How difficult is it to design a broadband transformer?

[hobbs125]

Hello everyone,

I am working on a project in which I need to make a broadband/pulse transformer.
The transformer will operate in the 20-80kHz range.

The transformer will use about 20 watts of power.
I need a turns ratio of 1:3.
20V input 60V output
It's just for hobby use. I don't have the money to pay an engineer to design it and it does not have to be perfect, I just need to get a good square wave on the secondary side.

I have tried to make this type of transformer in the past but I find it very difficult to get a good square wave on the secondary side. I usually end up getting AC on the secondary side. I know this could be due to a number of factors such as coil capacitance, core material, or leakage inductance.

So, my question is, how difficult is it to make a broadband/pulse transformer when one only has basic electronics knowledge?

What general rules should I go by?

Could anyone here offer any suggestions or advise...Is it even worth my time to try to figure out?

 

[Bobbywhy]

Transformers operate with AC voltages, normally sinusoidal waves pass through them. This means the voltages/currents change gradually and smoothly. A square wave changes instantaneously (or nearly) from low to high state and then the reverse. Transformer primary and secondary coils are big inductors with iron cores and will not pass a square wave from input (primary)side to output (secondary) side. Inductors, by their nature, resist any instantaneous change in current. Apply a square wave to the primary, expect a sine wave (or, at least not square) wave out. Your own results already show this.

You must re-think your design.

 

[Carl Pugh]

High voltage pulse transformers are difficult to design. Low voltage pulse transformers are usually easier to design.
If you would describe what you are trying to do, someone may be able to help you.

Pulse transformers if correctly designed will give a nearly square or rectangular output.

Are you sure that you require a pulse transformer? Pulse transformers are usually specified as pulses per second (PPS) and have a pulse width, input and output voltage and current, load resistance, and a maximum droop.

 

[hobbs125]

The transformer I need requires both a square wave input and output. It is different than a pulse transformer in the fact that it operates at a much lower frequency (20-80kHz).

What I need is coil that will output a decent square wave.
The coil will be used to charge a capacitor so I need a square wave that will not swing negative.
I am going to add a resistor to the secondary side to limit charging current.So, input voltage and current
20V @ 1A max
Output voltage and current
60V @ 300mA max
Turns ratio 1:3

My first question is, how do I determine the type of core material I need?, and how do I determine the secondary inductance I need?

 

[SunnyBoyNY]

Transformers operate with AC voltages, normally sinusoidal waves pass through them. This means the voltages/currents change gradually and smoothly. A square wave changes instantaneously (or nearly) from low to high state and then the reverse. Transformer primary and secondary coils are big inductors with iron cores and will not pass a square wave from input (primary)side to output (secondary) side. Inductors, by their nature, resist any instantaneous change in current. Apply a square wave to the primary, expect a sine wave (or, at least not square) wave out. Your own results already show this.

You must re-think your design.

This is false. Once a voltage is applied across a transformer winding, magnetic flux starts increasing in the entire core, producing voltage on all other windings. Voltage is the time derivative of magnetic flux divided by number of turns. Since materials are not perfect, their do not possesses infinite permeability. Thus, a small magnetizing current exists in the core with non-zero magnetic flux.

Isolated converter topologies such as flyback, half-bridge, full-bridge, etc. are all based on square-voltage waveforms.

What you need to do is to calculate the positive volt-seconds during one cycle of the primary windings, select a core size, calculate N1 of turns based on allowable flux density, determine other turns, and allocate window area to different windings.

Make sure you use a high-frequency magnetic material such as 3c94.

Transformer size is limited only by the required winding current.

SunnyBoy

 

[SunnyBoyNY]

High voltage pulse transformers are difficult to design. Low voltage pulse transformers are usually easier to design.
If you would describe what you are trying to do, someone may be able to help you.

Pulse transformers if correctly designed will give a nearly square or rectangular output.

Are you sure that you require a pulse transformer? Pulse transformers are usually specified as pulses per second (PPS) and have a pulse width, input and output voltage and current, load resistance, and a maximum droop.

What's difficult about pulse transformer design? The only problem is the usual high required turns ratio that allows quite large core flux leakage and thus leakage inductance proportional to magnetizing inductance.

