A power supply system for a plurality of electronic devices or units on board a motor vehicle

The present invention relates to a power supply system for a plurality of electronic devices and/or units installed on board a motor vehicle.

More specifically, the invention relates to a supply system including a non-stabilised DC voltage source (battery) and a plurality of voltage regulators with a respective input connected to the said source and a respective output intended to supply a stabilised voltage of predetermined value to an associated on-board electronic unit or device.

Electronic control systems and devices utilised on board motor vehicles are becoming ever more numerous.

This involves problems also from the point of view of electrical supply of such systems and devices, and for the dissipation of heat energy.

Currently, for the supply of a plurality of electronic systems or devices on board a motor vehicle, arrangements of the type schematically illustrated in Figure 1 of the attached drawings are adopted. In these drawings, U1, U2 and Un indicate n electronic control units installed on board a motor vehicle. Each of these units is associated with a respective stabilised supply or linear voltage regulator LR1, LR2 ... LRn. These linear voltage regulators have respective inputs connected to the battery B mounted on board the motor vehicle.

It is well known that the on-board battery is a non-stabilised DC voltage source.

The individual voltage regulators LR1 ... LRn are in general rather expensive in that they must be able to resist high input over-voltages and must be of "low drop-out" type in that the voltage delivered by the battery can fall, for example, down to 6V during starting.

The use of ever faster and more complex digital circuits has lead to an increase in the currents consumed and therefore the power dissipated by the various electronic control units. Therefore, the individual voltage regulators LR1 ... LRn must often be associated with heat dissipators, with problems such as dimensions, weight and cost.

The object of the present invention is to provide a supply system for a plurality of electronic devices or units on board a motor vehicle, which makes it possible to reduce the current consumed from the battery, reduces the heat dissipation, in particular at the level of the individual voltage regulators, as well as reduce the weight, the volume and the overall cost of the system.

This and other objects are achieved according to the invention with a supply system of the previously specified type, characterised in that between the said voltage source and the inputs of the voltage regulators there is interposed a centralised voltage regulator arranged to provide a stabilised output voltage of lower value, greater than the (maximum) value of the stabilised voltages intended to be delivered by the said voltage regulators, and in that the said voltage regulators are dimensioned in such a way as to be able to delivery their respective predetermined stabilised voltages when their respective inputs receive the stabilised voltage of lower value delivered by the centralised voltage regulator.

Further characteristics and advantages of the invention will become apparent from the following detailed description given purely by way of non-limitative example, with reference to the attached drawings, in which: <ul id="ul0001" list-style="none" compact="compact"><li>Figure 1, already described, is a block diagram which illustrates the structure of a prior art supply system;</li><li>Figure 2 is a block diagram which shows the structure of a supply system according to the invention; and</li><li>Figure 3 is a diagram which shows in greater detail one embodiment of a centralised voltage regulator incorporated in a supply system according to the invention.</li></ul>

As is seen in Figure 2, in a supply system 1 according to the invention, between the battery B and the inputs of the individual voltage regulators LR1, LR2 ... LRn there is interposed a centralised switching voltage regulator SMPS arranged to provide a stabilised output voltage V_R of lower value, less than the voltage V_B delivered by the battery B, but nevertheless greater than the (maximum) value of the stabilised voltages V_cc1, V_cc2 ... V_ccn intended to be delivered by the said voltage regulators.

The voltage regulators LR1, LR2 ... LRn in the system according to the invention are dimensioned in such a way as to be able to deliver their respective stabilised voltages V_cc1, V_cc2 ... V_ccn when their respective inputs receive the stabilised voltage of reduced value V_R delivered by the centralised voltage regulator SMPS.

Thus, for example, if the battery voltage V_B has a nominal value of 14V, and if the maximum value of the stabilised voltages required by the regulators LR1 ... LRn is for example 5V, the stabilised voltage output from the centralised regulator SMPS is equal for example to 7V.

With the system according to the invention all the protection functions (against polarity inversion of the battery voltage, against pulse voltage overloads, drops or sudden "holes" in the battery voltage etc) can be incorporated in the centralised voltage regulator SMPS. The local supplies LR1, LR2 ... LRn can now be voltage regulators of standard, low input voltage type and therefore of low cost.

