Control circuit for the current switch edges of a power transistor

Abstract: A battery-charging electronic device comprises a current generator (2) adapted to supply a charging current to a battery (3) and a controlled current edge switch having a circuit (1) for controlling the switching edges of current (I O ) being flowed through a power transistor (PW). The switching edge control circuit (1) comprises: a controlled edge variable voltage generator for generating a controlled edge voltage signal (VA); a voltage/current converter (11) for converting the voltage signal (VA) to a controlled edge current signal (I R ); a driver circuit (12) for the power transistor (PW) being input the controlled edge current signal (I R ) to mirror, onto the power transistor (PW), an output current (I O ) which is proportional to the controlled edge current signal (I R ).

Description: This invention relates to a circuit for controlling the switching edges of current delivered from a power transistor. Field of the Invention In particular, this invention relates to a controlled on/off switch which is connected between a battery charger and a battery, and incorporated to a portable apparatus equipped with rechargeable batteries, such as a cellular telephone set. This switch is intended for protecting the battery against possible voltage peaks resulting from connection/disconnection of the battery to/from the battery charger. In addition, with the current edges controlled as the load current is being turned on and off, electromagnetic radiations from excessively steep switching edges are suppressed which could interfere with communications in the instance of a cellular telephone, and in general with the apparatus performance. Background Art Switches of this kind are currently formed of discrete components including a bipolar transistor, which is driven by a constant voltage edge when turned on and off. This rather crude technique has a disadvantage in that the propagation times are long and the switching edges of the output current are dependent on the transistor beta. An embodiment of a prior art switch adapted to connect the battery to the battery charger in a "soft" manner, is shown in Figure 1. The operation of the circuit device of Figure 1, and its applications, will now be described in better detail. The battery can be modelled by a voltage generator whose value varies typically from a few hundred millivolts, in the exhausted condition, to 5-10V when fully charged, and a series resistor of very low value (100 to 200 milliOhms). The battery charger can be represented schematically by a current generator 2 which is saturated at a predetermined voltage. In this particular instance, the values of current and voltage would be 1.2A and 16V, respectively. The device depicted in Figure 1 complete of its served circuits, operates in either of two modes: a start-up mode and a PWM mode. It enters the start-up mode when the battery is exhausted, and is held there for a few tens seconds, enough time to have the battery charged to an intermediate level (about 3 Volts). The transistor P1 is off, and the voltage regulator 4 is activated to limit the current at 180mA. During this initial phase where the battery is in the exhausted condition, it is important that the battery be charged with a smaller current than that supplied by the battery charger, to avoid damaging the battery. Upon a battery voltage being reached at which all the circuitry to be powered by the battery can begin to operate (for example, 3 Volts in this case), the start-up regulator 4 is turned off and the device allowed to operate in the PWM mode. During the latter phase, the cellular telephone is able to operate properly, and the battery charging is handled by a microprocessor, outside the device 1 in question, which controls the transistor P1 through its PWM pin. Hence the need to control the battery charging current edges so as to prevent steep switchings from disturbing the communications. Thus, it can be appreciated from Figure 1 that this conventional approach has the following disadvantages: the propagation time of the turn-on command is quite long and dependent on the voltage at the VCH pin, to which the charging current from the generator 2 is applied, and on the base-emitter junction voltage of the transistor P1, i.e. T ON = R1 * C1 * In(V CH /V CH -V BE ); the current edge is a coarse value, and dependent on the voltage at the pin VCH and the beta of the transistor P1. Known from the US Patent No. 5,420,499 to DeShazo is a current rise and fall time limited voltage follower for controlling a power transistor. In particular, according to such a patent, a voltage signal is provided to a current driver connected to a bus. The load at the bus may vary, in particular decreasing in resistance and increasing in conductance. As the load decreases, the current into the bus increases. So the current into the load and the current rise time are inversely related to the load. The underlying technical problem of this invention is to provide a circuit for controlling the switching edges of a power transistor, which can operate at low propagation