SMPS float capacitor question

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  • #76
mheslep
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the device is not run from a 7812, its run from a small transformer , rectifier , and two 12 volt zeners in series., in other words from a regulated 24 volt supply.
Your schematic indicates a 7812 off a 15V (?) transformer.
 
  • #77
Baluncore
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as I switch the power on the lightbulb in the ac mains barely glows for a little while might have been the split second then I hear a crak noise and the light bulb lights fully.
The design was for kilowatts, but you were only testing 100 watt. Obviously, the energy stored in the full capacitance was capable of destroying the MOSFETS. When doing initial testing you should have been using much less storage capacitance after the line rectifier. You were warned.

the device is not run from a 7812, its run from a small transformer , rectifier , and two 12 volt zeners in series., in other words from a regulated 24 volt supply.
So you have no 7812 = +12V regulated floating supply, but instead use a 24 volt zenner clamp? Do you have a limiting resistor before the zenners ? Does that explain why the half-bridge driver may be unable to correctly drive the MOSFET gates?

How many other things are different from the original design?
 
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  • #78
mheslep
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Another good application note here.
http://cds.linear.com/docs/en/application-note/an19fc.pdf

The PWM controller is another make but the math is all applicable, the discussion is accessible. Especially note the discussion on how fast current rises in an inadequate magnetic core, on the order of amps per microsecond, and here the switch period is up to 20us.
 
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  • #79
Averagesupernova
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Hmmmm.
How many other things are different from the original design?
Yep, it just keeps getting better.
 
  • #80
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, thanks to all who are still interested even though I see its turning more into an amusement thing rather than a thread here.
well you may think im dumb but it's not that bad.just because i didint use something mentioned in the schematic doesnt mean its not working , I had a small transformer , and a few zeners so i thought i will use them and not buy the regulator.the transformer supplies 28v DC after rectifier and smoothing, i thought ok will give me some headroom.
i used two 12 zeners in series with a 10 ohm 2w resistor. if my math is about right this gives me an output of about 24v and close to 500 mA.
after all the chips can take up to 35v for the sg3525 and 25 for the IR2110, and since i have two IR2110 each driving a half bridge the extra voltage doesnt do bad.
now why would this be a problem here? as I said I probed the gates , the square wave is nice with no sparks and spikes.its about 16v in level on average.
i did probe the power supply for the IC's under load. it drops from the 24volts which i get with no IC's inserted to a steady 20 volts.the difference between driving a light load and a heavy load was only 0.5 to max 1 volt drop in the IC power rail.
again given that the FCH devices have less gate capacitance I fail to see how this could be a problem. Now ofcourse you can go the simple way and just blame a bad design from an amateur but up to the point when im writing this i really cant see why they failed.
and noone also mentioned the fact that they are rated almoust twice the IRFP's power in amps and a 100 volts more in volts and they fail so easily, I can't understand that.
i guess i should have tried using one of them simply as a light switch , just turn it fully on and leave it there and see what happens.

you may say the gate voltage was too high but the gate voltage to both these devices are standard +-20 volts so if one failed the other should too.
just please dont tell me that the IRFP is a foolsafe thing :D


sadly the time it took for them to fail is so short i can't physically manage to probe the gates with a scope but my guess here is that either the gates are driven too weakly and something goes wrong even though i find it weird as already mentioned a million times the capacitance is smaller and should be lighter to drive or yes the other scenario call me names all you want but maybe there is something wrong with the mosfets, as in the gate shoots through like breaks down.and then it shorts out on all three pins.a rather rare but not uncommon failure for a mosfet.I mean i tested them on two transformers each a bit different, also with current limut etc , i doubt anything in the circuit except for the gate drive could have caused teir mysterious falure.especially since the high current and voltage ratings im pretty sure by now there is something wrong with the gates or with how they were driven , but i cant still figure out what , I will measure the gate voltage next time on scope to very precise levels at the IRFP gates , maybe just maybe because the FCH had less gate charge the IR2110 were driving the gates to higher voltage than the IRFP's and that could have caused the gate to break down and short itself with the mosfet's current channel.

