SMPS float capacitor question

Baluncore
You seem to enjoy courting disaster. Instead, select a device that exceeds the original specifications.
The IXFH24N50 are only; 24A not 32A, 230mR not 150mR, 300W not 360W.

I would consider a more recent device with better specs and at a lower price than the poor choices.
For example; Fairchild FCH104N60. 600V, 37A, 104mR, 357W. It is less expensive than the IXTH24N50.

thanks for giving me an example I will run it through the databases of some of my local shops and see whether they haev it or not, I agree that my previous examples were not the best choices it's just that sometimes local distributors ask a cosmological scale price for a device which is not that worthy or even obsolete. like the IXFH32N50 mentione din the schematic. one supplier even asked about 24 euros a piece which is about 27 dollars a piece, the other store had the same device at exactly half the price.

Ok I will check out the fairchild device.

Baluncore
Take a look at these global suppliers.
FCH104N60 digikey.com have 480 in stock at AU$8.35 FCH104N60 mouser.com have 378 in stock at AU$11.06

I don't know about digikey but I just registered at mouser and yes the FCH104N60 itself was cheap about 6.3 euros but then I got to the shipping part and shipping alone costs 40 euros so I kinda thought no thanks.

the best I came up with now is to buy FCH35N60 from US through ebay , out of the many hong kong fake picture ads there seems to be a decent guy from the US selling electronics with a good track record. He offers 4 of these FCH35N60 for about 12 USD which is a good price as even at the l;ocal stores I usually can get thr standard IRFP's for such a price.
Are you from Australia ? I am from northern europe by the way , Latvia to be exact.

Also maybe I could just build a smart little circuit which could use the transformer as a magnetic amplifier at startup , to limit the current sort of failsafe soft satrt circuit, just say I added a small extra winding on the core. at powerup a capacitor would get charged up through this winding say the capacitor would charge for 1 second.as it would charge DC current would be flowing through the winding altering the cores saturation limit so that only half or even less of the AC could get through the core this would limit the current that could be drawn by the secondary at heavy startups if a 500w bulb is attached as load or anything else demanding, what do you think could it save an otherwise decent device from blowing up?
probably the IRFP 460 would have somehow driven the bulb i attached given that it would have been soft started or preheated.

still I doubt I would ever need this for the purposes on which I intent to use this smps ,

oh thanks for showing me this site , looks really good, I even checked up and started to order to see the shipping prices and they are reasonable atleast compared to what I saw elsewere, the problem is that I cannot finish the deal because it seems they are still developing their business and the homepage is temporarily closed so i cannot proceed beyind the input your data stage for odering , maybe will call them tomorrow to see what can be done.

so i got the fch104n60 from the RS company 9by the way thanks for telling me about them)
Now I wonder what kind of a startup load i could use i mena the minimum for a test?
i kinda dont feel like putting the 500w bulb which had 10 ohms when cold. since the use for the smps will be in an amplifier it would rarely see such low resistance (typical speaker load would be 4 to 8 ohms but thats an impedance load not a purely resistive one and one rarely plays speakers to max)

i have a 230w 15 ohm bulb , i dont know what else I could use to see the output power of whatever i have built there?
or maybe i should take the 500w bulb and just use a reostat to slowly bring it to full power.
also I wonder does the soft start capacitor on the sg3525 acts only as a time delay depending on the capacitance used or does bigger capacitance actually startup the mosfet driving IC's slower? like more gradually.

Baluncore
Take the output and connect across it several filament globe sockets with switches. That enables you to switch in a variable load during your tests. You should select and design your load by using the cold resistance of the filaments.

Your +/–70V represents a 140V DC output. That is insufficient to fully light a 230V filament lamp. The cold resistance of a globe will be about 1/10th that of a bright globe. We know that because resistance is proportional to absolute temperature. Room temperature is 300K while an incandescent filament will be closer to 3000K.

I have 240VAC and use an equation to estimate Rcold = 3600 / Filament wattage. So a 100W filament globe will have a cold resistance of 36R. 75W will have 48R. Each switched filament will be a current load. The maximum current will be less than Vout / Rcold since the filaments will be warm, radiating IR. It will not be the cold or the hot resistance, it will be something in between. You can always measure the current and voltage to determine the filament temperature.

