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Working with fixed inductors

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Arpit Gupta

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I am working on designing power supplies of 5v/3.3v/15v/-15v using power management ics with variable unregulated DC input.. Right now i am considering using L4973 from stmicroelectronics which is a buck SMPS. Now it uses a fixed inductor - "SMT Power Inductors – DO5040H". My question is how to make a transformer out of it to make multiple power supply outputs with different stepped up voltages.
 

SunnySkyguy

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Since pin 12 is the Voltage Feedback input with an internal reference of 5.1V all you need is variable voltage divider to scale to 5,1V to pin 12 to select the voltage. Other factors such as ripple, stability , efficiency require care on all other choices in the design as decribed in the CONVERTER datasheet ( not transformer)
 

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I am working on designing power supplies of 5v/3.3v/15v/-15v using power management ics with variable unregulated DC input.. Right now i am considering using L4973 from stmicroelectronics which is a buck SMPS. Now it uses a fixed inductor - "SMT Power Inductors – DO5040H". My question is how to make a transformer out of it to make multiple power supply outputs with different stepped up voltages.
Buck regulator can only decrease the input voltage, voltage step up is not possible.
+3.3v, +5v, and +15v are no problem, as long as your input voltage is suitably higher than +15v.
-15v is not going to be possible with a buck regulator.
 

Arpit Gupta

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an output inductor is also placed at the output pin, if i want to create multiple power supplies, a transformer type action is required to be applied, the datasheet has an application note which uses an output inductor commercially available as mentioned in the original post, so i wanted to ask how to use such inductors to apply the transformer application?
 

Orson Cart

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Yes, you can use additional windings on the buck choke to create other o/p's at higher voltages, but the main output needs to be in continuous mode with a fairly substantial load on it for this to work very well.

So if the main output is 5v0 and 10 turns, it will have 5.6volts on it say during the main switch off time (to allow for the diode drop - if using a diode) so a 3v3 winding will need about 7 turn to give just over 3v3
15V will need 28 turns on the same inductor,
but the regulation will be poor depending on loads and the main 5v0 loading...
 

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I can see a whole lot of problems with that, unless this supply is built into a piece of purpose built equipment that has fairly constant and predictable loads on all outputs.

I rather gather that the original poster was thinking more in terms of some kind of multi purpose bench power supply, which would need to operate over very wide and varying load conditions.
 

Arpit Gupta

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I can see a whole lot of problems with that, unless this supply is built into a piece of purpose built equipment that has fairly constant and predictable loads on all outputs.

I rather gather that the original poster was thinking more in terms of some kind of multi purpose bench power supply, which would need to operate over very wide and varying load conditions.

"i will be powering mosfet gate drivers , microcontrollers ,sensors using this power supply, so load will not vary much, what i am asking is how to make multi secondary windings using the inductors-

https://www.inductors.ru/pdf/do5040h.pdf

pls have a look at it and suggest how to add secondary windings to such inductors, as i believe additional windings cannot be wound to such indructors in the above link"
 

Orson Cart

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The man asked if it was possible to convert the choke to a Tx and derive other o/p's - the answer is yes, not very elegant or suited to many apps, but still yes. Completely wrong for multiple stable o/p's, but he's a newbie so....
 

Arpit Gupta

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The man asked if it was possible to convert the choke to a Tx and derive other o/p's - the answer is yes, not very elegant or suited to many apps, but still yes. Completely wrong for multiple stable o/p's, but he's a newbie so....

"how to make multiple outputs then?? i am reluctant to use a voltage divider using resistances because of the copper losses as i am drawing a high order current of 3.5A...??
I am also considering making my own transformer with suitable no. of secondaries and using that in place of that output choke as shown in the datasheet???"
 

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The only converter configuration that generates multiple linked voltages is a flyback topolgy, respectively inverted buck-boost. Using parallel windings to a buck converter inductor can only work if the main output has sufficient base load and the additional outputs are connected in flyback.

Practically, the leakage inductance makes the voltage linkage in a multple flyback configuration insufficient for applications with tight regulation requirements.
 
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