also this begs the question which will probably be left unanswered in the light of some funny and " smart' jokes about my circuit :D but does the IR2110 drives the gates with the voltage level it is being supplied or does it has a built in drive mechanism which adjusts the voltage even though i dont see any circuitry on the ic's periphery which would be doing that, so could it be that if the IR2110 gets supplied by 22 or at most say 23 volts DC it would also drive the mosfet with that voltage level?
 
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  • #81
mheslep
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If the FETs are repeatedly failing, instantly, then, either 1) their breakdown voltage is being exceeded, or 2) the current ratings are being exceeded while gated on. Period. If the failure occurs over slightly longer than instantly then include 3) exceeding the heat dissipation of the package. There is nothing "mysterious" about the failures, other than determining which of the various things you think are so are actually not. Given the conservative nature of IC ratings they are probably being exceeded by a high margin. So resolve to methodically investigate which.

For instance, what's the highest possible ring voltage against the FET, without the snubber? At FET shutoff, V=(1/2)Ldi/dt. If, say, L=200uH, di = 10A and dt is 0.1uS, Vpeak=1000. To investigate current problems, disconnect the AC and run the primary off, say, 18VDC from a couple of nice and safe 9VDC batteries ( scaling your feedback network accordingly.) Try some of those industry references supplied above for help.

Also, if you manage to get this switcher working, your layout and assembly shown in your picture is as likely to render it a big unstable 50 khz radio and spike noise feed into your AC mains as it is a power supply. Because of the current parameters in the example above, i.e. 100A/usec, one can not easily throw a SMPS assembly together on the bench as is possible with other electronic projects, i.e. long wires and no ground planes on the PCB (both of which are the case in your photo).
 
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  • #82
Baluncore
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could it be that if the IR2110 gets supplied by 22 or at most say 23 volts DC it would also drive the mosfet with that voltage level?
That is the case.
Recommended Operating Conditions. IR2110. VCC Low side fixed supply voltage 10V to 20V
Absolute Maximum Ratings. IR2110. VCC Low side fixed supply voltage 25V

You were warned. In post #40.
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.
 
  • #83
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Ok here is the summary so far from what I have seen , tested , thought and read from your asnwers.
even though the smps looks like a mess it's surprisingly stable , i mena i havent probed the AC mains side to see any bad feedback thrown into the grid but as long as the secondary side dc and primary side gates are concerned everythings nice and square or nice and flat.

as for the mosfet destruction problem , I think I know the problem , too high IR2110 supply voltage. Can't be anything else beacuse if it were to do with the Drain source voltage being too high or having spikes or the transformer inducing some spikes upon switching then the IRFP's would have long failed because they are less powerful.but they havent.
Due to the IRFP's having three times the capacitance on their gates as the FCH ones the IR2110 has a harder time driving the IRFP460 and so its supply voltage sags to an acceptable value , but when i had the FCH104N60 it's gate charge is lower and the IR2110 probably were driving the gate with something about 21 to maybe 22 volts based on what I know about the capabillities of my small transformer zener supply.
the most logic explanation here and also the most likely one is that somehow the FCH devices were driven with too high gate voltage due to their lower capacitance rating and hence them being easier to drive by the PSU.
also the second time i tried the FCH ones i was having a light bulb in series with the AC mains so current was little and probably didin't cause the damage but voltage was still the same so the IR2110 still got the same voltage and that explains the same results obtained.also when i did the second test with the series AC lightbulb the current could not destroy the mosfet simply by going D-S because the mosfets capability is rated at 37 amps and I was no way near that even wtihout the globe.


should have been better to simply use the all in one linear voltage controller. or just get a transformer whose secondary is closer to what i need because messing with these small transformer to rewind them is a pain in the ***.
or should have changed my zeners to regulate to 20 volts with a 15 ohm resistor.

well i guess a mosfet's gate is a really sensitive thing , but now to prove my theory right i should order two more FCH104n60 or anything close to that and reduce my gate voltage levels.would love to the bad side is that they have to be ordered, while the IRFP's are here and three times cheaper and i have gotten good enough output power with them so far.I'll take my tikme to think about this.
 
