Thank you. I have downloaded it. hmmm. That is good info.

I mean no disrepsect when I say there are a few salient points missing here that may or may not be significant.
The variability of the permeability is not typically a factor in a gapless tx design for a forward converter as is shown by the equation: Vp = kfABN where k is 4 or 4.44 depending on the excitation waveform.
For a flyback which will use a gapped core things are a bit more 'complicated'. In a forward converter the idea is to limit the flux in the core to avoid saturation and heating of the core but in a flyback the idea is to store energy in the tx and transfer it to the secondary.
In an ungapped core, the permeability of air (or free space) is of little consequence (see above). In a gapped core it is of great significance. If you think of the core and gap as being a magnetic circuit which will have a certain reluctance made up of the core reluctance in series with the gap reluctance. Unless the gap is microscopically small, the bulk of the reluctance is in the gap. So the bulk of the energy storage is also, in the gap. That does not let the core 'off the hook' because the core still has to accommodate the magnetic flux (H) anyway. In both cases though, the permeability of the core really does not make a huge difference to anything in a first level abstraction. It obviously effects the magnetising current, the mutual inductance, the core losses and it can influence the leakage inductances depending on magnetic paths and their couplings, but these are not the initial concerns in a tx design. A 'safe value' for B and a likely geometry and size is the usual starting point and the design gets refined from there.

In your case, which I believe is a flyback, you will have a gapped core (or a very small power handling capability otherwise). I suggest you use one of the 50% max duty cycle UC384x parts (I can never remember which are the low or high start threshold voltage ones and which are the 50% or 100% duty cycle ones) because going beyond a 50% duty cycle requires careful design to ensure you balance the Vt product for primary and secondary and because the UC384x parts are current mode controllers if you go beyond 50% duty cycle you will have to contend with 'slope compensation' to keep it stable.

If you stay under 50% and use current mode control you can still get to 'continuous current' operation and the loop stability becomes far less challenging than for a voltage mode flyback because your entire power stage is reduced to a single order system at frequencies below the switching frequency but good frequencies for a fast compensator design.

Any luck with a getting a CRO?
Sorry to hear Digikey and Mouser are not options for you. An obvious question I hope does not offend you; are there no local equivalents to them?

 

I made a simple smps with uc3845. The output of the ic is connected to IRF840 and a EE30_15_7 ferrite transformer wound by me. The osc section seems to be working.

Vcc supplied is 16.5 v

Pin 1 connected to opto coupler

pin 2 to GND

Pin 3 to the 0.47ohm resister connected between Mosfet Source and GND

pin 4 to 10 nf cap to GND

pin 5 to GND

pin 6 to a 47 ohm resister connected to gate of mosfet

pin 7 to VCC 16.5 volt

Pin 8 to 10K multiturn pot to pin 4

Snubber circuit

Diode - Resister + 1nf capacitor connected between the primary windings.

I tried winding data calculated by the formula Np= Nv *10^8 / 4 * Bmax * Freq * Ae

Bmax was taken as 1650

Ae = core size in cm2

Secondary should get 13v as per winding data.

The supply was given through a 60 w filament bulb.

It lights up when switch on. If the turn the pot and increase the frequency more than 125khz ( using a arduino uno ) , the light dims. However the output is only less 1 volt. At 50khz the bulb lightup like a virtual short circuit.

I tried changing the turns and even the core. Nothing seems to work. Any idea what might be wrong What should I look for problems in this setup. Any help is welcome. I don't a oscilloscope to test the wave form.