Also, 80 kHz is a standard frequency for power converters.

 

[Carl Pugh]

Why do you require a square wave?

Using resistors when charging capacitors reduces efficiency.

There are many circuits that place DC on a transformer secondary. But a simple transformer will only have AC on it's secondary.

At these power levels, a flyback transformer is a good approach.
Flyback transformers are fairly complicated to design. If you cannot find instructions on designing them, I will look through my files to see if there is any information on designing flyback transformers in them.

Google may have some useful information if you search for "battery charger ic"

Ferrite is usually used for cores at this frequency.

 

[hobbs125]

Carl,

I need a square wave so I can charge the capacitor fully before it discharges.

Also, your right about resistive charging.

I wonder if it's possible to add the current limiting inductor to the same transformer core. This would reduce the needed secondary turns since the indcutor would double the output voltage and act as a secondary coil as well. Then I would not have to make a separate inductor.

Does anyone know if you could add an additional coil on the same transformer core to act as a current limiting inductor?

 

[SunnyBoyNY]

The coil will be used to charge a capacitor so I need a square wave that will not swing negative.

The volt-second balance must be maintained on all windings. Otherwise, the voltage component would have a DC component and your transformer will quickly saturate.

What you can do is to use a rectifier secondary topology for non-flyback converter:

1) Half-wave
2) Full-wave
3) Center-tapped
4) Current doubler

Flyback converter simply needs a rectifying diode on the secondary side that block voltage during the primary device on-time.

To charge a battery you could use any of those. However, the current is not limited in any of those topologies (they contain an inductor/inductors that allow current-mode control) so a resistor might be your only choice to reasonably limit transformer/device current.

 

[SunnyBoyNY]

Does anyone know if you could add an additional coil on the same transformer core to act as a current limiting inductor?

There are ways of adding inductive element on the same core; however, those are not easy to implement.

An inductor will limit the current only in current-control mode. In such a mode the duty ratio is dynamically changed to maintain constant input/output current.

 

[hobbs125]

SunnyBoy,

Thanks for the comments thus far, I really appreciate them.

So, if I used a 50% duty cycle could I get a constant output current?

Are there any examples I could look up where an inductive element is used on the same core as the transformer? That is something I have had interest in for a while but had no luck finding any information on. Even if I don't utilize it or it's too complicated for me I would like to know understand how it works.

 

[skeptic2]

Carl,
I need a square wave so I can charge the capacitor fully before it discharges.

Can you explain this comment further? Why would a square wave fully charge a capacitor better than any other waveform?

So, if I used a 50% duty cycle could I get a constant output current?

Why would the duty cycle determine whether the output current is constant or not?

Trying to obtain a square wave output will reduce your efficiency significantly. I can think of two relatively easy ways of getting a square wave but they both are rather inefficient.

1. Put a relatively high value resistor in both the primary and secondary circuits. This reduces the effect of the inductance on the bandwidth thus increasing it.

2. Drive the core into saturation which will result in a clipped output waveform.

In both cases I think you would be better off using a sine wave. Perhaps you can explain why you don't like this solution.

 

[Enthalpy]

A transformer always gives zero as a mean DC output. To charge a capacitor, what you need is a diode.

Adding a series inductor to a transformer is very easy. Just separate the primary from the secondary winding and you get a "leakage inductance" which has been used in the past to produce a constant current from a transformer.

BUT people try hard to REDUCE this inductance! If you have one, you can forget your square wave, as this one will be in series with the load.

In fact, a difficulty of broadband transformers is to minimize the leakage inductance be putting the windings close (which means: split the secondary to wind the primary between the half-secondaries, or split them even further) AND minimize the stray capacitance between them, which increases when you put them closer.

The product of the stray cpacitance and the leakage inductance is limited by the winding's size, the speed of light, the relative permittivity.

As a core material at 100kHz, it will be ferrite.

 

[AlephZero]

I need a square wave so I can charge the capacitor fully before it discharges.

I think you are misunderstanding something here. If you connect a capacitor and an inductor, you have made a tuned (resonant) circuit, which will defeat any attempt to drive it with a square wave.

