Amplifier drive circuit for inductive loads

A transformer coupled amplifier drive circuit (60) and method especially suited for providing a saw-tooth current waveform to an inductive load (32) for horizontal deflection applications in CRT monitors and the like are described. The transformer primary (48) is coupled between the power source connection (11) and the input of the amplifier (26) while the transformer secondary (50) has an opposite polarity terminal coupled to the input of the amplifier (26) through an inductor (52) and the other terminal of the secondary (50) coupled to the common terminal (15) of the amplifier (26) through a diode (56). When the input switch (44) is closed, the input of the amplifier (26) is energized by a current (65) through the series connected primary (48) and energy is stored in the transformer magnetizing field and in the amplifier (26). Conduction in the transformer secondary (50) is blocked by the diode (56). When the input switch (44) is opened, the input charging current (65) stops and the collapsing primary magnetizing field produces an oppositely directed current (67) which flows through the secondary winding (50), the diode (56), the series connected inductor (52) and the input to the amplifier (26). The demagnetization energy stored in the transformer (48) cancels the energy stored in the amplifier (26), thereby reducing total dissipation, and the inductor (52) controls the fall time independent of the rise time.

FIELD OF THE INVENTION 
The present invention relates to means and methods for power amplifiers 
and, in particular, power amplifiers capable of driving inductive loads 
and relatively high repetition rates. 
BACKGROUND OF THE INVENTION 
A power amplifier that is designed to drive inductive loads typically 
operates as a high-current switching device. Power amplifiers of this type 
are used, for example, to drive the yoke mechanism of the horizontal 
deflection circuitry of a display system employing a cathode ray tube 
(CRT). Typical prior art deflection amplifiers are described in U.S. Pat. 
No. No. 4,670,692, 4,642,533, 4,205,259, 3,501,672 and 3,480,826, which 
are incorporated herein by reference. 
The switching rate or operating frequency of the deflection amplifier is 
one factor that determines the degree of resolution of an image formed on 
the CRT. As the need has increased for progressively higher resolution, so 
has deflection amplifier operating frequency. Deflection amplifiers 
operating at 64-270 kHz or higher are now much desired. 
There are several reasons why it is difficult to operate CRT deflection 
amplifiers and other inductive load amplifiers at comparatively high 
frequencies. For example, (1) the effect of leakage inductance in the 
impedance transformer and the impedance of series inductors typically 
employed in such amplifiers increase with increasing frequency, making it 
harder to supply sufficient drive to the amplifier, and (2) prior art 
deflection amplifiers waste power and create dissipation and overheating 
problems in the amplifier and/or in the circuitry required to drive the 
amplifier which grow rapidly worse as frequency increases. A further 
problem with prior art circuits is that they do not provide independent 
control of the positive and negative current pulses usually employed in 
such amplifiers with the result that operation of the amplifier is not 
fully optimized. 
BRIEF DESCRIPTION OF THE INVENTION 
Accordingly, it is an object of the present invention to provide an 
improved means and method for driving inductive loads, particularly 
inductive loads in CRT deflection circuits or the like, and especially 
those operating at comparatively high repetition rates. It is a further 
object of the present invention to provide more efficient operation and 
independent control over the turn-off portion of the amplifier drive 
cycle. Additionally it is an object to reduce the sensitivity of the drive 
circuit to variations in individual amplifier characteristics. 
These and other objects and advantages are provided in a first embodiment 
by a circuit comprising: amplifying means for driving an inductive load 
and having a first output terminal coupled to the inductive load, a second 
output terminal coupled to a common terminal, and an input terminal; an 
inductor having first and second terminals wherein the first terminal is 
coupled to the amplifier input terminal; a transformer having primary and 
secondary windings each having first and second terminals, wherein the 
first terminal of the primary is coupled to a switch for supplying current 
to the primary and the second terminal of the primary is coupled to the 
second terminal of the inductor, and first terminal of the secondary is 
coupled to the second terminal of the primary and the second terminal of 
the secondary is coupled through a unidirectional device to the common 
terminal; and wherein the transformer windings are arranged so that, when 
the primary is excited, the first terminals of the primary and secondary 
have the same polarity. 