Hi,
I don't know if you are still working on this project. I just opened your cct for the first time and have a few points to offer (in the hope you have not given up). Some of these may now be irrelevant but I offer them knowing they may be redundant now;
1. Your shunt resistor (source to gnd) should be in the order of ohms or milliohms. Size it to get a peak voltage under the chip limit (0.9V) with your converter under full load.
2. The snubber resistor value is way too high. The general idea with a snubber is to dominate the parasitic L & C that cause the voltage spikes on the mosfet drain. The resistor is sized to set the damping factor of the 2nd order system formed by the snubber C and leakage L to a value that keeps the spikes under control. This also helps with EMI and stops the mosfet operating in avalanche. (If you find the mosfet suddenly getting a lot hotter with greater load, then avalanche is a very real possibility right before the mosfet goes bang.)
3, The diode in the snubber circuit changes things a bit. The configuration is known as a low loss snubber but they typically are not really that. I recommend, for development purposes that you remove the diode and replace it later in the proceedings. Low loss snubbers usually require much more C and so lose a lot of that 'lower loss'. There is also a lossless configuration that uses a choke to dump the energy back into the input and these need a lot more C and are much harder to get right.
Start with a simple RC is my advice.
4. Your tx polarities are wrong but I think you already got this advice. A flyback separates the power in and power out phases of the tx. Otherwise the topology is a forward converter and typically needs two diodes (one rectifier and one freewheeling) and a choke in the output.
5. The current sense signal needs an RC filter to avoid the leading edge spike in the current waveform. The spike is caused by the gate current spike at switch on as well as the capacitance of the tx primary at switch on. I suggest starting with 1k and 100pF. Don't make the R too big and keep the C down so as not to distort the body of the current waveform too much.

You may have issues with stability as well. It is good you are working on a current mode control converter as that makes stability a lot easier but somewhere along the line there must be a compensator that provides one pole and one zero in the transfer function. It is not impossible that you may have fluked this in which case you should buy a lottery ticket because luck is on your side. If you need to add the compensator I suggest a TL431 on the secondary and a check of appnotes etc. You want a pole to stop the compensator interfering at or near the switching frequency and the zero (usually made with just a resistor) to bring the phase margin back to a point of stability.
If it were not a current mode control scheme, stability and compensation gets a lot more difficult. In current mode control the entire power stage simplifies to a (noisy and switched) current source, more or less and therefore represents a single pole. In voltage more control nothing simplifies. The power stage is a double pole until the pole formed with the output cap ESR takes effect. And from this simplified voltage mode model it gets worse and worse. You did good to go with current mode control

Hope your project is going well. Did you get a CRO?

 

// 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 ?

Not at all. Try a MBR10100 or similar. The forward losses for higher reverse rating are higher but being schottky there is no reverse recovery time to worry about.

 

Hi,
I don't know if you are still working on this project. I just opened your cct for the first time and have a few points to offer (in the hope you have not given up). Some of these may now be irrelevant but I offer them knowing they may be redundant now;
1. Your shunt resistor (source to gnd) should be in the order of ohms or milliohms. Size it to get a peak voltage under the chip limit (0.9V) with your converter under full load.
2. The snubber resistor value is way too high. The general idea with a snubber is to dominate the parasitic L & C that cause the voltage spikes on the mosfet drain. The resistor is sized to set the damping factor of the 2nd order system formed by the snubber C and leakage L to a value that keeps the spikes under control. This also helps with EMI and stops the mosfet operating in avalanche. (If you find the mosfet suddenly getting a lot hotter with greater load, then avalanche is a very real possibility right before the mosfet goes bang.)
3, The diode in the snubber circuit changes things a bit. The configuration is known as a low loss snubber but they typically are not really that. I recommend, for development purposes that you remove the diode and replace it later in the proceedings. Low loss snubbers usually require much more C and so lose a lot of that 'lower loss'. There is also a lossless configuration that uses a choke to dump the energy back into the input and these need a lot more C and are much harder to get right.
Start with a simple RC is my advice.
4. Your tx polarities are wrong but I think you already got this advice. A flyback separates the power in and power out phases of the tx. Otherwise the topology is a forward converter and typically needs two diodes (one rectifier and one freewheeling) and a choke in the output.
5. The current sense signal needs an RC filter to avoid the leading edge spike in the current waveform. The spike is caused by the gate current spike at switch on as well as the capacitance of the tx primary at switch on. I suggest starting with 1k and 100pF. Don't make the R too big and keep the C down so as not to distort the body of the current waveform too much.