But without seeing the whole circuit, we are guessing what you are realliy trying to do.

 

[hobbs125]

Here's a diagram of what I have so far. The diode will prevent any AC from forming between L and C.

So far from the feedback I have received here (Thanks SunnyBoy) I need to:
-Calculate volt-seconds/Webers of primary coil during one cycle
-Select a core size and material
-Calculate Primary turns based on allowable core flux density
-Determine secondary turns and window area
-Ensure volt-second balance is maintained on all windings
I definitely have some studying and math to do, but it seems possible. I always like a new challenge to learn and build something!

I still would like to have more information on adding the current limiting inductor to the same core, but I have not found any instances where it's utilized.

Is there anything else I should consider in the design?

 

[SunnyBoyNY]

A couple of things to do in case you would like to proceed with a flyback converter:

* You can ditch the diode in series with the BJT.
* Replace the BJT with a MOSFET (100V / 10A should suffice)
* Output diode must be fast-recovery diode. SiC carbide diode should do wonders.
* The primary winding should not be shorted by the diode but have an RCD snubber instead. Place a resistor in parallel with a capacitor between the cathode of your diode and the upper winding node. R & C depend on leakage inductance and switching frequency.
* A flyback converter does not need an output inductor. Its output impedance is large enough not to cause excessive currents when in current-control mode.
* You can search for a flyback driver chip. Its datasheet should contain a detailed description on how to measure and limit the input current, which is a scaled version of output current.

With regards to the current limiting inductor: I suggest you take this learning experience one step at a time. Only once you have designed a functional prototype of a current-limited flyback converter, then you should proceed to a different kind of converter. Converters with inductors on the secondary (or primary) are called direct converters and are generally more complex than indirect converters such as flyback converter.

Btw. the flyback converter transformer is not really a transformer as much as a coupled inductor because the core stores converter energy. An air-gap will be needed.

SunnyBoy

 

[Carl Pugh]

I agree with everything that SunnyBoyNY says.
The only thing that might be added is that polarity markings should be added to the flyback transformer. If the flyback transformer is connected with incorrect polarity, the circuit will not work.

 

[cabraham]

A flyback xfmr is indeed a xfmr. It is also a pair of coupled inductors as well. A transformer is merely 2 inductors with a high degree of coupling. They are 1 & the same thing, so I don't know how one can say it can be 1 but not the other, since xfmr & "coupled inductor" are the same.

The only exception is multiply wound inductors for ripple current filtering where the flux is a small ac ripple plus a large dc pedestal. Here we have a coupled inductor since several windings share the same core & filter several power supply outputs. But since most of the flux is dc (non-varying wrt time) there is little to no xfmr action, since xfmr action cannot happen with static flux.

As far as energy storage goes, that itself does not define xfmr behavior. A forward converter transformer does not store much energy, but transfers it to the secondary while the primary is conducting. Some energy is stored in the form of leakage inductance, an undesirable loss element. But a forward xfmr is gapped, since operation is limited to 1st quadrant, & w/o gap the core would saturate.

So a forward xfmr has the capability to store substantial energy, but does not do so because the forward network allows transfer to take place. When the primary power switch is on, the secondary rectifier allows secondary current, so that energy transfer takes place with little stored in the xfmr.

With flyback topology, the core is gapped just as in the forward case. This gap prevents core saturation since flybcaks also operate in 1st quadrant. But the flyback topology does not allow primary & secondary conduction at the same time. When the primary power switch is closed, the primary current builds up, increasing flux, & stored energy in the gap. But the secondary cannot conduct since the rectifier is reverse biased.

When primary power switch turns off, the secondary voltage inverts polarity, diode conducts, & energy is transferred. Both cases involve gapped cores which possesses energy storage capability. Whether the magnetic element stores energy then releases it next half cycle, vs. instant transfer, is determined by the circuit topology, flyback or forward.

The flyback xfmr obeys Faraday's law, just like in the forward case. It also obeys Ampere's law when averaged over a cycle. It also provides isolation just as good as a forward converter xfmr. It also exhibits core loss which varies with ac flux swing just like its forward counterpart. The flyback xfmr is a xfmr in every sense of the word.

