GB1561450A - Power switching circuit - Google Patents

Description

(54) POWER SWITCHING CIRCUIT (71) We, XEROX CORPORATION of Xerox Square, Rochester, New York, United States of America, a corporation organized under the laws of the state of New York, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to power switching circuits.

Many power switching circuits are known for switching transistors under fixed load and line or supply conditions. So far where variable loads are to be supplied and where the circuit arrangement has to be operable over a range of line voltages, it has been customary to use variable phase control or series controlled transformers. Phase control which relies on triac controllers tends to introduce voltage harmonic distortions into the line which are particularly difficult to remove, especially as increasingly requirements are demanded by the power supply authorities, for example, VDE regulations. In general, whereas series-controlled transformers do not generate difficult harmonic distortions, they tend to be expensive and inefficient.

It is an object of the present invention to provide improved power switching circuits.

According to the invention there is provided a power switching circuit including a pulse transformer having a primary and secondary windings, said secondary winding having extreme first and second terminals and a third intermediate terminal, a transistor having its emitter connected to said third terminal and having a connection at its collector for connection to a load, a pair of uni-directional devices each connected to said first terminal and respectively connected to the collector and base of the transistor to allow current to flow towards said transistor, a resistor and capacitor connected to the second terminal and respectively connected to the base and emitter of said transistor, and means for applying driving pulses to the primary winding which control in use the supply of power to said load.

A power switching circuit according to the invention will now be described by way of example with reference to the drawings in which: fig. 1 shows a circuit diagram of the power switching circuit; and fig. 2 shows schematically a layout of a switching system.

Referring to the drawings in figure 1, a pulse transformer 10 has a secondary winding with three terminals 11, 12 and 13. An n-p-n transistor 14 has its emitter connected to the terminal 13 and its collector connected to the primary of a load transformer 15 which primary is connected in turn to a voltage supply. The output of the circuit is identified as the supply to a load connected across the secondary of the transformer 15.

A pair of diodes 16 and 17 are each connected through a resistor 18 to the terminal 11 and connected respectively to the base and collector of the transistor 14. A resistor 19 and a capacitor 20 are both connected to the terminal 12 and respectively connected to the base and emitter of the transistor 14. The primary of the transformer 10 is supplied with a high frequency supply and is connected in series with a transistor 21.

The base of the transistor 21 is connected through a stabilising resistor 22 to an input terminal 23 to which driving pulses are applied in use.

A capacitor 24 is connected between the terminal 11 and the base of the transistor 14 and a resistor 19 is connected between the base of the transistor 14 and via a capacitor 20 to the emitter of the transistor 14.

A conventional snubber circuit, comprising a parallel connected resistor 26 and diode 27 which is connected in series with a capacitor 28, is connected between the collector and the emitter of the transistor 14. The snubber circuit as is well-known limits the rate of rise of the voltage at the collector to within limits as required by the specification of the chosen type of transistor 14.

A safety circuit (not shown in fig. 1.) is connected to the secondary of a transformer 29. The primary of the transformer 29 is connected between the collector of the transistor 14 and the load transformer 15. The safety circuit is arranged to inhibit the supply to the circuit in the event of any transient collector current approaching the operating limitations of the transistor 14.

Values of the components are given below for illustrative purposes. The specific circuit is designed to supply a load at 24 volts with output powers between 16 and 300 watts. The voltage supply to the load transformer 15 can vary between about 200 to 400 volts with respect to the base of the transistor 14. This is to take into account variations in the line voltage of the overall supply or to provide the possibility of using the power switching circuit, without adjustment, on a wide range of different line voltages. The high frequency supply to the primary of the transformer 10 is at 30 KHz + SKHz.

The output to the load is controlled in any convenient manner (by means not shown in fig.

1.) by altering the pulse width, up to 15 microseconds, of synchronised pulses applied to the terminal 23.

Component Values Transistor 14 BUX 80 supplied by Mullard Limited Diodes 16 A114F Diode 17 A 114M Resistor 18 0.82 Q 2.5 W Resistor 19 4.7 n 2.5 W Capacitor 20 16 MF, 16V Capacitor 24 0.5 MF, 16 V Resistor 25 56 Q 0.5 W Resistor 26 1 K 10 W Diode 27 A114M Capacitor 28 6800 pF. 1000 V A problem existing in earlier power switching circuits occurred in controlling the circuit during low power conditions. As there is generally a requirement for saturation of the transistor 14 under full load, the base current must always be sufficient to achieve saturation when desired. This requirement in earlier circuits tended to cause the transistor 14 to be over-driven under low power conditions.

Thus, after switch-off the transistor tended to continue to conduct for an indeterminate time period during which time the power continued to be supplied to the load. The addition of the diodes 16 and 17, ensures that during low and medium load conditions the transistor 14 does not saturate. This is because unrequired base drive is diverted through the diode 16.

