Circuit for limiting the maximum current value supplied to a load by a power MOS

Abstract: The invention concerns a circuit for limiting the maximum current to be supplied to a load (Z) through a power Mos (Mp), being in particular of the regulated type using an equalizing capacitor (C). The addition of circuit means (2) with a one-way current flow between a terminal of said equalizing capacitor (C) and the gate terminal of (Mp) is effective to lower the voltage across the capacitor (C) and to speed up its charging process, thereby making the current limiting action expected from the circuit a timely one. The circuit means (2) may include a second Mos (M) of the same type as the power Mos (Mp). In this way, any deviations of (Mp) from its designed operation, as due to its manufacturing process and thermal drift phenomena, can be also compensated.

Description: This invention relates to a circuit for limiting the maximum current to be supplied to a load from a power Mos, in particular of a type which is regulated by means of an equalizing capacitor. Specifically, the invention concerns a circuit for limiting the maximum current to be supplied to a load from a power Mos, where said load is connected to a terminal of an external voltage supply and to the drain terminal of said power Mos, being of a type which comprises: a current generator connected to the gate terminal of the power Mos, a resistance connected to the source terminal of the power Mos and to the ground terminal, an amplifier which consists of, an input stage comprising a pair of transistors whose collectors are connected to the output terminals of a current mirror circuit, and whose bases are connected to each other through a series of two resistances having the same value, the emitter of the first transistor being connected to the ground terminal, the emitter of the second transistor being connected to the source terminal of the power Mos, and the terminal shared in common by said resistances of equal value being connected to a terminal of a voltage reference, an output stage comprising a bipolar transistor of the pnp type having its emitter connected to the gate terminal of said power Mos, collector connected to ground, and base connected to the collector of the first of said pair of input stage transistors,and an equalizing capacitance connected between the collector and the base of the first of said input stage transistors of said amplifier. There are several ways of limiting the current rating to a load through an output terminal of a power Mos. The most widely used one in the art sees recourse to a feedback network which comprises an amplifier having one input connected to a reference voltage, the other input connected to a resistor which senses the current being flowed through the load, and the output connected to the gate terminal of the power Mos, see for example WO 89/01255. That amplifier basically performs a comparison between the reference voltage and that applied to the other of its inputs which increases with the current across the load. As these two voltages tend to equal each other, the amplifier operates by its output to limit the voltage at the gate of the power Mos, thereby also limiting the current at the output terminal connected to the load. A major problem to be addressed in such negatively feed back arrangements is that of stability, especially where the connected load is of the inductive type. In fact, the power Mos would tend to enter oscillation as soon as the amplifier is operated. To obviate this drawback, the prior art has provided an equalizing capacitor in the circuit structure of the amplifier which is designed to stabilize the overall arrangement. The introduction of that capacitor delays, however, the amplifier intervention, i.e. the limiting action on the load current, until the capacitor is fully charged. In the meantime, the load current may well rise above its desired strength with nothing to limit it. This particularly holds if the intrinsic input capacitance of the power Mos becomes charged ahead of the equalizing capacitor. In fact, since any Mos is bound to possess a certain input capacitance, its gate voltage, and hence its output current, would increase according to a time constant which is effectively tied to that input capacitance. Should the equalizing capacitor become charged at a slower rate, there would be a time period when the output current from the power Mos is no longer controlled. This may result in load current overshooting, that is load current values which exceed the rated value that the limiter circuit is to maintain. Another factor which may practically defeat an accurate and effective limitation of the output current from a power Mos is a spreading of its (output) characteristic slopes due to manufacture processing (process spread) and to temperature variations (thermal drift). The manufacturing processes frequently result in an actual device whose effective characteristic slopes depart from the theoretical ones set by its designer. This means, for example, that if a given potential difference Vgs is applied between the source and the gate terminals, the current Id at the drain terminal will not be as expected, i.e. as inferable from the characteristic output slopes supplied by the manufacturer, and