Starting circuit, MOS transistor using the same and corresponding applications

Abstract: The principle on which the start up circuit of this invention operates is that of causing the MOS transistor (M2) to be turned on by sensing local electrical quantities thereof, specifically the potential at the drain terminal (D) of the MOS transistor (M2). The basic idea is to inject a small current into the control terminal (G) when the potential at the drain terminal (D) is high. For the purpose, an electric network (SN) is arranged to couple these two terminals together.

Description: This invention relates to a start up circuit for use with MOS transistors, a MOS transistor provided with such a circuit, and applications of both. There are circuit arrangements in which, at initial or renewed power-on, at least one transistor of the MOS type is always in a cut-off state due to the stable initial state of the driver circuit of its control terminal, and in which it matters that the transistor be made to conduct immediately afterwards, so that the desired system operation can begin. This is the situation with self-oscillating bridge converters, for example, as used in driver arrangements for fluorescent lamps such as that depicted in the accompanying Figure 1. The driver system of Figure 1 is a traditional one having two input terminals AC1 and AC2 for a mains supply, e.g. at 220 Volts and 50 Hertz in many European countries, and 110 Volts and 60 Hertz in the United States, and comprises a bridge rectifier BR coupled to the terminals AC1 and AC2, filtering circuitry FT coupled to the output of the bridge BR for suppressing the AC component residue, and self-oscillating half-bridge circuitry for driving a fluorescent lamp LA which is coupled to the output terminals, VDD and GND, of the circuitry FT, with the terminal VDD functioning as a supply potential reference and the terminal GND as a ground potential reference. A fuse FS and/or a resistor RES may be connected between the terminals AC1 and AC2, on the one side, and the bridge BR on the other, for protection purposes. The load on the half-bridge circuitry is comprised of a primary winding B3 of a transformer, an inductor L1, and the lamp LA, connected in series. A first terminal of this connection is led to the output of the half-bridge, and a second terminal is connected to the intermediate tap of a first capacitive divider; this first divider being connected between the terminal VDD and the terminal GND and comprising two capacitors, C3 and C4. Connected in parallel with the lamp LA is a second capacitive divider comprising two capacitors, C1 and C2. The intermediate tap of this second divider is connected, through a thermistor PTC coupled to the lamp LA, to the intermediate tap of a serial connection of two diodes, DP1 and DP2. The anode of the diode DP2 is connected to the terminal GND, the cathode of the diode DP2 to the anode of the diode DP1, and the cathode of the diode DP1 to the terminal VDD. The half-bridge circuitry comprises a first power transistor M1 of the MOS type and a second power transistor M2, also of the MOS type, which have main conduction paths connected in series between the terminal VDD and the terminal GND. Each MOS power transistor needs a driver circuit connected between its control and source terminals. The transistor M1 is coupled to a first driver circuit NT1 which has an input connected to a first secondary winding B1 of a transformer. The transistor M2 is coupled to a second driver circuit NT2, being identical with the circuit NT1, which has an input connected to a second secondary winding B2 of a transformer; the windings B1, B2, B3 belonging to the same transformer, and the windings B1 and B2 being substantially similar and connected to the respective driver circuits to supply them with voltages in phase opposition. Associated with the half-bridge circuitry is start-up circuitry comprising a resistor RS and capacitor CS, connected in series between the terminals VDD and GND, a diode DS of the junction type which has its anode connected to the intermediate tap of that series connection and its cathode to the source terminal of the transistor M1, a diode DD of the DIAC type which has a first terminal connected to the intermediate tap of that series connection and a second terminal to the control terminal of the transistor M2, and a resistor RR connected in parallel with the main conduction path of the transistor M1. At system power-on, the control terminals of the transistors M1 and M2 are at zero potential and the capacitor CS begins to be charged. The diode DS will not be conducting because its cathode is at a very high potential due to the resistor RR provided. Upon the voltage across the capacitor CS exceeding the breakdown voltage of the diode DD, the transistor M2 begins to conduct and operation in the oscillating mode sets in. A prior art driver circuit is illustrated by the