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Sunday, December 29, 2013

Output Transformer Design



Output Transformer Design

Would you like to know how to design output transformer in step by step?

1st, calculate the inductance required.  For higher primary impedance, larger inductance is needed and therefore larger core and / or more turns are needed. This also affects the high frequency response.

               L = Z / (2 * pi * f)

2nd, calculate the primary voltage and current. If the power (Watt) and impedance are know, then you can use Ohm’s law to calculate the voltage and current needed.

E = sqrt (Z x W)
I = E / Z

3rd, calculate the secondary voltage and current. Secondary voltage can be calculated from impedance ratio.

               E2 = sqrt ((E12 x Z2) / Z1)
               E2 / E1 = I1 / I2
   I2 = (I1 x E1) / E2

4th, determine the core size and material. Since the core size will be dependent of the materials used and manufacturer electrical properties, we will not go into details. For small low level unit, material can be 50% or 80% nickel. For others, perhaps 4% silicon steel is needed. One can refer to the manufacturer datasheet that is usually specified for 50-60Hz operation, and de-rate it for 20Hz audio usage. The manufacturer datasheet should provide formula for inductance calculation. Follow that and choose the right core size to meet the inductance requirement. 

5th, determine primary / secondary wire size. This can also be found from manufacturer datasheet of the wires used. 

6th – calculate if the core size can fit the wires needed, for both primary and secondary turns included. Primary wire should take up 50% and vice versa. 

7th – determine the high frequency response. High frequency response is controlled by the impedance and the leakage inductance. The leakage inductance is proportional to the square of the turns. Therefore, the higher the number of turns, the lower the high frequencies limit. The number of turns is already set to get the needed primary inductance for low frequency response. So, it’s a balance between low and high due to the law of physics. 

Leakage inductance can be reduced by interleaved primary and secondary windings. Interleave as in split the windings and wind primary first, the followed by secondary, and so forth. Some examples of interleaving could be 1:2, 3:4, and etc. 

The high frequency limit can be calculated with respect to the leakage inductance and the normalized impedance, as follow:
Zn = (N1/N2)2 x Z2 + Z1
F2 = Zn / (2 * pi * L-leakage)
Do note that slopping winding or improper winding implementation will increase the leakage inductance. The windings must be directly above each other. Commonality is called for in every case if the wires cannot fill out the core, like equal spacing, spiraling and etc. No cross over windings is allowed. Good and patient workmanship is required to obtain excellent results. 

That sums up the output transformer design introduction. There are many more steps involved and we shall continue again in future articles.

J&K Audio Design
29/12/2013


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* Finished amplifiers, DACs, audio gadgets, upgrades and repairs - this is not our core business and we do it out of passion. We do not have fixed models, fixed price and we customize for each individuals. The sky is limit of creativity.

* Our product lines are always improving and increasing. If you do not see what you want, contact us!

* Please email for volume discounts, distributor and OEM pricing.

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