times and control the output current with high accuracy, thereby overcoming the aforementioned limitations of the prior art. The technical problem is solved by a circuit as indicated, and as defined in the characterizing part of Claims 1 to 4. Summary of the Invention The solvent idea behind this invention is to use a voltage signal having controlled rising and falling edges, convert it to a current signal, and mirror it at the device output onto a power transistor, to thereby obtain an accurate control of the transistor current edges. The technique we are using can be fully integrated, utilizes a MOS transistor, and provides short propagation times and highly accurate control of both the output current and the switching edges. The features and advantages of the circuit of this invention will be more clearly apparent from the following detailed description of practical embodiments thereof, as illustrated by way of non limitative examples in the accompanying drawings. Brief Description of the Drawings Figure 1 shows a conventional circuit for controlling the switching edges of a power transistor; Figure 2 shows a circuit for controlling the switching edges of a power transistor, according to this invention; Figure 3 is a plot vs. time of certain voltage signals and current signals in the circuit of Figure 2; and Figure 4 is a plot vs. time of certain voltage signals and current signals as obtained by electrical simulation of the circuit shown in Figure 2. Detailed Description Figure 2 shows a circuit for controlling the switching edges of a power transistor, which embodies this invention. The circuit 1 is connected between a battery charger, schematically illustrated by a current generator 2 connected to the terminal VCH of the circuit, and a battery 3 connected to the terminal VBAT. The circuit 1 comprises the following circuit parts: a power transistor PW of the MOS type having a drain-source main conduction path connected between the terminals VCH and VBAT of the circuit; a driver circuit 12 for the power transistor PW, which consists of an operational amplifier OP1, being supplied a higher positive voltage VCP than the voltage VCH, and a transistor PS coupled to the power transistor PW; a voltage/current converting block 11, which consists of an operational amplifier OP2, a transistor N2, and a resistor R2, and functions to convert a voltage waveform VA at its positive input to a current waveform I R which is proportional to the resistance of the resistor R2; a generator of a controlled edge variable voltage VA, which comprises a controlled variable voltage generator VC1 and a low-pass filter block 10 operative to filter out the voltage variations of the voltage generator VC1. The transistor PS has a gate terminal connected to the gate terminal of the power transistor PW and to an output terminal of the operational amplifier OP1, and has a drain terminal connected to the drain of the power transistor PW. The operational amplifier OP1 has an inverting input connected to the source terminal of the transistor PS, and a non-inverting input connected to the source terminal of the power transistor PW at the circuit node VBAT. In combination with the transistor PS, this operational amplifier is to mirror the current I R from the voltage/current converting block 11 to the output current I O being flowed through the power transistor PW. The manner of generating the current I R will now be explained. The variable voltage generator VC1 can generate at least two different voltage levels, V1 and V2, as shown by curve (a) in Figure 3. The setting of this generator, e.g. at V1, is filtered through the low-pass filter block 10 to produce a controlled edge voltage signal VA, shown by curve (b) in Figure 3. Specifically, to obtain a fully linear signal edge, the resistor R1 in the filter block may be replaced with a constant current generator I1. The voltage VA applied to a non-inverting input of the operational amplifier OP2 is converted, through the voltage/current converting block, to a current I R having the same time pattern as the signal VA illustrated by curve (c) in Figure 3 and an inversely proportional value to the resistance of R2; i.e.: I R = V A /R2. The current I R is multiplied by the surface area ratio of the transistors PS and PW, thereby generating the output current Io for supply to the battery 3. The operational amplifier OP1 functions to drive the gating node VG of the transistors PS and PW so as to maintain the current ratio I O /I R , set by the surface area ratio n of PS to PW. Using this circuit, the edges of the battery charging current will be controlled, so that the slew rate can be adjusted to suit the application requirements. In fact, the slew rate of the current I O will be: ΔI O /Δt = n*I 1 /C1*R2 where n is the surface area ratio of PS to PW. The innovative solution proposed herein has been implemented with BCD technology in a device having the function of connecting the battery to the battery charger in a "soft" manner. In this instance, the battery charging current I O could take either of two values: in the start-up mode, the current was 180mA, and in