Detection circuit of the current in an MOS type power transistor

Abstract: The detection circuit of the current in an MOS type power transistor comprises a detection transistor (T2) connected with drain and gate in common to the power transistor (T1) and having characteristics such that the current (I2) flowing through it is equal to a fraction of the current (I1) flowing through power transistor (T1). Downstream from detection transistor (T2) there are arranged means (T6; T13) for comparing a first current (I3) equal to a fraction of the current (I2) flowing through detection transistor (T2) with a second reference current (Ig1) having a pre-set value and to produce a detection signal of the value of the current in power transistor (T1) in relation to the difference between said first current (I3) and the reference current (Ig1).

Description: The present invention relates to a current detection circuit for an MOS type power transistor. In such a device it is necessary to control the output current, say, during overload or shortcircuit situations, so as not to destroy the device itself. It is also necessary to provide for the possibility of having a diagnostic signal when the output current is lower than a pre-set minimum value, say, because the output load is disconnected, or of having a diagnostic signal when the output current is higher than a pre-set maximum value. On the basis of the prior art the control of the output current in an MOS type power transistor exploits the 'current detection' technique to ensure a minimum power dissipation and a minimum voltage drop across the power device. In the cases wherein the power device consists of n cells, the current in each individual cell is 1/n of the total. With the 'current detection' technique this current may be used to control the current flowing through the entire MOS power device. A known circuit embodiment for limiting the current flowing through a power transistor according to the 'current detection' technique provides for the use of a detection transistor having its drain and gate connected to the drain and gate of the power transistor and having characteristics such that the current flowing through it is equal to a fraction of the power transistor's current and of a control circuit which operates in relation to the voltage which is established at its input due to the current flowing through said detection transistor. The control circuit, according to the known art, comprises a pair of comparators having their inputs connected together, with a series of a resistance and of a voltage generator interposed between them. As the current flowing through the power transistor and thus through the detection transistor rises, the voltage across the resistance increases until it equals that of the voltage generator. The control circuit then becomes active and controls the common gate of the power transistor and of the detection transistor thus limiting the current flowing through them. This circuit embodiment can also be used to signal when the current through the power transistor falls below a pre-set threshold value, and in particular to signal when the load is disconnected. In this case the signal is produced by the output of a comparator one of whose inputs is kept at a constant threshold voltage and the other of whose inputs receives a voltage proportional to the current flowing through the power transistor. This technique has several disadvantages such as the high number of components, the complexity of manufacture and a comparatively low and barely controllable degree of accuracy. EP-A-0 227 149 discloses a current detection circuit according to the preamble of claim 1. GB-A- 2 186 140 discloses a current source circuit which makes use of two reference current sources. The object of the present invention is to provide a current detection circuit for an MOS type power transistor which has a lower number of components and at the same time is accurate and hardly sensitive to variations in the parameters external to the circuit. According to the invention such object is attained with a current detection circuit as defined in claim 1. In this way, by making the comparison in terms of current rather than of voltage, the number of components is reduced and with it the complexity of the circuit. At the same time a high accuracy is obtained which depends solely on the accuracy, including that for temperature, of the device used to generate the reference current which may be programmed internally. Advantageously such circuit may be used to control the current flowing through the power transistor, as well as only as a signalling circuit with the function of diagnosing when the current through the power transistor is higher than a maximum threshold value or falls below a pre-set minimum threshold value. The features of the present invention shall be made more evident by some embodiments illustrated as a non-limiting example in the enclosed drawings, wherein: Fig. 1 illustrates an embodiment of the detection circuit, used with functions of control ; Fig. 2 illustrates an embodiment of the detection circuit, used with functions of diagnostics. With reference to Fig. 1, an embodiment of the circuit according to the invention comprises a power transistor T1, whose drain is connected to power supply Vd, while the source is connected to load L at circuit node S and the gate is connected to a driving device P1. With transistor T1 there is associated a detection transistor T2, whose drain is connected to power supply Vd, while the gate is connected to driving device P1 and the source is connected through a diode D3 to the collector of a bipolar transistor T9 of the NPN type, in turn connected to the base of the same. The emitter of transistor T9 is connected to a circuit node A. Between circuit node A and circuit node S there is a series of a diode D2 and of a diode D1. To the base of transistor T9 there is connected the base of a bipolar transistor T10 of the NPN type. The emitter of transistor T10 is connected on one side to circuit node A, on the other side to ground through a current generator G2. The collector of transistor T10 is connected to the collector of a bipolar transistor T11 of the PNP type, in turn connected to the base of the same; the emitter of transistor T11 is connected to power supply Vd. The base of transistor T11 is connected to the base of a bipolar transistor T12 of the PNP type whose emitter is connected to power supply Vd, while the collector is connected on one side to ground through a current generator G1, on the other to the base of a bipolar transistor T13 of the NPN type. The collector of transistor T13 is connected with functions of control to driving device P1, while the emitter is grounded. If in power transistor T1 there flows the current I1, in the detection transistor T2 there flows a current I2 = I1/n, where n is the ratio between the number of cells of T1 and of T2. If A9, A10, A11, A12 are the emitter areas of T9, T10, T11, T12, we have: I2 = I1/n, I10 = I2x(A10/A9), I11 = I10, I12 = (A12/A11)xI11 = (A12/A11)x(A10/A9)x(I1/n) so that the normally quenched transistor T13 is brought to conduction, giving rise to the inhibition of power transistor T1, when I12 is greater than or equal to Ig1, the current of current generator G1, that is for a current I1 such that (A12/A11)x(A10/A9)x(I1/n) is greater than or equal to Ig1 and thus when I1 is greater than or equal to nxIg1x(A11/A12)x(A9/A10). If A12 = A11, A9 = 10xA10, n = 1000 and Ig1 = 1 mA, I1 is roughly equal to 10 A. The circuit diagram of Fig. 1 utilises NPN type transistors T9 and T10. This makes it possible, with equal output currents, to have a lower ratio of the number of cells of T1 with respect to T2, and thus a greater accuracy in the ratio between the current of transistor T1 and that of detection transistor T2, in view of the greater current delivery, for equal sizes, of the NPN's with respect to the PNP's. Diodes D1 and D2 are used to obtain a voltage drop at point A (with respect to the source of T1) equal to 2Vbe. This drop is then recovered by the voltage across D3 and by Vce (collector-emitter voltage) of transistor T9. In this way the sources of transistors T1 and T2 are at about the same potential. Since the latter also have the gate in common, Vgs is the same for both transistors, and there is thus a greater accuracy in the ratio between the primary current I1 and the current of detection transistor T2. The circuit shown in Fig. 2 is similar to that of Fig. 1, but is used with functions of diagnostics. The difference from the circuit of Fig. 1 consists in the presence of a further transistor T21 whose base is connected to the collector of transistor T13 whose emitter is grounded; in addition the collectors of both transistors T13, T21 are connected to power supply Vd through respective resistances R13, R21. The current Ig1 is set at a low value corresponding to the minimum threshold desired. When current I1 falls below said threshold I12 in turn falls below Ig1 causing the quenching of T13 and the conduction of T21 on whose output U the diagnostic signal appears.

