Semiconductor circuit having an excess voltage protection circuit

Abstract: A semiconductor circuit has d.c. supply lines (10/13,11/14) and an excess voltage protection circuit. The protection circuit comprises a switching transistor (T1) connected in series with one of the supply lines (10/13), a control circuit (15) which is responsive to a voltage detector circuit (16), coupled with the supply line (10/13), and which is effective to trigger the switching transistor T1 to a non conductive state upon detection of an excess voltage. The voltage detector circuit comprises a semiconductor element (T8), having a voltage breakdown characteristic correlated with one or more of the remainder of the semiconductor elements (T10). The semiconductor element (T8) is adapted to be driven to breakdown in response to a voltage across the supply lines (10/13,11/14) less than that which will cause breakdown of said one or more of the remainder of the semiconductor elements T10. The increased current drawn by the element (T8) at the onset of breakdown is effective to cause the control circuit (15) to trigger the switching transistor (T1) to its non conductive state.

Description: This invention relates to a semiconductor circuit having an excess voltage protection circuit and more particularly to such a circuit suitable for semiconductor integration in circuits operated from rectified a.c. mains voltages. Such a circuit is known from DE-A-3 402 222. The present invention results from a consideration of the requirements for powering miniature fluorescent lamps intended to form substitutes for conventional incandescent lamps, but is applicable to other applications. Examples of such fluorescent lamps are the so called SL and PL types. Such lamps are required to be operated directly from a mains power source via for example an input filter, a clamping circuit and rectifier. Problems arise in such circuits in that in addition to 230 volts RMS the mains also carries voltage fluctuations of up to 60% together with transients of kilovolt and kiloamp magnitude. With a view to miniaturisation and cost saving it is desirable to produce the drive circuits for a fluorescent lamp by an integrated circuit process and such a circuit must accordingly be capable of withstanding voltages higher than 230V.RMS. The voltage supply from the mains can be limited to 750V peak in a relatively cheap manner by means of a resistor (10 ohms) in series with the mains followed by a voltage dependant resistor clamp across the input to the subsequent circuit. Although transistors capable of operating at 750V are producible it is difficult to provide them in integrated form. One way of providing a degree of protection is to compare the voltage with a reference voltage and depending on the comparison to take appropriate steps for example disable the supply until the over voltage has disappeared. To provide adequate protection in mass production of such circuits it is necessary to take the worst case component tolerances into account so that the circuit is designed to operate for components at the bottom end of the distribution curve for a specific performance e.g. breakdown voltage, switch off characteristics and reference voltage. Accordingly, the supply is disabled more frequently than might be desirable because protection is provided against lower over voltages than are necessary in many cases. This is a particular problem for systems where large voltage fluctuations can occur in which case it is desirable to sustain the operation as long as possible. This is particularly problematical in fluorescent lighting systems fed from the rectified mains source as disablement of the supply even for the duration of a half cycle of the a.c. supply can cause the lamp to flicker. Accordingly, the present invention seeks to provide a semiconductor circuit including a protection circuit which enables limitation of voltage and which avoids unnecessary switch off of supply. According to the invention there is provided a semiconductor circuit having d.c. supply lines and an excess voltage protection circuit, said protection circuit comprising a switching transistor connected in series with one of the supply lines, a control circuit which is responsive to a voltage detector circuit coupled with the supply line and which is effective to trigger the switching transistor to a non conductive state upon detection of an excess voltage, wherein the voltage detector circuit comprises a semiconductor element, having a voltage breakdown characteristic correlated with one or more of the remainder of the semiconductor elements of the circuit, adapted to be driven to breakdown in response to a voltage across the supply lines less than that which will cause breakdown of said one or more of the remainder of the semiconductor elements and in which the increased current drawn at the onset of breakdown of the semiconductor element is effective to cause the control circuit to trigger the switching transistor to its non conductive state, characterized in that said semiconductor element has the same breakdown characteristic as said one or more of the remainder of the semiconductor elements but is arranged to be exposed to a voltage component in addition to that which occurs across the supply lines such that it is exposed to a voltage greater than that of said one or more of the remainder of the semiconductor