In a second embodiment there is provided a further improved circuit for 
driving an inductive load, comprising: amplifying means having a first 
output terminal coupled to the inductive load, a second output terminal 
coupled to a common terminal, and an input terminal; an inductor having 
first and second terminals wherein the first terminal is coupled to the 
amplifier input terminal; a transformer having primary and secondary 
windings each having first and second terminals, wherein the first 
terminal of the primary is coupled to a switch for supplying current to 
the primary and the second terminal of the primary is coupled to the 
amplifier input terminal, and first terminal of the secondary is coupled 
to the second terminal of the inductor and the second terminal of the 
secondary is coupled through a unidirectional device to the common 
terminal; and wherein the transformer windings are arranged so that, when 
the primary is excited, the first terminals of the primary and secondary 
have the same polarity. 
it is desirable that there also be a rectifying means and capacitance means 
coupled between the first and second amplifier output terminals and a 
resistance coupled between the amplifier input and common terminals. 
There is also provided an improved method for driving an inductive load 
coupled to the output of an amplifying means whose input is coupled to a 
transformer primary and secondary through an inductor, comprising: 
energizing the input of the amplifier through the primary circuit coupled 
to the amplifier input through the inductor while the secondary circuit of 
the same transformer coupled to the same amplifier input through the same 
inductor is substantially blocked from conducting, and thereafter 
deenergizing the amplifier input through the inductor and transformer 
secondary while the transformer primary circuit is substantially blocked 
from conducting. 
There is also provided a further improved method for driving an inductive 
load coupled to the output of an amplifying means whose input is coupled 
directly to a transformer primary and coupled to the transformer secondary 
through an inductor, comprising: energizing the input of the amplifier 
through the primary circuit coupled to the amplifier input while the 
secondary circuit of the same transformer coupled to the same amplifier 
input through the inductor is substantially blocked from conducting, and 
thereafter de-energizing the amplifier input through the inductor and 
transformer secondary while the transformer primary circuit is 
substantially blocked from conducting. 
These and other aspects, objects and advantages of the present invention 
will be more fully understood by considering the attached drawings and 
explanation thereof that follows.

DETAILED DESCRIPTION OF THE DRAWINGS 
Prior art CRT horizontal deflection amplifier circuit 10, shown in FIG. 1, 
comprises input power supply connection 11, current limiting resistor 12, 
switch 14, transformer 16 having primary winding 18 and secondary winding 
20, series inductor 22, shunt resistor 24, amplifier 26, snubber diode 28, 
energy storage capacitor 30 and deflection coil 32. Power supply 
connection 13 is provided for forcing a current through coil 32 and 
charging capacitor 30 in response to whether amplifier 32 is conducting or 
not. Common connection 15 is also provided. Switch 14 may be any type of 
rapidly operating switch well known in the art for providing an 
interruptible current path to power supply connection 11. For CRT 
deflection applications, switch 14 usually operates in a periodic manner 
with a switching frequency equal to the desired deflection frequency. 
Amplifier 26 is typically a high power bipolar transistor, but other 
amplifying devices and circuits may also be used, as for example, power 
MOSFETS, Darlingtons and/or BiMOS devices or circuits. As used herein the 
term "amplifier" is intended to refer to these and other alternatives as 
will occur to those of skill in the art based on the disclosure herein. 
For convenience of explanation, the operation of circuit 10 is described 
using a bipolar type transistor as amplifier 26, however, the principles 
of operation apply to other amplifiers as well. 
When switch 14 is closed current flows through resistor 12 and transformer 
16, thereby producing current 25 in the circuit of transformer secondary 
20, flowing through inductor 22 into the base of transistor 26. This 
causes transistor 26 to conduct, discharging capacitor 30 and permitting a 
current to flow through deflection coil 32 and amplifier 26 to reference 
terminal 15. The build-up of current through coil 32 is limited by its 
inductance so that the desired rising current ramp is obtained through 
deflection coil 32. This corresponds to the "sweep" portion of the CRT 
deflection cycle. 
When switch 14 is opened, the collapsing magnetization field in transformer 
16 causes the polarity at the terminals of secondary 20 to reverse so that 
oppositely directed current 27 now flows through the input of amplifier 
26, inductor 22 and secondary winding 20. Amplifier 26 turns off and any 
charge stored in the input of amplifier 26 is removed. Inductor 22 limits 
the magnitude of current 27 so as to prevent damage to amplifier 26. 
