b. A pulse transformer has a ferro-magnetic core, is closely coupled,
and has few turns in its windings.
c. It couples the high-voltage modulator pulse to the magnetron with
minimum power loss by matching the low-impedance PFN to the high impedance
magnetron.
d. It has bifilar secondary to
isolate
the
magnetron
filament
transformer from the high-voltage pulse.
e. It steps up the pulse voltage to allow the PFN to operate at a lower
voltage than the magnetron.
Learning Event 7:
MAGNETRON IMPEDANCE
1. You know that when the load (magnetron) impedance equals the pulse-
forming network impedance, the voltage across the pulse transformer primary
equals approximately half the voltage on the PFN at the time of the
discharge. However, even though the pulse transformer does match impedance
to some degree, it is impossible to keep a perfect impedance match between
load and PFN at all times.
The reason is that the load presented by the
magnetron to the PFN varies widely from pulse to pulse. Magnetron impedance
is very high until it starts to conduct.
Then, impedance drops to a
comparatively low value (but still higher than the impedance of the PFN).
2. If the magnetron impedance does not match the impedance of the PFN,
reflections occur on the PFN.
This impedance mismatch causes part of the
pulse energy to be reflected back into the modulator from the magnetron.
Also, each time the magnetron misfires, a large part of the pulse energy is
reflected back into the modulator. When reflections occur, the voltage on
the PFN does not remain constant. The reason is that the polarity of the
reflected energy is the same as that of the modulator output pulse. Part A
of Figure 66 shows that the pulse transformer now acts as the source instead
of the load. As a result, a reverse current flows into the PFN at the end
of the discharge cycle. This reverse current flows in the same direction as
the charging current and thus charges the PFN.
Consequently, there is
already a voltage on the PFN at the start of the next charging cycle, so the
PFN charges to a higher voltage than normal. Thus, with impedance mismatch,
the voltage on the PFN may get so high that it breaks down the THYRATRON
switch and even the PFN itself.
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