Low voltage fed driving circuit for electronic devices

Abstract: A driving circuit comprising a low-voltage power supply line (V IN ), a MOS transistor (M1) having a driving terminal and adapted to feed a load and a control stage (10) connected to the power supply line (V IN ) and to the driving terminal of the MOS transistor (M1) to feed it. In order to allow the driving of the MOS transistor (M1) even when the power supply line voltage is lower than the threshold voltage of the MOS transistor (M1), the driving circuit comprises a bipolar transistor (T1) connected in parallel to the MOS transistor (M1) and a voltage multiplier circuit (14) connected at its input to a common terminal of the MOS and bipolar transistors and at its output to the control stage (10), the latter comprising comparators (20,21) adapted to initially enable the bipolar transistor (T1) to feed the multiplier circuit (14), which is thus capable of generating a voltage higher than the available supply voltage, and to subsequently deactivate the bipolar transistor (T1) and enable the MOS transistor (M1) when the voltage generated by the voltage multiplier (14) has reached at least the threshold voltage of the MOS transistor.

Description: The present invention relates to a low voltage fed driving circuit for electronic devices. In particular the invention relates to the driving of MOS transistors in the presence of a supply voltage insufficient to drive said MOS transistor. As is known, the use of switching MOS transistors has some advantages with respect to the use of bipolar transistors. In particular the use of MOS transistors allows operation at higher switching frequencies because of the absence of the charge storage phenomena, and the driving power of said MOS transistor is lower. In particular this second aspect can be very important when operating with high switching currents. Despite these advantages it is not always possible to use MOS transistors to drive electrical loads or other circuits, since the driving of the gate terminal of MOS transistors requires higher voltages than the driving of bipolar transistors. In fact, as is known, DMOS transistors for example require a driving voltage higher than 3 volts, as compared to the 0.7 volt base voltage required by bipolar transistors. This is a severe limitation for the use of MOS transistors in systems fed at low voltage. Reference is made now to figure 1, illustrating an exemplifying diagram of an normal circuit employing a MOS transistor to drive an inductive load. As can be seen, the MOS transistor, indicated by M1, is connected in series to an inductor L arranged between M1 and the supply voltage V IN . The circuit furthermore comprises a control stage 1 also connected to the supply voltage and to the gate terminal of the transistor M1 so as to drive said MOS transistor, by feeding a gate voltage according to the intended switching sequences. This circuit can operate only if the supply voltage V IN is at least greater than the threshold voltage V T of the MOS transistor (which, as mentioned, is equal to 3 volts for a DMOS transistor). If the input supply voltage V IN is lower it is necessary to renounce the diagram of figure 1, replacing the MOS transistor with a bipolar transistor, thus renouncing the above mentioned advantages related to the usable switching frequencies and to the power consumption required to drive the transistor. A further known circuit is disclosed by US Patent US-A-4,360,855. This known circuit includes a power transistor and a switching MOS transistor which are alternatively actuated by a control circuit. However, in a similar manner to the first circuit, this known circuit can also just operate if the supply voltage is at least greater than the threshold voltage of the MOS transistor. Similar considerations apply to the circuit described in the article "A switching circuit composed of BJT and MOS-FET" by OHASHI et. al., published in the TRANSACTIONS OF THE IECE OF JAPAN, Vol. E69, No. 4, April 1986, pages 509-511. A drive circuit for a single bipolar switching transistor is disclosed by US Patent US-A-4,201,928. The signal output by the bipolar transistor is conveyed to a pulse transformer, which in turn feeds back the multiplied signal to the drive section. However,due to the charge storage phenomenon of the bipolar transistor, the drive circuit can not operate at high frequencies and it has a higher power consumption with respect to that of MOS devices. Given this situation, the aim of the present invention is to provide a circuit for driving electronic devices fed at low voltage, capable of ensuring the driving of said electronic device, typically a MOS transistor, even if the supply voltage value is lower than the minimum driving voltage of said electronic device. Within this aim, a particular object of the present invention is to provide a driving circuit for MOS transistors which operates even with a supply voltage lower than the threshold voltage of said MOS transistor, so as to fully exploit the advantages of the MOS technology, in terms of the possibility of using higher switching frequencies and of reducing the driving power consumption with respect to the use of bipolar devices. Another object of the present invention is to provide a said driving circuit which is simple in concept and easy to manufacture, so as to not entail substantial increases in the cost of said driving circuit. Not least object of the present invention is to provide a driving circuit which by virtue of its structural and functional characteristics is capable of giving the greatest assurances of safety and reliability in use. This aim, the objects mentioned and others which will become apparent hereinafter are achieved by a circuit for driving electronic devices fed at low voltage as defined in the accompanying claims. Further characteristics and advantages of the invention will become apparent from the description of a preferred but not exclusive embodiment, illustrated only by way of non-limitative example in the accompanying drawings, wherein: figure 1 is a general circuit diagram of a known driving circuit; figure 2 is a general circuit diagram of the driving circuit according to the invention; and figure 3 is a general electric diagram of the control stage provided in figure 2. Figure 1 is not described hereinafter; reference is made to the above for its description. With reference thus to figure 2, the driving circuit according to the invention comprises a bipolar transistor T1 connected in parallel to the MOS transistor M1. In detail, the collector terminal of T1 is connected to the drain terminal of M1, while the emitter terminal of T1 is connected to the source terminal of M1 and to the ground. The base terminal 11 and gate terminal 12 of the transistors T1 and M1 are connected to a control circuit 10 adapted to appropriately enable and disable the transistors T1 and M1 as will be explained in further detail hereinafter. The