Maximizing Heat Dissipation in High-Power SMPS Design

[Salvador]

ups sorry yes , i was writing late at night got mixed up , both devices failed at the short circuit position showing less than 1 ohm on all legs no mater how you measure.
my bad i didint put the lightbulb in series with the incomming mains , maybe because i wasnt expecting something to happen since the load was small and it worked just fine with the previous IRFP's as before I changed them to the FCH I tested and it worked fine , after minutes of glowing the heatsink wasnt even warm it was cold and everything was fine.

I guess I'll just solder back my two leftover IRFP's and see it it works , after changing the IR2110 and probably the SG3525 which are blown everytime they mosfets get blown.
if it works then i will go to my friend , an electrician he has some lab instruments at work and just probe the waveforms coming out of the IR2110 into the mosfet gates and the transformer primary waveform and see maybe there is too small deadtime , maybe something else.
anyways it kinda weird.

 

[Salvador]

now I know it's hard to tell anything if one has to answer about a circuit thousands of miles away but , everything in my device is like in the schematic except the zener that feds the octocoupler is higher voltage and secondly the shunt across the primary winding is a ceramic 660pF with a 39 ohm resistor in series , instead of the 47 ohm and 470 pF ones in the schematic.
my question is is the shunt needed at all and if so is a series LC the best one to use in this case? maybe some high voltage spike developed in that shunt or something like that?maybe i could just use the 39ohm resistor and the 660pF caps in parallel?

even though i kinda doubt this was the cause but who knows.

 

[meBigGuy]

I think you need to test with a more constant resistive load, rather than a light bulb. The surge at startup coupled with a subtle issue could cause destruction.
Slower switching due to increased gate capacitance, for example.

Could even be the difference between typical specs and real world worst case values.

I'm not sure what the minimum load would be. Too light a load can cause failure also.

 

[Salvador]

thanks for coming into the discussion, yes i also kinda though of that maybe the fch104n60 devices have a higher gate capacitance since their also faster and higher rated devices than the IRFP's caused this.but then again was it because the IR2110 power supply sagged or the IR2110 can't or isn't menat to drive these devices (doubt this one)
i have to say I haven't seen in many smps a circuit that could or would sense frequency stability.

but since the lightbulb attached never came on yet the devices failed I have a feeling that maybe the IR2110 wasnt able to drive the mosfets , maybe the first was was left half open and once the second one opened it created a path and as all of the rectified mains current and voltage started to flow through it created a sudden and fast heat up and destruction of both devices.I sort of get that feeling because at the secondary side there was nothing at turn on not even a flash , so likely the transformer didint even see a flux set up.

I will check I actually have an old electrical heater made out of the same alloy wire used in high wattage big size wire wound resistors. I will measure the resisatnce and see if it could be used as a load.

speaking of too light of a load , i mean an amplifier with no input signal is basically almoust an open circuit except for some bias current especially in class A or AB. but mine is AB so would it even be safe to power an amplifier with this?
but then again what do commercial smps use for this as the basic principle is the same there is a transformer whose core if left with no load can act like an boost inductor causing spikes in voltage and so on.

 

[Baluncore]

my question is is the shunt needed at all and if so is a series LC the best one to use in this case? maybe some high voltage spike developed in that shunt or something like that?maybe i could just use the 39ohm resistor and the 660pF caps in parallel?

The series RC snubber looks like 47R for the high frequency transitions but appears to be open circuit when not switching. The 47R dampens the switching transitions, the 470pF reduces power consumption by the 47R at all other times.

If you used the R & C in parallel, the combination will look like a short circuit during transitions and like a heavy load at other times. The C will cause a current spike that will destroy the mosfet. Only the 1uF will limit the 47R resistor current, the 47R should be rated at maybe 2 watt * (1uF / 470pF) = 4.25 kW. Keep to the series circuit.