You may have issues with stability as well. It is good you are working on a current mode control converter as that makes stability a lot easier but somewhere along the line there must be a compensator that provides one pole and one zero in the transfer function. It is not impossible that you may have fluked this in which case you should buy a lottery ticket because luck is on your side. If you need to add the compensator I suggest a TL431 on the secondary and a check of appnotes etc. You want a pole to stop the compensator interfering at or near the switching frequency and the zero (usually made with just a resistor) to bring the phase margin back to a point of stability.
If it were not a current mode control scheme, stability and compensation gets a lot more difficult. In current mode control the entire power stage simplifies to a (noisy and switched) current source, more or less and therefore represents a single pole. In voltage more control nothing simplifies. The power stage is a double pole until the pole formed with the output cap ESR takes effect. And from this simplified voltage mode model it gets worse and worse. You did good to go with current mode control

Hope your project is going well. Did you get a CRO?

Thank you very much for taking time and effort in writing this. I have not given up. In fact I have been successful in making the smps. I am going in a research mode, making the ferrite transformer and noting down the voltages and other performance parameters to optimize the circuit.

The advice and points you have raised is very valid. The tx polarities are actually reversed as need for flyback topology. But I couldn't get an appropriate version in the proteus version that I am using.

I did purchased a new oscilloscope Siglent SDS1204X-E , just yesterday. This is my first scope. So I will have a lot to learn on how to use it first. I can't afford differential probe now So I have ordered an isolation transformer. I hope it would be OK (from various videos of actual testing of smps circuits ). But I wonder ? Won't this make the ground of scope floating ? How accurate the readings would be ? is it really safe?

Snubber circuit : You might be right on the value of the resistor. But it will depend on the voltages and switching frequency etc right ? I think I can arrive at a more accurate value after deciding on the transformer parameters. However I think the diode needs to a fast switching type rather than the ordinary IN4007 rectifier. I just received fs107 diode for testing.

I choose CC mode after going through various articles, circuit diagrams and datasheets of lot of switching ic's. One thing that I find good about UC3845 is we can change the switching frequency. This is very helpful when using a hand wound transformer.


I am afraid I really did not understand this // but somewhere along the line there must be a compensator that provides one pole and one zero in the transfer function. // Are you refering to the output voltage stabilization ? I am designing that secondary to get 16 to 20 v for a stable 12v, Hope it will work out.

Meanwhile I am making a AC current meter with stm32 to get a better reading of current consumption at zero load. The present clamp meter I have is not very accurate with low current values. I have completed software and voltage section. I need a Rail-to Rail op-amp which was not readily available. Got it today. I can complete the working modal in a few days. I hope this would be helpful.

One thing I am happy about is , I am blowing fewer components now. Very interesting and members in this forum has helped a lot. Thank you once again.

 

I've made lots of switched-mode supplies, and never felt that I needed a differential probe. I just use the standard x10 that comes with the scope and a x100 for the high voltages.
I would recommend discontinuous current mode for a flyback supply. The peak currents are greater so that produces more loss, but there is virtually no diode recovery loss. The vast majority of flyback supplies work in discontinuous current mode.
You can replace R1 and C1 with a 200V TVS diode. It might not be the best for interference, but it is simple and it works.
You can sort out the right values of R1 and C1 at a later date. The TVS diode is more efficient, as there is no continuous loss as there would be from the resistor.

I would recommend the UNI-T UT201E clamp meter. It has a 2A range. I was a bit sceptical about it, but it was recommended by @Ya’akov . I compared it to a Fluke with an in-circuit current meter and it read the same, right down to milliamps.

 

Not at all. Try a MBR10100 or similar. The forward losses for higher reverse rating are higher but being schottky there is no reverse recovery time to worry about.