  • #84
Baluncore
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The simple resistor–zener regulator really must go. It is very inefficient in that power supply application. It would require an emitter follower to improve efficiency.
Much better to replace it with a 7812 / LM340-12, you might find one in the output of an old desktop computer power supply. The 7812 can handle a maximum input of 35V, so the transformer secondary must be below 24VAC.

There is no advantage in switching MOSFET gates through more voltage than needed as it takes more time to move more charge with no reduction in Rds. A power MOSFET has thousands of gates in parallel. Only one has to fail at 20V to destroy the device.
 
  • #85
mheslep
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A comment on cost concerns of a SMPS project like this.

If the primary object is to DIY and thus save money over a complete supply purchase, then that goal is pretty much impossible, here and in eastern Europe, unless all the parts are somehow already on the bench. One can simply not buy and ship all the piece parts of a kW supply, even with zero value on labor and the necessary test equipment, for less than the cost of an off the shelf supply. I see $40/1000W, new, including cooling fan, chassis with safety and FCC approvals, connectors. Used, less still. Otherwise, an effective goal here must then be self-education.
 
  • #86
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It's basically the fun of building something yourself , the money is not that much of a concern here.Also I really don have most of the parts on the desk , the only ones i have to buy are the power mosfets and driving IC's.
I agree the linear regulator would be better, I just checked my local store (no shipping time) and they have in stock the UA7815, I think I will just go for that , regulated 15 volts @1.5 amps max.the output after bridge and capacitors from my small transfomer is 28v DC so the regulator should be fine.
One question though, ok the zener regulator isnt that effective but since this is a linera regulator wouldn't it produce quite some heat considering it would have to drop bit more than 10 volts on an average of 500mA?
or maybe it has some advanced little circuitry inside and that's not the case.Because usually when I imagine a classical linear regulator I see a half conducting BJT which then is only suited to low current applications due to the excessive heating.

as for the mosfets to take in place , i've been doing some search and I found basically these , IRFP32N50K, is in stock at my local store and costs about 7 euros., but then from the RS catalog I can get SiHG32N50D-GE3 for bit less than 3 euros a piece , I think it's a good price and the datasheets tells me the parameters are good.
surely the fairchild devices may have been better but they also cost more and for this particular application the power I'm getting even with the 460's is almsout enough.
Or maybe the Vishay products are less quality ? Also I dont quite understand the reason for pricing , as from the datasheets i see that amon similar rating mosfets some cost for example in the 7-10 euro range and some others just about 3 to 4 per device. Why is that why would some of otherwise similar rating devices cost more than half less?

Also I see some good high current but lower voltage rated devices.Now heres a question , for example if I used a 50a 200v device for both mosfets in the half bridge , as long as the deadime is kept in check and no spikes occur , the devices should work because the voltage any of the device has to whitstand is only half the rectified mains so about 162 volts in my case.I do realize the device would have a higher chance of failing under some unwanted events like spikes from the primary or else but what do you think?
 
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  • #87
Baluncore
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One question though, ok the zener regulator isnt that effective but since this is a linera regulator wouldn't it produce quite some heat considering it would have to drop bit more than 10 volts on an average of 500mA?
What is this? The zenner or the 7812? Where does that average 500mA come from? Only current pulses are needed when MOSFETs are switching. Resistor with Zener must carry current maximum needed over the full cycle. It will waste energy whenever current is below peak of cycle, = very poor for power pulses.
Why use a 15V regulator when 12V is more than enough. Higher gate voltages take longer to switch, so overlap of conduction is more likely. Your refusal to follow the original design suggests you must be an anarchist. Come the revolution, things will be different. Not better, just different.