accompanying Figure 2 and comprises a resistor R0 which has a first terminal connected to a terminal G and a second terminal connected to a node A, a capacitor C0 which has a first terminal connected to a terminal S and a second terminal connected to the node A, a diode D0 which has its anode connected to the node A and its cathode connected to the control terminal of the MOS transistor (transistor M2 in Figure 2) to be driven, and a bipolar transistor TR0 of the PNP type which has its emitter connected to the control terminal of the MOS transistor, its collector connected to the terminal S, and its base connected to the node A via a resistor RB; also provided is a diode D6 of the Zener type for protecting the MOS transistor against overvoltages. Also shown in Figure 2 for clarity are the transistor M2, having its drain terminal connected to a terminal D and source terminal to the terminal S, and a diode DD, having a terminal connected to the control terminal of the transistor M2, although these parts do not belong, strictly speaking, to the driver circuit. It matters that the two transistors are not turned on simultaneously, if connection of the terminal VDD to the terminal GND is to be avoided which would cause a huge flow of current through the two transistors M1 and M2 -- these forming a conduction path of very low impedance -- which is apt to inflict damage on them from overcurrent. Also important is to avoid, during normal operation when both transistors M1 and M2 are simultaneously turned off, too long a current re-circulation time through the re-circulating diodes, often inherently present between the drain and source terminals of MOS power transistors; dissipation through such diodes can be quite high, in fact. The driver circuit of Figure 2 is effective to turn on and off the MOS transistor connected to it at appropriate times; that is, the driver circuit of the transistor M1 should only turn it on after the driver circuit of the transistor M2 has turned it off, and vice versa. As is well recognized, the on and off states of the transistors M1 and M2 depend on the potential difference between the control and source terminals, and on their threshold voltage. The delaied turn-on is provided by the phase shift introduced by the phase shifter consisting of the resistor R0 and the capacitor C0, although an accurate computation ought to also take account of the intrinsic capacitance which exists between the control and source terminals of the MOS transistor. The advanced turning-off is provided by turning on the transistor TR0, which would occur upon the voltage across the capacitor C0 dropping by some 1,4 Volts below its maximum value consequently to the capacitor being discharged through the resistor R0. In a driver circuit as outlined above, the circuit components must be accurately sized to ensure utmost likeness in the performance of the various drivers, even with changing temperatures, for example. This likeness should be adequate to keep the duty cycle very close to fifty percent, and should prevent so-called "cross-conduction". In addition, the circuit is unsuitable for integration, particularly on account of the capacitor C0 therein. In general, as well as in the converter of Figure 1, the start-up condition is sensed by a suitable start up circuit directly connected to the power supply. At power-on, the start up circuit causes current to flow into the control terminal of one of the two MOS transistors and turn on the transistor, whereas in the steady state, the start up circuit has negligible or no effect. Such start up circuits are usually selected to suit their intended application, require circuit elements with storage capabilities, usually capacitors, and are difficult to integrate to the MOS-type transistor, due both to design diversity of the circuits, the presence of the capacitors, and the inherent circuit spread-out. It is the object of this invention to provide a start up circuit for MOS transistors which can obviate the drawbacks of prior art. This object is achieved by a start up circuit having the features set forth in Claim 1. Further advantageous aspects of this invention are set forth in the subclaims. The invention further concerns a novel transistor of the MOS type comprising the start up function at power-on, a self-oscillating bridge converter, and a driver arrangement for glow lamps whereto this transistor can be advantageously applied. The invention can be better appreciated from the following description, when read in conjunction with the accompanying drawings, in which: Figure 1 illustrates a driver arrangement for fluorescent lamps which includes a self-oscillating bridge converter, both of which are conventional; Figure 2 is a circuit diagram of a conventional driver circuit usable with the converter of Figure 1; Figure 3 is a combined block diagram of a driver circuit and a start up circuit according to this