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Old 19th June 2012, 09:14 AM   #1
Gunders is offline Gunders  Norway
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Default Variations in transformer inductance

Hi
I have a problem with a transformer design, my supplier says that the cores they have in stock are not within the specs given in the datasheet. The Al is higher than the specified 286nH +/- 25%. The result is that the primary windings have an inductance of 0,018 mH instead of 0,014 mH and the secondary windings have an inductance of 0,08 mH instead of 0,056 mH.

How do these variations in transformer inductance affect the performance of the transformer? Will it limit the maximum output current or power?
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Old 19th June 2012, 11:26 AM   #2
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by Gunders View Post
How do these variations in transformer inductance affect the performance of the transformer? Will it limit the maximum output current or power?
It depends very much on the topology they are used in: in a symetrical converter, it would be a bonus, in a resonant or ZVS topology, it could cause problems, in a flyback, it would reduce the output power.
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Old 19th June 2012, 12:31 PM   #3
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Which is the transformer designed for? Higher inductances in audio are good, in a Fly Back SMPS can cause lost of regulation at high load currents.
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Old 19th June 2012, 02:51 PM   #4
Gunders is offline Gunders  Norway
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The transformer is used in a PSU and the input is a square wave. Parts of the schematic is shown in the attachment. It's used to get an unregulated isolated voltage for a RS485 chip. Output current from the secondary windings are low, about 15 mA.

My biggest concern is whether the increased inductance will limit the power output or not.

Thank you for your help
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File Type: png PSU.png (7.2 KB, 42 views)
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Old 19th June 2012, 03:08 PM   #5
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Definitively, NO! If the inductance is too low, it will draw much power that will be wasted in generating the magnetic pah in the core, but it power is UNUSABLE. Higher primary inductance in this kind of job is a goal, and the higher, the best trafo and the lower the no-load self consumption.

Sorry if my English is not good, and I can´t see more specific.
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Old 19th June 2012, 04:20 PM   #6
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by Gunders View Post

My biggest concern is whether the increased inductance will limit the power output or not.
No, it will reduce the magnetizing current and will ease the task of the driver IC.
In fact, in this configuration, you can use ungapped cores (or minimally gapped cores, to prevent flux-walking issues).
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Old 21st June 2012, 09:35 AM   #7
Gunders is offline Gunders  Norway
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Great, exactly what I wanted to hear. Thank you for help
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