rather have a slightly different value. Thus, the Mos might fail to go on, or operate within the resistive range. Likewise, temperature variations from the design working value bring about changes in the characteristic slopes of the Mos which may perturb its expected performance. The above factors alter the operating conditions of the limiter circuit for the output current of a power Mos from those anticipated at the designing stage, which are based essentially on theoretical characteristic slopes. This may again result in inaccurate and/or untimely action of the limiter circuit, thereby dwarfing the benefits that the latter could bring about for the overall supply circuit of a load. It is an object of this invention to provide a circuit for limiting the maximum current to be supplied to a load through a power Mos, in particular a circuit of the type which comprises an equalizing capacitor, wherein the latter is charged at a faster rate than the intrinsic capacitance of the power Mos, such that it can intervene before the current at the load exceeds the maximum desired value. A further object of this invention is to provide a circuit as specified above, which is unaffected by process spreads and thermal drifts of the power Mos, such as would otherwise make its limiting action inaccurate and/or untimely. The first-mentioned object is achieved by a circuit as defined in the first claim, and specifically in the characterizing portion thereof. Said further object is achieved, along with the first-mentioned object, by a circuit as defined in the characterizing portion of Claim 4. For a clearer understanding of this invention, reference can now be made to a detailed description thereof, to be taken in conjunction with the accompanying drawings, in which: Figure 1 is a block diagram of a prior art circuit for limiting the maximum current to be supplied to a load through an output terminal of a power Mos; Figure 2 shows in diagram form the limiter circuit of this invention; and Figure 3 shows a particular embodiment of the circuit arrangement shown in Figure 2. The block diagram of Figure 1 is representative of the arrangement commonly adopted in the prior art for limiting a current I being supplied to a load Z and fixing it at a constant value using a power Mos. As can be seen, the load Z is connected between a terminal of an external voltage supply Va and one of the two output terminals of a power Mos, Mp, which is represented by its drain terminal in the drawing figure. Respectively connected to the two inputs A and B of amplifier 1 are a terminal of resistor Rs and the positive pole of a reference voltage Vr. The amplifier output is connected directly to the gate terminal of Mp to which a current generator Ig is also connected. Flowed through the resistor Rs is the same current I as is flowed through the load Z, and accordingly, the ground-going voltage drop which occurs across the resistor will increase with I. As this voltage tends to exceed the reference voltage Vr, the amplifier 1 intervenes with its output F to tap current off the generator Ig. Thus, the voltage across the gate of Mp will be lowered, and along with it will the current to the load via the drain. Shown diagramatically in Figure 2 is a limiter circuit according to the invention. But for circuit block 2, the circuit represents a known embodiment of the block diagram shown in Figure 1. In particular, it represents the embodiment mentioned at the outset. Thus, bipolar transistors Q4 and Q5 represent the input stage of the amplifier 1, bipolar transistor Q6 of the pnp type represents the output stage of the amplifier 1, capacitor C represents the equalizing capacitance, resistors R1 and R2 have the same resistance value, and transistors Q1, Q2 and Q3 constitute the current mirror circuit connected by its output terminals to the collectors of the transistors Q4 and Q5 in the input stage of the amplifier 1. The output stage of the amplifier 1 may well be implemented by the Darlington connection of a pnp transistor and an npn transistor between the same terminals of Q6. The terminal in common between said resistors R1 and R2 constitutes the input A of the amplifier 1, that is, the terminal connected to the reference potential Vr, whilst the emitter of Q5 constitutes the other input B of that amplifier. By placing the capacitor C between the collector D and the base E of the transistor Q4, the value of C required to regulate the overall circuit can be reduced. That capacitor would be charged by the base currents of Q4 and Q5 which, as is well recognized, are quite small, and since the voltage across it can be a relatively high value, it can become charged after the intrinsic capacitance of the Mos Mp is charged, with the attendant disadvantages mentioned in the foregoing. Specifically, as the voltages across Rs increases, i.e. as the load current I increases, the voltage Vbe between the base and the emitter of Q5 will decrease, with the potential at point A being fixed. Upon the voltage across Rs reaching the value of the potential Vr, said Vbe becomes very small and the transistor Q5 goes off. The input stage of the amplifier 1 is, therefore, unbalanced, and Q4 would tend to draw more current through the base of Q6, turning the latter on. In this way, some of