Transistor chopper protection circuit

A circuit for protecting the transistor of a high current transistor chopper from damage due to inductive energy stored in the chopper circuit when the transistor is biased nonconductive. The inductive energy charges a capacitor connected in parallel with the transistor, and a return circuit returns the energy stored in the capacitor to the source while the transistor is nonconductive and to the load when the transistor is subsequently biased conductive.

This invention relates to high current DC chopper circuits, and more 
particularly to transistor chopper circuits wherein the transistor is 
protected from damage due to inductive over-voltage at turn-off. 
DC chopper circuits generally comprise a solid state switch for controlling 
the application of a source of direct current to a load. The solid state 
switch is typically either a silicon controlled rectifier (SCR) or a power 
transistor. For many applications the transistor chopper is considered to 
be superior to the SCR chopper since the transistor chopper may be 
operated at higher frequencies and since a commutation network is 
unnecessary. However, the transistor chopper must include a protection 
circuit to prevent destruction of the transistor due to inductive 
over-voltage at turn-off unless the transistor is designed to withstand 
the turn-off voltages. In many high current applications, such as 
operating electric vehicle traction motors, transistors capable of 
withstanding the large turn-off voltages may not even be available. 
Various transistor protection circuits are disclosed in the prior art but 
such circuits are generally inefficient in operation and therefore not 
suitable for high current applications. 
Accordingly, it is an object of this invention to provide a transistor 
chopper circuit for controlling the application of power from a DC source 
to an electrical load wherein inductive energy stored in the source and 
the chopper circuit at transistor turn-off is absorbed to protect the 
transistor and returned (1) to the DC source while the transistor is 
turned off, and (2) to the electrical load when the transistor is 
subsequently turned on. 
It is a further object of this invention to provide a multiple phase 
transistor chopper circuit for controlling the application of power from a 
DC source to an electrical load wherein inductive energy stored in the 
source and in the chopper circuit at the turn-off of a transistor 
associated with a respective phase is returned to the electrical load when 
the transistor of another phase is subsequently turned on. 
These objects are carried forward with a protection network comprising a 
steering diode, a catch capacitor, and an energy return circuit path. The 
inductive energy stored in the chopper circuit when the transistor is 
turned off is diverted by the steering diode to charge the catch 
capacitor, and the return circuit permits the energy thereby stored in the 
catch capacitor to be used for charging the battery while the transistor 
is turned off and for energizing the load when the transistor is 
subsequently turned on. According to various embodiments, the return 
circuit may comprise an inductor or the series combination of an inductor 
and a diode. 
According to a further embodiment, current is supplied to the load through 
a multiple phase transistor chopper having a protection network associated 
with each phase. In this case, the inductive energy stored in a respective 
catch capacitor is substantially reduced so that most of the energy stored 
therein is returned to the load (through another phase transistor) rather 
than to the source. Increased efficiency is thereby achieved since 
charge/discharge losses in the DC source are substantially reduced.

Referring now to FIG. 1, reference numeral 10 generally designates a 
vehicular traction motor having an armature winding 12 and a series 
connected field winding 14. Alternately, a separately excited motor may be 
employed. Reference numeral 16 generally designates a plurality of 
serially connected storage batteries which provide a source of direct 
current for energizing motor 10 via conductor 17 and the collector-emitter 
circuit of power transistor 18. Switch 42 is provided in conductor 17 in 
order to disable the chopper circuit during periods of non-operation. The 
DC source 16 is comprised of twenty 12-volt batteries serially connected 
to provide a 240-volt supply voltage. Transistor 18 is periodically biased 
conductive by single phase (1) control circuit 20. It will be appreciated 
that the manner in which control circuit 20 operates does not form a part 
of this invention but that it may comprise circuitry effective to control 
the conduction period of transistor 18 for energizing motor 10 according 
to a desired schedule. Choke 22 is connected in series with traction motor 
10 in order to reduce armature current ripple, and free-wheeling diode 24 
is connected across traction motor 10 in a well known manner to circulate 
the inductive energy stored in motor 10 and choke 22 when transistor 18 is 
biased off. Reference numeral 28 designates a lumped inductance which 
represents the equivalent inductance of DC source 16 and the various 
circuit conductors. It will be appreciated that inductance 28 is not a 
physical circuit element and that its inductance value is a function of 
the circuit configuration and the conductor lengths. In high current 
applications such as delivering current to a vehicular traction motor, 
inductance 28 becomes a significant concern. For example, if transistor 18 
switches 400 amps of load current, and inductance 28 is only 8 uH, a 
typical transistor turn-off duration of 1 uS could produce a 3200-volt 
inductive surge. This induced voltage is more than capable of destroying 
transistor 18 at turn-off, and the remaining circuit elements (designated 
generally by reference numeral 30) comprise a protection network for 
preventing such destruction. Diode 32 and capacitor 34 form a series 
circuit connected in parallel with the collector-emitter circuit of 
transistor 18. Diode 32 is poled in a manner to allow excessive voltages 
appearing at the collector of transistor 18 to charge capacitor 34 but to 
prevent the discharge of capacitor 34 through the collector-emitter 
circuit of transistor 18. Inductor 36 and diode 38 form a second series 
circuit for returning the energy stored in capacitor 34 following the 
turn-off of transistor 18 to DC source 16 and to traction motor 10. 
