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Ferrite Core & Power capability 1
- Thread starter zacky
- Start date
- Thread starter
- #21
Possibly the preferred way is to epoxy everything together into one solid lump. Another method might be to place a clamping band right around the outside periphery of both U cores, rather like a giant hose clip. That could also be used to secure some appropriate mounting feet to the core.
Or maybe be a couple of flat plates with corner holes holding the whole thing together in compression with some long screws or all thread. Many possible ways to do it, but nothing really neat unfortunately. I have even used large rubber bands on a prototype.
One difficulty with the large ferrite U and I cores is the lack of commercial bobbins to fit the cores. That may have changed, but finding something onto which the windings can be placed is another difficulty to be solved.
Or maybe be a couple of flat plates with corner holes holding the whole thing together in compression with some long screws or all thread. Many possible ways to do it, but nothing really neat unfortunately. I have even used large rubber bands on a prototype.
One difficulty with the large ferrite U and I cores is the lack of commercial bobbins to fit the cores. That may have changed, but finding something onto which the windings can be placed is another difficulty to be solved.
zacky,
For your battery charger you will almost certainly need a current limiter measuring the DC output current. A hall-effect transducer or a shunt resistor and diff. amp are the conventional ways to do this. For simplicity the hall effect device from, say, LEM would be the easiest to interface to.
For joining large U cores together with minimal air gap, cyanoacrylate adhesive - superglue - has very low viscosity which will not add an appreciable air gap to the core design. Thicker adhesives may do this and cause the performance to deviate from the design expectations. Obviously adhesives do not make for a good solution when prototyping!
Aluminium angle and stainless steel band clamp may be a possibility for holding it all together. Thin rubber sheet fitted between the angle and the ferrite prevents the ferrite suffering compression fractures. I've never seen commercial hardware for these cores. Do not use standard steel hardware.
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One day my ship will come in.
But with my luck, I'll be at the airport!
For your battery charger you will almost certainly need a current limiter measuring the DC output current. A hall-effect transducer or a shunt resistor and diff. amp are the conventional ways to do this. For simplicity the hall effect device from, say, LEM would be the easiest to interface to.
For joining large U cores together with minimal air gap, cyanoacrylate adhesive - superglue - has very low viscosity which will not add an appreciable air gap to the core design. Thicker adhesives may do this and cause the performance to deviate from the design expectations. Obviously adhesives do not make for a good solution when prototyping!
Aluminium angle and stainless steel band clamp may be a possibility for holding it all together. Thin rubber sheet fitted between the angle and the ferrite prevents the ferrite suffering compression fractures. I've never seen commercial hardware for these cores. Do not use standard steel hardware.
----------------------------------
One day my ship will come in.
But with my luck, I'll be at the airport!
- Thread starter
- #24
Yes, that's what I would do. The mating faces of the ferrite are machined to a very high standard to give a near-zero air gap when assembled. I found the cores were dimensionally not particularly accurate. Tolerances of about +/- 0.5mm on nominal was about typical. Your clamping arrangement needs to accomodate this, hence the suggestion to use rubber sheeting or similar. Tolerances may have improved over the past few years, I simply don't know.
----------------------------------
One day my ship will come in.
But with my luck, I'll be at the airport!
----------------------------------
One day my ship will come in.
But with my luck, I'll be at the airport!
Agree with all of the above comments, but a very small controlled deliberate airgap can sometimes actually have advantages even in forward converters. With zero airgap in a push pull forward converter for example, there can be problems with staircase flux saturation where the push and the pull are not identical. Flux doubling at turn on from stored remnant flux may also create a massive uncontrolled inrush current spike unless very fast current limiting is incorporated.
Current mode control will solve a lot of these problems.
While not a complete solution by itself, a very small deliberate airgap will considerably soften the saturation characteristic of the core material, and add just a little bit more safety for very high power operation. It may not always be needed or even a good idea, but it is something to think about.
Epoxy adhesive if heated, and the parts tightly clamped will squeeze out extremely thinly. The cyano "super glues" have been known to give up and let go after a time, and may not be reliable. They are excellent for holding parts together while the epoxy cures.
Current mode control will solve a lot of these problems.
While not a complete solution by itself, a very small deliberate airgap will considerably soften the saturation characteristic of the core material, and add just a little bit more safety for very high power operation. It may not always be needed or even a good idea, but it is something to think about.
Epoxy adhesive if heated, and the parts tightly clamped will squeeze out extremely thinly. The cyano "super glues" have been known to give up and let go after a time, and may not be reliable. They are excellent for holding parts together while the epoxy cures.
