Radar System Engineering

SEC.10.11] MISCELLANEOUS COMPONENTS 385
the input and output ends of Fig. 10”45b should be avoided. Leakage
inductance “can be kept to a reasonable value by winding the secondary
and primary as close together as voltage breakdom will permit, for the
space between these two windings is responsible for most of the leakage
inductance. The shunt inductance and its magnetizing current constitute
an additional load on the generator which must be minimized by
making L. large.
This last consideration sets a lower limit on the number of primary
turns and also calls for a laminated core with high permeability at high
frequencies. Laminations bet ween 0.001 and 0.005 in. thick are necessary
to maintain a core permeability of several hundred up to the frequencies
of several megacycles per second present in a steep wave front.
Special core materials for pulse transformers were developed during the
war.
A square voltage wave of magnitude VO and duration t applied to the
inductance L. will build up a current of approximately IF = VO. t/L.
amperes during the early part of its exponential rise. If this current is
not to exceed a few per cent of the desired load current for pulse lengths
around 1 psec, L, must have a value between 10 and 20 mh. With this
information, the number of turns in the primary and the required core
area can be obtained from tables showing the effective permeability of the
core at the flux densities and rates of rise anticipated. 1
Since the voltage drop across the leakage inductance is approximately
LI . ~) the maximum permissible value of leakage inductance can be
estimated by assuming that the current in RL must reach 90 per cent of
its final value, VO/RL, in a time of about to/10. The current which flows
must satisfy the equation
V,= LZ. $+i. R..
To a sufficient approximation, LL = R&t. . ~, where 1/10 is the fraction
of final current built up in the rise time t,. If t, = 0.1 X 10-’ see,
R. = 1000 ohms, and I/10 = 0.9, L1 must be less than 100 ph.
Satisfactory high-power (100-kw to 5000-kw) pulse transformers
have been designed which pass good wave shapes down to tO= 10–7 sec
and up to to = ltiK see, but it is quite difficult to design a pulse transformer
to pass a wide range of pulse lengths. Although a long pulse calls
for a large value of L,, large L, magnifies the difficulties of securing the
small value of Ll required to pass a very short pulse. Transformers have
been designed which satisfactorily pass pulse widths varying by a factor
of 10; pulses of shorter or longer duration than the optimum suffer either
1 See Puke Gewrators, Vol. 5, Radiation Laboratory Series, Chaps. 12to 15.