The published circuit you have is good. Do not make changes without good technical reasoning.
The problem you have is with implementation of that circuit. Maybe post a photo of your physical circuit layout.

If you want your components to survive testing.
1. Get a 100W filament globe in series with the AC supply.
2. Reduce the 4x 680uF to something like 2x 100uF during early low power testing. That will reduce the current spike on control failure.
3. Use a 40W filament globe as the load during early testing.

 

[Salvador]

I just checked the datasheets and if I am reading them correctly it turns out that the more powerful FCH104N60 actually has a lower gate charge to fully open the device.
both Qgs and Qgd are lower than the IRFP 460 , so I think it would be unlikely that the IR2110 which was driving the IRFP no problem could have had a hard time driving the FCH ones? I am correct in assuming this ?

 

[Baluncore]

Devices with lower gate charge are easier to drive and so can switch faster.
FCH104N60 was selected because it was better in all respects than original, older design.

 

[Salvador]

well that's what makes me wonder , the gate charge being lower and other aspects being better and it results in a short circuit.
well i forgot to mention that instead of 4.7 ohm in the gate drive path I have 10 ohm ones, i doubt though that it has any major effect on what's going on but what you think?

 

[Baluncore]

well that's what makes me wonder , the gate charge being lower and other aspects being better and it results in a short circuit.

Drain current and voltage specifications need to be greater than or equal to the original design. Higher current mosfets take a greater area of silicon and so they tend to have greater capacitance with a greater charge to move during transition. Minimising gate charge is required to get a faster transition. The design of new mosfets is to lower the gate charge while increasing the maximum drain current.

well i forgot to mention that instead of 4.7 ohm in the gate drive path I have 10 ohm ones, i doubt though that it has any major effect on what's going on but what you think?

The 4R7 is there to stop a parasitic gate oscillation. By using 10R you have still killed that parasitic but you have also slowed down the transition which could cause overlap of the conduction. So it could be a problem.

Good high frequency layout is essential and critical to reliable SMPS design. For example, you can expect parasitic RF oscillation of the gate drive circuit if the distance between the IR2110 and the mosfet gate is greater than about 5 cm. How long and how thick are your floating common reference traces or do you use a local ground plane?

You have still not provided a photo of the layout of your implementation. We can still only guess at the cause of your problem.

 

[Salvador]

I will take a photo as soon as I will get a camera to do so, maybe in the weekend.since i threw this thing together much from what I had in hand and a little bit of "macgyver" thinking , the whole device consists of three parts , firt part is the mainboard which has the mains rectifier , capacitors and a small transformer for the IC's power supply, and which also has the other parts attached to it with physical connections.
then i have a separate small board which has the oscillator and control IC (SG3525) and the driver IC (IR2110) with all the periphery those IC's need on that board.then i have the heatsink with the mosfets on it next to that board and the connections to that board are made with short wires., everything else is the transformer and then there is the secondary side with the rectifier and so on , not the prettiest looking board for sure but everythings there.

now here's the question , before i touch the 10ohm gate resistor , does it really matter here ? because today as I looked at the datasheets and I think you confirmed that , that the FCH has a lower total charge (both Qgs and Qgd) so if the IRFP's have a higher charge then it should be the other way around , the system should fail driving the IRFP's not the FCH yet it goes vice versa.

now this evening i did some more tests using just one of the two onboard half bridge smps (basically just two of these circuits on one piece of a board,)
i had put the IRFP 460 back just to see what happens and guess what , I now put two bulbs in parallel , one was 120w the other was 57w
measured the cold resistance of these parallel bulbs and it was about 22 ohms.when i turned on the smps it worked with no problems whatsoever.nothing got warm no voltage fluctuations, no nothing.
How can this be , there must be some hidden obstacle here why the FCh devices failed wthout even giving me a glow on the secondary sides atached bulb, the previous time the IRFP's failed due to the much too big 500w halogen bulb , atleast the bulb got a bit bright before the IRFP'S reduced themselves to the ash they became.
(If I keep on like this I'll soon have a full container of copper to go sell to the recyclers , atleast i'll get some money back :D)