"may not always be needed or even a good idea"
pace maker equipped persons need not apply.![[bugeyed]](/data/assets/smilies/bugeyed.gif "[bugeyed] [bugeyed]")
Yes, by the way at HP I made up to 16 layer printed circuit boards that were epoxy/pressed with thin insulator sheet that when complete were just standard 0.0625" thickness so I concur you can make epoxy thin!
pace maker equipped persons need not apply.
Yes, by the way at HP I made up to 16 layer printed circuit boards that were epoxy/pressed with thin insulator sheet that when complete were just standard 0.0625" thickness so I concur you can make epoxy thin!
hehehe, I well know that conventional transformers are NOT supposed to have an airgap, but in some circumstances it CAN be advantageous. Like any design work, all the factors need to be considered, and very slightly gaping a transformer definitely falls into the sneaky tricks department.
![[hairpull2]](/data/assets/smilies/hairpull2.gif "[hairpull2] [hairpull2]")
It also reduces remnant flux. With absolutely no gap, (ferrite toroid?) when you turn off the current, there can remain some residual magnetism in the core. With a high permeability material this remnant flux can be considerable. The BH loop can sometimes look almost rectangular.
If you then switch it back on in the same direction, the core can violently saturate causing severe distress in the switching transistor. A small gap makes the residual remnant flux fall to a much lower level at turn off. This really helps to stabilize the flux operating point around the centre of the BH curve.
Unfortunately it also reduces primary inductance, and increases the magnetising current. But there ain't no free lunch.
Some of these nasty saturation effects can be why a new prototype can seem to work perfectly, then on odd infrequent occasions spontaneously go bang with no apparent obvious cause. I strongly suspect is a contributing factor to why the switchmode guys invariably get premature grey hair.
If you then switch it back on in the same direction, the core can violently saturate causing severe distress in the switching transistor. A small gap makes the residual remnant flux fall to a much lower level at turn off. This really helps to stabilize the flux operating point around the centre of the BH curve.
Unfortunately it also reduces primary inductance, and increases the magnetising current. But there ain't no free lunch.
Some of these nasty saturation effects can be why a new prototype can seem to work perfectly, then on odd infrequent occasions spontaneously go bang with no apparent obvious cause. I strongly suspect is a contributing factor to why the switchmode guys invariably get premature grey hair.
- Thread starter
- #31
They certainly CAN be extremely reliable, provided they are designed properly with the correct insight. The magnetics needs to be designed to suit the topology. High frequency magnetics is an absolutely fascinating field of study, and experience is all.
Some of the very early thirty year old switcher designs even by such venerated and illustrious companies as HP were far from reliable. But these days far more is known.
Very high power designs are not the sort of thing a beginner should cut his teeth on. But switchmode power supplies are these days just about universal, and the reliability and efficiency is excellent.
Some of the very early thirty year old switcher designs even by such venerated and illustrious companies as HP were far from reliable. But these days far more is known.
Very high power designs are not the sort of thing a beginner should cut his teeth on. But switchmode power supplies are these days just about universal, and the reliability and efficiency is excellent.
ChaosIllumination
Automotive
Hey Warpspeed,
You piqued my interest talking about series trigger transformers. I'm just dabbling with a little hobby project to try to control a short arc xenon lamp that I have in some rare strobe lights, albeit quite a bit lower power than your novel monster (150W rated lamp, 100A peak flash current).
My idea is to replace the crude capacitor discharge circuit with an IGBT PWM current mode control so that I can control the current (brightness) and repeat rate. I was thinking of using the secondary of the trigger transformer to be the inductance and to control the current through it and therefore the lamp. I've used the original trigger transformer (5mH secondary) and successfully struck the arc and maintained 1A constant current at a 20KHz PWM freq. So 'all' I have to do is get 100 times more current capability!
The current pulses I want may peak 100A, but the average power has to be kept low for the lamp not to overheat. Am I correct in thinking that if I wind my own transformer I can use thinner than 100A rated wire for the secondary as it's not continuous current? But at the same time won't I have to calculate the core size so that it doesn't saturate based on 100A? I need a much lower inductance than original so that I can ramp the current up rapidly, but at the same time I need the secondary turns ratio to work as a trigger transformer. As I'm a relative newbie to all this I seem to be going in circles knowing where to start. I'm not sure if I'm trying to design a transfomer or a DC choke, or if it's even possible!
Oh, and I thought popular ferrite cores were generally below a few hundred watts rating and not that they could be stacked, so that's two things I've learned here today...so thanks.
You piqued my interest talking about series trigger transformers. I'm just dabbling with a little hobby project to try to control a short arc xenon lamp that I have in some rare strobe lights, albeit quite a bit lower power than your novel monster (150W rated lamp, 100A peak flash current).
