I already had a transistor hooked up i the same configuration on the original schematic.

Not on either schematic you've posted thus far!

Look, if you want to get some decent insight into why things aren't working, you're going to need to take the time to draw up a proper and complete schematic with all of the parts you're using. The more complete and accurate your schematic is, the better our help will be.

The less information in the schematic, the worse our guessing will get.

If you posted a complete and accurate schematic, we could run simulations on it to figure out where things were going wrong.

As it is, the 555 timer will be running at a very low speed, and current decay in the primary will be slow because you are not snubbing it, except for the Vf of the diodes and the resistance of the primary winding. If the diodes in the primary start to get warm, their Vf will drop, which will decrease their snubbing effect, which will hasten the onset of core saturation.

You'll wind up with an ever-increasing DC current flow through the primary as the diodes heat up, right up until the MOSFETs can't handle the current anymore. Disaster will strike quite suddenly.

 

Ok, i'll try to draw a proper schematic and post it today or tommorrow. Now off to showeling snow.

 

OK. Try to give as much information as you can about the transformer you wound; what the dimensions of the core are, did you use a bobbin/tape, wire gauge used, # of windings on primary & secondary, etc.

I'm not trying to be hard on you - I'm just trying to get as many facts as possible about your circuit.

 

I'm not trying to be hard on you - I'm just trying to get as many facts as possible about your circuit.

I understand. This should be everything you might need. Tell me if any additional info is needed.

I found a huge miscalculation, 7000 gauss is way over ferrite limits. It must have caused saturation of the transformer, or maybe it was the duty cycle over 50%.

 

Here's the schematic.

There's an error the odulator frequency equation is:

f = 1.44 / (R1 * C1)

 

I vaguely recall 0.2T (2,000G ??) was nominal ferrite limit at 100kHz, based on core loss - you will need to crosscheck datasheets.

Ans are you absolutely sure you have a 630nH Al factor coreset ! (ie. did the inductance measurement end up being equivalent to the design turns level)

 

I got the inductance factor by measuring the inductance, and calculating it, so i'm sure it's correct.. Would be more accurate if i had more turns though.

Ok, i'll redesign the transformer to work bellow 2000 Gauss flux density.
Hope this one won't cause a black out.

 

Perusing through Magnetics, Inc datasheets, the dimensions you give are a close match to a couple of E-cores; ETD 44 (basic P/N 0_44444EC, see page 10 in the 1st attached PDF) and EER 42 (basic P/N 0_44216EC, see page 16 in the 1st PDF).

[eta]
I'm attaching some Magnetics, Inc datasheets that I feel are relevant.

They use three types of materials; R, P and F. These materials have a flux density of 5,000 for R and P, and 4,900 for type F. (See page 4 in the 2nd PDF)

Following data from the 1st PDF, page 10:
Al for type R is 2,750 (mH/1000), for frequencies < 200kHz
Al for type P is 3,000 (mH/1000), for frequencies < 100kHz
Al for type F is 4,950 (mH/1000), for frequencies < 100kHz

Your E-cores may not be Magnetics, Inc. products. I simply had the data available, and am just throwing the numbers out there.

Did you use a bobbin when winding your E-cores? They'll help in delaying the onset of core saturation.

Your inductance factor - is that 630nH for a single turn, or how did you calculate that? [eta] You really need to use at least 10 turns to get a ballpark value. 20 turns or more is better.

I usually go by Al value.

 

I measured the inductance of the whole 62 turn winding, and the used the equation Al = ( L * 10^9 ) / N^2
It's inductance for one turn, and has is the same value as mH/1000 turns

My Inductance factor may be so low because there's an air gap.

I used a bobbin.

You mentioned the cores have their flux densities is that their maximum or their working value?

Also thanks for posting the datasheets.

 

OK, so what did you actually measure for the inductance with 62 turns?

 

Everythings written in the schematic. The inductance i measured was 0.0025H.

 

dOH! (in my best Homer Simpson voice...)

Yes, you did. 0.0025H = 2.5mH = 2,500uH; for 62 turns I get an Al value of ~650.4

So, your secondary should measure ~65.04uH.