With a suitable dimensioning of the centralised regulator SMPS it is moreover possible to achieve an appreciable reduction in the overall power dissipated as heat.

This can be established on the basis of the following considerations.

In Figures 1 and 2, I₁, I₂ ... I_n indicate the currents delivered by each linear regulator LR1, LR2 ... LRn.

Supposing, for simplification, that the voltages delivered by individual linear regulators are equal to one another and to a value V_cc; in the prior art supply system of Figure 1 the overall power dissipated is equal to:<maths id="math0001" num="(I)"><math display="block"><mrow><msub><mrow><mtext mathvariant="italic">P</mtext></mrow><mrow><mtext mathvariant="italic">D1</mtext></mrow></msub><mtext>=(</mtext><msub><mrow><mtext mathvariant="italic">V</mtext></mrow><mrow><mtext mathvariant="italic">B</mtext></mrow></msub><mtext>-</mtext><msub><mrow><mtext mathvariant="italic">V</mtext></mrow><mrow><mtext mathvariant="italic">cc</mtext></mrow></msub><mtext>)</mtext><mtext mathvariant="italic">x</mtext><mtext></mtext><apply>&lt;sum /&gt;<lowlimit><mtext mathvariant="italic">k</mtext><mtext>=1</mtext></lowlimit><uplimit><mtext mathvariant="italic">n</mtext></uplimit><mrow><msub><mrow><mtext mathvariant="italic">I</mtext></mrow><mrow><mtext mathvariant="italic">k</mtext></mrow></msub></mrow></apply></mrow></math><img file="EP0805540A1_D0001.tif" /></maths>

In the case of supply systems according to the invention shown in Figure 2 the overall power dissipated is, on the other hand, equal to:<maths id="math0002" num="(II)"><math display="block"><mrow><msub><mrow><mtext mathvariant="italic">P</mtext></mrow><mrow><mtext mathvariant="italic">D2</mtext></mrow></msub><mtext>=(</mtext><msub><mrow><mtext mathvariant="italic">V</mtext></mrow><mrow><mtext mathvariant="italic">R</mtext></mrow></msub><mtext>/η-</mtext><msub><mrow><mtext mathvariant="italic">V</mtext></mrow><mrow><mtext mathvariant="italic">cc</mtext></mrow></msub><mtext>)</mtext><mtext mathvariant="italic">x</mtext><mtext></mtext><apply>&lt;sum /&gt;<lowlimit><mtext mathvariant="italic">k</mtext><mtext>=1</mtext></lowlimit><uplimit><mtext mathvariant="italic">n</mtext></uplimit><mrow><mtext></mtext><msub><mrow><mtext mathvariant="italic">I</mtext></mrow><mrow><mtext mathvariant="italic">k</mtext></mrow></msub></mrow></apply></mrow></math><img file="EP0805540A1_D0002.tif" /></maths> in which η is the efficiency of the switching regulator SMPS.

The above expressions for the power dissipated are valid on the hypothesis that the currents consumed by individual linear voltage regulation stages are negligible.

From the above expressions (I) and (II) it results that the relative variation of the power dissipated between the arrangement according to the invention and the prior art system is equal to:<maths id="math0003" num="(III)"><math display="block"><mrow><mtext>(</mtext><msub><mrow><mtext mathvariant="italic">P</mtext></mrow><mrow><mtext mathvariant="italic">D1</mtext></mrow></msub><mtext>-</mtext><msub><mrow><mtext mathvariant="italic">P</mtext></mrow><mrow><mtext mathvariant="italic">D2</mtext></mrow></msub><mtext>)/</mtext><msub><mrow><mtext mathvariant="italic">PD</mtext></mrow><mrow><mtext mathvariant="italic">D1</mtext></mrow></msub><mtext>=</mtext><mfrac><mrow><msub><mrow><mtext mathvariant="italic">V</mtext></mrow><mrow><mtext mathvariant="italic">B</mtext></mrow></msub><mtext>-</mtext><msub><mrow><mtext mathvariant="italic">V</mtext></mrow><mrow><mtext mathvariant="italic">R</mtext></mrow></msub><mtext>/η</mtext></mrow><mrow><msub><mrow><mtext mathvariant="italic">V</mtext></mrow><mrow><mtext mathvariant="italic">B</mtext></mrow></msub><mtext>-</mtext><msub><mrow><mtext mathvariant="italic">V</mtext></mrow><mrow><mtext mathvariant="italic">cc</mtext></mrow></msub></mrow></mfrac></mrow></math><img file="EP0805540A1_D0003.tif" /></maths>