the current Ig would be drained off the emitter of Q6, thereby decreasing the voltage at the gate of Mp, and hence the load current I. But, if the capacitor C has not yet been charged, Q4 will tap current off the base of Q6, thereby leaving it in the "off" state, and the gate of Mp can continue to draw current from Ig while causing the load current I to increase. By introducing the circuit block 2 shown in Figure 2, the capacitor C can be charged at a faster rate and the amplifier is allowed to intervene timely with its output F to limit the current I. That circuit block 2, being connected between the collector of Q4 and the gate of Mp, is made up of circuit means exhibiting a one-way current flow from the collector of Q4 to the gate of Mp. Their function is one of lowering the potential at point D such that the voltage VDE across C can be made small, and C can be charged very rapidly. In fact, if the circuit means 2 were not there, the voltage VDE across C, as is readily inferred from the circuit diagram of Figure 2, would be given by the following relation: (a) VDE = Va - VecQ2 - VbeQ4, where, Va is the supply voltage (having a fairly high value on account of Mp being a power Mos), VecQ2 is the voltage between the emitter and the collector of the transistor Q2 included in said mirror circuit, and VbeQ4 is the voltage between the base and the emitter of the transistor Q4. That value of Vde may be sufficiently high to cause C to become charged at a slower rate than the intrinsic capacitance of Mp. Consider, for example, the integrated circuit embodiment of Figure 2, including no circuit means 2. In this embodiment, in order to make the voltage across Rs -- which depends on the difference between the Vbe's of Q5 and Q4 --sufficiently large, a transistor Q5 is provided which has an emitter area n times as large as that of Q4, where n is an integer (e.g. n=5). On the circuit being turned on, Q5 will tend to draw a current from the collector of Q3 which is n times as large as that drawn by Q4 from the collector of Q2. The current mirror circuit comprised of Q1, Q2 and Q3 becomes unbalanced and fails to set the same current (I1) at its output terminals, in fact the collectors of Q2 and Q3. Q2 and Q3 have the same base current, but the collector current of Q3 is n times that of Q2. It follows that Q2 will operate in saturation, and therefore, that VecQ2 is the saturating voltage. Thus, VDE will take a fairly high value, as brought out by relation (a). By providing the circuit means 2, the potential at point D is tied to that taken by the terminal D' of said means, and since the gate potential of the Mos Mp is zero, that potential will be, upon turning on, equal to the voltage drop that occurs therein as it starts conducting. Such circuit means comprise a diode whose anode is connected to the collector of Q4, i.e. coincident with point D', so that the potential at point D would become equal to the threshold voltage of that diode, or to the sum of a number of like terms where more than one diode connected serially together are provided. That voltage, that is the potential at point D, is at all events made sufficiently small, and therefore, the voltage VDE is made small. This ensures that the equalizing capacitor C is charged more rapidly than the intrinsic capacitance of the Mos Mp. Figure 3 illustrates the preferred embodiment of the circuit means 2. This preferred embodiment consists of a diode K and a Mos M connected to each other. Specifically, the diode K has its anode connected to the gate of M, as well as to the point D', while its cathode is connected to the drain of the Mos. The latter has its source connected to the gate of Mp. Upon the overall circuit being turned on, the diode K will tap current off the collector of Q2 (through the D-D' shunt) and turn on the Mos M. The potential at point D', or point D of C, will then equal the voltage Vgs between the source and the gate of M, such that VDE = VgsM - VbeQ4 and that value will be fairly small. Once the input stage of the amplifier 1 is unbalanced and the transistor Q6 begins to conduct, drawing current from the gate of Mp, the Mos M goes off because its Vgs has become equal to the voltage between the base and the emitter of Q6, and that voltage is too small to hold M on. The diode K prevents, inter alia, current from flowing between the gate of Mp and the terminal D through the intrinsic diode KM of the Mos M, should the former attain, during its charging process, a higher potential than the latter. If the Mos M is of the same type as the power Mos Mp, that is a power Mos itself and implemented in the same technology as Mp, then its presence in the limiter circuit ensures that the aforementioned variations -- originating from the manufacturing process and thermal drift phenomena -- of Mp from its theoretical operation as evaluated at the designing stage will be compensated for. This can be accomplished if the whole limiter circuit is implemented in integrated form on one silicon chip. For such an integrated embodiment of the limiter circuit, it is preferred that said diode D be provided by a bipolar transistor of the npn type having its emitter connected to its base and the base in turn connected to the gate of M, as well as its collector connected to the drain of M.

