Kapton is a well-known brand name). The yellow tape _is_ polyester (Mylar is a well-known brand name). The yellow is normally just from the adhesive. Polyimide is a lot more expensive than polyester.
With a gapped core almost 100% of the energy is stored in the air gap.
When a transformer is used as a true transformer, the flux in the core does not change appreciably with load current, since the primary ampere•turns are cancelled by the secondary ampere•turns. As power increases, for a given frequency, the core size must increase to allow enough window area to fit windings heavy enough to handle the currents.
Extra windings are not used to discharge leakage inductance. Leakage inductance is there because of imperfect magnetic coupling between the windings that are already there. You might split a winding for better coupling to reduce leakage inductance. A reset winding is used in a forward converter where there is no means of discharging the energy stored in the magnetizing inductance.
Normally you don't fill a bobbin fully end-to-end because of the need for safety margins for "creepage and clearance." Triple insulated wire can be a boon, though an expensive one, because the insulation removes the need for the c & c allowance, letting you use the full bobbin. Typically it is used either for the primary or the secondary(s), but not often both.
A copper foil external shorting band is normally used only on a gapped transformer, which means only (in general) for a flyback converter.
I've designed 1 kW switchers. I'd hang my head in shame if they were as horrible as only 80% efficient. I've achieved much better than that overall in designs with active power factor correction (so the overall circuit is essentially a specialized boost converter followed by a half-bridge). It *is* difficult to get really high efficiency with very low output voltage due in large part to diode losses. Synchronous rectifiers can be a big help there.
Ignoring the skin effect issues, wire size is based on RMS current, not peak or average, though of course they all go together, but the ratios depend on the waveforms.
I don't think the wire shown as litz wire actually is, but it's hard to tell.. It looks like it is merely a large bundle of fine strands each of which is individually insulated.
In true litz wire the design is much more complex and the strands are arranged to that all of them "spend the same time" at the available positions in the bundle. That is, a conductor will move from the centre of the bundle out to the periphery and then back toward the center. This isn't necessarily done for each and every individual conductor, though that is often the case for small litz wires, but for small bundles of conductors.
Litz wire is expensive, nice to wind because it is very flexible, and hard to terminate because each strand is insulated.
Copper foil for windings has some significant benefits, but like everything else in a switcher there are compromises that must be evaluated. I've used it in several designs. If you go to a contract manufacturer for transformers they may prefer using parallel strands of magnet wire because their equipment is set up to handle it well.
A copper foil external shorting band is normally used only on a gapped transformer, which means only (in general) for a flyback converter.
Hi. Thanks for taking the time to watch all the videos and make clear and concise comments. Its difficult to go into detail in videos while keeping them clear and simple for beginners but also accurate for advanced people. My aim is that the video serves as a introduction for beginners, while more advanced people can correct any mistakes I make in the comments, so thanks again. However, I am confused about one of your points : "When a transformer is used as a true transformer, the flux in the core does not change appreciably with load current, since the primary ampere•turns are cancelled by the secondary ampere•turns. As power increases, for a given frequency, the core size must increase to allow enough window area to fit windings heavy enough to handle the currents."...This sounds like a transformer such as forward converter will never enter saturation because the flux is always canceled...is that correct?
@@ElectricMonkeyBrain
You most definitely can saturate the core in true transformers.
Acting essentially in parallel with the transformer is the primary as a simple inductor. When you apply voltage to the primary winding the current in it ramps up according to its inductance, independent of any emery being transferred to the secondary. If there is insufficient inductance the flux can build to the point where the core saturates.
Keep in mind that if the input voltage to a forward converter were fixed and the output voltage were fixed, the duty cycle would be constant, regardless of load current, once the output inductor started operating in continuous current mode, ignoring losses (typically you'd set the output filter inductance to make the transition form discontinuous to continuous current at somewhere in the range of about 20% to 40% of the full rated output current of the supply). Of course losses do increase with increasing load current so the duty cycle must increase a bit to compensate.