invention; Figure 4 is a combined circuit diagram of a driver circuit and a start up circuit according to this invention; and Figure 5 illustrates a driver arrangement for fluorescent lamps which includes a self-oscillating bridge converter, both of which are in accordance with this invention. As pointed out in the introduction, the circuitry encompassed by a self-oscillating bridge converter is fairly complex and hardly integratable. This invention is directed to simplify that circuitry, specifically in the respect of the power devices which comprise the bridge. This has been achieved by providing a novel type of MOS transistor, such as that shown in Figure 3, which is endowed with unique features. It should be understood, however, that this novel transistor type may be used wherever useful. The transistor of Figure 3 has three terminals only, as denoted by the characters G, S, D, and comprises a transistor M2 of the MOS type having a drain terminal connected to the terminal D, a source terminal connected to the terminal S, and a control terminal "coupled" to the terminal G. A diode DI is connected in parallel with the main conduction path of the transistor M2, as is often the case with these inductive load applications, also because this diode is an intrinsic one with certain technologies of MOS power transistor fabrication. The diode DI functions to recirculate the current while the transistor is in its off state, and it is on this account that it has come to be known as the "re-circulating diode". Advantageously connected between the control and source terminals of the transistor M2 is a diode D6 of the Zener type serving a transistor protection function, as is often done with these applications. Also shown is a capacitor CGS, in phantom lines to indicate that this capacitor is intrinsic to the transistor M2. In addition, two blocks are shown: the start up circuit SN and driver circuit DN. The circuit SN couples the drain terminal of the transistor M2 to its control terminal. The circuit DN couples the control terminal of the transistor M2 to the terminal G. As shown in Figure 3, the circuit DN is comprised of three sub-circuits: a turn-on delay circuit ON1, a first turn-off circuit OFF1, and a second turn-off circuit OFF2. What has been described in connection with the transistor M2 and Figure 3 also applies to the transistor M1 and any other MOS transistor, including non-power and N- and P-channel ones. The above sub-circuits will now be described in greater detail with reference to Figure 4. The principle on which the start up circuit of this invention operates is that of having the MOS transistor turned on by sensing local electrical quantities thereof, specifically the potential at the drain terminal of the MOS transistor. The basic idea is one of injecting a small current into the control terminal when the drain terminal potential is high. For the purpose, an electric network is provided to couple both terminals together. In the embodiment of Figure 4, the electric network comprises resistors R1, R2, R7 and diodes D4, D5. It will be appreciated that the electric network may be embodied with varying degrees of complexity, using passive and/or active components; the simplest embodiment being that of a resistor with a suitable value. Since for the specific application of driver arrangements to fluorescent lamps the drain terminal potential of MOS transistors is to attain very high values, e.g. in excess of 300 Volts, compared to the threshold voltage, which may be 2.5 Volts, a resistive divider is used as being simple and avoiding excessively large current injections. The divider consists of the series coupling of resistors R7, R2 and R1, it being allowed to regard the series connection of the resistors R1 and R2 as a single resistor for the purposes of the start up circuit. A first end terminal, corresponding to a terminal of the resistor R7, is connected to the drain terminal of the MOS transistor. An intermediate tap, corresponding to a node included between the resistor R7 and the resistor R2, is coupled to the control terminal of the MOS transistor through a diode. The second end terminal of the divider, corresponding to a terminal of the resistor R1, admits at least two alternative connections. A first possibility is to connect the second end terminal to the source terminal of the MOS transistor. The second possibility is that of having the second end terminal connected to the terminal G, when the second terminal would function as the control terminal of the novel MOS transistor. In this case, the injected current would not only be dependent on the potential at the terminal D but also on the potential at the terminal G. Inserted in the circuit of figure 4 are unidirectional conduction circuit elements, in the form of diodes D4 and D5, in order to limit the steady state effects of the start up circuit. Stated otherwise, the aim is to have the divider