Claims: A circuit for limiting the maximum current to be supplied to a load (Z) from a power Mos (Mp) having respective gate, drain and source terminals, where said load (Z) is connected between a terminal of an external voltage supply (Va) and the drain terminal of said power Mos (Mp), being of a type which comprises: a current generator (Ig) connected to the gate of the power Mos (Mp), a resistance (Rs) connected to the source of the power Mos (Mp) and to ground, an amplifier (1) which consists of, an input stage comprising a pair of bipolar transistors (Q4) and (Q5) whose bases are connected to each other through a series of two resistances (R1) and (R2) having the same value, and whose collectors are connected to the output terminals of a current mirror circuit (Q1,Q2,Q3), the emitter of the first transistor (Q4) in said pair being connected to ground and the emitter of the second transistor (Q5) in said pair being connected to the source of the power Mos (Mp), and an equalizing capacitance (C) being connected between the collector and the base of said first transistor (Q4), the terminal shared in common by said resistances (R1) and (R2) being connected to a terminal of a voltage reference (Vr), an output stage comprising a bipolar transistor (Q6) of the pnp type having its emitter connected to the gate of said power Mos (Mp), collector connected to ground, and base connected to the collector of said first transistor (Q4), characterized in that it comprises circuit means (2) with a one-way current flow, connected between the base of said bipolar transistor (Q6) and the gate of said power Mos (Mp). A circuit as in Claim 1, characterized in that said circuit means (2) with a one-way current flow comprise at least one diode (K). A circuit as in Claim 2, characterized in that said diode (K) is forward biased to the base of said bipolar transistor (Q6). A circuit as in Claim 2, characterized in that said circuit means (2) with a one-way current flow further comprise a second Mos transistor (M) having its gate terminal connected to the anode of said diode (K), drain terminal connected to the cathode of the latter, and source terminal connected to the gate terminal of said power Mos (Mp). A circuit as in Claim 3, characterized in that said second Mos (M) incorporated to said circuit means is implemented in the same technology as said power Mos (Mp). A circuit as in either Claim 4 or 5, characterized in that said diode (K) is implemented by a bipolar transistor of the npn type having its base and emitter connected together, which base is also connected to the base of said pnp bipolar transistor (Q6) and the gate terminal of said second Mos (M), and collector connected to the drain terminal of the latter.

Inventors: COCETTA FRANCO, LUONI LIANA, MARCHIO FABIO, ROSSI GIORGIO, COCETTA, FRANCO, LUONI, LIANA, MARCHIO, FABIO, ROSSI, GIORGIO, COCETTA, FRANCO, C/O SGS-THOMSON MICROELEC., LUONI, LIANA, C/O SGS-THOMSON MICROELEC., MARCHIO, FABIO, C/O SGS-THOMSON MICROELEC., ROSSI, GIORGIO, C/O SGS-THOMSON MICROELEC.
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1993
Main classification: H03F1
IPC codes: H03K17, H03G11, H03F1