My idea is to replace the crude capacitor discharge circuit with an IGBT PWM current mode control so that I can control the current (brightness) and repeat rate. I was thinking of using the secondary of the trigger transformer to be the inductance and to control the current through it and therefore the lamp. I've used the original trigger transformer (5mH secondary) and successfully struck the arc and maintained 1A constant current at a 20KHz PWM freq. So 'all' I have to do is get 100 times more current capability!
The current pulses I want may peak 100A, but the average power has to be kept low for the lamp not to overheat. Am I correct in thinking that if I wind my own transformer I can use thinner than 100A rated wire for the secondary as it's not continuous current? But at the same time won't I have to calculate the core size so that it doesn't saturate based on 100A? I need a much lower inductance than original so that I can ramp the current up rapidly, but at the same time I need the secondary turns ratio to work as a trigger transformer. As I'm a relative newbie to all this I seem to be going in circles knowing where to start. I'm not sure if I'm trying to design a transfomer or a DC choke, or if it's even possible!
Oh, and I thought popular ferrite cores were generally below a few hundred watts rating and not that they could be stacked, so that's two things I've learned here today...so thanks.
Hi Chaos,
Your idea of usng a PWM system to control lamp current directly should certainly work, once the initial lamp discharge arc has been initiated. Running a lamp in "simmer mode" from a current source at low current is a very effective way to maintain a continuous lamp discharge at extremely low power.
This can most easily be done with a voltage source of several Kv dc, and a large high power series wire wound resistor. That could be quite independent of your PWM, and it may be easier to do it that way. The source impedance needs to be high enough to get a stable simmer action, otherwise getting it to start up may be problematic.
The lamp itself will have a very high negative resistance, and you need enough series impedance (positive resistance) to get a stable simmer current.
I doubt if the same inductor could be used for both trigger and pwm averaging, because the inductance/current requirements are quite different. The main PWM inductor will need to be air cored and maintain a reasonably constant inductance value over a very wide operating current range.
An independent series trigger transformer can then be placed between the main PWM inductor and the lamp. This need be only a couple of turns looped through several very high permeability toroids.
The idea here is that the trigger transformer will generate a very high voltage, with extremely fast rise-time, but from perhaps only a few, or tens of millijoules of trigger power. As soon as real lamp current begins to flow through the trigger transformer, the ferrite toroids massively saturate. The very high initial permeability and no gap will guarantee total saturation at perhaps a few hundred milliamps or less, and above that, the trigger transformer will have almost negligible inductance. Once the xenon lamp is conducting, the inductance of the trigger transformer effectively disappears from the circuit.
For your PWM inductor, consider a flat air cored pancake coil of rectangular section copper transformer wire. Several flat coils can be ganged in series for higher inductance.
A xenon lamp simmering with a single very long fine filament of plasma is a fascinating thing to see. The arc will twist and corkscrew from reaction to local magnetic fields. Rather like one of those decorative plasma ball displays.
I think you will find this is sure to be a fascinating and most memorable project.
Your idea of usng a PWM system to control lamp current directly should certainly work, once the initial lamp discharge arc has been initiated. Running a lamp in "simmer mode" from a current source at low current is a very effective way to maintain a continuous lamp discharge at extremely low power.
This can most easily be done with a voltage source of several Kv dc, and a large high power series wire wound resistor. That could be quite independent of your PWM, and it may be easier to do it that way. The source impedance needs to be high enough to get a stable simmer action, otherwise getting it to start up may be problematic.
The lamp itself will have a very high negative resistance, and you need enough series impedance (positive resistance) to get a stable simmer current.
I doubt if the same inductor could be used for both trigger and pwm averaging, because the inductance/current requirements are quite different. The main PWM inductor will need to be air cored and maintain a reasonably constant inductance value over a very wide operating current range.
An independent series trigger transformer can then be placed between the main PWM inductor and the lamp. This need be only a couple of turns looped through several very high permeability toroids.
The idea here is that the trigger transformer will generate a very high voltage, with extremely fast rise-time, but from perhaps only a few, or tens of millijoules of trigger power. As soon as real lamp current begins to flow through the trigger transformer, the ferrite toroids massively saturate. The very high initial permeability and no gap will guarantee total saturation at perhaps a few hundred milliamps or less, and above that, the trigger transformer will have almost negligible inductance. Once the xenon lamp is conducting, the inductance of the trigger transformer effectively disappears from the circuit.
For your PWM inductor, consider a flat air cored pancake coil of rectangular section copper transformer wire. Several flat coils can be ganged in series for higher inductance.
A xenon lamp simmering with a single very long fine filament of plasma is a fascinating thing to see. The arc will twist and corkscrew from reaction to local magnetic fields. Rather like one of those decorative plasma ball displays.
I think you will find this is sure to be a fascinating and most memorable project.
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