100v 10A is great. I used this as suggested. Thank you

 

I've made lots of switched-mode supplies, and never felt that I needed a differential probe. I just use the standard x10 that comes with the scope and a x100 for the high voltages.
I would recommend discontinuous current mode for a flyback supply. The peak currents are greater so that produces more loss, but there is virtually no diode recovery loss. The vast majority of flyback supplies work in discontinuous current mode.
You can replace R1 and C1 with a 200V TVS diode. It might not be the best for interference, but it is simple and it works.
You can sort out the right values of R1 and C1 at a later date. The TVS diode is more efficient, as there is no continuous loss as there would be from the resistor.

I would recommend the UNI-T UT201E clamp meter. It has a 2A range. I was a bit sceptical about it, but it was recommended by @Ya’akov . I compared it to a Fluke with an in-circuit current meter and it read the same, right down to milliamps.

// never felt that I needed a differential probe //

Primary side ? will it not damage scope or something in the line ?

Thank you for the suggestions.

 

Thank you very much for taking time and effort in writing this. I have not given up. In fact I have been successful in making the smps. I am going in a research mode, making the ferrite transformer and noting down the voltages and other performance parameters to optimize the circuit.

The advice and points you have raised is very valid. The tx polarities are actually reversed as need for flyback topology. But I couldn't get an appropriate version in the proteus version that I am using.

I did purchased a new oscilloscope Siglent SDS1204X-E , just yesterday. This is my first scope. So I will have a lot to learn on how to use it first. I can't afford differential probe now So I have ordered an isolation transformer. I hope it would be OK (from various videos of actual testing of smps circuits ). But I wonder ? Won't this make the ground of scope floating ? How accurate the readings would be ? is it really safe?

Snubber circuit : You might be right on the value of the resistor. But it will depend on the voltages and switching frequency etc right ? I think I can arrive at a more accurate value after deciding on the transformer parameters. However I think the diode needs to a fast switching type rather than the ordinary IN4007 rectifier. I just received fs107 diode for testing.

I choose CC mode after going through various articles, circuit diagrams and datasheets of lot of switching ic's. One thing that I find good about UC3845 is we can change the switching frequency. This is very helpful when using a hand wound transformer.


I am afraid I really did not understand this // but somewhere along the line there must be a compensator that provides one pole and one zero in the transfer function. // Are you refering to the output voltage stabilization ? I am designing that secondary to get 16 to 20 v for a stable 12v, Hope it will work out.

Meanwhile I am making a AC current meter with stm32 to get a better reading of current consumption at zero load. The present clamp meter I have is not very accurate with low current values. I have completed software and voltage section. I need a Rail-to Rail op-amp which was not readily available. Got it today. I can complete the working modal in a few days. I hope this would be helpful.

One thing I am happy about is , I am blowing fewer components now. Very interesting and members in this forum has helped a lot. Thank you once again.

Great news about the CRO!!! Now you can stop flying blind. By the time you get used to it you will also know just how fantastically useful they are. Very pleased for you about this.
With the isolation tx, be careful! Understand that at 50Hz or 60Hz the transformer will offer excellent isolation, but at your switching frequency, that may not be the case.
I recommend you use the tx to isolate your smps and not the cro. This has down sides but it obviates the risk of your cro being at a deadly potential with no observable indication of it being so.
If your cro has two or more channels then you can use the math function to get a plot of the difference of two channels and thus get a differential probe, kind of. If both channels and probes are set the same.
Beware the voltage rating of your cro input and the probes against the voltages in your smps.
I don't have much time right now so more later. In the mean time, enjoy your cro!
One last note of warning,isolation tx will separate the mains neutral from the mains earth but the earth may be continuous from input connector to output connector. I mention this because isolation tx are often misunderstood with significant and explosive results that usually come with huge smoke leaks from pipes and components.... and your cro.

 

// never felt that I needed a differential probe //

Primary side ? will it not damage scope or something in the line ?

Thank you for the suggestions.