You can buy 230V filament globes from 15W to 400W. I suggest 15W, 25W, 40W, 60W along with several 100W globes. You do not have to have them all plugged in at the same time. During early testing you might progressively switch in low values, later during testing they might all be 100 watt globes. 400W will only be needed at the end during reliability testing.

For 140Vdc output it would suggest …
watt, Rcold, Imax
15W, 240R, 0.6mA
25W, 144R, 1A
40W, 90R, 1.5A
60W, 60R, 2.3A
100W, 36R, 3.9A
400W, 9R, 15.5A. Avoid the 400W filament initially. Go for lower values, then several 100W globes.

The other thing you can do is put a light globe in series with the AC input during early testing. It should flash on momentarily while the storage capacitors initially charge, then go dull once running. If it stays on, then the load is too great or you have a short in the AC switching side of your inverter. The series globe should minimise the damage from catastrophic failures that take out multiple components. For example, if your half bridge fails, or one mosfet goes short, then the current through the remaining mosfet will be limited to below the series filament globe specification. You will clearly see if you have a problem.

I wonder does the soft start capacitor on the sg3525 acts only as a time delay depending on the capacitance used or does bigger capacitance actually startup the mosfet driving IC's slower? like more gradually.
See this application note, page 5/13;
http://freedatasheets.com/downloads... Supply And Power Management SG3525 AN250.pdf

On page 6 is the equation for “turn on time”, t = Css * 2V / 50uA. With a bigger Css It will take longer to start.

now heres what's happening as of lately between me playing football and repairing my car.
i changed the IRFP460 to the newly received FCH104n60, now take into consideration that just before the IRFP's were driven by the same board which has the SG3525 and IR2110 on it , a 115w 230 bulb glowed nicely and even though i didnt try higher powers with the IRFP'S yet i soldered in the FCH ones.
now the same 115w bulb attached that made no problems to the IRFP and guess what happens when i switch on the power with the fairchild devices , nothing , not even a crack or a bang , but the lights went out immediately, so as I went to replace my fuses, came back and measured and yep there it was , both devices blown open completely , not physically but electrically , shorts on all three legs G D and S.

everything else was the same as before i changed nothing except the two half bridge mosfets driving the transformer.Mystery.
All in all I have to say I have never been a fan of semiconductors in places were high power both volts and amps have to be dealt with as any solid copper wire and a piece of iron will take any abuse and tolerate almoust anything but a semiconductor will not even close.tahst what i hate , yes I know that almoust every household item is now powered through a SMPS and their reliable and so on but well ehh I just have no luck with them , maybe because for them to work properly they need a heavy and advacned circuit were one parts watches over the other and so on and so forth and everything is designed and built to the maximum precision.

the other option is that the devices I have been given are fake and hence cant stand up to the requirments , since the IRFP's worked in the same situation with being lower rating devices.

maybe this will sound a little childish but since it's all just a big experiment i guess i will stick with what works and buy myself some more IRFP'S and just see how much power such a psu can give to my amplifier board , both with the satisfaction of my ears and also a small oscilloscope amplifier output load test. if they will do no good well FFS , i'll just use a toroid , atleast that thing aint gonna break down in a hundred years.

Baluncore
, both devices blown open completely , not physically but electrically , shorts on all three legs G D and S.
That is a contradiction. Open or short ?
Did you have a 100W filament globe in series with the AC when you turned it on ?

I have never been a fan of semiconductors in places were high power both volts and amps have to be dealt with
High current is OK, high voltage is OK. But never both at the same time.
That may be your problem.
There is a problem somewhere in your circuit. I have insufficient information to guess where.

the other option is that the devices I have been given are fake
That is most unlikely.

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.

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
Gold Member
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.

thanks for coming in to 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 cant or isnt menat to drive these devices (doubt this one)
i have to say I havent 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.

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.

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.

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 heres 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 dont 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 cant 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 dont 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.

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 succesful , 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.

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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.

the actual running of the smps is poor, it doesnt 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 im correct.

Baluncore