common point 13 between the collector of T1 and the drain of M1, illustrated in figure 2, is connected to a terminal of an inductive load L connected with its other terminal to the input supply voltage V IN . The inductive load L, according to the illustrated embodiment of the invention, is a part of a transformer 14, the inductive load L constituting its primary winding, its secondary winding 15 having a winding ratio n:1 sufficient to generate the driving voltage of the MOS transistor M1. In detail, the secondary winding 15 is connected between the ground and the anode of a diode D1 connected with its cathode to a point 16 said point having the potential V AL . The common point 16 is furthermore connected to the input supply voltage V IN by means of a decoupling diode D2 and to a blocking capacitor C arranged between the point 16 and the ground. The operation of the circuit of figure 2 is the following. Assume the input voltage V IN is not sufficient to drive the MOS transistor M1 but is sufficient to drive the bipolar transistor. In this case when the circuit is switched on the control circuit immediately enables the bipolar transistor T1 which, allowing the flow of current across the inductive load L, allows the transformer to charge the capacitor C by means of the diode D1 to the voltage nV IN . If the winding ratio n of the transformer is sufficiently large, the voltage nV IN is sufficient to drive the MOS transistor. Consequently, as soon as the voltage V AL - nV IN reaches at least the threshold voltage of the MOS transistor M1, the control circuit disables the bipolar transistor and enables M1. Once the circuit has been started, it is self-sustaining, sending the intended switching control impulses to the gate 12 of M1 according to the present application. It should be noted that the diode D2 is used initially to feed the control stage 10 with the input supply voltage V IN and that it acts as a decoupler when the voltage on the capacitor has reached a higher value than the input supply voltage. An example of the control stage of figure 2 is schematically illustrated in figure 3. As can be seen, the control stage 10 of figure 3 comprises a pair of comparators 20 and 21 both adapted to receive at their positive input, the multiplied voltage generated at the output of said transformer 14 and a respective reference voltage V REF1 , V REF2 on their negative input. The output signal of the comparator 20 is sent to the first input of an AND gate 22, said AND gate receiving at its second input the output signal of the comparator 21, appropriately inverted by means of the inverter 23. The output signal of the AND gate 22 is fed to the first input of a further AND gate 24, said further AND gate receiving at its second input the switching signal generated by a control signal generator 26. The output signal of the gate 24 is sent directly to the base of the transistor T1. The output signal of the comparator 21 is also sent, non inverted to another logical AND gate 25 which also receives at its input the signal of the control signal generator 26. The output signal of the AND gate 25 is then sent to the gate terminal 12 of the MOS transistor M1. The voltage V REF1 has a low value adapted to ensure in any case the driving of the base of the bipolar transistor T1, while the voltage V REF2 has a greater value, at least equal to the threshold voltage of the MOS transistor M1, preferably equal to the driving voltage of the gate of M1. Said voltage can be directly obtained from the voltage V AL , as V AL is at this stage multiplied to a value sufficient to drive the MOS transistor, or can be reduced to a preset fraction of the driving voltage of said MOS transistor, in which case said voltage is compared with a fraction of the output voltage of the transformer, conveniently reduced for example by means of a divider. The operation of the circuit illustrated in figure 3 is very simple and is clearly apparent from the diagram. In fact when the circuit switches on, as soon as the voltage V AL has exceeded the voltage V REF1 the comparator 20 sends a logical "1" signal to the AND gate 22, said gate being enabled at this stage, by a logical "1" signal generated by the inverter 23. The driving voltage at the base 11 of the bipolar transistor T1 is consequently supplied through the AND gate 24 according to the control signal of the generator 26. As mentioned, the enabling of T1 allows current to flow across L and therefore the output voltage of the transformer 14 to rise. Consequently, when the voltage V AL has reached at least the threshold voltage set for the MOS transistor M1, the comparator 21 switches, disabling the gate 22 and feeding the gate 25 with the gate driving voltage required for M1. At this point the transistor M1 is capable of driving the load by itself according to the control sequence generated by the generator 26. As can be seen from the above destription, the invention fully achieves the intended aims. A driving circuit has in fact been provided which allows to drive an electronic device requiring a high driving voltage, typically a MOS transistor, even in the presence of an external supply voltage lower than the required value. In this manner it is possible to fully use the speed and low consumption advantages of the MOS transistor or the advantages related to the electronic device employed. The circuit is simple in concept and easy to manufacture, and in particular the fact is stressed that the provision of the transformer 14 does not entail a great expense or cost increase if the inductive element already present in the load circuit can be used. The circuit is furthermore reliable in all operating conditions. It should be furthermore noted that the dimensions of the bipolar transistor may be much smaller than those of the MOS transistor, since said bipolar transistor is supplying only the power required by the control circuit, while the said MOS transistor is supplying the load power. The invention thus conceived is susceptible to numerous modifications and variations. In particular, the fact is again stressed that though the illustrated example refers to the driving of a MOS transistor for supplying power to an inductive load, the same inventive concept can be used in low voltage fed systems, which initially need a higher turn on voltage, but are then able to generate suitable voltages. For example, the voltage multiplier circuit (transformer 14) can be replaced with a capacitive voltage converter able of generating the required start voltage. However this step is complicating the electronic circuit. The diode D2 can furthermore be replaced with a resistor if a higher energy dissipation due to the return of current from the capacitor C to the power supply V IN is acceptable. Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly such reference signs do not have any limiting effect on the scope of each element identified by way of example by such reference signs.