When amplifier 26 turns off, the collapsing magnetization energy stored in 
deflection coil 32 produces a current in the same direction through coil 
32 which is absorbed charging capacitor 30. The L and C values of coil 32 
and capacitor 30 are chosen to regulate the rate of change of the 
decreasing current ramp, which corresponds to the "fly-back" portion of 
the CRT deflection cycle. Resistor 24 is conveniently provided across the 
input terminals of amplifier 26 to aid in suppressing parasitic switching 
transients, but is not essential. 
One of the difficulties associated with prior art circuit 10 is that 
inductor 22 is in series with the input of amplifier 26 during both the 
turn-on and turn-off portions of the deflection drive cycle. During the 
turn-on portion of the input drive to amplifier 26, inductor 22 adds 
impedance to the input drive circuit, thereby requiring that larger drive 
voltages be provided through switch 14. Also, the series resistance of 
inductor 22 adds to the power loss in the circuit during turn-on. Further, 
the the energy stored in inductor 22 during the turn-on portion of the 
amplifier input drive, opposes the oppositely directed base current flow 
during the turn-off portion of the input drive cycle, thereby making it 
more difficult to turn off amplifier 26. 
A further difficulty is that turn-on current 25 and turn-off current 27 are 
not independently controllable. Also, substantial power is dissipated in 
current limiting resistor 12. In addition, the relatively square base 
input current (i.sub.b) pulse shape during turn-on (see FIG. 4A) increases 
the charge stored in the amplifier which must be extracted during turn-off 
and increases the power dissipated in amplifier 26. Further, the operation 
of the circuit of FIG. 1 is relatively sensitive to variation in amplifier 
characteristics. These are significant problems. 
Other prior art circuits are known to have various combinations of these 
difficulties, some more, some less. Accordingly, a need continues to exist 
for circuits and methods that avoid some or all of these problems. 
The circuit illustrated in FIG. 2 overcomes several of the above-noted 
problems. FIG. 2 shows inductive load drive circuit 40, according to a 
first embodiment of the present invention. 
Circuit 40 comprises input switching device 44 analogous to switch 14 of 
FIG. 1, parasitic suppression resistor 42, transformer 46 having primary 
48 and secondary 50, inductor 52, amplifier means 26, e.g., a bipolar 
transistor, resistor 54 and unidirectional conducting means 56, e.g., a 
diode or equivalent, coupled substantially as shown. Deflection coil 32, 
diode 28, capacitor 30 and power connections 11 and 13 and reference 
connection 15 are the same as in FIG. 1. Primary 48 and secondary 50 of 
transformer 46 have the indicated polarity relationship, i.e., the black 
dots adjacent the windings indicating the ends of the windings having the 
same polarity when, for example, the primary is excited. 
When switch 44 is closed, current 45 flows through primary winding 48, 
inductor 52 and into the input of amplifier 26. This causes amplifier 26 
to turn on. There is no current flow in transformer secondary 50 because 
conduction in the circuit of secondary 50 is blocked by diode 56. Resistor 
54 is analogous to resistor 24 and is provided to assist in suppressing 
parasitic switching transients. The operation of the output circuit of 
amplifier 26, including deflection coil 32, capacitor 30 and snubber diode 
28, is substantially the same as described in connection with this portion 
of FIG. 1. 
during the turn-on portion of the amplifier input drive when switch 44 is 
closed, transformer 46 acts as an inductor having an inductance determined 
by primary winding 48 which provides primary control of the rate of rise 
of input drive current 45. Thus, control over input current and amplifier 
turn-on is improved by virtue of the ability to separately select the 
primary inductance. The presence of diode 56 prevents transformer action 
during the turn-on portion of the input drive to amplifier 26. 
when switch 44 is opened (or the input drive otherwise interrupted), the 
collapsing magnetization in the transformer windings causes current 47 
having the opposite direction to input drive current 45 to flow in the 
secondary circuit comprising the input to amplifier 26, inductor 52, 
secondary winding 50 and diode 56 which is now forward biased. Inductor 52 
primarily limits the fall time of the turn-off portion of the amplifier 
input drive corresponding to current 47. 
