A circuit for connection between a car radio and a motor-vehicle battery

Abstract: The circuit comprises an anti-interference filter constituted by an inductor (L) in series with one of the supply terminals of the car radio (12) and by a capacitor (C) in parallel with the supply terminals, and a diode (Ds), connected in series with the inductor (L), for protection against reversal of the polarity of the battery (11). The connection circuit may also comprise an electronic switch (Mt) between the node (N) of the connection of the inductor (L) to the diode (Ds) and the other supply terminal, and a circuit (14) for controlling the switching of the electronic switch (Mt). A voltage raiser which utilizes the components (L and C) of the filter for its operation is thus produced.

Description: The present invention relates to a circuit for connection between the supply terminals of a car radio and a motor-vehicle battery, as disclosed in DE 4400093 A and in DE 4124616 and defined in the preamble of Claim 1. In this description, the term battery is intended to define the source of direct-current electrical energy of a motor vehicle, comprising recharging and stabilizing circuits in addition to the actual battery, and the term car radio relates not only to a radio receiver but also to any electronic apparatus, supplied by the battery, for reproducing sound or for other uses. As is known when the engine of the motor-vehicle is started, and also during normal operation of the motor vehicle, the direct voltage generated by the battery is subject to interference such as reductions in level and pulsed positive and negative transients which may cause malfunctioning or even breakdown of electronic devices connected to the battery.. In the case of a car radio, a reduction in level below a minimum permitted voltage may cause the apparatus to switch off and pulsed transients may cause annoying noises to be emitted by the loud speakers. It is also known that incorrect connection of the battery (reversal of polarity) can cause serious damage to the apparatus supplied by the battery. To limit interference, a filter is usually used in the circuit connecting the car radio to the battery, and to prevent damage due to accidental reversal of the battery polarity, suitable protection means are used in the connection circuit. A typical connection circuit comprising a filter and a protection device is shown in Figure 1. The circuit, indicated 10 in the drawing, is interposed between a battery 11 and a car radio 12 and is normally located physically in the container which houses the car radio. The circuit is constituted by an LC filter and by a power diode. The LC filter in this example is a two-pole, low-pass filter and is formed by an inductor L in series, that is, connected between the positive pole of the battery and a supply terminal of the car radio, and by a capacitor C in parallel with the car radio supply terminals. The diode, indicated Dp, is also connected in parallel with the car radio supply terminals, more precisely, for reverse conduction, that is, with its anode connected to the terminal of the circuit which is to be connected to the negative pole of the battery, so that the diode conducts only when the battery is connected with reversed polarity. A fuse 13 which intervenes and interrupts the supply in the event of overloading is normally provided between the positive pole of the battery and the connection circuit 10. It is known that, in order to increase the output power of the car radio or of other apparatus supplied by the motor-vehicle battery, it is possible to use a voltage raising circuit as described, for example, in US 6 307 357 based on Italian patent application No. ITMI990257 filed by the Applicant on 10th February 1999. When necessary, this circuit is connected between the filtering and protection circuit and the supply terminals of the apparatus. The known filtering and protection circuit of Figure 1 has some disadvantages, as a result of which it is not always reliable and effective in use. In particular, if the battery is connected with reversed polarity, the voltage drop in the diode is quite high, for example 2V, so that, between the supply terminals of the car radio, there will be a negative voltage (-2V) which may damage the apparatus before the fuse intervenes and interrupts the connection to the battery. Moreover, a negative transient in the battery voltage Vbatt is translated, at the output of the filter, into a voltage Vfil with oscillations of considerable amplitude which are also potentially damaging to the apparatus. For example, as is shown in the graph of Figure 3, relating to apparatus which absorbs a current of 2A, a transient of the battery voltage Vbatt which starts at 14V, falls to 0V and rises again to 18V before returning to 14V brings about a ripple of the filter output voltage Vfil which goes from -0.7V to 26V. An object of the present invention is to provide a circuit for connection between a car radio and a motor-vehicle battery that contains a filtering and protection circuit which is reliable and effective in all applications. Another object of the invention is to provide a circuit for connection between a car radio and a motor-vehicle battery which performs filtering and protection functions together with a voltage-raising function. These objects are achieved by the provision of the circuit which is defined and characterized in general in Claim 1 and various embodiments of which are defined in the dependent claims. The invention will be understood further from the following detailed description of two preferred embodiments thereof, given with reference to the appended drawings, in which: Figure 1 shows a known filtering and protection circuit, Figure 2 shows a known circuit for connection between a car radio and a battery, having filtering and protection functions, Figure 3 is a graph illustrating the operation of the known circuits of Figure 1 and Figure 2, and Figure 4 shows a circuit for connection between a car radio and a battery, having filtering, protection and voltage-raising functions according to an embodiment of the invention. As can be seen in Figure 2, in which elements identical to those of Figure 1 are indicated by the same reference numerals or symbols, the connection circuit differs from the circuit of Figure 1 in that the protection against reversal of the battery polarity is provided by a diode, indicated Ds, connected in the forward conduction direction in series with one of the supply terminals of the car radio, instead of a diode in parallel as in Figure 1. This different configuration leads to considerable advantages. In particular, if the battery is connected with reversed polarity, the diode Ds is reverse biased and thus blocks the supply to the car radio. A negative supply voltage can no longer be established between the supply terminals of the car radio. Moreover, any negative transient of the battery voltage Vbatt is attenuated by virtue of the series resistance of the diode Ds so that the voltage, indicated Vfil', filtered by the circuit, has an oscillation of smaller amplitude than that of the circuit of Figure 1. For example, in the case shown by the graph of Figure 3, the voltage Vfil' varies between 9V and 20V. Finally, the diode Ds may be of a smaller size and hence a lower cost than the diode Dp of the known circuit since its maximum reverse conduction current during operation with the polarity of the battery reversed is limited by the impedance of the load (the car radio). Naturally, during normal operation, the effective supply voltage of the car radio, that is, the filtered voltage Vfil', is lower than the battery voltage. It is in fact Vbatt-Vds, where Vds is the voltage drop in the diode Ds connected for forward conduction (typically 0.4-0.6V). This reduction may be negligible in many cases. However, if it is not negligible and it is in fact desired to have a supply voltage higher than the battery voltage, according to the invention, the connection circuit may be produced in a form which enables the voltage to be raised without the use of a special circuit downstream of the filtering and protection circuit. An embodiment of this type is shown in Figure 4, in which elements identical to those of Figure 2 are indicated by the same reference numerals or symbols. The connection circuit of Figure 4 comprises, in addition to the filtering components L and C and the protection diode Ds, an electronic switch Mt connected between the node N of the connection of the inductor L to the diode Ds and the negative supply terminal of the car radio, and a circuit 14 for controlling the switching of the electronic switch. In this embodiment, the electronic switch Mt is an n-channel MOS field-effect transistor (a MOSFET) having an intrinsic diode Md between its source and drain terminals. The gate terminal of the transistor Mt is connected to the output of the control circuit 14. It is intended that, instead of the MOSFET, there could be a bipolar transistor or another device with controllable conduction. The control circuit 14 is connected to the supply terminals of the car radio both in order to be supplied with the continuous voltage Vfil' which is established between them, and in order to generate switching signals in dependence on the voltage Vfil'. The control circuit 14 is preferably a pulse-width modulator (PWM) which can apply a pulsed voltage of predetermined frequency to the gate terminal of the transistor Mt. The width of the pulses varies in dependence on the voltage Vfil' and on the load constituted by the car radio 12. The control circuit 14 also has an enabling terminal EN connected to a device or to an external circuit, not shown, for applying to this terminal a two-level signal for activating or deactivating the control circuit. The operation of the circuit will now be considered. With the battery 11 connected correctly, if the signal EN is such as to keep the control circuit 14 inactive, the transistor Mt is cut off (the diode Md is reverse biased) so that the voltage Vfil applied between the supply terminals of the car radio 12 is equal to the battery voltage Vbatt reduced by the voltage drop in the diode Ds. If the signal EN is such as to keep the control circuit 14 active, the transistor Mt is made conductive and non-conductive alternately. During the conduction period, a voltage substantially equal to the voltage Vbatt is applied to the terminals of the inductor L. When the transistor becomes non-conductive, the electromotive force induced in the inductor L causes the potential of the node N to rise until the diode Ds is made conductive. A current thus flows from the inductor L to the capacitor C and to the load 12. Suitable selection of the values of the inductor L and of the capacitor C, as well as of the frequency of the switching signal generated by the control circuit, achieves an output voltage Vfil' of the connection circuit, that is, a supply voltage of the car radio, higher than the input voltage Vbatt. By way of example, with a voltage Vbatt=14V, the inductor L may have an inductance of 220µH, the capacitor C may have a capacitance of 2000µF and the switching frequency of the control circuit 14 may be 50kHz. According to this embodiment of the invention, the components necessary for the filtering functions (L and C) and for the protection function (Ds) are also utilized to perform the voltage-raising function for which, otherwise, according to the prior art, a separate circuit would be required. It should also be noted that, in the event of accidental reversal of the battery polarity, the diode Md in parallel with the transistor Mt is connected for forward conduction and thus protects the transistor. It will be noted that, in this case, the negative voltage drop which is present at the terminals of the diode Md until the fuse 13 intervenes is not applied to the supply terminals of the car radio 12 because it is blocked by the diode Ds which is reverse biased in this situation. Although only one embodiment of the invention has been described and illustrated, clearly many variations and modifications are possible within the scope of the invention as defined in the following claims.

