Designing High Power Ferrite Transformers for HF
First the core must be designed for the voltage that will be applied to the winding. This can be done in two ways, increase the number of cores used or increase the number of turns used. Many of the transformers that I design use 4 cores, in 43 material, and have two legs of .500 brass tube with pc board end plates. The back end plate is solid and connects the tubes together (can be considered a center tap).
The ferrite sleeves have a .560 ID and are about 1 inch long and are 1 inch in diameter. This type of construction has much better primary to secondary coupling than toroidal transformers because the secondary winding is inside the primary (near 100% coupling).
Since the ferrite companies cannot tell you how much power a core will handle the loss must be measured. I have found that this size core can handle a maximum of 2w/core. By resonating the 1 t, 4 core transformer at the frequency of interest and matching it to 50 ohms the total loss can be measured with a Bird wattmeter. Increase the measurement power to 8 watts (for 4 cores) and that will be 2w per core. Measure the voltage across the 4 cores with a scope or rf voltmeter and that will give you the voltage across 4 cores. If the voltage was 28 volts then one core would have 2w loss at 7v per core-turn
An important concept is core-turns and in my example 4 cores x 1t = 4 core turns. If you had a 5 turn primary the core-turns would be 20 and 140 volts across the primary would give a total core loss of 8 watts. Ferrite loss increases with decreasing frequency and I have not measured it at frequencies below 1.8 Mhz but at 1.8 it was 20v per core-turn
And at 3.8 it was 22.3v per core-turn. At 7.2 it was 24v per core-turn. Figure 1 shows a transformer with a 3 turn primary.
To be conservative you should design for less than 2 Watts per core. A 500 Khz, 1kw pa, running on 48 volts (up to 96 volts across the primary) needs a load of 8.68 ohms for 1kw with a primary to secondary ratio of 5:12. A 5 turn primary would run at 4.8 volts per core-turn.
With this low excition it should be possible to feed the dc in at the center tap of the primary without increasing the core loss.