operate in the unidirectional mode only, this being a useful choice in itself as well as where the start up circuit is associated with a driver circuit, as is the case in Figure 4. The diode D5, connecting the control terminal of the MOS transistor -- of the N-channel type in Figure 4 -- to the intermediate tap of the divider, is effective to prevent the control terminal of the transistor from being discharged at a time when this is undesired. The diode D4, connecting the resistors R7 and R2, functions to make unidirectional the flow of current from the drain terminal of the MOS transistor. A first principle on which the driver circuit of this invention operates is to cause the turning on of the MOS transistor to be delaied by utilizing the time-wise pattern of the circuit input signal, rather than generating a delay within the circuit itself. The basic idea is one of using a threshold type of circuit element and arranging for no current flow toward or from -- according to the type of MOS transistor -- the control terminal, until the circuit input voltage exceeds a predetermined value. This can be achieved, for example, by coupling a Zener diode D1 in series to the control terminal, as in the embodiment of Figure 4. Where the input signal is of a kind which increases with a degree of uniformity, the time required to exceed that threshold will correspond to the desired amount of delay. In this way, the driver circuit can be matched to the dynamic range of the input signal automatically. A second principle on which the driver circuit of this invention operates is to have the MOS transistor turned off by using a different conduction path from the turn-on path. The basic idea is to cause at least one junction, coupled between the circuit input and the control terminal, to be turned on. The circuit of Figure 4 uses two junctions, coupled serially together, which correspond to the transistors TR1 and TR2. Where this junction belongs to a bipolar transistor, such as the transistor TR1 in Figure 4, it can be of advantage if its main conduction path couples the control terminal and source terminal together. In this way, the MOS transistor would be turned off rapidly through a low-impedance conduction path and while seeing a very low fixed potential, i.e. the ground potential. The turn-off speed is also tied to the gain of the bipolar transistor. The above junctions belong to the first turn-off circuit OFF1, since their conduction is activated directly by changes in potential at the input of the driver circuit. Advantageously, the second turn-off circuit OFF2 can be inserted in the driver circuit for faster turning off, its conduction being activated by the first turn-off circuit OFF1. The second circuit OFF2 includes a third transistor, such as the transistor TR3 in Figure 4, which has a main conduction path coupling the control and source terminals together and is controlled to the on state by a first bipolar transistor of the first circuit OFF1, such as the transistor TR1 in Figure 4, through its main conduction path. In this way, the MOS transistor can be rapidly turned off while seeing a very low fixed potential, i.e. the ground potential. and through a conduction path having a very low impedance if the third transistor is brought to a state of deep conduction by the first transistor. When the first and third transistors are connected to form an SCR device, as shown in Figure 4, the turn-off effect that results from its activation will not be limited by the evolution pattern of the circuit input signal, and is practically instantaneous. In further detail and with reference to Figure 4, the driver circuit is comprised of the following. The terminal G is connected to a first terminal of a resistor R3 which has a second terminal connected to the cathode of the Zener diode D1. The anode of the diode D1 is connected to the anode of a junction diode D3 having its cathode connected to the control terminal of the transistor M2. The combination of all this provides the circuit ON1. The anode of the diode D1 is also connected to the anode of a junction diode D2 having its cathode connected to the emitter of the PNP-type transistor TR2 whose collector is connected to the terminal S and whose base is connected to the resistor R1. The transistor TR1, of the PNP type, has its emitter connected to the control terminal of the transistor M2, its collector connected to the terminal S via a resistor R5, and its base connected to the emitter of the transistor TR2. A resistor R4 is connected across the emitter and base terminals of the transistor TR1. The combination of all this provides the circuit OFF1. The transistor TR3, of the NPN type, has its emitter connected to the terminal S, its collector connected to the base of the transistor TR1, and its base connected to the collector of the transistor TR1. The combination of all this provides the circuit OFF2. A transistor TR4 of the