Claims: A circuit for connection between the supply terminals of a car radio (12) and the poles of a motor-vehicle battery (11) comprising an inductor (L) in series with one of the supply terminals of the car radio, a capacitor (C) in parallel with the supply terminals of the car radio, a diode (Ds) for protection against reversal of the battery polarity connected in series with the inductor (L), an electronic switch (Mt) connected between the node (N) of the connection of the inductor (L) to the diode (Ds) and the other supply terminal of the car radio, and a circuit (14) for controlling the switching of the electronic switch (Mt) in dependence on the voltage (Vfil') present between the supply terminals of the car radio (12), characterized in that the control circuit (14) is connected to the supply terminals of the car radio (12) to be supplied with the voltage (Vfil') present therebetween. A circuit according to Claim 1, in which the diode (Ds) is downstream of the inductor (L) and upstream of the capacitor (C). A circuit according to Claim 1 or 2, in which the control circuit (14) has an enabling terminal (EN) for activating or deactivating the control circuit (14) in response to a signal applied to the anabling terminal (EN). A circuit according to Claim 1, 2 or 3, in which the control circuit (14) comprises a pulse-width modulator (PWM). A circuit according to any one of Claims 1 to 4, in which the electronic switch comprises a field-effect transistor (Mt).

Inventors: BOTTI EDOARDO, CASSANI FABRIZIO, CRESPI LUIGI, BOTTI, EDOARDO, CASSANI, FABRIZIO, CRESPI, LUIGI
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1999
Main classification: H02H11
IPC codes: H02H9, H02H11, H02M3