If you are going to connect to the primary side, use an isolating transformer. That way, you can make the negative supply earth.
Just buy a 230V : 230V transformer and put it in a box with an output socket and a mains lead. That way you know exactly how it is connected.
You can probably get a transformer with two 115V secondaries for the same price. That way you can test 120V equipment as well.

 

Great news about the CRO!!! Now you can stop flying blind. By the time you get used to it you will also know just how fantastically useful they are. Very pleased for you about this.
With the isolation tx, be careful! Understand that at 50Hz or 60Hz the transformer will offer excellent isolation, but at your switching frequency, that may not be the case.
I recommend you use the tx to isolate your smps and not the cro. This has down sides but it obviates the risk of your cro being at a deadly potential with no observable indication of it being so.
If your cro has two or more channels then you can use the math function to get a plot of the difference of two channels and thus get a differential probe, kind of. If both channels and probes are set the same.
Beware the voltage rating of your cro input and the probes against the voltages in your smps.
I don't have much time right now so more later. In the mean time, enjoy your cro!
One last note of warning,isolation tx will separate the mains neutral from the mains earth but the earth may be continuous from input connector to output connector. I mention this because isolation tx are often misunderstood with significant and explosive results that usually come with huge smoke leaks from pipes and components.... and your cro.

Thank you very much for all your advice. Very nice of you.

// One last note of warning,isolation tx will separate the mains neutral from the mains earth but the earth may be continuous from input connector to output connector. I mention this because isolation tx are often misunderstood with significant and explosive results that usually come with huge smoke leaks from pipes and components.... and your cro.//

I was also thinking about the same point. I plan to remove mains earth from the output connector and use only the P & N from the Tx. Hope that is ok for the CRO. I have ordered a 2A Tx , so that I can use for isolating both the circuit and cro and other equipments on the lab bench.

Thank you once again.

 

isolating both the circuit and cro and other equipments on the lab bench.

You can isolate the scope, but you don't need to, because the scope live and neutral are not connected to anything on the scope other than its mains input connector. It may be best not to, as it becomes an IT earthing system which is single-fault-tolerant, meaning that it can become dangerous AFTER there is one short to earth. If there is only ONE piece of equipment connected to the isolating transformer then it is less likely to become dangerous.

 

You can isolate the scope, but you don't need to, because the scope live and neutral are not connected to anything on the scope other than its mains input connector. It may be best not to, as it becomes an IT earthing system which is single-fault-tolerant, meaning that it can become dangerous AFTER there is one short to earth. If there is only ONE piece of equipment connected to the isolating transformer then it is less likely to become dangerous.

Ok. Understood. Thank you very much.

 

You can isolate the scope, but you don't need to, because the scope live and neutral are not connected to anything on the scope other than its mains input connector. It may be best not to, as it becomes an IT earthing system which is single-fault-tolerant, meaning that it can become dangerous AFTER there is one short to earth. If there is only ONE piece of equipment connected to the isolating transformer then it is less likely to become dangerous.

And this is the isolation tx confusion I was talking about earlier. To be clear: an isolation tx dissociates the mains neutral from the protective earth. The protective earth is usually continuous from the input connector of the tx to the output connector of the tx. (At one point in every installation, there is a short and solid connection between the protective earth and the neutral. This is typically in a distribution box / cabinet and the link is made between the earth bus bar and the neutral bus bar and the purpose served is to ensure that in the event of an active to earth fault the fault current is sufficient to cause a rapid disconnection by the circuit breaker or fuse.)
The only reason to be using an isolation tx is so that the ground connection of your cro lead, which is connected to protective earth does not make things go bang when connected to some point in the mains powered part of your project. Just try to think what would happen if you took the neutral of the mains input to your project and started connecting it to various parts of the mains part of your project, because that is exactly what you would be doing if not for the isolation tx.
So, it makes no sense at all to use the iso tx on the cro unless you also break the protective earth connection of the cro at its mains input (which will also make the iso tx redundant because the cro internal power supply will already have equivalent mains isolation as the external iso tx). Breaking the cro earth connection is a bit dodgy and not recommended for anybody, but especially for a novice. It will allow the cro chassis and all of the cro probe ground leads to be at lethal voltages if just one probe ground lead is connected to a lethal voltage.
Use the tx to isolate the mains input to your project. That way you can connect the ground of your cro lead with less fear of things going bang. The cro ground connection can still cause problems depending on where you connect it, but you should be able to avoid the worst of the explosive effects.
I hope this makes the use of iso tx clear and obvious and understood. Some of the advice given here by others is downright wrong and dangerous. To those other respondents, please beware offering advice on subjects you do not properly understand and that have a distinct possibility of ending a life. You might also avoid the misuse of random technical terms such as the class of an earthing system. It conveys nothing to the uninitiated and the rest of us know it is a meaningless detail and invoked in error.