I don't have all of the library components you used, but I'll substitute some similar types in a simulation model.

 

Hey, i found another shot component, the output rectifier is shorting. Could this be the cause of the supply failure or it's just a victim?

 

If the output rectifier diode is shorted, it's a good bet your output capacitor C7 is either shorted or burned open as well.

Still not done with the simulation, the computer is being a dog today.

 

Don't hurry, and thanks . The capcitor is fine my load was about 2.4 ohms almost a short.

Hey, this makes sense, I remember, that when I was measuring inductance of the primary, shorting the secondary winding caused a huge decrease in primary inductance.

So this is why I got whacky readings from the lc meter after the boom. When the load was still connected to the transformer, I measured about 250uH instead of 2500uH. When I disconnected the load the meter gave me 2500uH. How could I missed this out?

Is my assumption that the diode caused supply failure may be correct?

 

Sounds like you appreciate that shorting the secondary will effectively cause you to measure the leakage inductance of the primary (which is a pertinent parameter to know, as it helps in the assessment of losses and transients). Although the best measurement is with the secondary shorted with the least extra resistance/inductance. Lower leakage inductance may be achievble by reviewing your winding configuration.

And yes, a shorted secondary would cause the fets to uncontrollably cross conduct.

Do you have any access to a proper control IC for the application - sometimes you can get samples - beats the hell out of a 555.

 

Could you you give me the definition of cross conduction? I googled a bit but didn't find anything useful or clear enough.

Do you have any access to a proper control IC for the application - sometimes you can get samples - beats the hell out of a 555.

Yeah, i have some from old equipment. A TL494 and various other.

The schematic i'm doing here is just a test to see if i can make it work without significant heating (power loss).

I recalculated the transformer.

On monday i'll have acces to an oscilloscope. So i'll finetune the pwm frequency.
And finally ditribute the load onto two windings. To decrease current flow through the rectifier.

I'll write to inform you how it works.

Thank you all for your input.

 

The FETs have to be turned off before their current skyrockets - eg. when the core saturates and the inductance plummets and dI/dt skyrockets. Given you have no feedback, or cycle by cycle current limit, then you are totally at mercy of have correct values and settings.

Did you calculate the peak diode turn-on current level and cross check the peak repetivive device rating with regard to heatsinking and junction temperature.

 

On your schematic above T2, you show 250V as the supply, which makes me curious how you came up with that.

If you have 220VAC 50Hz power coming in, you'd have more like 300v after rectification and filtering at that point with no load.

555 timer: With R1 being 6.8k and C1 being 10nF (0.01uF), you're running at around 10kHz which is pretty slow. The 680 Ohm resistors on the gates is making MOSFET turn-on/turn-off times VERY slow; I suppose the only reason they don't get fried right away is because they barely make it past the threshold.

What makes a transformer work is when the magnetic fields around the windings change.

What's happening with your circuit is that there is a large change (field expansion) when the MOSFETs turn on, but due to D4 and D5 having such a low VF, the current decay (and resulting field collapse) is very, very slow. After a certain threshold point (difficult to find, actually) the current flow through the primary never decays to zero; and each subsequent turn-on of the MOSFETs adds to this now-DC current flow through the transformer primary winding.

The addition of snubbers would cause a rapid decrease in current, and corresponding rapid collapse of the field, which is what you want.

 

Did you calculate the peak diode turn-on current level and cross check the peak repetivive device rating with regard to heatsinking and junction temperature.

The diode was a 5 Amp shottcky. So the current ( about 5A) might have exceeded it's limit. There was no heatsinking.

As for the frequency of the 555 chip. It's weird because i measured the period with an oscilloscope and i got a frequency of nearly 22kHz.

But wouldn't the snubbers you mentioned add power loss. I heard about some lossles snubbers. could you give me an egzample of how to implement one onto this circuit?

http://www.powerdesignindia.co.in/S...L_2008DEC19_SUPPLY_TA_01.pdf?SOURCES=DOWNLOAD
This document states that as long as the duty cycle is below 50%, the primary current reaches zero because the reversed current during the off time resets the winding.

The 250V was another error in the schematic it was measured under load. Without a load it's 300V.