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APPLICATION NOTE
Design of HF wideband power transformers
ECO6907
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Philips Semiconductors
Design of HF wideband power transformers
CONTENTS 1 2 3 3.1 3.2 4 4.1 4.2 5 5.1 5.2 6 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.8.1 6.8.2 7 7.1 7.2 8 INTRODUCTION TRANSFORMER SPECIFICATION INFLUENCE OF THE CORE ON PERFORMANCE Primary Inductance Core Losses INFLUENCE OF THE TRANSMISSION LINE ON PERFORMANCE Resistive Loss and Power Handling Mismatch loss COMPENSATION TECHNIQUES Compensation at Low Frequencies Compensation at High Frequencies TRANSFORMER CONFIGURATION Phase Reversing Transformer Balanced to Unbalanced Transformer Symmetrical 1 : 4 Impedance Transformer Asymmetrical 1 : 4 Impedance Transformer Symmetrical 9 : 1 Impedance Transformer Asymmetrical 1 : 9 Impedance Transformer Single-ended Hybrid Push-pull Hybrid Impedance Step-up Type Impedance Step-Down Type PRACTICAL EXAMPLES 12.5 Ω Balanced to 50 Ω Unbalanced Transformer 50 Ω Unbalanced to 5.55 Ω Balanced Transformer REFERENCES
Application Note ECO6907
1998 Mar 23
2
Philips Semiconductors
Design of HF wideband power transformers
1 INTRODUCTION
Application Note ECO6907
Transmission line power transformers can be used to perform a variety of functions, among which are phase reversal, balanced to unbalanced coupling, impedance transformation and hybrid functions. Such transformers find many applications in wide-band power amplifiers for both s.s.b. transmitters in the h.f. region and f.m. transmitters in the lower v.h.f. region. The properties of a practical h.f. power transformer are discussed here and their effect on transformer performance is analysed. Since losses must be kept low, in practice the transformer will use a ferrite core. Further, we have limited the discussion to cores without an air-gap since these have a low stray magnetic field, a high permeability, and can cover the power range (up to 80 W) dealt with here. Data (dimensions, permeability values etc.) on all core types can be found in our Data Handbook “Soft Ferrites”, MA01. A glance through the Handbook will show the wide range of materials, dimensions and types from which the designer may choose. It must be remembered, of course, that when cores constructed in two parts (pot-cores and cross-cores, for example) are used, the type without an air-gap must be selected. Throughout we have aimed at giving practical solutions to the problems posed by material and design limitations. In particular, compensating techniques for extending the frequency range of a number of transformer configurations are discussed. To give an idea of some application possibilities, practical examples in several transformer configurations have been worked, using transformer cores from our range of ferrites. 2 TRANSFORMER SPECIFICATION
The transformer design considerations dealt with in this publication are: • Maximum power level to be handled • Frequency range • Input and output impedance • Allowable reflection and resistive losses. How a transformer can meet the above considerations for a particular application is analysed in the following three sections. The first two sections deal with the influence of the core and transmission line respectively on transformer performance, and the third with mismatch compensation techniques. 3 3.1 INFLUENCE OF THE CORE ON PERFORMANCE Primary Inductance
This inductance determines the amount of reflection at the low frequency end of the band. It can be calculated using the formula: L = µoµrn2 A/l in which: L = inductance in H µo = 4 π 10-7 (rationalised M.K.S. units) µr = relative permeability A = average ferrite cross section in m2 l = average length of the lines of force in m n = number of turns between the input connections.
1998 Mar 23
3
Philips Semiconductors
Design of HF wideband power transformers
Application Note ECO6907
In a simple example, like the phase reversing transformer, this relation holds. Other cases may require a transformation (see Section 7.1). If degrading of performance at the high end of the band is to be avoided, the value of L must not be higher than really necessary. A good practical value is: L = 4R/ωmin in which: R = midband input resistance in Ω ωmin = 2π times the minimum frequency in Hz. Where requirements are severe the compensation technique described in Section 5.1 may be used. 3.2 Core Losses
The losses caused by the core material will be represented here as a resistance (Rp) in parallel with the input. This resistance depends on: • The sort of ferrite material • The frequency • The quantity L/µr • The maximum flux density Bmax. In the small signal case (Bmax → 0), Rp can be calculated with the aid of curves of the type shown in Fig.1(1). In these curves a comparison is made between different core materials based on equal core dimensions and equal number of turns. It can be seen that 4C4 and 4C6 are the best materials for frequencies above approxi.