Whether the gapped core stores energy & releases a moment later, vs. transferring instantly is determined exclusively by the external circuit topology. A xfmr from a forward & that from a flyback can be exchanged. Both are gapped to prevent saturation. Which stores energy & which transfers it instantly is purely a function of the circuit, not the core/winding assembly.

For well over half a century, flyback magnetic elements have been called "transformers". Because they are just that. Just because it stores substantial energy does not mean it cannot be a xfmr. A forward xfmr has the same capabiltiy to store energy, it having a gap. The forward circuit does not allow storage. The flyback circuit does not allow instant transfer.

The circuit topology determines which mode it operates in. Flyback transformers are indeed true transformers as are forward xfmrs. However, multiple windings on a common core to filter ac ripple from several dc outputs are not a true transformer because since the flux is predominantly dc, transformer induction action cannot occur at dc. This is truly a coupled inductor which cannot be called a transformer.

Claude

 

[SunnyBoyNY]

cabraham,

While I am not refusing your reasoning and I do not want to bicker over pedantry, I could not help myself to write at least the following paragraph. In case I am wrong, I will be happy to learn something new or correct my mistakes.

The only exception is multiply wound inductors for ripple current filtering where the flux is a small ac ripple plus a large dc pedestal. Here we have a coupled inductor since several windings share the same core & filter several power supply outputs. But since most of the flux is dc (non-varying wrt time) there is little to no xfmr action, since xfmr action cannot happen with static flux.

Xfrm action can happen with a flux DC offset just fine.

As far as energy storage goes, that itself does not define xfmr behavior. A forward converter transformer does not store much energy, but transfers it to the secondary while the primary is conducting. Some energy is stored in the form of leakage inductance, an undesirable loss element. But a forward xfmr is gapped, since operation is limited to 1st quadrant, & w/o gap the core would saturate.

Exactly which forward converter operates only in the first quadrant? Why would you operate a forward converter only in the first quadrant - the effective number of turns is doubled as opposed to the converter working in the 1st and 3rd quadrants.

Whether the gapped core stores energy & releases a moment later, vs. transferring instantly is determined exclusively by the external circuit topology. A xfmr from a forward & that from a flyback can be exchanged. Both are gapped to prevent saturation. Which stores energy & which transfers it instantly is purely a function of the circuit, not the core/winding assembly.

Standard HF XFRMs have ferrite cores and certainly do not have a gap.

For well over half a century, flyback magnetic elements have been called "transformers". Because they are just that. Just because it stores substantial energy does not mean it cannot be a xfmr. A forward xfmr has the same capabiltiy to store energy, it having a gap.

XFRMs for forward converters do store very little energy in comparison to gapped cores. See above. Forward converters such as full-bridge ZVT operate in the 1st and 3rd quadrant. Magnetizing current is reset to zero every two switching cycles.

The circuit topology determines which mode it operates in. Flyback transformers are indeed true transformers as are forward xfmrs. However, multiple windings on a common core to filter ac ripple from several dc outputs are not a true transformer because since the flux is predominantly dc, transformer induction action cannot occur at dc. This is truly a coupled inductor which cannot be called a transformer.

Claude

Flyback operating in the CCM certainly has a large DC flux offset. Btw. if inductor had static flux, it would be just a piece of wire. Thus, inductor also has AC flux.

A flyback converter working in the continuous conduction mode has a constant DC flux offset, just as a series inductor.

 

[cabraham]

cabraham,

1) While I am not refusing your reasoning and I do not want to bicker over pedantry, I could not help myself to write at least the following paragraph. In case I am wrong, I will be happy to learn something new or correct my mistakes.



2) Xfrm action can happen with a flux DC offset just fine.




3) Exactly which forward converter operates only in the first quadrant? Why would you operate a forward converter only in the first quadrant - the effective number of turns is doubled as opposed to the converter working in the 1st and 3rd quadrants.



4) Standard HF XFRMs have ferrite cores and certainly do not have a gap.



