MXPA06008118A - Dv/dt-detecting overcurrent protection circuit for power supply - Google Patents

Abstract

An overcurrent protection circuit for a power switching transistor wherein the power switching transistor has a control electrode and two main electrodes, the circuit comprising a circuii including a protection switch for sensing the rate of change of voltage with respect to time at one of the main electrodes of the power switching transistor and for controlling the protection switch to remove a control signal to the control electrode of the power switching transistor to turn off the power switching transistor if the rate of change exceeds a predefined value.

Description

- before ihe expiration or / the lime lim l for amending the F? R? Ve leller t? Dei and? Lher abbreviali? Ns refer l? íhe 'Guidlaims and to be republished in the Year of Reception of Notes on Codes and? bbrevialwns' appeanng at the beginning-, amendments no of each regular issue of the PCT Gazette DETECTION PROTECTION CIRCUIT FOR DV / DT CURRENT EXCESS FOR. POWER SUPPLY FIELD OF THE INVENTION The present invention relates to an over current detection protection circuit for electrical systems, such as power supplies, and in particular, a self-sufficient high-efficiency linear power supply.

BACKGROUND OF THE INVENTION In the EUA patent application identified above with serial number 10 / 458,608, a high efficiency autonomous power supply is described.

The power supply is designed to provide power to electronic circuits for a period of time when a reducing circuit for electric lamps is not drawing current. In particular, the power supply is designed to draw current to energize electronic circuits during periods of time when the bidirectional thyristor of the reduction circuit is turned off. With reference to Figure 2, this figure shows an AC waveform (dotted line) together with a power supply current draw waveform (solid line). When the dimmer is off, the bidirectional thyristor is off for the entire half-cycle time. In this case, it is during periods 1 and 3 that the power supply of the above-identified patent application provides energy to a storage capacitor, which, subsequently, is regulated by a linear regulator. The power supply does not draw current during period 2. Due to the distinctive "cat-ear" regions 1 and 3 during which the power supply provides charging current to the storage capacitor, sometimes it is referred to as an "ear of a cat" energy supply. When the dimmer is set at full intensity or some intermediate level between 0% and 100%, the bi-directional thyristor is off for a certain part of each half cycle, and on during the other part of the half cycle. Now, the power supply provides power to the storage capacitor only during period 1 and does not draw current during periods 2 and 3. In both previous cases, the power supply draws current when the bidirectional thyristor is off and there is available voltage at through the bidirectional thyristor to charge the storage capacitor. Because the dimmer will never be turned off all the time, the power supply preferably only draws current during period 1 in all cases. With reference to Figure 1, this figure shows a power supply similar to the high efficiency autonomous power supply described in the co-pending US patent application identified above. The energy is applied from an alternating current source to the input I, which is rectified by the DI diode to provide a rectified half-wave voltage level in the busbar V +. Alternatively, a full-wave rectified voltage of a full-wave bridge can be provided to bus V +. A Ql power switching transistor is provided in series with the busbar. The source of transistor Ql is provided to a unregulated voltage busbar capacitor C4. The regulator Ul supplies a regulated output voltage Vo. The power supply includes a gate voltage supply that includes resistors R1, diode D2, capacitor C1, and zener diode Z1, which operate essentially in the manner described in the co-pending patent application identified above to provide a difficult door voltage ignition for transistor Ql through resistor R3, diode D3 and resistance R5. The voltage provided to the gate of the transistor Ql by this circuit provides a difficult ignition of the transistor Q1, reducing the loss of energy in the transistor Q1 when the transistor Q1 is turned on. The transistor Q2 turns off the transistor Ql when the voltage level at its base, as defined by means of a voltage divider comprising the resistors R1 and R2, reaches the threshold to turn on the transistor Q2. This occurs when the busbar voltage in the busbar V + exceeds a predefined value, typically when the bidirectional thyristor of the associated dimmer is turned on and the bus waveform V + is located in region 2 of the figure 2. Transistor Q2 can also be turned on when the voltage at capacitor C4 exceeds a predetermined value set by Z2. When transistor Q2 is turned on, the gate drive is removed for transistor Q1 and transistor Q1 turns off. When transistor Q2 is turned off, for example, in region 3, transistor Q1 is turned on again. The circuit of Figure 1 includes a surge current detection protection circuit 100. That circuit includes a transistor Q3 and a low resistance resistor Rβ in series with the transistor Q1. The resistor R6 passes the full load current and consequently results in a loss of energy in the order of approximately 0.9 watts for current levels of approximately 3 amps. The over current detection protection circuit 100 operates so that if the current level through transistor Ql exceeds the predetermined value, transistor Q3 is turned on, thus shutting down the gate drive for transistor Q1 and preventing transistor damage. Ql. The over current detection protection circuit 100 of the power supply circuit of FIG. 1, wastes energy in the series resistor R6 and contributes to an unnecessary voltage drop for the unregulated busbar. It is desirable to provide an over current detection protection circuit that results in less energy loss but still adequately protects the power switching transistor.

