Field of the Invention

This invention relates to a transient suppressor for an electrical supply in which the suppressor cycles on and off to dissipate an overvoltage condition.

Background of the Invention

An electrical supply typically serves loads which have a varying demand. As the demand changes, the current, which represents kinetic energy, must change. This presents a serious problem wnen a supply is operating at maximum cur- rent and the load is suddenly removed. A voltage transient occurs which may damage system components. Even when the source voltage of the supply is actively regulated, the reg¬ ulator response time is finite and the voltage transient may not be completely controlled.

Transient suppression circuits have used a Zener diode clamp which conducts upon occurrence of an overvoltage condition. However, the typical Zener breakdown voltage varies with temperature. Where tight control of voltage transients is required, a Zener suppressor circuit is not satisfactory.

Nercessian U.S. patent 3,949,271 shows a transient suppressor circuit in which a differential amplifier is re¬ sponsive to system voltage and with the occurrence of an overvoltage condition turns on a switch which conducts cur¬ rent through a shunt resistor circuit. The differential amplifier turns off, opening the switch, when the voltage returns to normal. The Nercessian circuit will stay on con¬ tinuously as long as the voltage remains greater than

normal. This operation may result in damage to the shunt resistor or the transistor with higher energy transients.

s-t.-m-ma-ry of the Invention

In accordance with the invention, a transient sup- pressor has a resistor and switch connected across the ter¬ minals of the electrical supply power source. A transient sensor and control responsive to the source voltage turns the switch on at a first overvoltage level and turns the switch off at a second voltage level less than the first voltage level. More particularly, in the preferred embodi¬ ment, both the first overvoltage level and the second volt¬ age level are greater than the normal rated voltage for the source.

Another feature of the invention is that a tran- sient suppressor for a DC source has a resistor, switch and control while a suppressor for an AC source has a first re¬ sistor, switch and control responsive to a source voltage of one polarity and a second resistor, switch and control re¬ sponsive to source voltage of the opposite polarity. A further feature of the invention is that the sensor and control includes a comparator biased to cause conduction of the switch upon occurrence of the first overvoltage condition, with a feedback circuit which modi¬ fies the comparator operation so that the switch is turned off at a second voltage level, providing hysteresis in the switching operation.

And another feature of the invention is that the control includes a counter which counts switch transitions between on and off, and means responsive to a selected count for inhibiting operation of the transient suppressor.

Further features and advantages of the invention will readily be apparent from the following specification and from the drawings, in which:

Figure 1 is a block diagram of the transient sup¬ pressor for an electrical supply with a DC source;

Figure 2 is a block diagram of the transient sup¬ pressor for an electrical supply with an AC source; Figure 3 is a block diagram of the transient sup¬ pressor control;

Figure 4 is a plot of source voltage as a function of time, illustrating operation of the transient suppressor; and Figure 5 is a partial schematic diagram of the control illustrating a preferred embodiment of a comparator in the control.

An electrical supply having a DC power source 15 is shown in Figure l. The source has two terminals 16, 17, to which a load 18 is connected. A transient suppressor includes an energy dissipating resistor 20 connected in se¬ ries with the collector-emitter circuit of switching tran¬ sistor 21 across the power source terminals. A sense and control circuit 22 is responsive to the voltage from power source 15 and has an output connected with the base of tran¬ sistor 21 to turn the transistor on and off. When the tran¬ sistor conducts, current through resistor 20 dissipates transient energy causing the source voltage to drop. In accordance with the invention, the sense and control circuit 22 turns the switch on at a first overvoltage level, greater than the normal rated voltage of the power source and turns the transistor off at a lower voltage which may also be greater than the. normal rated voltage the source. This dif¬ ference in the turn on and turn off voltages is sometimes referred to herein as hysteresis in the transient suppressor switch.

Operation of the transient suppressor is illus¬ trated in the plot of voltage as a function of time. Figure 4. Normal voltage of the power source 15 is applied to load 18 as indicated at 25. When the load is removed, a tran- sient voltage increase occurs at 26. At the upper or turn on threshold 27, transistor 21 conducts. Energy is dissi¬ pated in resistor 20 and the voltage drops to the lower or turn off threshold 28 which is higher than normal voltage 25 but less than the turn on threshold 27. Following the turn off of transistor 21 , the voltage again increases. The cy¬ cle of turn on/turn off/turn on continues until the kinetic electrical energy of the system is insufficient for the voltage to reach the turn on level as shown at 30. The voltage then drops to the normal level 25, as may be deter- mined by the characteristics of the power source or by a power source regulator, not shown. The periods of suppres¬ sor conduction are indicated at 31. The turn off threshold 28 may, of course, be selected at or below the level of nor¬ mal voltage 25 as long as the average of the turn on thresh- old 27 and the turn off threshold 28 is greater than the normal voltage 25.