elements. Preferably, said semiconductor element and said one or more of the remainder of the semiconductor elements are fabricated on a single integrated circuit chip. Ideally all the elements of the circuit are fabricated on the same integrated circuit chip. In an advantageous construction the switching transistor is of the field effect type having a gate connected to a source of voltage, the drain to source path of the switching transistor is in series with one of the supply lines, the control circuit comprises a degenerate thyristor connected between the gate and the source of the switching transistor, the semiconductor element is connected between a triggering input of the thyristor and the other one of the supply lines, such that when there is an overvoltage the current drain thereby is effective to trigger the thyristor into conduction and cause the switching transistor to be triggered to a non conductive state. In order that the invention and its various other preferred features may be understood more easily, some embodiments thereof will now be described, by way of example only, with reference to the drawings in which:- Figure 1 is a schematic circuit diagram of one embodiment of semiconductor circuit with voltage protection circuit constructed in accordance with the invention, Figure 2 is a cross sectional view of part of an integrated circuit chip showing a double-acting resurf structure forming a switching transistor for the circuit of Figure 1, Figure 3 is a block diagram illustrating a particular application of the semiconductor circuit with voltage protection circuit in accordance with another aspect of the invention, and Figure 4 is a side view of a fluorescent lamp unit which incorporates the semiconductor circuit with voltage protection circuit. Referring now to Figure 1 the voltage protection circuit has a pair of input lines 10,11 for connection to a source of rectified a.c. mains. One of the input lines 10 is connected via a switching transistor T1, shown for example as a NMOS type, a resistor R1 and diode D1 to an output terminal 13 whilst the other input terminal 11 is connected directly to an output terminal 14. The gate to source path of the transistor T1 is bridged by a control circuit 15 which is formed by a degenerate thyristor circuit comprising in this example a pnp transistor T2 having its emitter coupled to the gate of T1 its collector connected via a resistor R2 to the source of T1 and its base connected to the collector of an npn transistor T3 of the thyristor. The transistor T3 also has its collector connected to the gate of T1 via a resistor R3 and its emitter connected to the source electrode of T1. The base of T2 is coupled to the supply line 11/14 via the series connected drain/source paths of field effect transistors T8 and T9. The transistor T9 is biassed at a low current level by a low gate to source voltage (e.g. 0.5 volts above the threshold). Transistor T9 is bridged by a zener diode Z4 of which the anode is connected to the input line 11. The gate of transistor T1 is connected via a blocking diode D2 to the drain of a field effect transistor T4 the source of which is connected to a 15 volt supply line. The circuitry coupled with the supply line 10/13 and fed from the line 10 floats in voltage up and down with the rectified mains voltage. The transistor T4 serves to switch on the transistor T1 in response to a control signal applied to the gate of transistor T4 either from a remote source or by a manual switch. In its normal state T4 is arranged to be on and applies a 15 volt switch on voltage to the gate G of T1 whilst the thyristor circuit is off. In order to switch the thyristor on and thereby turn off T1 there is provided a switching input 18 which is connected to the gate of transistor T8. In response to a control signal on the gate of T8 either provided remotely or via a mechanical switch the transistor T8 can be switched on causing the thyristor to switch on and the transistor T1 to be switched off. The gate to source voltage of T1 can be restored at each zero crossing of the mains by applying the proper gate voltages at the gates of T4 resp. T8. The transistor T8 in the off condition of the thyristor is exposed to a voltage greater than that on line 10/13 by approximately 15 volts. This enables the same transistor type to be used for T8 as T10 and T11 and in this way their breakdown voltages are correlated by being substantially equal. However, in view of the increased voltage exposure of T8 this will approach breakdown earlier than T10 or T11 and thereby provides advance warning of breakdown. If T8 breaks down and the voltage at the input increases, as long as T1 is not yet switched off, an increase in voltage can be sensed at the source of T8. An output current detector circuit 17 is also provided this comprising an npn transistor T7 in combination with the resistor R1. The base of T7 is connected to the side of R1 which is coupled to the transistor T1, the emitter of T7 is connected to the other side of R1 and the collector is connected to the triggering input of the thyristor on the collector of T3. When the current through R1 causes the voltage developed across it to be sufficient to trigger T7 into conduction then the thyristor is caused to conduct and T1 is switched off. The transistor T4 is provided with a 15 volt supply derived from the rectified a.c. input voltage. For