Bottom line, understand what the 3 pins of the mains connection are and how they work, then understand what the iso tx does do, what it does not do, what you need to do and how the iso tx might help you achieve that in the context of your mains supply, your project and your test bench equipment. Then before powering up, take a moment to consider the safety of the configuration of things as you have them. It is better than being electrocuted. By the way,your primary DC voltage of ~320V is way more lethal than 50Hz 230VAC which is way more lethal than 60Hz mains. (50Hz is a sweet spot for putting a human heart into ventricular fibrillation. In comparison, 60Hz is hopeless at doing that, but it will have a good go at it.)

 

(50Hz is a sweet spot for putting a human heart into ventricular fibrillation. In comparison, 60Hz is hopeless at doing that, but it will have a good go at it.)

Interesting. Could you cite your source?

 

Interesting. Could you cite your source?

I was told that by one of the pioneers of implantable pace makers. Having done a fair bit in cardio focused biomed myself I really should refrain from quoting these over generalized concepts. And I would except that they are so true so often. The mechanisms for inducing AF are not straight forward but if you put enough punch into the current you can fire up all sorts of electrophysiological effects and one of them is bound to work. For pacemaker studies in animals (sheep in this case) they had to induce cardiac problems for the pacemakers to detect and fix. Hence the practical knowledge of the best ways to do that.
Sorry I don't have a journal, academic paper or even a text book to cite.
I can say with confidence though, and you can quote me on this, avoiding large externally applied currents running through the cardiac area is an excellent way to avoid untimely death

 

Thank you very much for all your advice. Very nice of you.

// One last note of warning,isolation tx will separate the mains neutral from the mains earth but the earth may be continuous from input connector to output connector. I mention this because isolation tx are often misunderstood with significant and explosive results that usually come with huge smoke leaks from pipes and components.... and your cro.//

I was also thinking about the same point. I plan to remove mains earth from the output connector and use only the P & N from the Tx. Hope that is ok for the CRO. I have ordered a 2A Tx , so that I can use for isolating both the circuit and cro and other equipments on the lab bench.

Thank you once again.

You are welcome Sometimes, it can be satisfying and pleasing to help a person with a project and see them learn or succeed or both.

The 2A rating of your tx may be a bit light. The smallest one I use is 1kVa and up to 6kVa. The things to be aware of with lower power tx is that the output regulation is not so good due to the winding resistance and leakage inductance which are usually higher and can be a problem powering a small converter with a very high crest factor (peaky input current) or a larger converter with a PFC (power factor correction) input stage.
Just keep it in the back of your mind, if you encounter some problem, think about the iso tx and if it might be causing the problem.

 

I've made lots of switched-mode supplies, and never felt that I needed a differential probe. I just use the standard x10 that comes with the scope and a x100 for the high voltages.
I would recommend discontinuous current mode for a flyback supply. The peak currents are greater so that produces more loss, but there is virtually no diode recovery loss. The vast majority of flyback supplies work in discontinuous current mode.
You can replace R1 and C1 with a 200V TVS diode. It might not be the best for interference, but it is simple and it works.
You can sort out the right values of R1 and C1 at a later date. The TVS diode is more efficient, as there is no continuous loss as there would be from the resistor.