5) XFRMs for forward converters do store very little energy in comparison to gapped cores. See above. Forward converters such as full-bridge ZVT operate in the 1st and 3rd quadrant. Magnetizing current is reset to zero every two switching cycles.



6) Flyback operating in the CCM certainly has a large DC flux offset. Btw. if inductor had static flux, it would be just a piece of wire. Thus, inductor also has AC flux.

7) A flyback converter working in the continuous conduction mode has a constant DC flux offset, just as a series inductor.

My replies to the issues you raised, which I numbered.

1) Ok fine.

2) Of course xfmr operation can happen in the presence of dc flux offset. Reread what I stated. Only the ac component of flux can have xfmr action. The dc component of flux cannot induce a seconday emf/mmf.

3) Single switch forward converter. Some variations operate in 1/3 quadrant, but single switch must have an air gap. Check your smps references & it will be affirmed.

4) When you use an acronym like "HF", please define it once so I know what you mean. Is "HF" to mean "half forward"?

5) I already stated that forward converters store negligible energy in the xfmr. I stated it clearly. But being gapped, said xfmr has storage capability. But the forward topology does not allow for storage because of polarity. When primary power FET is on, secondary diode id conducting & energy is transferred instantly. No storage followed by release as in flyback network.

6) I've already acknowledged that flyback xfmr has a dc flux offset, hence gap is needed to prevent saturation in addition to energy storage.

7) Yes it does. But a series inductor does not transfer energy from 1 winding to another. The flyback xfmr transfers energy from primary coil to secondary via magnetic flux coupling. The flyback xfmr is consistent with laws of Faraday & Ampere just like forward xfmr. The fact that forward xfmr transfers energy w/o storage vs. the flyback which stores energy w/o transfer until next half cycle, is strictly a characteristic of the network driving it.

The flyback xfmr obeys the volts per turn relation per Faraday just like a forward, i.e. Vp/Np = Vs/Ns.

It obeys Ampere's law of balancing amp-turns when averaged over a full cycle, just like a forward, i.e. Np*Ip = Ns*Is.

It provides galvanic isolation between primary & secondary just like a forward xfmr. Again for a 1 quadrant forward, the xfmr is gapped. A flyback xfmr is gapped. If you have 2 converters, 1 forward & 1 flyback, designed for the same xfmr spec, you could interchange xfmrs. The xfmr in the forward converter is forced to transfer energy, not store it. The xfmr in the flyback converter is forced to store energy, transferring it later on the next half cycle.

Both devices can store an transfer. How they do it depends only on the network. Both obey the volts per turn relation, as well as the amp turns relation. Both provide galvanic isolation. Both are 2 inductors coupled on a common core. As I said, "coupled inductor", & "transformer", are one & the same thing. You cannot differentiate them. The operating mode regarding energy transfer vs. storage is a circuit characteristic, not the magnetic device characteristic.

With 2 or more inductor windings wound on a common core, where the flux is a small ripple plus a large dc offset, here is the scoop. The ac ripple exhibits xfmr action, since the ac flux couples the other winding. But the dc flux in 1 winding DOES NOT INDUCE a dc flux in the other. So this arrangement can rightfully be called a coupled inductor that is NOT a xfmr.

Make sense. Have I clarified it well? Feel free to ask for elaboration if something isn't clear. BR.

Claude

 

[SunnyBoyNY]

Claude,

Of course xfmr operation can happen in the presence of dc flux offset. Reread what I stated. Only the ac component of flux can have xfmr action. The dc component of flux cannot induce a seconday emf/mmf.

I think we both agree on this.

3) Single switch forward converter. Some variations operate in 1/3 quadrant, but single switch must have an air gap. Check your smps references & it will be affirmed.

To my knowledge, the single switch forward converters have a mechanism to reset the magnetizing current each cycle. Also, why would you store energy in a transformer core when the energy is not needed?

I do not deny the fact that one could use a gapped core for such transformer. My question is why would one do so?

4) When you use an acronym like "HF", please define it once so I know what you mean. Is "HF" to mean "half forward"?

High-frequency.

5) I already stated that forward converters store negligible energy in the xfmr. I stated it clearly. But being gapped, said xfmr has storage capability. But the forward topology does not allow for storage because of polarity. When primary power FET is on, secondary diode id conducting & energy is transferred instantly. No storage followed by release as in flyback network.