this purpose a 15 volt zener diode Z2 with a parallel smoothing capacitor C1 is connected between the source of T4 and a ground reference line. The source of transistor T4 is fed directly from the zener diode Z2. In order to provide some current flow from node 10 to node 13 when T1 is switched off (e.g. during start up) a resistor R6 (e.g. 10 k0hms) may be connected between drain and source of T1. A zener diode Z3 of say 22 volts may be connected between gate and source of transistor circuit T1 to protect the thyristor and subsequent circuitry against overvoltage. The output terminals 13,14 are connected in this example to a half bridge circuit formed of field effect transistors T10 and T11. Although the circuit of Figure 1 is shown feeding a half bridge circuit it will be understood that other circuits can be fed and the correlated protection can be provided if the semiconductor devices employed are correlated with the protection element T8. A particularly advantageous application is a switched mode power supply feeding a miniature fluorescent lamp where off time needs to be minimised to avoid flicker of the lamp. The basic principle of this invention does not require T8 to be a field effect transistor ; any other semiconductor device having a breakdown characteristic correlated with one or more subsequent semiconductor elements can be employed. It will be appreciated that although the circuit of Figure 1 is made up of transistors of specific polarity types, a similar arrangement can be produced with transistors of opposite polarity type and such arrangements are intended to fall within the scope of this invention. A particular advantage of the described circuitry is that it can be fabricated on an integrated circuit chip employing the resurf principle to form a lateral DMOS transistor for T1 together with transistors with a similar breakdown voltage, e.g. "vertical DMOS transistors" for the control and detector circuits. The resurf principle and its applications are documented in proceedings of the International Devices Meeting 1979 pages 238-41, 1980 pages 87-99 and 1982 pages 81 to 83. Further information is available in our British Patents Nos. 2013029B, 2026339B, 2026240B. Figure 2 shows a cross section of part of an integrated circuit version of the circuit of Figure 1 showing how the transistor T1 is fabricated so as to operate with the resurf effect. This is the only transistor that is required to carry the full peak voltage fluctuations provided by the rectified a.c. input and only the drain D has an electrical connection at that voltage. It is difficult to provide on the chip conductors which will carry such voltages so the resurf principle permits the drain D to be provided with a pad to which an electrical wire is directly connected. This wire forms the input 10 shown in Figure 1. It is also disadvantageous to provide the extreme breakdown voltage for all the rest of the circuitry. Figure 3 shows a further aspect of the invention where the protection circuit is employed in a high voltage power supply which derives its source of power from the a.c. mains. The power supply has a pair of input terminals 20,21 for connection to the a.c. mains which are connected to a voltage clamp and rectifier circuit 23. The voltage clamp formed by a series arrangement of resistor R₀ and a voltage dependant resistor VDR. VDR may be connected in parallel with the output of the rectifier circuit R, as shown in solid lines, or alternatively may be connected in parallel with the input of the rectifier circuit R as shown by dotted lines. This depends upon the relative prices of suitable protection components and the integration process. An output from the voltage clamp is coupled via a filter to a protection circuit 25 as described in connection with Figures 1 and 2. Alternatively the filter circuit could be connected before the rectifier circuit. The output of the protection circuit is connected to a switched mode power supply 26 the output of which is connected to a load L for example a fluorescent lamp. As already described the protection circuit 25 can be fabricated on a semiconductor chip and this and the remainder of the circuits 23 and 26 can be built into the housing of a miniature fluorescent lamp together with the tube itself. Such an arrangement is illustrated in outline in Figure 4 which shows the side view of a fluorescent lamp unit comprising a base 30 having a bayonet connector 31 for removeable connection of the lamp in a socket such as is employed for incandescent bulbs. The base unit contains the circuitry as described in relation to Figures 1 to 3 and the fluorescent tube 27 forming a load for the circuit which tube is contained within a housing 33. The supply circuits for a fluorescent lamp may alternatively be provided in a lamp socket or luminaire which is adapted to receive a removeable fluorescent tube. Although the latter part of this description has been directed to the provision of a miniature fluorescent lamp unit it will be appreciated that the invention has other applications for example the supply of power to:- Electronic bridge circuits suitable for providing a primary voltage for ferrite transformers in power supplies, Bridge circuits suitable for low power motors. Control electronics for discrete transistors of high power motors, Shavers, Chargers for batteries e.g. in nickle cadmiun rechargeable devices.