SUMMARY OF THE INVENTION It is an object of the present invention to provide improved over current detection protection that results in less energy loss than the over current detection protection circuit described above.

The above objective as well as others of the invention are achieved by means of an over current detection protection circuit for a power switching transistor, wherein the power switching transistor has a control electrode and two main electrodes, the circuit comprises: A circuit including a protection switch for detecting the rate of change of voltage with respect to time at one of the main electrodes of the power switching transistor and for controlling the protection switch for withdrawing a control signal for that the control electrode of the power switching transistor turns off the power switching transistor if the rate of change exceeds a predefined value. Other objects, features and advantages of the present invention will be apparent from the following detailed description.

BRIEF DESCRIPTION OF THE FIGURES Figure 1 shows a prior high efficiency self-contained linear power supply circuit incorporating an over current detection protection circuit; Figure 2 shows waveforms to explain the operation of the circuit of Figure 1; and Figure 3 shows a power supply incorporating the over current detection protection circuit of the present invention.

DETAILED DESCRIPTION OF THE INVENTION Referring now to Figure 3, a new over current detection protection circuit 300 is provided, according to the invention. The over current detection protection circuit includes a transistor Q4, a capacitor C8 and a resistor R8. This circuit operates in the following way. During normal operation, the voltage VBE between the base and the emitter of the transistor Q4 is less than about 0.3 volts corresponding to a maximum normal operating dV / dt current at the source of the transistor Ql, determined by the time in increase of the voltage on the wave in capacitor C4. This is insufficient for transistor Q4 to turn on. The circuit is designed so that at approximately twice the dV / dt current normally developed, the VBE of transistor Q4 is approximately 0.6 volts. This will be suitable for turning on the transistor Q4, thus removing the gate drive from the transient Q1 and turning off the power transistor Q1. Therefore, when dV / dt exceeds a predefined value, corresponding to an excess of current, the base drive for transistor Q4 is suitable to turn it on. If the current in transistor Ql exceeds a predefined limit, dV / dt or rate of change of wave voltage in capacitor C4 will be such that, a pulse passed through capacitor C8 because of dV / dt, will cause a voltage drop through resistor R8 of about 0.6 volts turning on transistor Q4. Under normal operating conditions, the dV / dt current present in capacitor C4 will develop only about 0.3 volts through resistor R8, insufficient to turn on transistor Q4. Capacitor C8 must be reset for each cycle whether a half-wave or full-wave rectified voltage is supplied to bus V +. The exemplary circuit uses a half-wave rectifier so that the capacitor must be restarted at the end of each complete wave of the AC cycle. Resistor R8 should normally be adequate to discharge capacitor C8, so that it will be ready to pass the next pulse during the next AC cycle. If resistance R8 is inadequate to discharge the capacitor, a diode can be provided between the base of transistor Q4 and the polarized ground with its anode to ground to discharge the capacitor before the next cycle. The protection circuit of over current detection dV / dt, according to the invention, provides benefits over the over current detection protection circuit described with respect to Figure 1. In particular, because there is no energy dissipation in a series resistance, such as resistance R6 of Figure 1, the energy consumption is reduced. At a current level of 3 amps, for example, the power dissipation in the R6 resistor is approximately 0.9 watts. In addition, because there is no series resistance element, there is no voltage drop of the element in series, allowing a high voltage to develop through capacitor C4 and thus preserve energy. The power supply will be charged more quickly and the peak currents can be reduced resulting in a lower voltage drop across the transistor Q1 and, therefore, less power dissipation in the transistor Q1. According to an alternative embodiment of the invention, the transistor Q4 can be replaced by a field effect transistor. In the circuit described, the capacitor C8 is approximately 0.01 uF and the resistor R8 is approximately 3.3 KOhms. Although the overcurrent detection protection circuit of the invention has been shown in connection with the protection of a power switching transistor of a power supply, the invention can be used in several circuits wherein the aim is to protect a transistor switching power or other electrical device against damage by excess current. For example, the overcurrent detection protection circuit of the invention could be used to protect the bidirectional thyristor of a light reducer if there is a shortage in the lighting load. Although the present invention has been described in relation to particular embodiments thereof, those skilled in the art will appreciate that there are other variations and modifications as well as other uses. Therefore, the present invention should be limited, not by the detailed description shown herein, but only by the appended claims.