Anyways since the IRFP'S acost 1/3 of the FCH devices I will test them some more to see what they can and maybe i'll just stick to them for now , I hope on lighting up the 230w halogen bulb which has a cold resistance of 15 ohms , it they can do that I'm satysfied.
now the other options for testing I have (please don't laugh) is a sandwitch toaster which measures about 80 ohms of resistance , and then i have an old wire heater (basically a high resistance wire wound around a ceramic tube) the resistance cannot be measured because the wire is old and the contacts being metal are corroded from the effects of heat. with a multimeter it shows about 6Kohms which i assume can't be true since then its power would be very small but its somewhere between 600 and 900w.
Ok one more question suppose my IRFP's work happily cooking me bread in a toaster or heating my room in a dummy heater load, but what would they do with driving an actual amplifier, now i don't know what te resistance is as seen by the psu, when one listens to music on low levels since the transistors are always only 1/3 open or so but what would happen if i used a powerful speaker at high volume, assuming the speaker is either 4 or max 8 ohms , i mean what would the PSU see as its resistance ?
because I know what it does when it's sees a cold 10 ohm filament for a split second , it disintegrates.

 

[Baluncore]

For SMPS design and construction help, download this pdf; http://www.ti.com/lit/ml/slup224/slup224.pdf
Take a look at topic 4=Layout, topic 1=safety, and 3=AC line.

 

[Salvador]

I am reading this pdf now, while at it I have a question that I couldn't find doing google searches.
when the psu is connected to an amplifier load, aka drives an amplifier, surely the load of an amplifier is not resistive only but it's an impedance load , so in this case waht would be the average load seen by the power supply driivng the load through an amplifier?
surely i understand it would change constantly since music is a signal with varying waveform, but just assuming some averages here , given that the amplifier board can take about 500+ watts at 4 ohms , assuming its driving a decent speaker load at those 4 ohms.
I guess its kinda complicated to arrive at the answer , yet it will determine if my psu will go up in smoke again if i turn some club hit loudly on a big sized woofer... just for example.

P.S. the 230w 230 volt halogen bulb test also comes out as successful , runs with no problems ,as much as i could measure the filament when cold it was about 16 ohms. which gives me the starting current at 140 volts DC of about 9.3 amps.

 

[Baluncore]

surely the load of an amplifier is not resistive only but it's an impedance load , so in this case waht would be the average load seen by the power supply driivng the load through an amplifier?

A SMPS sees it's output capacitance. The SMPS regulates the voltage on that capacitance, as best it can.
The SMPS must provide the required charge to maintain capacitor voltage.

The amplifier sees that same capacitance, with a fixed voltage, and so can draw the current it needs.
A regulated capacitor voltage Vc, with a variable amplifier current requirement i, represents a variable resistance load R = Vc / i.

The dynamic impedance of the amplifier is isolated from the SMPS by the intermediate storage capacitance.

 

[Salvador]

the actual running of the smps is poor, it doesn't drop volts when running with the lightbulb but that is only because its operational white hot filament resistance is high and it drawn only about 0.8 amps at 140 volts DC.
I was testing today using resistive dummy loads and an oscillator , the gate drive waveform appears to be good , nice symmetrical square wave , .firstly i tested loads from 40 ohms to 20 ohms and all of them made the output sag, at 20 ohms the output went from 140 vDC to about 110vDC, and the higher in resistance i went the lower it got yet still.
now in the schematic i measured the frequency putting the probes first for the upper transistor at leg7 and leg 5 on the IR2110, then for the lower one leg1 and ground.
in both cases i got about 1 square wave for about 20uS.

now if my maths is right there are a million uS in one second so dividing those million Us with the 20uS i got one period at the gate drive turns out to be 50khz , so the frequency is about right, also the duty cycle seems to be correct,
is it possible that having too many turns on the primary could result in less power considering this topology? i just saw an old paper after which i made the transformer and it has a little over 30 turns for the primary if I am correct.