the PWM mode, the current was equal to the maximum current (1.2A) supplied by the battery charger, and was limited internally to 1.8A. In either cases, the slew rate of the output current is 300mA/ms. This slew rate is obtained based on the following circuit parameters: surface area ratio of the transistors, PW and PS, = 1000; current generator I 1 = 10 microAmpere; a resistor R2 = 1 kOhm; an external capacitor C1 = 33 nF. To obtain the output current values of 180mA (start-up mode) and 1.8A (PWM mode) from the voltage generator VC1, the settings were V1=180mV and V2=1.8V, respectively. The internal limiting current has been set at a higher value than the maximum current that the battery charger can supply such that, with the device operated in the PWM mode, the transistor PW is operating within its linear range, with an equivalent resistance of 300 milliOhms. In fact, upon the current I O reaching the maximum current supplied from the battery charger, the voltage at the inverting input of the operational amplifier OP1 will tend to decrease, allowing the voltage VG at the gating node of the transistors PS and PW to rise up to VCP. In this way the transistor PW overdrive will be maximum (about 10 Volts) and allow it to operate in its resistive range. Figure 4 illustrates the behaviours of the voltage VA at the capacitor C1 and of the battery charging current I O , as obtained by electrical simulation of the circuit proposed. In this case, the slew rate of the current I O was set at 600mA/ms. The advantages of such a solution are, therefore, the following: the slew rate of the output current I O is controlled in conformity with relationship (2); it can be seen that it depends on parameters which can be highly accurate and changed in a simple manner to make the circuit fit different applications; the absolute value of the battery charging current I O can be set by suitably varying the variable voltage generator VC1, the resistance value of R2, and the surface area ratio n of the transistors PS and PW; the turn-on signal propagation time is only dependent on the threshold voltages of the transistors PS and PW.

Claims: An electronic device for battery (3) charging, comprising: a current generator (2) adapted to supply a charging current to said battery (3); a controlled current edge switch having at least one power transistor (PW) with at least one control terminal (G) and two main conduction terminals (D,S), a circuit (1) for controlling the switching edges of a current (I O ) flowing through said power transistor (PW), characterized in that said circuit (1) for controlling the switching edges further comprises: a controlled edge, variable voltage generator (VC1,10) for generating a controlled edge voltage signal (VA) comprising a controlled variable voltage generator (VC1) and a low-pass filter (10) adapted to filter the voltage variations of the variable voltage generator (VC1); a voltage/current converter (11) for converting said voltage signal (VA) to a controlled edge current signal (I R ); and a driver circuit (12) for the power transistor (PW) being input said controlled edge current signal (I R ) to mirror, onto the main conduction path (D-S) of the power transistor (PW), an output current (I O ) which is proportional to said controlled edge current signal (I R ). A device according to Claim 1, characterized in that the voltage/current converter (11) comprises: an operational amplifier (OP2) having a non-inverting input terminal (+) coupled to the variable voltage generator (VA), an inverting input terminal (-), and an output terminal; a first transistor (N2) having a control terminal connected to the output terminal of the operational amplifier (OP2), and having a conduction path in series with a resistor (R2) between an output terminal of the converter and a voltage reference (GND), with a node, intermediate the first transistor (N2) and the resistor (R2), being connected to the inverting input (-) of the operational amplifier (OP2). A device according to any of Claims 1 to 2, characterized in that the driver circuit (12) of the power transistor (PW) comprises: a second transistor (PS) having a control terminal coupled to the control terminal (G) of the power transistor (PW), a first conduction terminal coupled to a first conduction terminal (D) of the power transistor (PW), and a second conduction terminal connected to an output terminal of the voltage/current converter; an operational amplifier (OP1) having a non-inverting input terminal (+) coupled to a second conduction terminal (S) of the power transistor (PW), an inverting input terminal (-) coupled to the second conduction terminal of the second transistor (PS), and an output terminal coupled to the common node between the control terminals of the power (PW) and second (PS) transistors. A device according to Claim 1, characterized in that the power transistor (PW) is an N-channel transistor of the MOS type.

Inventors: MILAZZO PATRIZIA, PULVIRENTI FRANCESCO, MILAZZO, PATRIZIA, PULVIRENTI, FRANCESCO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1997
Main classification: H03K17
IPC codes: H02J7, H03K17, H03K4