The action of the diodes 16 and 17 is additionally relied upon because a negative defined current, supplied from the terminal 12 through the resistor 19, can be made available at the base to drive the transistor 14 to cut-off. The negative defirffiU current does not always have to drive the transistor out of saturation but "negative" pe ver is available to assist the action being provided by the capacitor 20. It will be noted that once the transistor has turned off, a small negative voltage remains on the base of the transistor 14 which is much desired to withstand secondary breakdown effects.

It will also be noted that the capacitor 24 provides an overdrive pulse at switch-on of the transistor 14.

It is believed that while the circuit described operates with mains derived frequencies of 30 KHz, the control provided by the circuit makes feasible the use of higher frequencies. At present operating frequencies have to be within the limitations of readily available switching transistors 14 but it is likely that faster transistors will soon be commercially available. In any event, the ability of this described circuit to operate under wide ranges of load and supply conditions is believed to be particularly advantageous.

It will be appreciated that such line transients as the described circuit may generate are confined to reflections caused by rectifying networks (see fig. 2) connected to the line and are generally not difficult to filter out where stringent user performance specifications exist.

Referring now to fig. 2, an overall power switching system is shown. Numerals of fig. 1 are used where like components are shown in fig. 2.

The system is supplied from a power line through a filter circuit 30, rectifier circuit 31 and a storage capacitor circuit 32 to provide power to the load transformer 15. A supply is taken from the filter circuit 30 to a transformer rectifier circuit 33 to provide a pulsed 18 volt supply at regulator circuit 34. The regulator circuit 34 supplies the pulse transformer 10. The secondary of the pulse transformer 10 supplies the transistor 14 which in turn controls the flow of current to transformers 15 and 29.

A sensor 35 is provided to respond to an excess current in transformer 29 to inhibit a drive transformer controller 36 and to supply control signals to a pulse width controller 37 indicative of the voltage being supplied to the load. The controller 37 responds to the signals from the sensor 35 to maintain the load voltage, in this example to 24 volts, by changing the pulse width of the signals supplied to the transformer 10.

WHAT WE CLAIM IS: 1. A power switching circuit including a pulse transformer having a primary and secondary windings, said secondary winding having extreme first and second terminals and a third intermediate terminal, a transistor having its emitter connected to said third terminal and having a connection at its collector for connection to a load, a pair of uni-directional devices each connected to said first terminal and respectively connected to the collector and base of the transistor to allow current to flow towards said transistor, a resistor and capacitor connected to the second terminal and respectively connected to the base and emitter of said transistor, and means for applying driving pulses to the primary winding which control in use the supply of power to said load.

2. A power switching circuit substantially as herein described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (2)

**WARNING** start of CLMS field may overlap end of DESC **. present operating frequencies have to be within the limitations of readily available switching transistors 14 but it is likely that faster transistors will soon be commercially available. In any event, the ability of this described circuit to operate under wide ranges of load and supply conditions is believed to be particularly advantageous. It will be appreciated that such line transients as the described circuit may generate are confined to reflections caused by rectifying networks (see fig. 2) connected to the line and are generally not difficult to filter out where stringent user performance specifications exist. Referring now to fig. 2, an overall power switching system is shown. Numerals of fig. 1 are used where like components are shown in fig. 2. The system is supplied from a power line through a filter circuit 30, rectifier circuit 31 and a storage capacitor circuit 32 to provide power to the load transformer 15. A supply is taken from the filter circuit 30 to a transformer rectifier circuit 33 to provide a pulsed 18 volt supply at regulator circuit 34. The regulator circuit 34 supplies the pulse transformer 10. The secondary of the pulse transformer 10 supplies the transistor 14 which in turn controls the flow of current to transformers 15 and 29. A sensor 35 is provided to respond to an excess current in transformer 29 to inhibit a drive transformer controller 36 and to supply control signals to a pulse width controller 37 indicative of the voltage being supplied to the load. The controller 37 responds to the signals from the sensor 35 to maintain the load voltage, in this example to 24 volts, by changing the pulse width of the signals supplied to the transformer 10. WHAT WE CLAIM IS:

1. A power switching circuit including a pulse transformer having a primary and secondary windings, said secondary winding having extreme first and second terminals and a third intermediate terminal, a transistor having its emitter connected to said third terminal and having a connection at its collector for connection to a load, a pair of uni-directional devices each connected to said first terminal and respectively connected to the collector and base of the transistor to allow current to flow towards said transistor, a resistor and capacitor connected to the second terminal and respectively connected to the base and emitter of said transistor, and means for applying driving pulses to the primary winding which control in use the supply of power to said load.

2. A power switching circuit substantially as herein described with reference to the accompanying drawings.