 

[Baluncore]

at 20 ohms the output went from 140 vDC to about 110vDC, and the higher in resistance i went the lower it got yet still.

Which indicates that the system is not regulating duty cycle correctly, or that there is a limitation in the energy flow path.
Maybe your transformer is limiting energy flow? Reduce the number of turns to detect if saturation of the magnetic core is a problem.

 

[Baluncore]

Attached here is a useful reference for use when designing SMPS transformers.

 

[Baluncore]

Here are more. Everyone has a different approach. Get all these files, then browse them until they start to make sense.

slup123 “Magnetics Design for Switching Power Supplies”; http://www.ti.com/lit/ml/slup123/slup123.pdf

slup171 "Magnetic Field Evaluation in Transformers and Inductors". http://www.ti.com/lit/ml/slup171/slup171.pdf

Attached is; slup205 "Transformer and Inductor Design for Optimum Circuit Performance"

 

[Salvador]

OK, now I have a lot of material will take me a few days to get through it all with a cool head.
meanwhile i can say that he first smps (two of them being on my board) is running with a "3c85" rated ferrite core, I am not sure i'll try to look up maybe judging by this ferrite index we can approximately know how many turns would be required for a given frequency since we know the ferrite brand.

also what makes me wondr is that I was using a 82v zener for the feedback, since i though that i could have more voltage on he rails since the amplifier can take it.
yet still I am only doing +-70 volts DC output at max. so maybe i should just get a 66 volt zener and see what happens , the octocoupler should work then and maybe youre right , maybe the pulse width isn't regulating.it probably isnt.
i will have more oscilloscope tests next week then i will also try to measure the duty cycle with different loads.
it has to be the transformer that limits the current , the output diodes used are MUR1560, the driving circit is working fine.
maybe the fault lies in the transformer design , still a mystery why the FCH devices failed

 

[Baluncore]

yet still I am only doing +-70 volts DC output at max.

If the output is not regulated then do not mess with the feedback circuit.
You should instead lower the test load and identify the reason why the supply has such poor power output that it will not regulate.

maybe judging by this ferrite index we can approximately know how many turns would be required for a given frequency

That core is for a wound transformer, NOT an inductor. Frequency is determined by the components on the PWM clock.
The important thing now with the transformer is the ampere*turns which could be causing current limiting by saturation of the core.
The inductance of the transformer determines magnetisation current.
You need as many turns as possible without core saturation at maximum output current, so as to keep the magnetising current as low as possible.

Keep reading the references until they start to make sense and you can put numbers in the equations.

 

[mheslep]

You must be careful testing that circuit. I use a 1:1 mains isolation transformer and Earth the common reference when working inside live switching power supplies.

Yes, that's step two or three for me. Before getting anywhere close to the mains voltage, I'll run the primary off of a low voltage AC bench supply and the control circuitry off a bench DC supply, with the feedback loop jiggered accordingly. Later with with the mains isolation transformer in place, first use a well below design rating fuse, and so on. Little by little.

 

[Salvador]

ok here's what i think i should do , take a 0.1 resistor and place it in series with primary winding then attach scope probes across the resistor and see the waveform under the condition when the secondary voltage sags abut 10 to 20 volts under load. this should show me whether the core is saturating or not, even though i read that in half bridge the core should't saturate under load if it hadnt saturated at idle already, kinda makes sense because as you add load to the secondary it becomes harder for the primary to keep the same magnetizing current as it did with no secondary load isn't this true?