Claims: A semiconductor circuit having d.c. supply lines (10/13,11/14) and an excess voltage protection circuit, said protection circuit comprising a switching transistor (T1) connected in series with one (10/13) of the supply lines, a control circuit (15) which is responsive to a voltage detector circuit (16) coupled with the supply line (10/13), and which is effective to trigger the switching transistor (T1) to a non conductive state upon detection of an excess voltage, wherein the voltage detector circuit comprises a semiconductor element (T8), having a voltage breakdown characteristic correlated with one or more of the remainder of the semiconductor elements (T10) of the circuit, adapted to be driven to breakdown in response to a voltage across the supply lines (10/13,11/14) less than that which will cause breakdown of said one or more of the remainder of the semiconductor elements (T10), and in which the increased current drawn at the onset of breakdown of the semiconductor element (T8) is effective to cause the control circuit (15) to trigger the switching transistor (T1) to its non conductive state, characterised in that said semiconductor element(T8) has the same breakdown characteristic as said one or more of the remainder of the semiconductor elements (T10) but is arranged to be exposed to a voltage component in addition to that which occurs across the supply Lines such that it is exposed to a voltage greater than that of said one or more of the remainder of the semiconductor elements (T10). A circuit as claimed in claim 1 characterised in that all the elements of the circuit are fabricated on the same integrated circuit chip. A circuit as claimed in any one of the preceding claims, characterised in that a current detector circuit (17) is provided which is responsive to the current flow in one (10/13) of the lines and is effective to trigger the switching transistor (T1) to a non conductive state upon detection of an excess current-flow in the supply line (10/13). A circuit as claimed in any one of the preceding claims, characterised in that the switching transistor (T1) is fabricated by the resurf principle and is integrated on a common semiconductor chip with the control circuit (15) and the or each detector circuit (16,17). A circuit as claimed in claim 4, characterised in that the switching transistor (T1) has a high voltage input provided via a supply wire (w) connected to a bonding pad on the integrated circuit structure, the transistor being formed by layers surrounding the pad. A circuit as claimed in any one of claims 1 to 5, characterised in that the switching transistor is of the field effect type having a gate (G) coupled with a source of voltage, the drain to source path of the switching transistor is in series with one (10/13) of the supply lines, the control circuit comprises a degenerate thyristor (15) connected between the gate (G) and the source (S) of the switching transistor (T1), the semiconductor element (T8) is connected between a triggering input of the thyristor and the other one (11/14) of the supply lines such that when there is an overvoltage the current drawn thereby is effective to trigger the thyristor into conduction and cause the switching transistor (T1) to be triggered to a non conductive state. A circuit as claimed in claim 6, characterised in that said semiconductor element (T8) is a field effect transistor. A circuit as claimed in claim 6, or 7 when dependant from claim 3, characterised in that the current detector circuit (17) comprises a resistor (R1) connected in series with the switching transistor (T1) in the supply line (10/13) and a transistor (T7) having its base coupled to the supply line (10/13) on the switching transistor side of the resistor (R1), its emitter coupled to the supply line to the opposite side of the resistor (R1) and its collector coupled with the triggering input of the thyristor (15) such that an excess current through the resistor (R1) triggers the transistor (T7) into conduction and causes the switching transistor (T1) to be triggered to a non conductive state. A direct current power supply circuit, characterised by the combination of a voltage clamp and rectifier circuit (23) fed from a pair of input lines (20,21) for a source of alternating current, which clamp and rectifier circuit (23) feeds a semiconductor circuit (25) as claimed in any one of the preceding claims. A fluorescent lamp unit, characterised by the combination of a fluorescent tube (27) a switched mode power converter (26) and a direct current power supply (23,25) as claimed in claim 9. A fluorescent lamp unit as claimed in claim 10, characterised in that the tube (27), power converter (26) and direct current power supply (23,25) are incorporated in a housing (33) which is equipped with a connector portion (31) that permits removeably electrical coupling with a socket for an incandescent light bulb. A fluorescent lamp unit as claimed in claim 10, characterised in that the power converter and direct current power supply (23,25) are incorporated in a lamp socket or luminaire which is adapted to receive a removeable fluorescent tube.

Inventors: SCHOOFS FRANCISCUS ANTONIUS CO, SCHOOFS, FRANCISCUS ANTONIUS CORNELIS MARIA
Applicants: PHILIPS ELECTRONICS NV, PHILIPS ELECTRONICS N.V.
Application year: 1989
Main classification: H02H3
IPC codes: H02H7, H02H3