Claims: Low power fed driving circuit for electronic devices, comprising: a low-voltage power supply line (V IN ); a first switching transistor (M1) driving an inductive load (L) connected to said low-voltage power supply line (V IN ); a second switching transistor (T1) driving said inductive load (L) connected to said low-voltage power supply line (V IN ), whereat said second switching transistor (T1) has a driving voltage that is lower than the driving voltage of said first switching transistor (M1), said second switching transistor (T1) being connected in parallel to said first switching transistor (M1); and a control stage (10) connected to said power supply line (V IN ), to a driving terminal (12) of said first switching transistor (M1) to drive it, and to a driving terminal (11) of said second switching transistor (T1) to drive it; said control stage (10) comprising means (20,21,22,23,24,25,26) which at turn-on initially enable said second switching transistor (T1), and subsequently deactivate said second switching transistor (T1) and enable said first switching transistor (M1), CHARACTERIZED IN THAT said driving circuit further includes a voltage multiplier circuit (14) connected at its input to a common terminal (13) of said first switching transistor (M1), said second switching transistor (T1) and said inductive load (L) and at its output to said control stage (10), said voltage multiplier circuit (14) generating a start voltage (V AL ) for said first switching transistor (M1) greater than said supply voltage (V IN ), whereat, at turn-on said control stage (10) enables said second switching transistor (T1), feeding thereby said voltage multiplier circuit (14), and whereat said control stage (10) deactivates said second switching transistor (T1), and enables said first switching transistor (M1) by providing said start voltage (V AL ) to the driving terminal (12) of said first switching transistor (M1) when the start voltage (V AL ) output by said voltage multiplier circuit (14) has reached at least the minimum voltage value necessary to drive said first switching transistor (M1). Low power fed driving circuit according to claim 1, characterized in that said first switching transistor (M1) is a MOS transistor having gate, drain and source terminals, and said second switching transistor is a bipolar transistor (T1) having base, collector and emitter terminals, said drain terminal being connected to said collector terminal, said source terminal being connected to said emitter terminal. Low power fed driving circuit according to claim 2, characterized in that said multiplier circuit (14) comprises a transformer the primary winding thereof being constituted by said inductive load (L), its secondary winding (15) being connected to a blocking capacitor (C) and to said control stage (10), said transformer having a winding ratio set to generate at least the threshold voltage of said MOS transistor (M1) on said secondary winding (15). Low power fed driving circuit according to one of the preceding claims 2-3, characterized in that a first terminal of said inductive load (L) is connected to said first common terminal (13) defined by the collector terminal of the bipolar transistor (T1) and the drain terminal of the MOS transistor (M1), and in that a second terminal of said inductive load (L) is connected to said low-voltage power supply line (V IN ), a first diode (D2) connecting said power supply line (V IN ) and a second common point (16) to said capacitor (C) and said control stage (10), a second diode (D1) connecting said secondary winding (15) of said transformer and said second common point (16). Low power fed driving circuit according to one of the preceding claims 2-4, characterized in that said control stage (10) comprises at least one comparator (21), the inputs thereof being respectively connected to said second common point (16) and a first reference voltage (V REF2 ), the output thereof being connected, through an inverter (23), to the base terminal of said bipolar transistor (T1), and through a logical AND gate (25) to the gate terminal of said MOS transistor (M1), when said AND gate is enabled by a switching signal. Low power fed driving circuit according to claim 5, characterized in that said control stage (10) comprises a second comparator (20) the inputs thereof being respectively connected to said second common point (16) and a second reference voltage (V REF1 ), the output thereof being connected to an input of a second logical AND gate (22), a further input of said second AND gate (22), receiving the output signal of said inverter (23), the output of said second gate (22) being connected to an input of a third logical AND gate (24) which also receives at a further input, said switching signal, and the output of said third logical AND gate (24) being connected to the base terminal of said bipolar transistor (T1). Low power fed driving circuit according to claims 1 or 2, characterized in that said voltage multiplier circuit (14) comprises a capacitive voltage converter.

Inventors: CHIELI DAVIDE, CHIELI, DAVIDE
Applicants: SGS THOMSON MICROELECTRONICS, SGS-THOMSON MICROELECTRONICS S.R.L.
Application year: 1988
Main classification: H03K17
IPC codes: H03K17