Exactly. See my response #3. Gapped core also have much less inductance with the same number of turns due to higher reluctance and thus allow much higher magnetizing current, which increases converter losses.

6) I've already acknowledged that flyback xfmr has a dc flux offset, hence gap is needed to prevent saturation in addition to energy storage.

We both agree.

7) Yes it does. But a series inductor does not transfer energy from 1 winding to another. The flyback xfmr transfers energy from primary coil to secondary via magnetic flux coupling. The flyback xfmr is consistent with laws of Faraday & Ampere just like forward xfmr. The fact that forward xfmr transfers energy w/o storage vs. the flyback which stores energy w/o transfer until next half cycle, is strictly a characteristic of the network driving it.

It provides galvanic isolation between primary & secondary just like a forward xfmr. Again for a 1 quadrant forward, the xfmr is gapped. A flyback xfmr is gapped. If you have 2 converters, 1 forward & 1 flyback, designed for the same xfmr spec, you could interchange xfmrs. The xfmr in the forward converter is forced to transfer energy, not store it. The xfmr in the flyback converter is forced to store energy, transferring it later on the next half cycle.

Both devices can store an transfer. How they do it depends only on the network. Both obey the volts per turn relation, as well as the amp turns relation. Both provide galvanic isolation. Both are 2 inductors coupled on a common core. As I said, "coupled inductor", & "transformer", are one & the same thing. You cannot differentiate them. The operating mode regarding energy transfer vs. storage is a circuit characteristic, not the magnetic device characteristic.

I believe the fundamental problem here is the difference between the definitions of coupled inductor and transformer.

My definition of a coupled inductor is that it stores significantly more energy than transformer. Other than that, there is no physical difference. Since we do not require transformers to store energy as those are parallel devices (as opposed to inductors that are series devices), I consider a flyback transformer a coupled inductor.

With 2 or more inductor windings wound on a common core, where the flux is a small ripple plus a large dc offset, here is the scoop. The ac ripple exhibits xfmr action, since the ac flux couples the other winding. But the dc flux in 1 winding DOES NOT INDUCE a dc flux in the other. So this arrangement can rightfully be called a coupled inductor that is NOT a xfmr.

This underscores your argument above.

 

[cabraham]

Claude,
I think we both agree on this.

1) To my knowledge, the single switch forward converters have a mechanism to reset the magnetizing current each cycle. Also, why would you store energy in a transformer core when the energy is not needed?

2) I do not deny the fact that one could use a gapped core for such transformer. My question is why would one do so?

---

3) Exactly. See my response #3. Gapped core also have much less inductance with the same number of turns due to higher reluctance and thus allow much higher magnetizing current, which increases converter losses.

4) I believe the fundamental problem here is the difference between the definitions of coupled inductor and transformer.

My definition of a coupled inductor is that it stores significantly more energy than transformer. Other than that, there is no physical difference. Since we do not require transformers to store energy as those are parallel devices (as opposed to inductors that are series devices), I consider a flyback transformer a coupled inductor.

---

1) Some have a reset winding, but I've seen them without it. The core stays in the 1st quadrant & needs a gap. A reset winding can mitigate this issue, & I generally use such a winding. With symmetrical forward converters like full bridge, half bridge, push-pull center-tapped, the core is driven in both directions & no gap is used. Of course, if a gap is not needed, I avoid using one to keep the magnetizing current as small as possible.

2) One would do so to avoid saturation. Otherwise, one can reset the core with an additional winding & drive circuit, which is what I do, or use a symmetrical forward topology to eliminate the gap. If no reset mechanism is employed, then a gap is needed.

3) Again, smallest gap possible, such as incidental gap when mating 2 core pieces, is most desirable. But if no reset winding is available, core operating in 1st quadrant forces us to use a gap to avoid saturation. Increases magnetizing current is the price paid for this approach. I don't like gaps if they are not needed, & only use them if I have to.