The voltage levels 27, 28 are selected so that excessive voltage is not applied to components of the load and to avoid exceeding the current handling and dissipation capabilities of resistor 20 and transistor 21. For example, in a VSCF electrical system as shown in Krinickas SN , filed and assigned to the assignee of this application (docket B02936) a DC transient suppres¬ sor may be connected with the DC link between the rectifier and the DC to AC converter. The DC link has a nominal oper¬ ating voltage of 380 volts. The transient suppressor may be designed to turn on at 420 volts and to turn off at 380 volts. The peak current through the suppressor is of the

order of 700 amperes; the peak power dissipation is of the order of 200 kilowatts.

A transient suppressor for an electrical system having an AC power source is shown in Figure 2. The AC pow- er source 35 has terminals 36, 37. Electrical power is sup¬ plied to load 38. Two suppressor circuits 39, 40 are con¬ nected across the power source terminals 36, 37. The sup¬ pressor circuit 39 operates when power source terminal 36 is positive and terminal 37 is negative. Suppressor circuit 40 operates when terminal 37 is positive and terminal 36 nega¬ tive. The transistor 39a and series connected diode 39b of suppressor circuit 39 are poled oppositely from the transis¬ tor 40a and diode 40b of suppressor 40.

The sense and control circuit 22 is illustrated in a simplified block diagram, Figure 3. Transient sensor 45 has inputs 46, 47 connected with the terminals of the power source. A voltage reference 48 provides a stable reference with which the system voltage is compared. Upon occurrence of a transient overvoltage condition, a signal is coupled through optical isolator 49 with a control initializing cir¬ cuit 50 which is operative upon start up of the system to ensure that the transient suppressor is nonconductive. The control signal is connected with a level shift circuit 51 which conditions the control signal to operate driver ampli- fier 52. Transistor switch 21 is controlled by driver am¬ plifier 52 to turn on and off the transient suppression cur¬ rent through resistor 20. If the system conditions require, two or more transistors 21 or resistors 20 may be connected in parallel. Where the ratings of resistor 20 and transistor 21 do not permit unlimited continuous operation, a suppressor inhibit circuit 55 is provided. A counter 56 connected with the output of transient sensor 45 counts the repetitions of

turn on - turn off for a transient occurrence. When the transient is dissipated, the counter is reset. If the count exceeds a selected number, inhibit circuit 55 blocks the control signal through optical isolator 49 and turns off the transient suppressor. At the same time a signal at 57 from inhibit circuit 55 shuts down the power source to avoid dam¬ age to components of the electrical supply or load.

The output of transient sensor 45 may also be con¬ nected with a timer 59 as shown in broken lines. The timer provides a minimum off time for the switching transistor 21 so that the transistor is not turned back on before current flow through it is completely turned off.

The transient sensor 45. is shown in more detail in the schematic diagram of Figure 5. A differential amplifier comparator 65 has noninverting and inverting input terminals 66, 67 respectively. Voltage reference 48 has a stable pos¬ itive output connected through a voltage divider of series connected resistors 68, 69 and 70 with the comparator input terminals. In a DC transient control system, for example, power source terminals 16 and 17 are connected with sensor input terminals 46, 47, respectively, through series resis¬ tors 73, 74. A feedback resistor 75 is connected from the output 76 of comparator 65 with inverting input 67.

With the system voltage less than the turn on threshold 27, the output of comparator 65 is low. - When a transient voltage condition occurs and the system voltage rises to the turn on threshold, the output of comparator 65 goes high. This provides a control signal at 77 to optical isolator 49 and through isolation transistor 78 to counter 56. Counter 56 is a commercially average binary counter with preset. The high voltage at the output 76 of comparator 65 is coupled through feedback resistor 75 to the inverting input 67 shifting the operating parameters of the

comparator so that the voltage at terminals 16, 17 must drop to the turn off threshold 28 before the comparator output goes low. The repetition of suppressor turn on, turn off then continues as described above until the transient is dissipated.

In the AC system of Figure 2, the inputs of the sensor for transient suppressor 39 are connected to respond to a high positive potential at terminal 36 while the con¬ nections for the sensor of suppressor 40 respond to the high positive potential at terminal 37.