Claims: Current detection circuit for an MOS type power transistor (T1), comprising an MOS type detection transistor (T2) having drain and gate terminals connected to drain and gate terminals of the power transistor (T1) and having characteristics such that the current (I2) flowing through it is equal to a fraction of the current (I1) flowing through the power transistor (T1), comparison means comprising a first transistor (T13) for comparing a first current (I12) equal to a fraction of the current (I2) flowing through the detection transistor (T2) with a second, reference, current (Ig1) produced by a current generator (G1) and having a pre-set value and to produce a detection signal of the value of the current (I1) in the power transistor (T1) in relation to the difference between said first and second currents (I12, Ig1), and current generating means (G2) for generating a further reference current (Ig2) of a value such as to ensure the equality of the voltages between the gate and the source of both the power and detection transistors (T1, T2), characterized in that in series with the source-drain channel of the detection transistor (T2) there is a diode (D3) having the cathode connected to the collector and to the base of a second transistor (T9) having the base connected to the base of a third transistor (T10) arranged in a branch parallel to the series connection of said detection transistor (T2), said diode (D3) and the collector-emitter path of said second transistor (T9), this branch also containing the collector-emitter path of a fourth transistor (T11) having the base connected to the base of a fifth transistor (T12) the collector-emitter path of which is connected in series with said current generator (G1). Circuit according to claim 1, characterized in that said first transistor (T13) is arranged to control a driving circuit (P1) for the gate voltage of the power and detection transistors (T1, T2). Circuit according to claim 1, characterized in that said first transistor (T13) is arranged to control a sixth transistor (T21) arranged for supplying a diagnostic signal related to the rise of the current in the power transistor (T1) above a pre-set maximum threshold or to its drop below a pre-set minimum threshold.

Inventors: PALARA SERGIO, TAGLIAVIA DONATO, PALARA, SERGIO, TAGLIAVIA, DONATO
Applicants: SGS THOMSON MICROELECTRONICS, SGS-THOMSON MICROELECTRONICS S.R.L.
Application year: 1990
Main classification: G01R19
IPC codes: G05F3, G01R31, G01R19