also i think i should insert the 66v zener instead of the 82 volt one and then drive the smps under the same load and monitor the signal on the gates of the mosfets with the scope to see if the waveform chages under load (whether PWN works or not)

also if my calculations are correct the supply did 110 volts instead of 140 idle when it had a 20 ohm resistor as its secondary load , a simple calculation shows that V/R = 110/20 =5.5 amps , now take 110 volts times 5.5 amps turns out to be just a little over 600w of power delivered to those resistors at that time and load, which isn't exactly small and maybe isn't my goal either , so right now it seems that either core saturation or non working PWN could be the cause for the drop , as if i would maintain 140 volts with a 20 ohm load , the power would be 980 watts , and quite frankly that would be way too good for two IRFP 460 driving a homemade half bridge.
so maybe its not that bad after all.
ok I will continue on reading the materials provided and other information and also measure and test my supply to see where the falt may lie, also the destruction of the FCH devices is still an open case.
any information from you folks is much appreciated and also I want to say thank you for the help already given on this topic.

 

[Salvador]

also , i think I will increase or at first for experimental purposes use a reostat, in place of the resistor between sg3525 pins 7 and 5, now there is a value of 33 ohms as shown in the original schematic , but with this low value the deadtime mayb be too short and given that i have more turns than originally shwn on primary may have been the cause for the FCH untimely death.
i also saw on scope when i probed the gate signals that there is close to no deadtime.my plan is to increase deadtime with this resistor and then after tests increase frequency with the decreasing of the other resistor for which i already have a reostat inserted and then will measure the voltage response to applied load.

 

[mheslep]

What kind of scope probe do you intend to connect to the primary?

 

[mheslep]

Jim Williams of Linear Tech has the definitive smps design guide, imo.
http://www.linear.com/docs/4120
See appendix C in particular for a safe method of testing out your magnetics and switches for saturation prior to assembling the entire design.

 

[Salvador]

I see that in the west you are extremely concerned about safety , I mean it's not like I or anyone else I know measures the potentials with his fingers or probes a live switching circuit while washing his body in a bathtub full of water and a nice long earthing cable attached to it.
and while on this topic , quite frankly I have seen safety regulations being abandoned much more by professionals than amateurs like me , because when i get to a place where once in a while I am not certain what might be waiting at the other side i use a safety tool instead of my hand to check things or sometimes when i have worked with live AC at the wall installation just those rubber gloves that insulate from electricity also stepping on a rubber pad combined with the gloves does the trick quite nicely.as for the question you asked , I will be using as I said alow value resistor in series with the primary so that i can attach an oscilloscope and see the waveform , i mena i could do it otherways but then i have to use some resistors in other way because its an old but simple soviet made scope and the max you can see in it is about 50 volts +-.
the man with whom I am working with has repaired tons of tv's back in the day using that and other scopes so we will just probe the primary to see what going on with the waveform.
also the deadtime , i have to make it longer at first to put less stress on the mosfets and to save them from a potential current shooting through.
also will see whether the length of each pulse has to do with driving the core into saturation which maybe limiting the power output.

 

[Baluncore]

I see that in the west you are extremely concerned about safety ,

Are you suggesting that those in the East are not, or the West should not be extremely concerned about safety?

PF is seriously concerned both for everyone's health and the liable for any injury or death to anyone. PF does not have to carry that liability, the thread could simply be locked. I know that you will continue to experiment even if this thread is locked. Personally I consider it safer to remain in contact and keep warning you than to lock the thread. Others may read this thread who are neither as experienced nor as careful as you. They definitely need the warnings. You are not the only reader of this thread.

When probing the active part of a SMPS you really should be using a 1:1 isolation transformer and earthing the common reference that would otherwise be connected to the live mains voltage.

You really do need to acknowledge the concern expressed and agree to take the advice seriously, or you may find this thread being locked like so many threads like this have been locked in the past.

