Abstract:
In a motor control system of the type having a plurality of transistors connected in series between a power supply and ground, a protection circuit is utilized to prevent the transistors from turning on simultaneously. The protection circuit introduces time delay into the turn on signals applied to the bases of the serially connected transistors, thereby preventing simultaneous operation and preventing excessive currents from damaging the transistors.

Description:
This application is a continuation-in-part application of patent application Ser. No. 547,637 filed Nov. 1, 1983, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to protection of transistors connected in a switching bridge configuration and more specifically to protection for the series connected transistors therein. 
     Transistors connected in series between terminals of a power supply are often found in switching bridge control systems for brushless DC motors. Typically, brushless motors are constructed with stationary windings and a rotating permanent magnetic field. The windings are commutated by the operating transistor pairs which are controlled in response to signals from rotor position sensors. Since the brushes and commutator of the normal DC motor are eliminated, and since commutation is controlled by the series connected transistors, brushless motors require sophisticated electronic motor control systems. A representative example of a DC motor having an electronic control system is disclosed in U.S. Pat. No. 4,368,411, entitled &#34;Control System For Electric Motor&#34;, issued Jan. 11, 1983, to H. Keith Kidd. 
     In brushless motor circuits there are typically six transistors connected in a bridge configuration of three pairs of series connected transistors which are connected between terminals of a power supply. The series connected transistors should never be turned on together, since simultaneous operation causes a short circuit across the power supply. Bipolar transistors have a time delay between the time that they are commanded to turn off and the time that they actually do turn off. This time delay is caused by the charge stored in the base-emitter junction of the transistor. The time delay is commonly referred to as the storage time of the transistor. The turn on time of the transistor is generally much shorter than the turn-off time because of the storage time. If a series connected pair of transistors is switched rapidly from the state where one transistor is on to the state where the other transistor is on, the difference between storage time of the transistor being turned off and the turn-on time of the transistor being turned on will cause both transistors to be on for the period of the time difference, resulting in excessive currents which can damage the transistors. 
     An object of the present invention is to provide a protection circuit for such series connected transistor pairs. 
     Another object of the present invention is to provide a motor control circuit in which time delays are introduced where needed to prevent excessive currents and consequent damage to the series connected transistor pairs. 
     SUMMARY OF THE INVENTION 
     The present invention provides a protection circuit for transistors connected in series within a switching bridge configuration. The switching transistors typically include two transistors connected in series across the terminals of a power supply. 
     In a control system for a brushless DC motor, the protection circuit of the present invention can be used to protect the series connected transistor pairs in the switching bridge used to commutate the stator windings. As the motor rotates, position signals are generated which the motor controller utilizes to provide synchronous control signals to the switching bridge. The protection circuit compares the present state of the control signals to their previous state to detect dangerous switching transitions. The transistor pair protection circuit according to the invention introduces time delays where needed to prevent the simultaneous operation of a series connected pair of transistors. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic block diagram of a motor control system utilizing the apparatus of the present invention; 
     FIG. 2 is a schematic block diagram of the series connected transistor pair protection circuit of FIG. 1; and 
     FIGS. 3 and 4 are illustrations of waveforms useful in explaining the operation of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 is a schematic block diagram of a control system 100 for an electric motor 101 according to the present invention. The present invention in its broadest aspects is not limited to motor control systems. Although the present invention is described herein in conjunction with a brushless DC electric motor, it is equally applicable to other inverters using switching bridge configurations which have series connected transistor pairs across the power supply terminals. 
     Motor 101 is preferably a brushless DC electric motor including a permanently magnetized rotor and a three phase stator winding excited by currents supplied on lines designated φA, φB, φC. Position signals from position sensors disposed in an operative relationship to the rotor provide position signals on lines 102, 103 and 104 to a motor controller 105. The motor controller includes a plurality of transistors Q A+ , Q A- , Q B+ , Q B- , Q C+ , Q C-  forming a six transistor switching bridge connected between terminals of a power supply V cc . The motor controller may be of the type disclosed in U.S. Pat. No. 4,368,411. Series connected transistor pair protection circuits 106, 107, and 108 are coupled between the motor controller 105 and its associated transistors Q A+ , Q A- , Q B+ , Q B- , Q C+ , Q C- . 