Or maybe the Vishay products are less quality ?
Buy on specification, not a guess of quality. The quality of your circuit construction is nowhere near as high as quality of Vishay products.

Also I see some good high current but lower voltage rated devices.Now heres a question , for example if I used a 50a 200v device for both mosfets in the half bridge , as long as the deadime is kept in check and no spikes occur , the devices should work because the voltage any of the device has to whitstand is only half the rectified mains so about 162 volts in my case.I do realize the device would have a higher chance of failing under some unwanted events like spikes from the primary or else but what do you think?
When one mosfet is on the other has full voltage of -162V to +162V = 324V total.
 
  • #88
Averagesupernova
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One question though, ok the zener regulator isnt that effective but since this is a linera regulator wouldn't it produce quite some heat considering it would have to drop bit more than 10 volts on an average of 500mA?
or maybe it has some advanced little circuitry inside and that's not the case.
A zener regulator with a series resistor is less efficient than a 7815 since the series resistor with the zener is ALWAYS conducting. A 7815 will not operate in this manner. The 7815 only wastes power when there is current passing through it.
Because usually when I imagine a classical linear regulator I see a half conducting BJT which then is only suited to low current applications due to the excessive heating.
There are plenty of linear power supplies out there that are high current supplies. 50 amps is not uncommon.
 
  • #89
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fun thing tho about the anarchists , i see you like political philosophies, me too, I could call you some interesting names but i doubt that would be productive for this thread, no offence.and after all, i've followed the desing in everything except this thing , sadly i didint rethink it through about the voltage exceeding the mosfets gate capabillity, might have saved the fairchild's.After all im not that rich, and the local metal recyclers are having enough copper already so my two cents are not necessary.

I see the thing right i forgot that once the high mosfets turns on the only thing between the +ve and ground is the low side mosfet so it then has to be able to whitstand the full ractified mains.

by this i was reffering to the 7812.I think of buying the 7815 because 15 volts is in the acceptable voltage level for a mosfets gate and mainly because the 7812 is not in my store but the 7815 is.after all ive been driving my mosfets with 20 volts and they have stood except for the fairchilds which took a bit more than those 20 and lost the game, 15 vmax should be good.i will see if i can get the 12 volt version but if not ill go with the 15v one.

as for the quality of my circuit, it's not like i cant do nice and high quality stuff it's just that the board im using here i was actully making it for the very first smps i had years ago which used a simple IR2153 driving a pair of mosfets.I was just too lazy to go all over the board making stuff again and since i had the amplifier boards already and so i just built some periphery stuff and made this smps.
if you have any interest whatsoever I will post some pics of the box my friend welded for me from aluminum which i got at a warehouse.the idea is simple to make the box smaller and requiring less space it's made from aluminum and that also is the heatsink , if thre smps would have been not the first one of such kind i would have made its heatsink also together with the chassis.


oh and by the way I see you are a bit eccentric yourself or atleast that's the impression one would get from you r avatar picture which I assume is painted by Salvador Dali, maybe some other not sure , after all Dali is my personal favourite much of his work resonates within me, also the reason why I picked this username. :D Sadly havent yet had enough will to push myself to put an avatar that would go togeher with this username.
 
  • #90
Averagesupernova
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by this i was reffering to the 7812.I think of buying the 7815 because 15 volts is in the acceptable voltage level for a mosfets gate and mainly because the 7812 is not in my store but the 7815 is.after all ive been driving my mosfets with 20 volts and they have stood except for the fairchilds which took a bit more than those 20 and lost the game, 15 vmax should be good.i will see if i can get the 12 volt version but if not ill go with the 15v one.
Why not look into a 317 adjustable regulator if your source does not have the 7812? Even a 7805 can be used as an adjustable regulator. Changing a design may be acceptable when you consider all of the consequences.
 
