Note: Descriptions are shown in the official language in which they were submitted.
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10~711)7
BRIEF DESCRIPTION OF THE DR~WING,
Fig. 1 is a simplified schematic of a prior art converter,
F'ig. 2 is a simplified schematic of a converter according to
the invention~ ¦
B~CKGROUND OF THE'INVENTION;
1 Fiel~ of the Invention;
The invention reIates to DC to DC voltage converters
and particularly to those in which an arnplifier in the primary
circuit is driven by autotransformer action in a portion of a
transformer primar~ winding to produce self-oscillation.
2, Description of the Pr'io~ Art;
For a low to hi~h voltage conversion it is common to
build up current in a transEormer winding to a maximum and then
interrupt it suddenly so that the magnetic flux field collapses
abruptly across the secondary winding to produce a high voltage
pulse in the secondary~ One known way of doing this is depicted
in Fig~ 1 where primary winding 10 of transformer 11 has tap 12
for divlding it into main portion 14 and auxiliary portion 15.
Transistor 16 has its emitter-base circuit connected in series
with portion 15~ The emitter-collector circuit of transistor 16
is connected in a series loop with portion 14 and source 17.
Start-up bias is provided by resiskor 18 from the base of tran~
sistor 16 to the negative side of source 17, When switch Z0 in
series with source 17 is closedr transistor 16 starts conducting
providing current through portion 14 and effectively isolating
portion 15 from source 17~ Autotransformer action between
portlons 14 and 15 then increases the drive to -the base of trans-
istor 16 producing a rapid turn-on of transistor 16 lnto saturatiol.
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As transistor 16 reaches saturation the current in portion 14
begins to stabilize reducing .the autotransformer drive to the
base of transistor 16 which'then starts to turn off. Again
autotransfornler reaction due to current drop in portion 14
occurs. This time the drive to the base of transistor 16 is
reversed turning transistor 16 off in opposition to the bias
from resistor.l8~ Upon stabilization~ the bias from resistor 18
starts the cycle anew so that the circuit is free running. The
. tuxn-off o transistor 16 is sufficiently abrupt to dump most of
I0 the energ~ stored in core material 21 into secondary winding 22.
To the extent the turn-off is not square~ there is a loss of
e~flciency.
SUMM~RY OF THE INVENTION: i
_ , . I
i Now, in accordance with the invention, an auxiliary
switching circuit is provided to the converter of Fig. 1 to
speed up the turn.off of'the current switching'amplifier. A
capacitor connected between the tap'on the transformer primary
and a source is also connected to a voltage triygered switch
conneated in a circuit between the source and the drive to the
current switching amplifier. In operation, the capacitor begins ¦
to charge as soon as the current .s~7itching amplifier starts to
turn on and charges for an interval determined by the current
source until it reaches the trigger voltage for the switch.
Drive from the current source is then transferred to the current
switching amplifier driving it immediately into cutoff. Thus,
the object of the invention is to provide a DC to DC voltage
converter of improved efficiency due to faster s~itching cir-
cuitry
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Further objects and features o the invention ~ill
become apparent upon reading the following description together
with the drawing.
DESCRIPTION OF THE PREFERRED'EMBODIMENT: ¦
_ _
Fast turn-off of transformer current is provided by
the circuit shown in the simplified schematic diagram of Fig. 2.
; For ease of comparison~ those components of Fig. 2 that are
direct'counterparts of components in the prior art circuit of
Fig, 1 are numbered the same. Thus in Fig. 2, source 17 is
connected to primary winding 14 by means of switch 20 and a
current switching ampliier 16 depicted as PNP transi.stor 16,
The base o~ transistor 16 is connected to the negative side of
source 17 by resistor 18 and to the end of trigger 15 by resis- ¦
tor 24. While primary winding 14 and trigger winding 15 are
depicted as a single tapped winding~ they may be separate
windings and it is simpler to discus~ them as separate windings
for this description~ ¦
The main components of the added circuitry according
to the invention are a current source using PNP transistor 25,
capacitor 26 and a voltage sensitiVè switching element depicted
as switching diode 27. The emitter and base of transistor 25,
are connected to switch 20 by means of resistors 28 and 30
respectively~ The base of`transistor 25 is also connected to
the negative si~e of source'17 by`res~tor 31. The collector o
transistor 25-is connected to the anode of diode 34, the cathode
of which is connected both to one end of capacitor 26 and to the
anode switohing diode 27 . ~he emitter o l transistor 16,
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which is connected to tap 12 at the ~uncture of primary winding
14 and trigger windin~ 15, is also connected to the second end
o~ capacitor 26 and to the cathode of diode 35, Resistor 36
connects the anode o~ diode 35 to the cathode of diode 27 and
to the base oP transistor 16. The secondary circuit of trans-
~ormer 11 is not critical to the invention and is shown in a
very simple arrangement. Secondary windlng 22 has capacitor 40
in series with rectifier 41 connected across it, Load 42 is
connected across capacitor 40~ Load 42 may, for example, be a
flash tube.
The operation o~ the circuit o~ Fi~, 2 is as ~ollows:
Upon closure of switch 20, resistor 18 biases the emltter-base
Junction of transistor 16 in the ~orward dlrection so that a
conduction path is provided frorn source 17 through primary
winding 14, transistor 16 and back to source 17, This conduction
path e~fectively isolates trigger winding 15 ~rom source 17, but
induction between windin~s 14 and 15 provides additional forward
bias to transistor 16 through current limitin~ resistor 24,
As a result the current through winding portlon 14 rapidly in-
creases until transistor 16 nears saturation, This operation lsthe same as rOr the prior art ~'ig, 1. In the circult o~ ~ig. 2,
conduction Or transistor 16 also establishes a path ~rom the
negative side of source 17 throu~h capacitor 26, diode 34,
the collector~emitter circuit of transistor 25 and resistor 28
to the positive side of source 17. Transistor 25 is biased
into conduction by resistor 31. Transistor 25 now acts as a
current source charging capacitor 26 at a constant rate deter-
~ined by the voltage divider network of resistors 30 and 31.
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The voltage across capacitor 26 is also imposed across diode
27 which eventually triggers into its conductive state Diode
27 is suitably a pnpn or oth0r multilayer diode having a fast
switching characteristic Other types of ~ast voltage sensitive
switches may be used. The time that diode 27 switches into
conduction is determined by the size of capacitor 26 and resis-
tors 28, 30, & 31 The values are desirably selected so that
diode 27 triggers prior to saturation o~ transistor 16. Diode
27 transfers a positive bias to the base of transistor 16
providing abrupt sharp turn-off of transistor 16. Upon abrupt
turn-off or transistor 16, current through winding portion 14
halts, the magnetic field collapses, and the energy, largely
stored in core 21, is trans~erred to the secondary circuit
through winding 22. At the same time diode 35 and resistor 36
discharge capacitor 26 restoring diode 27 to the "open" state.
The cycle then begins again automatically and continues. It
will be recognlzed that once diode 27 triggers and starts
turning transistor 16 off, induction between windings 14 and 15
again comes into operation aiding turn-o~f o~ transistor 16.
While the invention has been described with relation
to a sin~le simplified embodiment, it is applicable to all con-
verter and related circuits having the basic theoretical approach
of Fig. 1. Thus it is the intention to protect those aspects
of the inventlon set forth within the full scope of the
appended claims,