I would recommend the UNI-T UT201E clamp meter. It has a 2A range. I was a bit sceptical about it, but it was recommended by @Ya’akov . I compared it to a Fluke with an in-circuit current meter and it read the same, right down to milliamps.

Just to clarify a few points here:
1. the voltage rating of a cro probe is not determined by the attenuation factor. A100x is usually higher rated than a 10x but a relative rating is not useful when confronted with a circuit that has a specific high voltage you want to measure. At that point, you need the actual rating.
2. I am not sure our OP is at the point of making a conscious decision about the continuity of the tx currents. Being a fixed frequency flyback it will start discontinuous but may move into continuous mode at higher loads. It is better if it does work this way.
3. the vast majority of flyback converters DO NOT work in discontinuous mode. Traditionally, a great many used a current mode control but there were still plenty of voltage mode control flybacks around too. I wonder if you are confusing the mode of control with the operating mode of the converter?
4. Discontinuous current mode will not guarrantee no reverse recovery in the rectifier diode. That PN junction still needs to adapt to the reverse bias and it takes effort to expand that depletion region.
5. The TVS is a good idea for getting this off the ground but be sure it is sized to suit. I have assumed 230VAC power so a 400V TVS would be about right maybe. Check data sheets (carefully).
6. There is no continuous loss from the resistor in the snubber. In a low loss snubber (one with R, C and D) the function of R changes from a damping factor control element, ie sympathetic to the resonance of the snubber C and tx leakage inductance, to a cap discharge function. There should not be any continuous resistor current unless the current is too small. Lode the diode and go traditional RC snubber. Good honest snubber.
7. The problem with the input current measurement, especially a converter with a simple rectifier / peak detector input stage and very light load is that the input current is not just small,but it is also quite wide band. The waveform will be a little, very short pulse of current near the middle of the peak of the mains waveform. (This is where crest factor comes in.) So the measurement device needs to be sensitive and have a reasonably wide bandwidth to have any hope of accuracy. To check the quality of your readings, I would suggest a waveform or spectrum capture and manual calculation from the capture. Messy and painful but honest method.

 

Thank you very much for taking time and effort in writing this. I have not given up. In fact I have been successful in making the smps. I am going in a research mode, making the ferrite transformer and noting down the voltages and other performance parameters to optimize the circuit.

The advice and points you have raised is very valid. The tx polarities are actually reversed as need for flyback topology. But I couldn't get an appropriate version in the proteus version that I am using.

I did purchased a new oscilloscope Siglent SDS1204X-E , just yesterday. This is my first scope. So I will have a lot to learn on how to use it first. I can't afford differential probe now So I have ordered an isolation transformer. I hope it would be OK (from various videos of actual testing of smps circuits ). But I wonder ? Won't this make the ground of scope floating ? How accurate the readings would be ? is it really safe?

Snubber circuit : You might be right on the value of the resistor. But it will depend on the voltages and switching frequency etc right ? I think I can arrive at a more accurate value after deciding on the transformer parameters. However I think the diode needs to a fast switching type rather than the ordinary IN4007 rectifier. I just received fs107 diode for testing.

I choose CC mode after going through various articles, circuit diagrams and datasheets of lot of switching ic's. One thing that I find good about UC3845 is we can change the switching frequency. This is very helpful when using a hand wound transformer.


I am afraid I really did not understand this // but somewhere along the line there must be a compensator that provides one pole and one zero in the transfer function. // Are you refering to the output voltage stabilization ? I am designing that secondary to get 16 to 20 v for a stable 12v, Hope it will work out.

Meanwhile I am making a AC current meter with stm32 to get a better reading of current consumption at zero load. The present clamp meter I have is not very accurate with low current values. I have completed software and voltage section. I need a Rail-to Rail op-amp which was not readily available. Got it today. I can complete the working modal in a few days. I hope this would be helpful.