FIG. 4B shows the direction and relative magnitude of the amplifier input 
current versus time for the circuit of FIG. 2. It will be seen that a 
positive going ramp is obtained during amplifier input turn-on and a 
negative going saturated ramp is obtained during turn-off. Further, 
because the shape of the turn-on portion is primarily controlled by the 
inductance of primary winding 48 and the shape of the turn-off portion is 
primarily determined by the inductance of inductor 52, the ability to 
separately control the turn-on and turn-off portions of the amplifier 
input drive is improved. Further, the saturated portion of the turn-off 
input drive may be adjusted by selecting the turns ratio of transformer 
46, providing additional control over operation of the power amplifier. 
Even though the performance of the circuit of FIG. 2 is improved over that 
of the prior art, some difficulties still remain. For example, the turn-on 
charging current still passes through inductor 52, so that its internal 
losses waste energy during both turn-on and turn-off. Further, its 
reactance during the turn-on portion still contributes to increasing the 
required drive voltage. Finally, the energy stored in inductor 52 during 
turn-on makes it more difficult to turn off transistor 26, since the 
collapsing magnetization field of inductor 52 produces an EMF tending to 
maintain current 45 and opposing current 47. 
A further improved circuit is shown in FIG. 3, which is preferred. The 
waveform associated with the amplifier input drive current 65, 67 of the 
circuit of FIG. 3 is shown in FIG. 4C. Drive circuit 60 of FIG. 3 is 
similar to that of FIG.2 and has substantially the same components, except 
that primary winding 48 is coupled to the input of amplifier 26 without 
passing through inductor 52. The operation of circuit 60 of FIG. 3 differs 
from that of circuit 40 of FIG. 2 in that, during input turn-on when 
switch 44 is closed, input current 65 flows through primary winding 48 
into the input of amplifier 26 without passing through inductor 52. As 
before, transformer 46 functions as an inductor determined by the 
inductance of primary winding 48. Thus, the rate of rise of the input 
drive current during the turn-on portion of the amplifier input may be 
substantially independently selected by choosing the inductance of primary 
48. It should also be noted that there is no series resistance in the 
input circuit other than the parasitic resistance of the transformer 
primary and circuit wiring, which may be made relatively small. The series 
resistance of inductor 52 is not in the circuit during turn-on. Thus, 
power dissipation is reduced. 
When switch 44 is opened, the circuit of FIG. 3 behaves substantially as 
that of as FIG. 2. Turn-off current 67 flows in the opposite direction, 
driven by the collapsing magnetization field of the transformer primary 
inducing current flow through secondary winding 50. Since the direction of 
current flow is opposite to that of charging current 65, diode 56 is now 
forward biased. Inductor 52 substantially controls the fall time of 
turn-off current 67 and the turns ratio of transformer 46 may be adjusted 
to control maximum (saturation) negative value 70 of turn-off current 67 
(see FIG. 4C). Further, since inductor 52 in FIG. 3 is not conducting 
during turn-on, there is no magnetization field whose collapse would 
generate an EMF opposing turn-off current 67. Thus, turn-on rate of rise 
71, turn-off rate of fall 72 and maximum turn-off drive 70 are decoupled. 
These features provide substantial advantages in optimizing the 
performance of amplifier 26 so that improved results are obtained for the 
same intrinsic transistor capabilities. 
A further advantage of the circuit of FIG. 3 is that the energy which was 
previously stored in inductor 52 in the circuits of FIGS. 1-2, and which 
opposed removal of the charge stored in the input of amplifier 26 and 
which was wasted, is no longer present. This energy is given by 1/2 
L.sub.52 [I(25).sub.peak ].sup.2 or 1/2 L.sub.52 [I(45).sub.peak ].sup.2, 
where L.sub.52 stands for the inductance of inductor 52 and I(25).sub.peak 
and I(45).sub.peak indicate the peak values of turn-on input charging 
currents 25 and 45, respectively. Hence a much smaller fall time and an 
independently adjustable fall time is achieved without interference with 
other performance characteristics. In addition, dissipation is reduced. 
Further, the magnitude of the circuit components may be arranged so that 
the energy needed to turn off amplifier 26 is provided by the energy 
stored in primary winding 48 of transformer 46 substantially independent 
of the other components. This stored energy is 1/2 L.sub.p [I(65).sub.peak 
].sup.2, where L.sub.p stands for the inductance of primary winding 48 of 
transformer 46 and I(65).sub.peak stands for the peak value of turn-on 
input charging current 65. The amount of energy needed to turn-off 
amplifier 26 is frequently substantial. By using the energy temporarily 
stored in transformer 46 during turn-on to provide the turn-off drive, the 
total energy is substantially conserved and overall dissipation is 
significantly reduced without sacrifice of other performance 
characteristics. Other than parasitics, the input drive is substantially 
lossless. This is a great advantage. 