From this expression it is observed that if the efficiency η of the centralised voltage regulator SMPS is greater than V_R/V_B the system according to the invention allows an appreciable reduction in the overall power dissipated.

In effect, with the values of V_R and V_B indicated above (V_R = 7V and V_B = 14V) a reduction in the power dissipated with a centralised voltage regulator having an output greater than 50% is achieved.

The above relation (III) shows, moreover, that the power dissipated falls more the lower is V_cc. It is therefore convenient that the electronic system supplied by means of the linear regulators LR1 ... LRn operate with relatively low supply voltages, for example equal to 3.3V.

The advantages which can be achieved with the supply system according to the invention are therefore evident.

The centralised voltage regulator SMPS can be formed as a multi-stage regulator, that is to say comprising for example a plurality of parallel DC/DC convertors, in such a way as to ensure a high redundancy in view of the particular reliability which in general such a system needs to have. The centralised regulator can be arranged to perform diagnostic detection and signalling of anomalous operating conditions or failures.

Hereinafter, with reference to Figure 3 of the attached drawings, there will be described an embodiment of a centralised voltage regulator suitable for use in a system according to the invention.

In Figure 3 is shown a centralised voltage regulator SMPS with an input terminal 2 directly and stably connected to the battery B, and an input terminal 3 connected to a line 4 which is connected to the battery B and in which is interposed a manually operable switch 5, for example by means of the key 6 for controlling the ignition and starter switch of the motor vehicle.

The centralised regulator SMPS includes a DC/DC convertor 7 the input of which is connected to the terminal 2 through a protection device 8 and the contact 10a of a power relay 10. The protection device 8 is for example a varistor for overload protection.

The relay 10 comprises an excitation winding or solenoid 10b which is connected to the protection device 8 through a protection diode 11 for protecting against polarity inversion of the battery B. In series with the winding 10b of the relay 10 is disposed a control device 12 such as, for example, a transistor controlled by a logic circuit 13.

The logic circuit 13 has one input connected to the terminal 3 and another plurality of inputs possibly intended to be connected to corresponding "power latch" outputs of electronic control systems or units supplied by the system according to the invention.

The logic circuit 13 is arranged to render the control device 12 conductive to cause current to flow in the winding 10b of the relay 10 when the voltage delivered by the battery B is present on the terminal 3, or when a power latch signal is present on one of the other inputs of the circuit 13, that is to say a signal requesting the supply of electrical power even after the switch 5 has been opened.

The flow of current in the winding 10b causes closure of the contact 10a of the relay 10 and coupling of the input of the DC/DC convertor 7 to the battery B. The DC/DC convertor 7 is arranged to provide a stabilised DC output voltage V_R the value of which is suitably predetermined on the basis of the previously explained considerations.

The centralised voltage regulator SMPS can further include a lower power voltage regulator, indicated 14 in Figure 3, substantially in parallel with the DC/DC convertor 7. The lower power voltage regulator 14 is intended to allow possible supply of electronic control units which need to be permanently supplied, and for which it is not convenient to maintain the DC/DC convertor 7 in permanent operation.

If the outputs of the convertor 7 and the regulator 14 are coupled together by two diodes 15 and 16 as is shown in Figure 3, the standby voltage V_SB obtainable from the regulator 14 is slightly less than the regulated voltage V_R delivered in operation by the convertor 7.

Naturally, the principle of the invention remaining the same, the embodiments and details of construction can be widely varied with respect to what has been described and illustrated purely by way of non-limitative example, without by this departing from the ambit of the present invention as defined in the attached claims.