NPN type is also provided which has its collector connected to the collector of the transistor TR1, its emitter connected to the terminal S, and its base connected to the terminal G via a resistor R6. All this, in combination with the diodes D2 and D3, is effective to prevent the SCR device from being turned on when this is not desired. Finally, it should be emphasized that, with the driver and start up circuits associated together in the circuit of Figure 4, account must be taken of their mutual influence in selecting the parameters for the circuit elements as well as in certain topology options. For example, the resistive value of the resistor R2 should be such that, upon turning on, the transistors TR1 and TR2 will not enter conduction, or the resistive divider of the start up circuit may have, additionally to an intermediate tap for the control terminal of the MOS transistor, another intermediate tap corresponding to a node included between the resistor R1 and the resistor R2 for driving the transistor TR2. Advantageously, circuitry like that just described can be integrated on a single chip and inserted into a package provided with but three terminals, G, S and D. A transistor such as that described above can be used to advantage in a self-oscillating bridge converter, whether of the full or half-bridge type, specifically of a near-resonant type. A converter such as this has several applications, including driver arrangements for fluorescent lamps. A transistor of the kind described above may also be used to advantage in a generic driver arrangement for fluorescent lamps. Shown In Figure 5 is a driver arrangement for fluorescent lamps which comprises a self-oscillating bridge converter, both being in accordance with this invention. The arrangement in Figure 5 is similar to the arrangement in Figure 1, but comprises a half-bridge composed of a first transistor NM1 and a second transistor NM2, both in accordance with this invention. The terminal D of the transistor NM1 is connected to the terminal VDD, the terminal S of the transistor NM1 is connected to the terminal D of the transistor NM2, and the terminal S of the transistor NM2 is connected to the terminal GND. The secondary windings B1 and B2 are connected across the terminals G and S of the transistors NM1 and NM2, respectively. Also provided is a resistor RR which is connected across the terminals D and S of the transistor NM1. This resistor allows the output potential of the half-bridge to be set at the system power-on. which potential will be fairly high, e.g. not far below the potential at the terminal VDD.

Claims: A start up circuit for a MOS transistor (M2), the circuit comprising an electric network (SN) coupling the drain terminal of the MOS transistor to its control terminal for injecting a small current into the control terminal when the potential at the drain terminal is high, characterized in that the electric network (SN) comprises: a first resistance (R7) connected between the drain terminal (D) and a first node; a second resistance (R2) coupled between the first node and the control terminal; and a diode (D5), whose anode is connected to the first node and its cathode is connected to the control terminal. A circuit according to Claim 1, wherein the first resistance is coupled at a first end terminal to the drain terminal, and wherein the second resistance is coupled at a second end terminal at the control terminal. A circuit according to Claim 2, wherein the second end terminal is adapted to be coupled to the source terminal, instead of coupled to the control terminal. A circuit according to Claim 2, wherein the second end terminal is adapted to function as the control terminal of the transistor. A circuit according to one of the preceding claims, wherein said network includes at least one unidirectional conduction circuit element (D4) adapted to make unidirectional the flow of current to/from the drain terminal. A MOS-type transistor, characterized in that it is provided with a start up circuit (SN) as claimed in one of the preceding claims. A transistor according to Claim 6 and further provided with a driver circuit (DN) of the threshold type. A transistor according to either Claim 6 or 7 and made in an integrated form as a single chip having three terminals (G,S,D). A self-oscillating bridge converter including a start up circuit for at least one transistor (NM1,NM2) as claimed in one of the preceding claims. A driver arrangement for fluorescent lamps, including a start up circuit for at least one transistor (NM1,NM2) as claimed in one of the preceding claims.

Inventors: GRAZIANO VITO, PALARA SERGIO, GRAZIANO, VITO, PALARA, SERGIO
Applicants: CONS RIC MICROELETTRONICA, ST MICROELECTRONICS SRL, CO.RI.M.ME. CONSORZIO PER LA RICERCA SULLA MICROELETTRONICA NEL MEZZOGIORNO, STMICROELECTRONICS S.R.L.
Application year: 1995
Main classification: H05B41
IPC codes: H05B41, H02M7, H03K17