Confused about the transformer Design

anuj · ‎2017-02-25

Posted on February 25, 2017 at 23:11

I am using ST's eDesign Suite for designing power supply. (AC-DC Converter)

I contacted the transformer design firm to get my transformer designed. I gave them all the information regarding the number of turns and length of wire etc..

Here is my power supply specs

Input: 95 Vac - 265 Vac - 50 Hz - Operating Vdc [96.9 V - 374.8 V]

Output Power: 14.96 W

Output -: 17 Vdc (38 mVpp ripple) - 880 mA max - 14.96 W - Regulated

Switching Frequency (fsw): 60 kHz

Transformer Specs :-

fsw: 60 kHz
Prim/Aux turn ratio 6.39
Prim/Out 1 turn ratio 4.77. (Np/Ns)

Design Specs :-

Type EE20/10/6 (EF20) TH-V 10-Pin
Core Gap: 0.28 mm expected

Primary: Turns 105 - Layers 6 - Paralleled wires 3

Secondary: Turns 22 - Layers 3 - Paralleled wires 3

Auxiliary: Turns 16

Power Supply Layout :-

Attached

Now I need to order the design for transformer and the transformer design centre asking for dot polarity/Start-End point of pins. According to circuit, there are only 4 pins used on AC side and 2 pins on DC Side. The ST recommends EE-10 size Bobbin.

Wurth Electronics Transformer Winding Layout :- Attached

Do I need to use Pin 2 for interleaving/CenterTapping, though in schematic it is not mentioned, but only in transformer layout. Or should I go with simple 3 windings (without Center Tape) as mentioned in schematic i.e For Primary (Pin 1 -> Pin3),

For Aux (Pin 4 -> 5),

For Secondary(Pin 7 & Pin 8 -> Pin 9 & Pin 10).

I only need 6 pin for my circuit, but the layout for Transformer shows 10 Pins. I am confused about the it.

#ac-dc-convertor #viper #edesignsuite #smps

Emil Nierges · ‎2017-03-04

Posted on March 04, 2017 at 23:31

Hi Mattoo Anuj,

I will try to answer to your questions:

''

Do I need to use Pin 2 for interleaving/CenterTapping, though in schematic it is not mentioned, but only in transformer layout. Or should I go with simple 3 windings (without Center Tape) as mentioned in schematic i.e For Primary (Pin 1 -> Pin3),

''

- In the transformer schematics that you provided, pin 2 is for internal use of the transformer - helping in the internal construction. For analogy, just like we could see in some chips IC diagrams, the IC have pins with not connected (NC) notations and the user should not connect/use those pins.

- Here is an example on how to connect the transformer for the given pinout: connect pin 3 to positive DC input voltage; connect pin 1 to Drain IC; pin 2 should not be connected.

''

For Aux (Pin 4 -> 5),

'' - correct

''

For Secondary(Pin 7 & Pin 8 -> Pin 9 & Pin 10).

'' Correct

Here you can find a handmade sketch on the schematics (Wurth Electronics Transformer Winding Layout - Custom Capabilities Catalog) on how to do the transformer connections:

Please note that once you have the effective datasheet of the transformer you could judge the connections in the circuitry. Why? Because the design engineer of the transformer may find better pinout for your particular request than the initial catalog proposal.

As for - Why the transformer has more pins that required - , I would say that this is because of leaving options for the same transformer package/size to be reused in other applications that may require more windings like for multiple outputs or maybe more current therefore the output windings are winded separately and connected in parallel and so on...

If getting a custom transformer for you is not an option, I would recommend you to use Smart Transformer Selector (STS) online tool from Wurth Electronics Midcom, link:

http://www.we-online.com/sts

Based on your parameters, using STS, I was able to find already some available transformers that may fit in your application:

http://katalog.we-online.de/ctm/datasheet/7508112330.pdf

http://katalog.we-online.de/ctm/datasheet/7508111417.pdf?__hstc=147049192.33943b4f54495b05b9c25fbb0dede7f4.1444318122157.1487668925918.1488659063437.61&__hssc=147049192.2.1488659063437&__hsfp=2519724436&_ga=1.82521046.1123980246.1444318115

http://katalog.we-online.de/ctm/datasheet/7508116313.pdf?__hstc=147049192.33943b4f54495b05b9c25fbb0dede7f4.1444318122157.1487668925918.1488659063437.61&__hssc=147049192.2.1488659063437&__hsfp=2519724436&_ga=1.107226050.1123980246.1444318115

Please note that the proposed existing transformers are at the suggestion level and further investigations and adjustments may be required.

Hope this give some clarifications on the transformer topic.

Regards,

Emil Nierges

Product Applications Engineer - Europe

http://katalog.we-online.com/en/midcom/