One thing I am happy about is , I am blowing fewer components now. Very interesting and members in this forum has helped a lot. Thank you once again.

I missed your point about the compensator.
Yes, it is about the regulaton. The compensator is usually formed around the error amplifier and the compensation is to account for the phase shift and gain between the output of the compensator / error amplifier and the feedback signal from the converter output. If this is new to you all I can say is that it is a big topic which TI and others have several papers on. In your case though, because you are using CC mode it is a LOT easier than for VC mode.
To be frank, I am not sure how you might implement a compensator in a zener based regulator. I will likely only confuse you if I try to run through a few options, so again, I would point you at TI for an app note or some such. If you get into real trouble with it though, reach out and I will do what I can to help.

 

And this is the isolation tx confusion I was talking about earlier. To be clear: an isolation tx dissociates the mains neutral from the protective earth. The protective earth is usually continuous from the input connector of the tx to the output connector of the tx. (At one point in every installation, there is a short and solid connection between the protective earth and the neutral. This is typically in a distribution box / cabinet and the link is made between the earth bus bar and the neutral bus bar and the purpose served is to ensure that in the event of an active to earth fault the fault current is sufficient to cause a rapid disconnection by the circuit breaker or fuse.)
The only reason to be using an isolation tx is so that the ground connection of your cro lead, which is connected to protective earth does not make things go bang when connected to some point in the mains powered part of your project. Just try to think what would happen if you took the neutral of the mains input to your project and started connecting it to various parts of the mains part of your project, because that is exactly what you would be doing if not for the isolation tx.
So, it makes no sense at all to use the iso tx on the cro unless you also break the protective earth connection of the cro at its mains input (which will also make the iso tx redundant because the cro internal power supply will already have equivalent mains isolation as the external iso tx). Breaking the cro earth connection is a bit dodgy and not recommended for anybody, but especially for a novice. It will allow the cro chassis and all of the cro probe ground leads to be at lethal voltages if just one probe ground lead is connected to a lethal voltage.
Use the tx to isolate the mains input to your project. That way you can connect the ground of your cro lead with less fear of things going bang. The cro ground connection can still cause problems depending on where you connect it, but you should be able to avoid the worst of the explosive effects.
I hope this makes the use of iso tx clear and obvious and understood. Some of the advice given here by others is downright wrong and dangerous. To those other respondents, please beware offering advice on subjects you do not properly understand and that have a distinct possibility of ending a life. You might also avoid the misuse of random technical terms such as the class of an earthing system. It conveys nothing to the uninitiated and the rest of us know it is a meaningless detail and invoked in error.

Bottom line, understand what the 3 pins of the mains connection are and how they work, then understand what the iso tx does do, what it does not do, what you need to do and how the iso tx might help you achieve that in the context of your mains supply, your project and your test bench equipment. Then before powering up, take a moment to consider the safety of the configuration of things as you have them. It is better than being electrocuted. By the way,your primary DC voltage of ~320V is way more lethal than 50Hz 230VAC which is way more lethal than 60Hz mains. (50Hz is a sweet spot for putting a human heart into ventricular fibrillation. In comparison, 60Hz is hopeless at doing that, but it will have a good go at it.)

Very nice of you to explain the issue. Thank you.

 

You are welcome Sometimes, it can be satisfying and pleasing to help a person with a project and see them learn or succeed or both.

The 2A rating of your tx may be a bit light. The smallest one I use is 1kVa and up to 6kVa. The things to be aware of with lower power tx is that the output regulation is not so good due to the winding resistance and leakage inductance which are usually higher and can be a problem powering a small converter with a very high crest factor (peaky input current) or a larger converter with a PFC (power factor correction) input stage.
Just keep it in the back of your mind, if you encounter some problem, think about the iso tx and if it might be causing the problem.

Ok. I shall keep this in mind. Thank you.