 

[Salvador]

by the " West" i was reffering more to a kind of political/economic /cultural part of the worls rather than specific places , I'm sure you took it as that.well there's a lot of talking involved in this and this is not the place for it , all I can say is where I come from (the former USSR) nowadays Eastern europe (badly misrepresented in the world media) we just do things (yes sometimes with much less care) differently.Yes the things you mentioned are correct no doubt but since I don't have an isolation transformer , but i do have non conducting floor (non conducting with respect to domestic voltage levels) automatic circuit breakers , a fuse before the smps and working with tools that are made of non conducting materials , I personally am safe. the advice to put a light bulb in series with the ac mains entering the smps is on the other hand a really good advice as to save the components from blowing in case something goes wrong and saving extra money and work.

I think it boils down to the difference in understanding who is to blame if someone does something foolish, in the " western society" you feel and maybe even are physically responsible under law to bear responsibility for someone who had an accident if one proves that he did it because someone told him on the internet , in our side of the world no one would ever be held responsible for that.More like each man for himself ,
in no way I am implying that simply because of this me or anyone else should give false safety instructions or no safety instructions at all, I am just saying that not always the laws of physics and the safe handling with them go together with what PF or any other human made website or organization considers safe or not safe.
I have felt many times this limits the amount or especially depth of information one could go into sharing all kinds of know how and tricks but you see I can't because the moderators of this forum think I'm beyond law or telling others unsafe practice even though that is far from truth.
My personal opinion is such that everyone is responsible only for his own actions and not those of someone else unless he is physically or in other ways pushing that person to do the things he wants. If someone in the world decides to do something crazy he will do it anyways no matter what you tell on some internet forums.Ok for the sake of this thread let's forget about this philosophical issue and Baluncore , youve been a help so far , could you please share some opinion on the questions posed in my previous two replies? like for example the pwm part of the half bridge etc.
personally i think i would be better of reducing the duty cycle length and increasing frequency , as to give the transistors more deadtime , since the core saturates probably quite fast anyways so the longer duty cycle is useless beyind saturation yet the higher frequency could push more useful power through the core.

 

[Baluncore]

personally i think i would be better of reducing the duty cycle length and increasing frequency , as to give the transistors more deadtime , since the core saturates probably quite fast anyways so the longer duty cycle is useless beyind saturation yet the higher frequency could push more useful power through the core.

I am not going to answer your endless streaming thoughts on modifications to a tested SMPS design if you are not prepared to focus on the real issues. So long as you avoid those critical issues, you are lost. You are reacting emotionally to the circuit. I expect you will mess it up even more by going off on further “reality avoidance tangents”.

It is clear that you have not resolved a fundamental transformer problem. Timing is irrelevant if the transformer saturates with the required output current. That will quickly destroy mosfets. Your PWM FB loop is not regulating, maybe it is the transformer, or maybe you just used cheap output capacitors with high ESR. We have no idea because you have not posted a single photo of the layout. If you vary component values for any reason you will almost certainly fail to meet the specifications of what was once an optimised design.

You cannot expect to successfully prototype a SMPS without an isolation transformer. If you need to, you can make an isolation transformer out of two identical microwave oven power transformers. Knock out the current shunts, throw out the secondaries, put both identical primary windings on the same core, you then have a 1:1 isolation transformer rated well above 1kW.

 

[Salvador]

first things first.By putting a reostat for both deadtime and frequency control of the SG3525 I am in no way 'messing' anything up, rather it could be considered as an extra option to stabilize or improve the circuit considering its put together using many leftover parts and maybe not in the best engineering sense.

Not the first time people start "hatin" on me here on PF.probably because i always ask side questions and talk much , yes I do have a weird way of exploring things including the ones i like but eventually I get there either with or without the help of others or established learning methods.

Ok just for a little relief , here comes some pictures.
http://foto3.inbox.lv/girtsliep

in this link there should be two folders , one is a folder with some pics of a few of my home made and repaired amplifiers, the other one should be called 'smps'
a few things I want to note, when viewing the mainboard circuit trace side , don't worry before this application i made it originally for other intents so the only traces used here are the mains input rectifier and filter caps , everything else is either on other boards or uses wires.
then above the filter caps there is the small board which has the SG3525 and both IR2110 on it., from that board there extends a bunch of wires no longer than 5cm to the gates of the mosfets.
everything else well is what it seems, two ferrite e core transformers , aluminum heatsink for the high frequency rectifier diodes etc.

