Flyback Converter-Need Help

hi all,
we are designing flyback converter with following parameter:

• input voltage:230V
• output voltage:5v
• frequancy:70Khz
• switch MOSFET:IRFP250N
• Diode: Fast Recovery Diode B860
• transformer: primary turns:88
• secondary turns:2

problem is that switch or transformer get heated when we apply voltage above 90(at that time current is 0.95A). we intechange secondary terminal also. Increase Frequancy up to 120Khz to avoid core saturation. but nothing improvement.

suhelshaikh21 said:

problem is that switch or transformer get heated when we apply voltage above 90(at that time current is 0.95A).

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At the same ratio (carried to your intended 230V) it suggests your primary will carry 2.4 A at 230 V. The primary winding should be 17 AWG, to carry this amperage safely.

The power will be 559 W, roughly. Some of it is resistive, creating heat. Some of it is reactive. Expect 10 to 15 percent losses. The heat could be equivalent to a 60W incandescent bulb.

You can expect the primary to carry peaks of 4 or 5 A. Did you construct the transformer with sufficient metal mass, to be certain it will not saturate at this current?

Do you plan to draw 559 W from the secondary (at 5 or 6V)? That will be 93 amperes. I'm not sure it will be feasible. The resistance in the secondary loop cannot be greater than .064 ohms.

Or does your secondary need less power? Then it might help if you design your primary so it does not draw so many amperes, either at 90V or 230V.

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Links to charts of wire gauges showing safe amp-carrying capacity:

http://amasci.com/tesla/wire1.html

http://www.technifest.com/wire_gauge.htm

http://www.powerstream.com/Wire_Size.htm

Here's a link to another wire gauge table. Its ampacity values are much different than the other tables.

http://www3.telus.net/chemelec/Projects/Wire/Wire-1.png

Possible reason for the disparity: One source may base ampacity on a higher temperature which makes the wire get too hot to hold, while the other source may use a lower temperature where copper's electrical characteristics start to change even though it is only warm.


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http://www.helukabel.de/pdf/english/technik/X_081_Current_ratings_for_UL-CSA_cables.pdf

How much current do you need on the secondary side? Is this just to charge a mobile phone?

burt007 said:

How much current do you need on the secondary side? Is this just to charge a mobile phone?

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we need 2 to 3A on secondary side

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we calculate secondary current 2 to 3 A primary current less than 0.5A but some how current is increase beyond the limit.
wire guage we select is 23 & 27SWG

we intechange secondary terminal also

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Don't you know what a flyback converter is or aren't you sure about transformer wiring?

In any case you should get clear about the correct wiring instead of using trial-and-error method.

Essential informations are missing.
- how do set the duty cycle? What's the duty cycle in your test?
- is it a properly designed flyback transformer with air gap? Did you calculate and/or measure the primary inductance?

duty cycle i am changing from 20% to 80% using programming.
I am having 4-5 different transformer that what we got from SMPS or Cellphone charger.
One transformer we design by local manufacturer in which there is no airgap(when we de-assemble its winding & core )

Duty cycle you don't increase but need to decrease, increase the frequency. What is the frequency you are driving at?

suhelshaikh21 said:

duty cycle i am changing from 20% to 80% using programming.
I am having 4-5 different transformer that what we got from SMPS or Cellphone charger.
One transformer we design by local manufacturer in which there is no airgap(when we de-assemble its winding & core )

Click to expand...

A flyback transformer usually needs an airgap to achieve sufficient transformer current without saturation. Alternatively a low permeabiity transformer core material could be used, but that's really uncommon.

Thus I assume that you experience saturation with the ungapped core.

A transformer from an existing flyback switcher should work, if it's used under suitable operation conditions.

An additional comment to your initial post. You are reporting to use IRFP250, which has a maximum Vds of 200V. Thus it's pretty expectable that the circuit won't work with higher input voltages.

FvM said:

A flyback transformer usually needs an airgap to achieve sufficient transformer current without saturation. Alternatively a low permeabiity transformer core material could be used, but that's really uncommon.

Thus I assume that you experience saturation with the ungapped core.

A transformer from an existing flyback switcher should work, if it's used under suitable operation conditions.

An additional comment to your initial post. You are reporting to use IRFP250, which has a maximum Vds of 200V. Thus it's pretty expectable that the circuit won't work with higher input voltages.

Click to expand...

but we are having 2 to 3 transformer that we are de-assemble from computer sMPS, cell phone charger,
so I assume that in that core there is an airgap.
second thing my MOSFET get heated when i am applying 50-70V its not even reach 100V

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burt007 said:

Duty cycle you don't increase but need to decrease, increase the frequency. What is the frequency you are driving at?

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I am also change my frequancy from 20Khz to 70Khz

second thing my MOSFET get heated when i am applying 50-70V its not even reach 100V

Click to expand...

You have to look for maximum Vds during flyback phase.

I have change MOSFET IRF840 with maximum voltage rating 500.
& also check Vds it was around 300 when i applied 100V. but at that time mosfet getting too hot.
So still no solution. but its batter than last time that i can applied atleast 100V