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SMPS Transformer for DC-DC Converter 12 to 400V

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Dimitrisvlamis

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Greetings everyone :D ,

I am facing a problem 1 month now and i cant understand why. I am following a guide (https://microcontrollerslab.com/ferrite-transformer-turns-calculation/) this one. and i cant work the circuit out. At first i tried this circuit circuit 50kHz.png
but i burned out all of my mosfets (more than 10 so far), i added a snubber circuit wich didnt do anything.
I calculated the transformer turns from the datasheet of the core and i got 3+3 turns and 150turns to step up 8-16v to 400v. After i gave up i tried to do a different circuit which they sell on ebay and stuff. its this one Screenshot from 2018-02-20 16-41-25.png
with no luck as well! i winded 2 transformers one with 4+4 and 75 turns for 200v (EI33) and one 3+3 and 150turns for 400v (EC42). Nothing seems to work even in an open circuit with no feedback as the second one. When i put load the transistors get hot and nothing happens. They even get hot with no load. I am beggining to think i winded wrong the transformer, but i did it same way i was when i bought it. it had in the bottom the primary with 90turns and above it a primary of 3+3 with center tap with copper foil instead of wires. I am wiring the coils in the same direction and i get nothiing. Any suggestions??
 

The ec42 is not that small. i had a small scope 20dollar one but i burnt it trying to see something on the circuit. I will go check it out in a real scope. I didnt calculate primary inductunce, i did it with a program for calculating turns for push pull transformer. but when i tried to measure the primary inductunce i got from the rcl meter 0.3uH which is not accepptable i think. ABout the zeners. I connect them how? +of zener to S and - of zener to D ?
 

Hi,

first picture:
* connect a 33V/1W zener across S-D of each IRFZ44 very close to the mosfet pins.
* maybe your transformer core is too small and gets into saturation. What primary inductance did you calculate?

**
Do you have a scope to show the signal waveforms?
They even get hot with no load.
Sounds like saturation.

Klaus

- - - Updated - - -

Hi,

I connect them how? +of zener to S and - of zener to D ?
There are only two options. One causes a short circuit.

If you really need someone to tell you how, then designing a power supply is not a good idea...


*****
Inductance: 0.3uH. This would explain everything.
Then the current rises from 0 to 100A within 2.5us...

Klaus
 

One big problem with push pull converters is staircase saturation.

This in addition to the fact that you calculated correctly the turns-volts-seconds correctly such that the resulting flux density does not exceed the maximum for your core material and magnetic cross section.

Do you know how to calculate flux density?
 

One big problem with push pull converters is staircase saturation.

This in addition to the fact that you calculated correctly the turns-volts-seconds correctly such that the resulting flux density does not exceed the maximum for your core material and magnetic cross section.

Do you know how to calculate flux density?

Isnt this the formula?

Bmax = (Vin * 10^8) / (4 * f * Npri * Ac)

The problem is I used same amount of turns as it came with. I bought it winded with 90turns in bottom and then 3+3 on top with copper tape. All I did was add more turns to the secondary.
Putting numbers on the formula above it gives numbers from 900 to 1900 for Bmax. Which is acceptable for most cores as I read. The core data is here.
https://en.tdk.eu/inf/80/db/fer_13/er_42_22_15.pdf
 
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The problem is I used same amount of turns as it came with. I bought it winded with 90turns in bottom and then 3+3 on top with copper tape. All I did was add more turns to the secondary.
Putting numbers on the formula above it gives numbers from 900 to 1900 for Bmax. Which is acceptable for most cores as I read. The core data is here.
Re-read the second part of post #3.
 

Is it possible to get some help or suggestions and not smartass answers? The part that i asked about the diode in which way to connect is because of the internal one. whats the point to connect 2 diodes in parallel. i am not new in electronics trust me its my 4th year, i know when a zener diode is connected as zener or not. Thanks for the irony but i think thats not the point of the forum, i have a problem and i wanted to find out what i am missing!
 

Back in the 1990s we used an almost identical circuit as yours, and eventually we found it was staircase saturation.

Unfortunately the only way to know for sure is to measure the drain current of both transistors simultaneously.
This means two identical and properly calibrated current probes.

The current waveform should be trapezoidal, but in the transistor which is suffering saturation the ramp segment instead of being a straight line becomes an exponential.

Simplistic contollers like the 3525, which was designed perhaps 40 years ago, does not incorporate a mechanism to prevent cycle by cycle volt-second unbalance, which leads to staircase saturation.

Later the Unitrode semiconductor company did bring out such a specialized push pull controller. But either the power supply engineers had become tired of push pull failures, or Texas Instruments which bought Unitrode did not promote the IC, but it never acquired widespread acceptance.

ADDITIONAL INFO:
Since you are using a variable duty wave form, you need to replace f with 1/Ton.
And most important, for N you use half the number of total primary turns, because that is how much you use each half cycle.
 
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What are all the specifications? 400 V at what power? Switching frequency?

It is very important to know the leakage inductance of the primary windings as to determine the peak voltage spikes on the MOSFET's. Do you have any specifications for the purchased transformer? If you have a good inductance meter, you can measure it easily.
 

Are you talking about leakage inductance, because 0.3uH is too low for primary inductance? I would expect a value around 5-6 uH for the EC42 core.

Furthermore, at 300 W you will be drawing high peak currents in the primary, where excessive **broken link removed** can kill your MOSFET's quickly without protection.

- - - Updated - - -

Below is an illustration what to expect in your case when you have leakage inductance to deal with. You should appreciate that it is very important to know this value. The EC41 part number is for a gapped core.

The one set of curves uses an ideal transformer which gives you textbook curves. The other is with typical leakage inductance that can happen. With sloppy transformer construction and connections, it can be worse.

As you can see you can exceed the MOSFET or diode breakdown ratings. That is why it is essential to have at least a good LCR meter and a scope for this type of work. You can be blowing up many devices if you don't know what you are dealing with.


Another thing is to make sure that you have a long enough soft-start time to deal with the initial start transient when the filter capacitor is discharged. On the SG3525 it is just picking a large enough start capacitor value.
 

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