 

[mheslep]

so that i can attach an oscilloscope and see the waveform

I asked the question because, if you use the scope probe ground on one side of that inline test resistor on the primary, without an isolation transformer, you may well short the high voltage primary to Earth ground through the scope. That won't go well for the scope lead or your primary circuit.

 

[Salvador]

ok mheslep, I see your point ,. but the scopes my friend has have no grounding at all in the socket , nor are they otherwise grounded, nor the scope chassis has any ground connection, . most of our sockets don't have any grounding , just the phase and the neutral , even those who have the separate ground connection are useless because the older cables had only two wires in most home AC installations.
the only places were a grounding wire extended to the local ground spot, were the ones installed in bathrooms or kitchens for washing machines , water heaters etc.Also nothing bad can happen to the scope otherwise because the voltage will be only that which is dropped across the resistor hence very small.

P.S. a theme for a whole other topic but a little mentioning, separate grounding in places like bathrooms in apartment buildings in cities is kinda trivial , I once measured the resistance of the cold water pipe in my 4 story appartment building (most of it metal some of it plastic) I made the measurment such that one probe was attached in the bathroom the othe in the kitchen , the pipe in the kitchen extends all down the house through the basement up at another place into the bathroom , I was surprised i found about 2 ohms of resistance and 1.2 of them came from my copper wire extension which went for about 3 meters.
so technically in cities were miles of pipes are all connected together they basically act like a big very low resistance grounding wire.I'm pretty sure one couldn't make a better lower resistance ground path than the one that naturally occurs in such a pipe.
So if my electrical water heater developed a ground fault at some point it would be hard to tell whether the ac would be cut by my (nonexistent) ground fault circuit breaker or by the disconnection of the typical automatic circuit breaker(there is one) in series with the heater due to the sudden large current running through the attached pipe down into the city's pipeline.
in the end of the day i assume it's all about which one of these devices are faster in cutting the AC supply.faster in terms of the splits of a second.

 

[Salvador]

I have a question specifically for Baluncore, while reading your given documents i came upon a place were it is said quote

In practice, the flux density change, ΔB, is
limited either by core saturation or by core loss,
thereby limiting the volt-seconds per turn that
can be applied to a specific core cross-section
area. To fully utilize any core, the design should
result in ΔB close to the saturation or core loss
limit, whichever governs, by adjusting either the
core area, the number of primary turns, or the
ON time.

so this is basically saying the same thing i said earlier about having a reostat to control the duty cycle manually, or to see whether the feedback is working , even though I already have the close maximum duty cycle as i saw in the scope while measuring gates.
anyhow i can only be certain after i probe the primary and see whether its saturating or not , spekaing of which would probing the secondary also work for determining core saturation ?

And since I didint get a reply on this question I would like to restate it, when the core saturates , does the saturation decreases with a heavy load on the secondary or not?
my reasoning here is such that in order to drive a core into saturation one needs energy applied for a given amount of time to the primary but under heavy secondary loads most of that energy goes into the secondary load so why would a core saturate under heavy load even if it did that at light or no load on the secondary? reading further i think i found the answer

The flux density in the core (which links both
windings) is determined solely by volt
seconds per turn applied to the primary
(Faraday’s Law), independent of load
current.

thank you.

 

[Baluncore]

ok mheslep, I see your point ,. but the scopes my friend has have no grounding at all in the socket , nor are they otherwise grounded, nor the scope chassis has any ground connection, . most of our sockets don't have any grounding , just the phase and the neutral , even those who have the separate ground connection are useless because the older cables had only two wires in most home AC installations.