Claims (14)

1. NOVELTY OF THE INVENTION Having described the present invention, it is considered as a novelty and, therefore, the content of the following is claimed as a priority: CLAIMS 1. - An over current detection protection circuit for a power switching transistor, wherein the power switching transistor has a control electrode and two main electrodes, the power switching transistor provides power to a load, the circuit comprises: a circuit that includes a protection switch to detect the rate of change of voltage with respect to time through the load on one of the main electrodes of the power switching transistor and to control the protection switch to remove a control signal for the control electrode of the power switching transistor to turn off the power switching transistor if the rate of change exceeds a first predefined value.
2. 2. The circuit according to claim 1, characterized in that the detection circuit comprises a capacitor coupled to a main electrode on the load side of the power switching transistor and a resistor coupled to receive a pulse from said capacitor and to develop a voltage across the resistor to turn on the protection switch if the voltage across the resistance exceeds a second predefined value.
3. 3. The circuit according to claim 2, characterized in that the protection switch comprises a transistor.
4. 4. The circuit according to claim 3, characterized in that the protection switch comprises a bipolar junction transistor.
5. 5. The circuit according to claim 4, characterized in that the resistance is coupled through the junction of the base-emitter of the protection transistor.
6. 6. The circuit according to claim 4, further comprising a diode coupled through the junction of the base-emitter protection transistor for discharge to the capacitor. 7. - The circuit according to claim 3, characterized in that the protection switch comprises a field effect transistor (FET) 8. - The circuit according to claim 1, characterized in that the power switching transistor comprises a field effect transistor (FET). 9.- An over current detection protection circuit for a power switching transistor, where the power switching transistor has a control electrode and two main electrodes, the power switching transistor provides power to a load , the circuit comprises: a circuit comprising a protection transistor, the circuit comprises an RC circuit for detecting the rate of change of voltage with respect to time through the load on one of the main electrodes of the power switching transistor and for controlling the protection transistor to withdraw a control signal so that the control electrode of the power switching transistor turns off the power switching transistor if the rate of change exceeds a first predefined value. 10. The circuit according to claim 9, characterized in that the RC circuit comprises a capacitor coupled to a main electrode on the load side of the power switching transistor and a resistor coupled to receive a pulse from said capacitor and to develop a voltage across the resistor to turn on the protection transistor if the voltage across the resistance exceeds a second predefined value. 11. The circuit according to claim 10, characterized in that the protection transistor comprises a bipolar junction transistor. 12. The circuit according to claim 11, characterized in that the resistance is coupled through the junction of the base-emitter of the protection transistor. 13. The circuit according to claim 10, characterized in that the protection transistor comprises a field effect transistor (FET) 14. - The circuit according to claim 9, characterized in that the power switching transistor comprises a field effect transistor (FET).