  • #91
Baluncore
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oh and by the way I see you are a bit eccentric yourself or atleast that's the impression one would get from you r avatar picture which I assume is painted by Salvador Dali, maybe some other not sure ,
My photo of a picture in Tiroler Volkskunstmuseum, unknown 17thC European artist, there are several copies and later versions by different people.
Title is usually “Bird of self knowledge” or “Do not take yourself by the nose”. Google images “bird of self knowledge”.
 
  • #92
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as to what average supernoca said, since i already modified the circuit and was driving the gates with almoust their maximum and sometimes over that, I think the 15v regulator could barely be called " considering the consequences"
7805? but the last two digits indicate it's a 5v regulator?

thanks Baluncore for the info on that very interesting avatar of yours.Will check out his works.
 
  • #93
Baluncore
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LM340-xy = 78xy = positive xy voltage regulator.
If you put a 7V zenner in the reference lead of a 7805 it becomes a 12V regulator.

Depending on the MOSFET design, 5V is usually insufficient to guarantee gates are fully conducting. 12V is sufficient. 15V is unnecessary and delays switching.

Will check out his works.
Let me and the Museum know if you work out who the artist was. She was well ahead of her time.
 
  • #94
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:D:D, yeah right I said it so that it sounds like the unknown author is someone I know :D
you say she? so they think it was a woman?
 
  • #95
Baluncore
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You assumed male, but the best way of being written out of history was to be a woman.
Yes, I know it is sexist, but I contend that an unknown artist is therefore more probably a woman than a man.
 
  • #96
Averagesupernova
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7805? but the last two digits indicate it's a 5v regulator?
You can also wire it like an adjustable regulator. We know that the regulator will do what it needs to in order to get 5 volts between the ground pin and the output pin. So if we make a voltage divider between the output and ground and tie the ground PIN to the node between the resistors of our voltage divider the output voltage will be determined by the ratio of the resistors in the divider.
 
  • #97
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oh interesting so technically , if I make a voltage divider in which the middle point is say at 50 volts measuring to the actual circuit ground then putting the ground pin of the 7805 to that divider's middle point the output from the regulator would be 55 volts?
in other words the regulator only regulates between the two points it is being attached to, no matter that the point which the regulator uses as the reference might actaully be way above the actual common circuit ground or lowest circuit point?

that's quite handy , even though when using the regulator with a resistor voltage divider to achieve higher regulated voltage the current would then be limited to that of the voltage divider correct?
by the way I got my 7815 regulators. will try them soon.
 
  • #98
Averagesupernova
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The current in the divider has nothing to do with the current that the regulator can source. This part has not changed. The divider is in parallel with the load. I don't like to take a regulator very far away from the design spec when using that trick. The heat sink tab of a 78xx is connected to the ground pin so this also needs to be kept in mind.
-
Just a quick google: https://www.google.com/#q=using+a+7805+as+an+8+volt+regulator
 
  • #99
Baluncore
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The 78xx series requires a good solid reference. Once you mess with the ground reference terminal weird things start to happen. For example, a LED in the ref will lift the output by a few volts, dependent on the colour, but in multi-rail supplies it may obstruct the start-up sequence and the output of a regulator may be pulled to reverse polarity.

If you use a 7805 as a 55V reg then all seems OK for steady state, but do not forget that the maximum ref to input voltage is only about 35V. That configuration cannot be started or stopped without risking device failure. An output short circuit will destroy an otherwise protected regulator. By the time you have provided enough protection it has become uneconomic.

Protection circuits are inherently unreliable because they are not always exercised and so cannot be easily tested.
 
  • #100
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I asked more about the 55 volts as a thought experiment rather than an actual idea of doing something like that since these days one can get all kinds of semiconductors without the need to make something from scratch
 

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