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TECHNICAL PUBLICATION
Power transformers for the frequency range of 30 − 80 MHz
ECO7703
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Philips Semiconductors
Power transformers for the frequency range of 30 − 80 MHz
CONTENTS 1 2 3 4 5 6 7 8 8.1 9 ABSTRACT SUMMARY INTRODUCTION CHOICE OF CORE MATERIAL POWER HANDLING CAPABILITY DETERMINATION OF THE NUMBER OF TURNS WINDING LOSSES PRACTICAL EXAMPLE Reference: APPENDIX
Technical Publication ECO7703
1998 Mar 23
2
Philips Semiconductors
Power transformers for the frequency range of 30 − 80 MHz
1 ABSTRACT
Technical Publication ECO7703
In this report design information is given for transformers with a power handling capability up to 300 W in the frequency range of 30 − 80 MHz. The most suitable core material is ferrite type 4C6. The efficiency of these transformers is typically 98%. 2 SUMMARY
In this frequency range only transmission line transformers can be used. For the windings coaxial cables with P.T.F.E. isolation are recommended. The size of the core is based on a 1% power loss and a dissipation of 350 mW/cm3 corresponding with a flux density of 6 Gauss at 80 MHz. The required number of turns is determined by the ratio RP/L = 860 Ω/µH in which RP is the loss resistance and L the inductance in parallel with the input or output terminals. In the appendix the relation between the above mentioned quantities is derived. In the report a practical example is given of a symmetrical 1 : 4 impedance transformer with a power handling capability of 120 W. 3 INTRODUCTION
In Ref.1 information was given on the design of power transformers mainly intended for the frequency range of 1.6 to 28 MHz. In this report some additional information will be presented for the frequency range of 30 to 80 MHz. 4 CHOICE OF CORE MATERIAL
The best available ferrite for this frequency range is 4C6. In this material a series of toroids can be obtained in different sizes according to Table 1. Table 1 D(1) (mm) 36 23 14 9 Notes 1. Outside diameter. 2. Inside diameter. 3. Height. 4. Cross-section. 5. Average length of the lines of force. 6. Volume. d(2) (mm) 23 14 9 6 h(3) (mm) 15 7 5 3 A(4) (mm2) 97.7 31.5 12.5 4.51 A/1(5) (mm) 1.06 0.552 0.351 0.193 V(6) (mm3) 8 500 1790 445 105
1998 Mar 23
3
Philips Semiconductors
Power transformers for the frequency range of 30 − 80 MHz
5 POWER HANDLING CAPABILITY
Technical Publication ECO7703
An important question in the design of a power transformer is how much R.F. power can be handled by a given toroid. Restricting ourselves to the core losses at this moment it can be said that these losses are highest at the maximum frequency of operation i.e. 80 MHz. From practical experience we have found that a core dissipation of 350 mW/cm3 can be allowed without excessive rise of the core temperature. As it is a realistic target to keep the core losses below 1% of the power handled by the transformer we come to the following recommendations for the power handling of the different toroids (see Table 2). Table 2 D×d×h (mm3) 36 × 23 × 15 23 × 14 × 7 14 × 9 × 5 9×6×3 300 60 15 3 PRF (W)
The core dissipation of 350 mW/cm3 mentioned above corresponds with a flux density of 6 Gauss at 80 MHz as can be found in earlier versions of Data Handbook MA01 of Philips series on Magnetic Products: Soft ferrites. 6 DETERMINATION OF THE NUMBER OF TURNS
In the frequency range of 30-80 MHz the number of turns is entirely determined by the loss resistance in parallel with the input or output terminals of the transformer being caused by the core losses. According to the Appendix the core loss figures given Chapter 5 can be expressed in another way, viz:
2 2 RP ω B max V - × ---------- = --------------------2 µo µr PL L
in which: Rp = loss resistance in parallel with input or output terminals L = inductance in parallel with input or output terminals Bmax = maximum flux density µr = relative permeability being typ. 120 for 4C6 material V = volume of transformer core PL = power loss in core. Using the figures given in Chapter 5 we get: RP ------ = 860 Ω ⁄ µ H at f = 80 MHz. L This ratio is hardly dependent on the flux density and therefore it is very useful for defining the number of turns as will be shown by a practical example in Chapter 8. Applying the above mentioned criterion ensures a sufficiently high reactance in parallel with the input or output terminals at the lowest frequency of operation. So this reactance caused by the inductance of the winding needs no compensation.
1998 Mar 23
4
Philips Semiconductors
Power transformers for the frequency range of 30 − 80 MHz
7 WINDING LOSSES
Technical Publication ECO7703
In this frequency range conventional transformers can not be used because of their stray-inductance. The only suitable type is the transmission line transformer. For the windings we can choose a.o: Twisted enamelled copper wire Miniature twin lead Coaxial cable with P.T.F.E. isolation, available in several d.