     The collector of transistor Q A+  is connected to the positive terminal of a voltage supply designated V CC . The base of transistor Q A+  is connected to an output terminal of the series protection circuit designated A&#39;+. The emitter of transistor Q A+  is connected to line φ A and the collector of the transistor Q A- . The base of transistor Q A-  is connected to the other output terminal of the series protection circuit 106 designated A&#39;-. The emitter of transistor Q A-  is connected to the negative terminal of the voltage supply V CC . The series connected pairs of transistors Q B+ ,Q B-  and Q C+ ,Q C-  are connected to series protection circuits 107 and 108 in a similiar fashion. For purposes of simplicity, only the series connected transistor pair Q A+ ,Q A-  and series connected transistor pair protection circuit 106 will be described in detail, but the description is applicable to the other series connected transistors Q B+ , Q B- , and Q C+ , Q C-  and their respective series connected transistor pair protection circuits 107 and 108. 
     Controller 105 provides output signals designated A+,A- which are useful for turning on transistors Q A+ ,Q A- . If the controller is simply providing output signals which successively turn transistor Q A+  on and off in a pulse width modulating fashion, for example, there is no danger that there will be simultaneous operation of transistors Q A+ ,Q A- . If the controller 105, however, has to make a transition from the state in which transistor Q A+  is on and transister Q A-  is off to a state in which transistor Q A-  is on and transistor Q A+  is off. As illustrated in the first set of waveforms A+,A- of FIG. 3, the charge stored in the base-emitter junction of transistor Q A+  causes both transistors to be turned on for a period of time causing a short circuit across the voltage supply V CC , thereby causing damage to the transistors. This simultaneous turning on is avoided by introducing a time delay t d  into the output from the protection circuit as shown in the second set of waveforms A&#39;+, A&#39;- of FIG. 3. When there is a transition from turning on transistor Q A-  to turning on transistor Q A+  the same short circuiting also results unless there is a time delay as illustrated in FIG. 4. The series connected pair transistor protection circuit 106, therefore, prevents such damage, by introducing time delays t d  in the signals A&#39;+,A&#39;- whenever a transition is detected where one of the transistors Q A+  and Q A-  is turned on before the other has completely turned off. 
     FIG. 2 is a schematic block diagram which depicts the series connected protection circuit 106 of FIG. 1. The hardware necessary to detect a transition in input signals A+,A- as well as introducing time delays into output signals A&#39;+,A&#39;- includes a read-only memory (PROM) 110, a plurality of flip flop latch circuits 111, and a 1 mhz clock 112. Preferably the flip flops or latch circuits 111 consist of an integrated circuit available from commercial sources under device number 74LS374. 
     PROM 110 is an eight bit addressable read only memory capable of storing 256 bytes of data at the addressable locations. The PROM is addressed by binary signals applied to the eight address inputs A1 to A8, bit A8 being the most significant bit. The output of the PROM appears as an eight bit word on the output terminals Q1 to Q8, bit Q8 being the most significant output bit. 
     Latch circuits 111 includes eight flip flop circuit each connected to receive its input control signal from a different one of data input terminals D1 to D8 and provide signals at output terminals Q1 to Q8, respectively, in accordance with the state of the flip flop circuit. Clock 112 provides clock pulses to the latch circuits 111. The occurrence of a clock pulse causes the respective flip flop circuits to assume a state corresponding to the input signals at terminals D1 to D8, and hence, the same signals thereafter appear at the output terminals Q1 to Q8. In the period between clock pulses, changes in the signals applied to the data input terminals have no effect on the states of the flip flop circuits. Thus, a clock pulse transfers the eight bit binary word appearing at input terminals D1 to D8 to the output terminals Q1 to Q8 and thereafter maintaining those output signals until occurrence of the next clock pulse. 
     The signals A+ and A- from motor controller 105 (FIG. 1) are supplied to input terminals A8 and A7, respectively, of PROM 110. Output terminals Q8 and Q7 of latch circuits 111, respectively, supply the signals A&#39;+ and A&#39;- for controlling series transistors QA+ and QA- in the switching lo bridge. Outputs Q1 to Q6 of latch circuits 111 are connected to address inputs A1 to A6, respectively of PROM 110. PROM outputs Q1 to Q8 are connected, respectively to data inputs D1 to D8 of latch circuits 111. 
     The data stored in the PROM is as shown in Table I. The addresses (A8 to A1), in hexadecimal form, are to the right of the vertical lines and the corresponding stored values which appear at PROM outputs Q8 to Q1 are to the left of the vertical lines and are also indicated in the hexadecimal form. 