Typically what you would do is set the number of turns for the primary winding high enough so that the maximum volt-second product you would apply wouldn't ramp the magnetizing current up to the saturation point. When the converter is operating normally, the volt•second product will be essentially constant with the output inductor in CCM. However, things get messy if you have a wide input voltage range. Normally you still have constant volt-second product when the converter is regulating, but "large signal" perturbations can result in transient increase. You have to be careful. Converters that employ cycle-by-cycle peak current limiting can allow less conservative design with fewer primary turns (lower primary inductance). The current limiting ends any switching cycle that might saturate the core before saturation can happen, assuming, of course, that it is properly designed. Without such current limiting you might be forced to use the maximum possible ON time and the maximum possible instantaneous input voltage to determine how many turns you need for the primary so that you can be sure, not matter what happens, you won'd drive the core to saturation. (the current limiting can't distinguish between over current due to load current or due to the core approaching saturation, but that's OK)
Generally you don't want any more turns in any winding than you really need since you have to fit the wire and consider the resistive losses. Sometimes you do need to adjust the number of primary turns to get an integer number of secondary turns because half turns for the secondary are not easy. This is more likely to occur if you are designing for a very low voltage output where the total winidng might be only 2 or 3 turns.
Forward converters bring another saturation problem. A forward converter operates in only one quadrant of excitation of the core. You apply voltage to the primary and store some energy in its inductance. You want to get all of that energy out and drive the core back to essentially zero residual magnetism. If you don't do that, each successive cycle will drive the residual magnetism higher and higher until the core saturates. There is no inherent path to get that energy out - it has nowhere to go on the primary side and it can't go into the secondary circuit because of the diode. This is where the "reset winding" comes in. It allows for the energy stored in the inductance of the primary to be discharged, "resetting" the core. Typically the reset winding has the same number of turns as the primary and the maximum duty cycle is limited to 50%. That assures that there is sufficient time to discharged the stored energy in the worst-case condition. There are ways to allow higher duty cycle but care is required. Many control ICs don't allow easy precise control of maximum duty cycle - you can get nearly 50% easily or nearly 100% easily, but something like 70% might be difficult with any consistency unit-to-unit.
Even with converters that operate in two flux quadrants (push-pull, bridge, half-bridge) you can get into trouble with staircase saturation if some persistent imbalance in volt-seconds between the two working quadrants arises.
Switchers are horrible beasts! There are all sorts of opportunities for things to go wrong. I often describe switchers as something in which everything is in conflict with everything else.
Thanks a lot, that much more clear. I will pin this comment. I will also ask for your advice if I decide to design a SMPS in the future, if you dont mind of course.
While I have your attention I would like to ask you a different question....do you know of any chips/micro-controllers suitable for controlling multiple switches? I have a little research project where I need to control the phase between two full bridges driving resonant transformers. I have done this using analogue chips with some limited success but it always contains some nonlinearity in frequency and pulse width control, further complications occur when I want to introduce pulse packets, like interruption or amplitude modulation. Then I moved to an Arduino which also worked, but the min pulse width is limited because of the internal computation which is going on. Know any chips suitable for this? thanks@@d614gakadoug9
Absolutely wrong: With a gapped core almost 100% of the energy is stored in the air gap. No, it depends on the magnetic path to gap ratio. Using a gap in a transformer core is a bad practice.
Hello, there is no way to message you, but I want to ask one question which prevents me from designing my own SMPS. It's the frequency. Do I just pick the frequency myself or the core needs to be compatible at my frequency or how would I choose it? Can you help me? Thanks in advance
well done, thank you. Great detailed explanation without all the math, very practical.
glad you liked it
Interesting video to someone like me relatively new to, or not that deep in this subject. Thanks!
glad you liked it
Very interesting video. Does the copper tape around the edge to prevent EMI actually need to be connected to earth?
12:43 they will also use a single winding that doesn't connect to anything inside the transformer and the only connection is to ground.
yes I think you are right. There really is lot of different techniques used in transformers.
Great explanation thank you
Great BASICS! :)
Are pot cores less sensitive regarding the placement of the primary coil close to the center as in 7:48 ?
I would guess the answer is yes, but the difference is probably small....so the primary is always best near the core center.
Very nice video. Can you show in future videos the topology of transformer and how to winding them? Thanks
sure, i will try
10:28 this allows for better magnetic coupling.
correct
Why don't all these experts in the comments make their own videos? You did just fine.
What is described here is not a transformer at all, it is a multi-winding inductor. Inductors store magnetic energy. For transformers storing magnetic energy is calamity.
Yellow transformer tape is mylar iirc
lol. No ones perfect I guess. Thanks for the correction.
The tape shown is polyimide (Kapton is a well-known brand name). The yellow tape _is_ polyester (Mylar is a well-known brand name). The yellow is normally just from the adhesive. Polyimide is a lot more expensive than polyester.