Inductor 36 operates to dampen the current passing therethrough so that 
the energy is returned at a substantialy constant rate. Diode 38 serves to 
prevent capacitor 34 from ringing to twice the supply voltage when switch 
42 is initially closed. It will be appreciated, however, that diode 38 may 
be omitted if transistor 18 is capable of withstanding an applied voltage 
of this magnitude when it is first biased on. 
The operation of the chopper circuit illustrated in FIG. 1 will be 
described in reference to FIG. 2 which graphically illustrates the battery 
current (Ib) with respect to time. When transistor 18 is rendered 
conductive, energization current for traction motor 10 flows out of 
battery 16 as indicated by the arrow labeled Ib. Battery current (Ib) 
increases as a function of the circuit impedance as indicated by reference 
numeral 46 until transistor 18 is turned off at time T1. At this point 
current continues to flow out of DC source 16 due to the influence of 
inductance 28. During the time period T1-T2, the energy stored in 
inductance 28 charges capacitor 34 through a circuit path comprising 
battery 16, traction motor 10, choke 22, and diode 32. At time T2 
capacitor 34 reaches its peak voltage and charging current begins flowing 
out of capacitor 34 through inductor 36 and diode 38 into the positive 
terminal of battery 16. Battery charging continues until time T3 when Ib 
swings positive again due to circuit resonance. Accordingly, shaded area 
52 depicts the amount of energy that protection network 30 returns to 
source 16 while protecting transistor 18. At time T4, transistor 18 is 
biased on again and further discharges capacitor 34 through traction motor 
10, completing the cycle. 
FIGS. 3 and 4 illustrate a second embodiment of this invention employing a 
multiple phase transistor chopper. As illustrated in FIGS. 1 and 2, a 
single phase chopper circuit made in accordance with this invention 
energizes traction motor 10 through a single transistor 18 and returns a 
substantial amount of stored inductive energy to DC source 16 while 
transistor 18 is biased off. A multiple phase chopper, on the other hand, 
distributes the motor current between two or more successively actuated 
phases. Instead of using the inductive energy absorbed by capacitor 34 to 
charge DC source, 16, the stored energy in a multiple phase chopper is 
applied directly to traction motor 10 through a power transistor 
associated with the other phase. For many applications it has been found 
that the multiple-phase chopper is preferable to the single-phase chopper 
since battery charge/discharge efficiency losses may be eliminated or 
substantially reduced, depending on the chopper duty cycle. 
Specifically, FIG. 3 illustrates a two-phase transistor chopper for 
energizing traction motor 10, the transistor each phase being protected by 
a protection network such as the one illustrated in FIG. 1. Circuit 
elements corresponding to those depicted in FIG. 1 have been assigned the 
same reference numerals. A first phase designated generally by reference 
numeral 60 includes a power transistor 62, a free-wheeling diode 64, and a 
protection network comprising capacitor 66, inductor 68, and diodes 70 and 
72. A second phase designated generally by reference numeral 76 includes 
power transistor 78, free-wheeling diode 80, and a protection network 
comprising capacitor 82, return inductor 84 and diodes 86 and 88. 