     
                       TABLE I______________________________________   ADDR         PROM______________________________________   00           00   01           1F   02           2F   03           00   04           03   05           04   06           05   07           06   08           07   09           08   0A           09   0B           0A   0C           0B   0D           0C   0E           0D   0F           0E   10           00   11           10   12           11   13           12   14           13   15           14   16           15   17           16   18           17   19           18   1A           19   1B           1A   1C           1B   1D           1C   1E           1D   1F           1E   20           00   21           20   22           21   23           22   24           23   25           24   26           25   27           26   28           27   29           28   2A           29   2B           2A   2C           2B   2D           2C   2E           2D   2F           2E   30           0F   31           30   32           31   33           32   32           32   35           32   36           32   37           32   38           32   39           32   3A           32   3B           32   3C           32   3D           32   3E           32   3F           32   40           01   41           41   42           2F   42           01   44           02   45           03   46           04   47           05   48           06   49           07   4A           08   4B           09   4C           0A   4D           0B   4E           0C   4F           0D   50           0E   51           01   52           01   53           01   54           01   55           01   56           01   57           01   58           01   59           01   5A           01   5B           01   5C           01   5D           01   5E           01   5F           01   60           01   61           20   62           21   63           22   64           23   65           24   66           25   67           26   68           27   69           28   6A           29   6B           2A   6C           2B   6D           2C   6E           2D   6F           2E   70           0F   71           30   72           31   73           32   74           32   75           32   76           32   77           32   78           32   79           32   7A           32   7B           32   7C           32   7D           32   7E           32   7F           32   80           02   81           1F   82           82   83           02   84           03   85           04   86           05   87           06   88           07   89           08   8A           09   8B           0A   8C           0B   8D           0C   8E           0D   8F           0E   90           02   91           10   92           11   93           12   94           13   95           14   96           15   97           16   98           17   99           18   9A           19   9B           1A   9C           1B   9D           1C   9E           1D   9F           1E   A0           02   A1           02   A2           02   A3           02   A4           02   A5           02   A6           02   A7           02   A8           02   A9           02   AA           02   AB           02   AC           02   AD           02   AE           02   AF           02   B0           0F   B1           30   B2           31   B3           32   B4           32   B5           32   B6           32   B7           32   B8           32   B9           32   BA           32   BB           32   BC           32   BD           32   BE           32   BF           32   C0           32   C1           32   C2           32   C3           32   C4           32   C5           32   C6           32   C7           32   C8           32   C9           32   CA           32   CB           32   CC           32   CD           32   CE           32   CF           32   D0           32   D1           32   D2           32   D3           32   D4           32   D5           32   D6           32   D7           32   D8           32   D9           32   DA           32   DB           32   DC           32   DD           32   DE           32   DF           32   E0           32   E1           32   E2           32   E3           32   E4           32   E5           32   E6           32   E7           32   E8           32   E9           32   EA           32   EB           32   EC           32   ED           32   EE           32   EF           32   F0           32   F1           32   F2           32   F3           32   F4           32   F5           32   F6           32   F7           32   F8           32   F9           32   FA           32   FB           32   FC           32   FD           32   FE           32   FF           32______________________________________ 
    
     The operation of the system according to the invention can best be understood by reference to Tables IIA, IIB and IIC which correspond to portions of Table I giving both the hexadecimal values and the binary values. 