EXAMPLE 
By way of example, an amplifier drive circuit suitable for driving 
inductive loads such as are encountered in CRT horizontal deflection is 
implemented with voltage of typically 6 volts at power supply connection, 
resistor 42 of 470 Ohms (1 Watt), transformer 46 of Type 1811P-3C8 
manufactured by Feroxcube of Saugerties, N.Y. with a turns ratio of about 
12:4 and primary inductance of 50 microHenries, inductor 52 of about 2 
microHenries, resistor 54 of about 22 Ohms (1 Watt), diode 56 of Motorola 
Type MUR110 rectifier and amplifier 26 of Motorola Type MJH16206 bipolar 
transistor manufactured by Motorola, Inc. of Phoenix, Ariz. Other 
components are conventional. The voltage of supply connection 13 and 
output components 28, 30, are chosen depending upon the desired properties 
of deflection coil 32 using means well known in the art. 
The circuits of FIGS. 1-3 were compared using the same amplifier transistor 
and operating at the same frequencies under substantially the same 
conditions. Three important parameters are considered; the storage time 
t.sub.s (see FIGS. 4A-C) during which the charge stored in the amplifier 
is being extracted, the switching power loss (SWPL) in the circuit and the 
variation .DELTA.t.sub.f in collector current fall time t.sub.f observed 
due to variations in individual transistor characteristics. The results 
are presented in the following table, normalized to the behavior of the 
circuit of FIG. 1. 
______________________________________ 
RELATlVE AMPLIFIER DRIVE CIRCUIT PERFORMANCE 
CIRCUIT 
AMETER FIG. l FIG. 2 FIG. 3 
______________________________________ 
t.sub.s 1.0 0.7 0.5 
SWPL 1.0 0.7 0.4-0.5 
.DELTA.t.sub.f 
1.0 0.3 0.1 
______________________________________ 
These results show that the invented circuits provide significantly 
improved (smaller) storage times and substantially improved (smaller) 
power dissipation. These improvements are very important in practical 
applications. 
The .DELTA.t.sub.f values shown in the table illustrate how the fall time 
t.sub.f changes in the several circuits when transistors having varying 
individual characteristics are plugged in as amplifier 26. For example, if 
two transistors yield differences in t.sub.f amounting to .DELTA.t.sub.f 
=100 nanoseconds in the circuit of FIG. 1, then according to the data in 
the table, the same two transistors substituted in the circuits of FIGS. 2 
and 3 would show .DELTA.t.sub.f values of 30 and 10 nanoseconds, 
respectively. The reduction in .DELTA.t.sub.f is extremely important 
because it indicates that the invented circuits are much more tolerant of 
unavoidable device characteristic variations. This is greatly to be 
desired. 
Having thus described the invention, those of skill in the art will 
appreciate that the invented circuit and method provide substantial 
improvement over the prior art by increasing the degree of freedom in 
independently setting the input operation of the amplifier for driving 
inductive loads during amplifier input turn-on, turn-off and turn-off 
saturation, that power dissipation is substantially reduced, and that the 
sensitivity to variations in transistor characteristics is much reduced. 
The invented means and method are especially well suited for driving 
inductive loads such as are encountered in high speed CRT deflection power 
amplifiers, 
While the present invention has been illustrated in terms of an elementary 
input switching means and of a bipolar transistor output amplifier driving 
an inductive load coupled to a capacitor and diode and of various 
resistors for parasitic transient suppression, it will be apparent to 
those of skill in the art based on the teachings herein that various 
modifications and additions may be made to the present invention without 
departing from the spirit thereof, as for example and not by way of 
limitation, by employing other types of output amplifying devices and/or 
circuits and other types of input switching means and other components or 
combinations for suppressing switching transients and other circuit 
configurations for coupling the amplifier output to the inductive load. 
Accordingly, it is intended to incorporate these and other variations as 
will occur to those of skill in the art based on the teachings herein in 
the claims that follow.