Two-phase control circuit 90 biases either or both transistors 62 and 78 
to a conductive state in accordance with a selected duty cycle, whereby 
the current through traction motor 10 may be viewed as the sum of the 
current through inductors 92 and 94. It will be appreciated that the 
manner in which control circuit 90 operates does not form a part of this 
invention, but that it may comprise circuitry effective to control the 
conduction periods of transistors 62 and 78 for energizing motor 10 
according to a desired schedule. The conduction periods are typically 
equal in duration and 180.degree. out of phase with each other--they may 
or may not overlap, depending upon their duration. Inductors 92 and 94 
function to reduce armature current ripple so that the average current 
supplied to motor 10 is substantially the same as for the single-phase 
chopper described in reference to FIGS. 1 and 2. 
The operation of the chopper circuit illustrated in FIG. 3 will now be 
described in reference to FIG. 4 which graphically illustrates the battery 
current (Ib) with respect to time for the same load point conditions as in 
FIG. 2. When transistor 62 is rendered conductive, DC source 16 supplies 
energization current for traction motor 10 through field winding 14, 
armature winding 12, inductor 92 and the collector-emitter circuit of 
transistor 62. Battery current increases as illustrated by reference 
numeral 96 as a function of the circuit impedance until transistor 62 is 
biased nonconductive at time P1. It should be noted that the battery 
current at transistor turn-off for the two-phase chopper circuit is 
substantially less than for the single-phase chopper circuit, decreasing 
inductive stress at turn-off. Accordingly, the energy stored in catch 
capacitors 66 and 82 is correspondingly reduced, and the size of the 
various elements in the protection networks may be decreased. When 
transistor 62 is biased nonconductive, the inductive energy stored in 
inductance 28 charges capacitor 66 through diode 70 and motor 10. It 
should be noted that for the load point illustrated in FIG. 4, no energy 
is returned to source 16. Rather, the energy stored in capacitor 66 is 
returned solely to traction motor 10 when second-phase transistor 78 is 
biased conductive at time P2. It will be appreciated that for short 
duty-cycles (less than 20%) a small amount of the energy stored in 
capacitor 66 may be returned to DC source 16 due to circuit resonance as 
in FIG. 2 between time periods T2 and T3. However, the amount of energy 
returned to source 16 would be much less than is returned with the 
single-phase circuit since much less energy is stored in the catch 
capacitor of the two-phase circuit. Battery current rises from time P2 as 
indicated by reference numeral 98 until transistor 78 is biased 
nonconductive at time P3. The energy stored in inductance 28 charges 
capacitor 82 through diode 86 upon turn-off of transistor 78, and the 
energy thereby stored in capacitor 82 is returned to traction motor 10 
through transistor 62 at time P4 when transistor 62 is biased to a 
conductive state. It will be appreciated that if the conduction periods of 
transistors 62 and 78 overlap, the energy stored in the catch capacitor 66 
or 82 associated with the transistor 62 or 78 that is first biased to a 
nonconductive state will be returned to motor 10 through the conducting 
transistor 62 or 78 as soon as the voltage across the respective catch 
capacitor is sufficient to forward bias its associated diode 72 or 88. 
FIGS. 5 and 6 illustrate alternate chopper circuit arrangements in which 
the power transistor(s) is (are) connected between the motor and the 
positive terminal of source 16 in lieu of the circuit arrangements 
illustrated in FIGS. 1 and 3. For certain applications the alternate 
circuit arrangement may be desired, and the protection circuit of this 
invention works equally well with either arrangement. The reference 
numerals used in FIGS. 5 and 6 are primed but otherwise correspond 
directly to the reference numerals used in FIGS. 1 and 3, respectively. 
Although FIGS. 3 and 6 illustrate two-phase transistor choppers, the 
protection network of this invention equally applies to a transistor 
chopper having three or more phases. It will be appreciated, of course, 
that the optimum design for a given application involves at trade off 
between system efficiency and circuit complexity. It will also be 
appreciated that the NPN power transistors shown might be replaced with 
PNP transistors, although such replacement is thought to be unlikely due 
to the limited power handling capability of currently available PNP 
transistors. 
While it will be appreciated that various component values may be used 
depending upon the application, the circuit values according to the 
preferred embodiment of this invention (with reference to FIG. 1) are as 
follows: 
Source 16--240 volts DC 
Inductor 36--169 uH 
Capacitor 34--150 uF 
Transistor 18--Toshiba 2SD648 
While specific embodiments have been illustrated in order to fully disclose 
this invention, it will be understood that this invention is not limited 
thereto and that various modifications may be made therefrom without 
departing from its spirit and scope.