     
                                           TABLE II__________________________________________________________________________A8   A7  A6    A5      A4        A3          A2            A1    Q8                    Q7                      Q6                        Q5                          Q4                            Q3                              Q2                                Q1__________________________________________________________________________CONTROL STATE &#34;00&#34;00 0 0 0 0 0 0 0 0  00 0 0 0 0 0 0 0 001 0 0 0 0 0 0 0 1  1F 0 0 0 1 1 1 1 102 0 0 0 0 0 0 1 0  2F 0 0 1 0 1 1 1 103 0 0 0 0 0 0 1 1  00 0 0 0 0 0 0 0 004 0 0 0 0 0 1 0 0  03 0 0 0 0 0 0 1 105 0 0 0 0 0 1 0 1  04 0 0 0 0 0 1 0 006 0 0 0 0 0 1 1 0  05 0 0 0 0 0 1 0 107 0 0 0 0 0 1 1 1  06 0 0 0 0 0 1 1 008 0 0 0 0 1 0 0 0  07 0 0 0 0 0 1 1 109 0 0 0 0 1 0 0 1  08 0 0 0 0 1 0 0 00A 0 0 0 0 1 0 1 0  09 0 0 0 0 1 0 0 10B 0 0 0 0 1 0 1 1  0A 0 0 0 0 1 0 1 00C 0 0 0 0 1 1 0 0  0B 0 0 0 0 1 0 1 10D 0 0 0 0 1 1 0 1  0C 0 0 0 0 1 1 0 00E 0 0 0 0 1 1 1 0  0D 0 0 0 0 1 1 0 10F 0 0 0 0 1 1 1 1  0E 0 0 0 0 1 1 1 010 0 0 0 1 0 0 0 0  00 0 0 0 0 0 0 0 011 0 0 0 1 0 0 0 1  10 0 0 0 1 0 0 0 012 0 0 0 1 0 0 1 0  11 0 0 0 1 0 0 0 113 0 0 0 1 0 0 1 1  12 0 0 0 1 0 0 1 014 0 0 0 1 0 1 0 0  13 0 0 0 1 0 0 1 115 0 0 0 1 0 1 0 1  14 0 0 0 1 0 1 0 016 0 0 0 1 0 1 1 0  15 0 0 0 1 0 1 0 117 0 0 0 1 0 1 1 1  16 0 0 0 1 0 1 1 118 0 0 0 1 1 0 0 0  17 0 0 0 1 0 1 1 119 0 0 0 1 1 0 0 1  18 0 0 0 1 1 0 0 01A 0 0 0 1 1 0 1 0  19 0 0 0 1 1 0 0 11B 0 0 0 1 1 0 1 1  1A 0 0 0 1 1 0 1 01C 0 0 0 1 1 1 0 0  1B 0 0 0 1 1 0 1 11D 0 0 0 1 1 1 0 1  1C 0 0 0 1 1 1 0 01E 0 0 0 1 1 1 1 0  1D 0 0 0 1 1 1 0 11F 0 0 0 1 1 1 1 1  1E 0 0 0 1 1 1 1 0CONTROL STATE &#34;01&#34;40 0 1 0 0 0 0 0 0  01 0 0 0 0 0 0 0 141 0 1 0 0 0 0 0 1  41 0 1 0 0 0 0 0 142 0 1 0 0 0 0 0 0  2F 0 0 1 0 1 1 1 143 0 1 0 0 0 0 1 1  01 0 0 0 0 0 0 0 144 0 1 0 0 0 1 0 0  03 0 0 0 0 0 0 1 145 0 1 0 0 0 1 0 1  04 0 0 0 0 0 1 0 046 0 1 0 0 0 1 1 0  05 0 0 0 0 0 1 0 147 0 1 0 0 0 1 1 1  06 0 0 0 0 0 1 1 048 0 1 0 0 1 0 0 0  07 0 0 0 0 0 1 1 149 0 1 0 0 1 0 0 1  08 0 0 0 0 1 0 0 04A 0 1 0 0 1 0 1 0  19 0 0 0 0 1 0 0 14B 0 1 0 0 1 0 1 1  0A 0 0 0 0 1 0 1 04C 0 1 0 0 1 1 0 0  0B 0 0 0 0 1 0 1 14D 0 1 0 0 1 1 0 1  0C 0 0 0 0 1 1 0 04E 0 1 0 0 1 1 1 0  0D 0 0 0 0 1 1 0 14F 0 1 0 0 1 1 1 1  0E 0 0 0 0 1 1 1 050 0 1 0 1 0 0 0 0  01 0 0 