The Coroners verdict will be that "the deliberate use of an oscilloscope with a live common chassis" was tantamount to the murder of those who responded to your suicide. I need not say more.

so this is basically saying the same thing i said earlier about having a reostat to control the duty cycle manually, or to see whether the feedback is working ,

NO. Live controls are dangerous to you and the circuit. You can adjust it to the point where it destroys itself while you are trying to optimise output. It is one thing creeping up on an optimum operating point, but you must also be able to get there and back safely when applying, removing or momentarily interrupting power to the circuit at any time. In aerodynamics that region of operation is called "coffin corner".

You must first design everything by the numbers. The data and equations are there in the references. A SMPS is too complex a system to rely on luck.

Contrary to your belief, the celebration of a “Wild West” approach to engineering represents engineering incompetence. Not only is your engineering reasoning unsound, but so is your critical thinking about your approach to engineering. If you cannot analyse your approach to engineering then you cannot expect to analyse the technical engineering issues.

 

[Averagesupernova]

Also nothing bad can happen to the scope otherwise because the voltage will be only that which is dropped across the resistor hence very small.

Ok, the above comment to me shows this thread is on the verge of needing to be shut down. There is no place in the primary circuit you can put a series resistor and have one side of it already grounded simply due to the nature of the design of the circuit. Sticking a scope probe ground on either side of a series resistor in the primary circuit guarantees disaster. One way to 'see' the voltage across a resistor of this sort using a scope is to have a 2 channel scope that you can invert one channel and use the summing function of the scope. The difference in voltage from one side of the resistor to the other will then show up in the scope display.
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If you want to test for proper feedback vary the line voltage up and down slightly with no load. If a power supply can't regulate with no load you cannot possibly expect it to regulate with a load. My impression throughout this whole thread is that you are not troubleshooting this in a manner that yields any useful information. A lot of shoot-in-the-dark here.

 

[Salvador]

ok I have usually quite a lot of enthusiasm but even with all of that I have to say I am done in this thread.Instead of answering a few simple questions about core saturation etc I read only about that damn scope thing.I do realize it's not among the safest of practices.And whne i come to think of it maybe there is some other way to probe the primary involving a capacitor but not in series.
the man who has the scope is turning 61 this year he works as an electrician and repaired crt's back in the day for a living, also keep in mind that this scope came from the factory with no grounding wire and such scopes don't have their input test ground also directly connected to their chassis.otherwise when making any kind of measurment you would almost always spoil it because your body capacitance affects the readings and everyone knows this since i assume we all have touched the measuring wires with hands at some point to see the waveform on the screen being distorted.

i'm not an expert on smps's but I also don't like or expect to have a " wild west' approach.Enthusiasm and wild west in not exactly the same thing.
My second question was totally disregarded in the light of my first one, I don't consider that fair.PF becomes more and more useless by the day since now I don't even see the old staff coming by to comment except for a few , and then there are a bunch of people with pretty good feedback and even some titles (aint going to call names even though i would love to) which just cruise by and every thread that somehow is dangerous to their mind or where they spot some possible inconsistency with the rules they just report and sometimes give their "golden" advice in a few lines of comment which are sometimes on the border of being intellectually disrespectful, I mean maybe they should just consider their candidacy for the local police since police is meant to watch the law but a forum for physics is meant to learn and explore.
and after all you have to realize that no matter jow much you enjoy pointing out various legal issues about giving some safety advice which let's be honest you don't really care about someone at the other side of the world that much might as well just answered his question with explaining the possible drawbacks of the situation he is in.

if i'll get any positive results I will come here maybe to write a few short lines (even though i dotn see much use of it)

as for what averagesupernova said , yes indeed I would tend to agree that it cannot regulate itself if even at small loads it would " walk" a little.
I actually measured this when i was driving a light load , the mains voltage drifted some 5 volts and my output with a constant resistive load also did some 2.5 volts lower and then back again.
i've made some nice resistive loads of various resistances and after a bit of adjusting and testing will see what goes on.and what this thing is eventually capable of.