The have used the UC3845 from when I got the very first prototypes. I really like the UC384x parts. Do you understand that the 3845 is limited to 50% duty cycle? The osc runs at 2x the frequency of the output. The rest of the family will to 95%.

The output is rated at +/-1A. (a little questionable) I know it will drive 600mA at 2V from ground. When I was using them we were running 60khz through 120khz. Driving 2 to 10A MOSFETS. I would not use the part at 500khz but defiantly at 250khz.
The part is old. I would not use it with a MOSFET with very low Gate turn on voltage. It is a very good part for a "normal" MOSFET.
View attachment 338266

Finally started to get some results. I connected the primary of the transformer and it started working. When the secondary is connected, all hell started. That is when I checked the components on the secondary. The schottky diode was short. Least expected.

Now getting a steady 12.53 volt as expected. It is far from over. I need to perfect the transformer to work at the correct frequency and optimize the current consumption and stability on load.

The discussion gave me some more pointers and time to think. Any suggestions are most welcome.

 

Finally started to get some results. I connected the primary of the transformer and it started working. When the secondary is connected, all hell started. That is when I checked the components on the secondary. The schottky diode was short. Least expected.

Not unexpected at all.
If you have a 325V DC supply, and a primary flyback voltage of 125V, and a turns ratio of about 10:1, you will get a peak inverse voltage of 45V on the secondary diode.

 

Off-the-shelf transformers simply are not available that work at 10kHz.
Start at 50kHz.
Buy a scope. You can probably get a usable scope for the cost of a few dozen dead MOSFETs

//Off-the-shelf transformers simply are not available that work at 10kHz.//

Very true. Off the shelf ? they are not available. One has to get them wound. They are asking for oders in 500's minimum qty.
//Start at 50kHz.// I am working on 50Khz to 132 Khz range.

//Buy a scope. You can probably get a usable scope for the cost of a few dozen dead MOSFETs //

Not possible here. I can get two mosfets for 1 dollar in India. But a used good scope will cost almost 150 to 200 dollars.

If buying, I think would need a 200Mhz scope , since I work with microcontrollers too. And they don't come cheap.

 

Not unexpected at all.
If you have a 325V DC supply, and a primary flyback voltage of 125V, and a turns ratio of about 10:1, you will get a peak inverse voltage of 45V on the secondary diode.

// you will get a peak inverse voltage of 45V on the secondary diode.// That was my bad. I just used a diode that was lying around and. I left that side of circuit for later scrutiny once the switching side works perfectly. I didn't expect it to giveup so soon.

I was planning to use a 60v or 100v diode on the secondary side later. Or is that too much ?

 

One problem I am facing is, I have to buy ferrite core through retailers. They don't provide any data about the make or material. I can only guess that it would be from TDK. But they atleast three type of core with the same spec N27, N87 and N97. These core's have different Bmax values. Or they might be Chinese or Korean make. Because of this I have to fiddle with different Bmax values and so the winding data.

The issue is how to recognize the core ? is there any way to know which type a core is from unknown source.

 

One problem I am facing is, I have to buy ferrite core through retailers. They don't provide any data about the make or material. I can only guess that it would be from TDK. But they atleast three type of core with the same spec N27, N87 and N97. These core's have different Bmax values. Or they might be Chinese or Korean make. Because of this I have to fiddle with different Bmax values and so the winding data.

The issue is how to recognize the core ? is there any way to know which type a core is from unknown source.

A few things to note:
If if fits a bobbin that is generally used in power transformers then it is most likely a power grade.
Core loss is proportional to the two-and-a-halfth power of peak-to-peak flux excursion, so if you use a high flux density, things get warm very fast.
The reluctance will be dominated by the reluctance of the gap, so the permeability of the core isn’t really that important.
So, proceed with a value of Bmax=200mT and you won’t be far wrong. More than that and you willl have a really toasty transformer.

 

A few things to note:
If if fits a bobbin that is generally used in power transformers then it is most likely a power grade.
Core loss is proportional to the two-and-a-halfth power of peak-to-peak flux excursion, so if you use a high flux density, things get warm very fast.
The reluctance will be dominated by the reluctance of the gap, so the permeability of the core isn’t really that important.
So, proceed with a value of Bmax=200mT and you won’t be far wrong. More than that and you willl have a really toasty transformer.

Quite true. As the Bmax is raised, The number of turns decrease , which should put a lot of stress on the core and Mosfet too (?).

I am using 1650 Gauss for calculation.
And I am using
Np = Vp * 10^8 / 4 * Bmax * freq * Carea.

I have seen the same formula with 2 instead of 4 .
Np = Vp * 10^8 / 2 * Bmax * freq * Carea.
I wonder if this for EI core and the former for EE core.

and the Vp is taken as the average of Min and max Ac voltage converted to DC ( x 1.41).

Or is it better to use the Min voltage ?

Thank you.

 

No, no, no.
The magnetic structure in a flyback IS NOT a transformer, but a coupled inductor.
Very different behavior.
As such, the core characteristics are calculated differently, namely the size of the air gap required to prevent saturation. Have a read on the attached image.
Since you have chosen to ignore this fact, This is my last post on this thread.