0 0 0 0 0 151 0 1 0 1 0 0 0 1  01 0 0 0 0 0 0 0 152 0 1 0 1 0 0 1 0  01 0 0 0 0 0 0 0 153 0 1 0 1 0 0 1 1  01 0 0 0 0 0 0 0 154 0 1 0 1 0 1 0 0  01 0 0 0 0 0 0 0 155 0 1 0 1 0 1 0 1  01 0 0 0 0 0 0 0 156 0 1 0 1 0 1 1 0  01 0 0 0 0 0 0 0 157 0 1 0 1 0 1 1 1  01 0 0 0 0 0 0 0 158 0 1 0 1 1 0 0 0  01 0 0 0 0 0 0 0 159 0 1 0 1 1 0 0 1  01 0 0 0 0 0 0 0 15A 0 1 0 1 1 0 1 0  01 0 0 0 0 0 0 0 15B 0 1 0 1 1 0 1 1  01 0 0 0 0 0 0 0 15C 0 1 0 1 1 1 0 0  01 0 0 0 0 0 0 0 15D 0 1 0 1 1 1 0 1  01 0 0 0 0 0 0 0 15E 0 1 0 1 1 1 1 1  01 0 0 0 0 0 0 0 15F 0 1 0 1 1 1 1 1  01 0 0 0 0 0 0 0 1CONTROL STATE &#34;10&#34;80 1 0 0 0 0 0 0 0  02 0 0 0 0 0 0 1 081 1 0 0 0 0 0 0 1  1F 0 0 0 1 1 1 1 182 1 0 0 0 0 0 1 0  82 1 0 0 0 0 0 1 083 1 0 0 0 0 0 1 1  02 0 0 0 0 0 0 1 084 1 0 0 0 0 1 0 0  03 0 0 0 1 0 0 1 185 1 0 0 0 0 1 0 1  04 0 1 0 0 0 1 0 086 1 0 0 0 0 1 1 0  05 0 0 0 0 0 1 0 187 1 0 0 0 0 1 1 1  06 0 0 0 0 0 1 1 088 1 0 0 0 1 0 0 0  07 0 0 0 0 0 1 1 189 1 0 0 0 1 0 0 1  08 0 0 0 0 1 0 0 08A 1 0 0 0 1 0 0 0  09 0 0 0 0 1 0 0 18B 1 0 0 0 1 0 1 1  0A 0 0 0 0 0 0 1 08C 1 0 0 0 1 1 0 0  0B 0 0 0 0 0 0 1 18D 1 0 0 0 1 1 0 1  0C 0 0 0 0 1 0 0 08E 1 0 0 0 1 1 1 0  0D 0 0 0 0 0 1 0 18F 1 0 0 0 1 1 1 1  0E 0 0 0 0 0 1 1 090 1 0 0 1 0 0 0 0  02 0 0 0 0 0 0 1 091 1 0 0 1 0 0 0 1  00 0 0 0 0 0 0 0 092 1 0 0 0 0 0 1 0  11 0 0 0 1 0 0 0 193 1 0 0 1 0 0 1 1  12 0 0 0 1 0 0 1 094 1 0 0 1 0 1 0 0  13 0 0 0 1 0 0 1 195 1 0 0 1 0 1 0 1  14 0 0 0 1 0 1 0 096 1 0 0 1 0 1 1 0  15 0 0 0 1 0 1 0 197 1 0 0 1 0 1 1 1  16 0 0 0 1 0 1 1 098 1 0 0 1 1 0 0 0  17 0 0 0 1 0 1 1 199 1 0 0 1 1 0 0 1  18 0 0 0 1 1 0 0 09A 1 0 0 1 1 0 1 0  19 0 0 0 1 1 0 0 19B 1 0 0 1 1 0 1 1  1A 0 0 0 1 1 0 1 09C 1 0 0 1 1 1 0 0  1B 0 0 0 1 1 0 1 19D 1 0 0 1 1 1 0 1  1C 0 0 0 1 1 1 0 09E 1 0 0 1 1 1 1 0  1D 0 0 0 1 1 1 0 19F 1 0 0 1 1 1 1 1  1E 0 0 0 1 1 1 1 0__________________________________________________________________________ 
    
     Table IIA gives the lower sixteen values for the condition where A+=0 and A-=0, i.e., address inputs A8 and A7 to PROM 110 are both &#34;0&#34;. Table IIB gives the lower sixteen values for the condition where A+=0 and A-=1, i.e., address inputs A8 and A7 are &#34;0&#34; and &#34;1&#34; respectively. Table IIC gives the lower sixteen values for the condition where A+=1 and A-=0, i.e., address inputs A8 and A7 are &#34;1&#34; and &#34;0&#34;, respectively. 