4) It's all about definitions. My understanding that xfmr & coupled inductor are the same thing is based on Ampere & Faraday per Maxwell. You suggest that your definition is based on storing energy vs. instant transfer. We are now down to who defines when a coupled inductor is actually a xfmr. Nothing personal but neither of us have the authority to arbitrarily make definitions based on our own intuition as to how it ought to be defined.

Many practitioners before me, & since, have already established that a xfmr obeys the laws of Maxwell. The only difference between our definitions is as follows.

The forward xfmr stores energy, but it is incidental energy, a loss component that must be dealt with by snubbing, or energy recovery methods. Leakage reactance stored energy does not couple from primary to secondary, & ideally we wish it vanished. Forward xfmrs can & do store a little energy, but not useful to us, only a burden.

Forward xfmr transfers energy from pri to sec instantly. Flyback also transfers energy, but not instantly, but on alternate half cycles. That is the difference. In both cases, primary power source energizes xfmr, with forward xfmr transferring energy immediately to secondary, flyback xfmr storing energy half cycle then transferring it to secondary.

From an external viewpoint there is little difference, only the time it takes. From the control loop viewpoint, there is no difference. The control loop bandwidth is typically one sixth or one fifth the switching frequency, so the energy transfer is virtually instantaneous for flyback & forward as far the the control loop can react.

I don't think that considering the flyback magnetic element as not being a true xfmr helps us at all. It isolates same as a true xfmr. It obeys Faraday the same, Ampere the same except for the half cycle time lag. All known xfmr laws are equally valid w/ the flyback device. Main difference is that the gap allows the xfmr function & storage inductor function to be merged into 1 device. A forward converter needs a separate inductor along w/ the xfmr. The flyback uses its single xfmr to do both functions.

Telling people that the flyback device is not a true xfmr does not provide any useful insight at all. To say that it differs from an ungapped device in its ability to store energy is correct. That provides insight. I always emphasize that when agency requirements mandate isolation (UL, CSA, VDE, etc.), the flyback xfmr is as approved as the forward xfmr. The flyback device meets xfmr isolation requirements same as forward.

Adding a gap does not make it a non-xfmr. It still meets agency isolation reqmts, still meets all xfmr laws. It's a modified device capable of energy storage. Such storage is usually undesirable, but for a flyback, we make good use of it. Can we have agreement that it does differ from ungapped xfmrs? It can store energy more so than ungapped. But in all other respects it is identical.

I cannot label it a mere coupled inductor & deny it is a xfmr for the reasons above. It is a true xfmr with an added gap allowing energy storage. But adding the gap never tales away its xfmr attributes.

Claude

 

[Mike_In_Plano]

I'd drop in a pre-made transformer, the VP5-0083-R from Digi-Key. I'd run two windings in series for the primary, three in series for the secondary, and leave one out.

For the control, I'd consider the LT3750 from Linear Tech. It's made for charging capacitors and will be better behaved than a regular flyback control.

This chip is easy on the output rectifier, so an MUR120 (or MURS120) from ON would be my first choice.

Linear Technology has a free spice package at their site and it's one of the easiest to use I've ever seen. It's got a nice graphical interface and will likely have the part and an example circuit for you to play with.

One bit of warning. In the LT datasheet, they didn't treat the issue of leakage inductance in the transformer. This will manifest itself as large voltage spikes on the Drain of the MOSFET at turn off. Generally, I use and RC snubber from the Drain to ground to deal with this. 330pF in series with 50 ohms is usually a good neighborhood to start with.

Best of Luck,

Mike :)

 

[hobbs125]

Ok,

First off, thanks everyone (espically sunnyboyNY) for your input. I have since designed a broadband transformer which has a clean output with little distortion and no backswing.

But I have more to learn...

The transformer has a 1:8.5 turns ratio. When I apply 7V pulses to the primary coil the load across the secondary (which should be about 60V) only shows about 19V. I know this is because I am overdriving the coil, but my question is; Where did I go wrong in my design that is preventing me from getting full voltage across the load?

I can increase the primary voltage and current, but it does no seem to change anything on the secondary side.

I am thinking it has something to do with the primary or secondary inductance, but if I knew for sure I would not be here.

Any help or suggestions would be greatly appreciated?