 

Quite true. As the Bmax is raised, The number of turns decrease , which should put a lot of stress on the core and Mosfet too (?).

I am using 1650 Gauss for calculation.
And I am using
Np = Vp * 10^8 / 4 * Bmax * freq * Carea.

I have seen the same formula with 2 instead of 4 .
Np = Vp * 10^8 / 2 * Bmax * freq * Carea.
I wonder if this for EI core and the former for EE core.

and the Vp is taken as the average of Min and max Ac voltage converted to DC ( x 1.41).

Or is it better to use the Min voltage ?

Thank you.

You have the equations for a AC driven transformer.
You calculate flyback transformer starting with the current. From the current you calculate the magnetomotive force.
Flux equals magnetomotive force divided by reluctance. Flux divide by cross sectional area equals flux density.
Note that you need to know the reluctance, and the reluctance comes from the GAP.

 

You have the equations for a AC driven transformer.
You calculate flyback transformer starting with the current. From the current you calculate the magnetomotive force.
Flux equals magnetomotive force divided by reluctance. Flux divide by cross sectional area equals flux density.
Note that you need to know the reluctance, and the reluctance comes from the GAP.

Thanks a ton.

Just getting the hang of this.

Ip = 4* Iout / Eff * Cbulk

Ipri_rms = Ip * Sqrt( duty cycle /3

Lpri = Cbulk *500 / Ip * freq
Np = 0.1 * Lpri * Ip / Acore * Bmax

Core Gap =(( 0.6 * Np * Ip) / (Bmax * 1000) * 25.4

Isn't this what you wrote about ?

 

No, no, no.
The magnetic structure in a flyback IS NOT a transformer, but a coupled inductor.
Very different behavior.
As such, the core characteristics are calculated differently, namely the size of the air gap required to prevent saturation. Have a read on the attached image.
Since you have chosen to ignore this fact, This is my last post on this thread.

//No, no, no.
The magnetic structure in a flyback IS NOT a transformer, but a coupled inductor.
Very different behavior. //

Thank you for the clarification.

// Since you have chosen to ignore this fact, //

I had not ignored it. Just toying with , to understand this better.

 

I don’t have a definitive answer. But from the symptoms posted, it seem the coupled inductor is going into saturation. When this happens it stops being an inductor and just becomes a short circuit.
The design of the coupled inductor is tricky. And you really need a scope to measure voltages and currents. Otherwise you’re just guessing whether you got the design correct.

 

//No, no, no.
The magnetic structure in a flyback IS NOT a transformer, but a coupled inductor.
Very different behavior. //

Thank you for the clarification.

// Since you have chosen to ignore this fact, //

I had not ignored it. Just toying with , to understand this better.

 

I've attempted several amps designs and never completed the designs.
For diy constructors, I found it difficult to get specs on the cores available to me, or alternatively the cost of cores from major suppliers was prohibitive ( and you may need to buy a few in order to experiment and get a final design)
I just resort these days to getting modules from aliexpress or similar. Whilst it's not the perfect design at least I get it at a cheap price and I know it works.

 

I've attempted several amps designs and never completed the designs.
For diy constructors, I found it difficult to get specs on the cores available to me, or alternatively the cost of cores from major suppliers was prohibitive ( and you may need to buy a few in order to experiment and get a final design)
I just resort these days to getting modules from aliexpress or similar. Whilst it's not the perfect design at least I get it at a cheap price and I know it works.

Choose a bobbin that is obviously designed for power conversion: EF or ETD. That means that the cores available will be made in a suitable material for power conversion. You can get most of the parameters you need to know from a pair of calipers. The relative permeability may vary from core to core, but the permeance of the final core is dominated by the reluctance of the gap.

 

The circuit looks like a flyback converter. if you intended this, you should change the polarity of the output winding. AND there must be an air gap in the transformers magnetic core. Wile the Mosfet is switched on, the transformer acts like an inductor, letting the primary current ramping up, while se output rectyfier is blocking the output. As soon an the current threshhold is reached (V over the source resistor), the FET switches off an lets the inductor current change over to the secondary side through the output rectifyer. One frequent problem is the missing low pass filter after the current sense resistor, to filter out the corrent spike an switching on of the mosfet.
If you really intended a foreward design, you will definitely need a additional inductor in the output path.
Unfortunately i just dropped my old Unitrode (te first manufacturer of that chip) data book with a lot of basics and sircuit examples... Good Luck. Hartmut

 

Unfortunately i just dropped my old Unitrode (te first manufacturer of that chip) data book with a lot of basics and sircuit examples... Good Luck. Hartmut

Have to look if I still have that one.

 

Have to look if I still have that one.

I think most of them are on TI's website.

 

4.70Ω in the grid and 0.47Ω in the source and it will also work at 200kHz if you coil with litzwire. Don't forget the gap between the ferrite core.

 

I don’t have a definitive answer. But from the symptoms posted, it seem the coupled inductor is going into saturation. When this happens it stops being an inductor and just becomes a short circuit.
The design of the coupled inductor is tricky. And you really need a scope to measure voltages and currents. Otherwise you’re just guessing whether you got the design correct.

Yes , you are on dot.

Thank you.