     Assume initially that A8 and A7 are both &#34;0&#34; and that A6 to A1 from the latch circuits are also all &#34;0&#34;. This PROM address corresponds to the top line in Table IIA and results in a PROM output &#34;00&#34; in hexadecimal or &#34;00000000&#34; in binary. Upon occurrence of a clock pulse, the PROM output is transferred to the latch output. Outputs Q8 and Q7 are both &#34;0&#34; and therefore A&#39;+=0 and A&#39;-=0. As a result transistors QA+ and QA-=0. As a result transistors QA+ and QA- are both off. Latch outputs Q6 to Q1 are &#34;000000&#34; binary and these outputs are supplied to PROM address inputs A6 to A1. Thus, so long as A8 and A7 are both &#34;0&#34;, the input to the PROM remains &#34;00000000&#34; and Q8 and Q7 from the latch circuits maintains both transistors turned off. 
     If the signals from the motor controller 105 now change so that A8 remains &#34;0&#34; but A7 becomes &#34;1&#34;, the PROM address becomes &#34;01000000&#34; (40 hexadecimal) which appears on the top line of Table IIB. The PROM output becomes &#34;00000001&#34; and, after the next clock pulse, the six least significant bits are transferred to the PROM address inputs so that the address input becomes &#34;01000001&#34; (41 hexadecimal) corresponding to the second line on Table IIB. This results in a PROM output &#34;01000001&#34; (41 hexadecimal) and, after the next clock pulse, the latch outputs include Q8=0 and Q7=1 thereby maintaining transistor QA+ turned off and transistor QA- turned on. Since the PROM output (A1 hexadecimal) is the same as the PROM address, the system remains locked in at this address until the next change in A8, A7. 
     Assume next that the signals from the motor controller change so that A8 becomes &#34;1&#34; and A7 becomes &#34;0&#34;, a transition which could result in both transistors of a series pair being momentarily conductive. According to the invention, this transition is detected and an appropriate time delay is provided to delay the turn on of transistor QA+ sufficiently for transistor QA- to turn off. When the motor controller signals become A8=1 and A7=0, the PROM address changes to &#34;10000001&#34;  (81 hexadecimal) resulting in a PROM output &#34;00011111&#34; (1F hexadecimal). On the next clock pulse Q8=0 and Q7=0 and therefore both transistors receive a &#34;0&#34; (turn-off) signals. The lower six bits of the latch output, &#34;011111&#34; are supplied to the PROM address which then becomes &#34;10011111&#34; (9F hexadecimal) as appears in the bottom line of Table IIC. The PROM output then becomes &#34;00011110&#34; (1E hexadecimal). After the next clock pulse the lower six bits are supplied to the PROM address which therefore moves up one line in Table IIC. The values in Table IIC are such that each successive clock pulse moves the address up one line on the table and maintains the values Q8=0 and Q7=0 so that both transistors receive a turn-off signal. Thus, both transistors receive turn-off signals during fifteen clock pulses as the address works up the table until reaching address 91 (hexadecimal) corresponding to a PROM output &#34;00000000&#34;. Transfer of the lower six bits to the address input on occurrence of the next clock pulse results in an address &#34;10000000&#34; (80 hexadecimal) on the top line of Table IIC. The PROM output becomes &#34;00000010&#34; (02 hexadecimal) and, after the next clock pulse, the PROM address changes to &#34;10000010&#34; (82 hexadecimal). At this point the PROM output is the same as the PROM address and therefore the system locks in at this location until A7 or A8 changes. Since Q8=1 and Q7=0 at this location, transistor QA+ begins to receive the time delayed turn-on signal. 
     If the command state changes from &#34;01&#34; (i.e., A+=0 and A-=1) to &#34;00&#34; (a transition which does not result in an overlap of transistor conduction states) and then to &#34;10&#34; (a condition which could result in an overlap of transistor conduction states), the system nonetheless provides the necessary time delay. In command state &#34;01&#34; the address is 41 (hexadecimal). If the command state changes to &#34;00&#34;, the PROM address changes to 01 (hexadecimal) and then to 1F (hexadecimal) to begin the fifteen step counting sequence. If, for example, after five counts (at address &#34;1B&#34; in Table IIA), the command changes to &#34;10&#34;, the PROM address changes to &#34;9B&#34; on Table IIC and the fifteen step count continues. Thus, a turn-on signal is not supplied to the series transistor in less than fifteen counts (15 microseconds) after turn-off of the other series transistor. 
     While the invention has been described in its preferred embodiments it is to be understood that the words which have been used are words of description rather than limitation and that changes may be made within the purview of the appended claims without departing from the true scope and spirit of the invention in its broader aspects.