WO2016120103A1

WO2016120103A1 - Electric circuit comprising a plurality of resistive elements connected in parallel

Info

Publication number: WO2016120103A1
Application number: PCT/EP2016/050855
Authority: WO
Inventors: Bertrand Puzenat
Original assignee: Valeo Systemes Thermiques
Priority date: 2015-01-27
Filing date: 2016-01-18
Publication date: 2016-08-04
Also published as: FR3032071A1

Abstract

The invention concerns an electric circuit comprising an array of resistive elements (1₁, 1₂,... 1₈) connected in parallel, an electronic switch (2₁, 2₂,... 2₈) in series with each resistive element (1₁, 1₂,... 1₈) of said array capable of allowing the selective powering of the resistive elements of said array with a DC voltage, and control means (5) capable of determining, in said array, a final combination of at least one resistive element to be powered by closing the associated switch in order to satisfy a received setpoint power. According to the invention, the resistive elements of the array have different resistance values constituting a geometric progression of common ratio 2 between a minimum resistance value and a maximum resistance value, and the control means (5) are capable of controlling several successive cycles each comprising the switching of one or more electronic switches (2₁, 2₂,... 2₈) from a closed state to an open state or from an open state to a closed state in order to correspond with an intermediate combination of the resistive elements (1₁, 1₂,... 1₈) of said array, until said final combination is obtained, in order to reach the setpoint power by power levels, each power level corresponding to each cycle being equal to the power value produced by the resistive element of said array having said maximum resistance value.

Description

ELECTRIC CIRCUIT COMPRISING A PLURALITY OF RESISTIVE ELEMENTS

CONNECTED IN PARALLEL

The present invention generally relates to an electrical circuit having a plurality of resistive elements connected in parallel.

In particular, the invention relates to electric heaters for electric or hybrid vehicles, used to heat the passenger compartment of the vehicle by heating the water of the cooling circuit. Such heaters generally comprise several heating resistive elements connected in parallel so as to be powered by a power current from the high voltage battery of the vehicle (of the order of 400 volts), called traction battery. Each heating resistive element is conventionally controlled by a control circuit including an electronic switch placed in series with each heating element so as to allow or prevent a current to flow through the associated resistive element according to its closed or open state.

By way of example, FIG. 1 schematically illustrates a known electric heater comprising three heating resistive elements li, 1 ₂ and I ₃ with their associated electronic switch 2 ₁ , 2 ₂ , 23 connected in parallel between a high voltage power supply 3. and an electrical mass 4 of a motor vehicle. Each electronic switch 2i, 2 ₂ , 23 is preferably an IGBT power transistor. A control circuit (not shown) selectively controls the electronic switches 2i, 2 ₂ , 23 so as to respond to a request to obtain a heating power corresponding to a desired power.

In the state of the art, an electric heater such as that shown in Figure 1 uses three resistive elements li, 1 ₂ and I ₃ identical, each capable of generating the same power, for example a power of 2000 Watts, and the control circuit drives a power modulation at a few hertz by the pulse width modulation technique applied to the control of the electronic switches. The electronic switches 2i, 2 ₂ , 2 ₃ are switched with a phase shift of 2Π / 3, so that the currents II, 12, 13 in the resistive elements li, respectively 1 ₂ and I ₃ have in time the appearance shown diagrammatically in FIG.

Since the total current Itot corresponds to the sum of currents II, 12 and 13, this makes it possible to adjust the average heating power to the target power by sequentially conducting and then blocking the current of each resistive heating element.

The disadvantage of this type of control lies in the current calls corresponding to the turning on or off of a resistive element li, 1 ₂ or I ₃ , which, although minimized by the phase shift of 2Π / 3 periods switching between them, remain important. Typically, at a supply voltage of 250 Volts corresponds a inrush current of 2000 W / 250 V, or 8 amps, which has a negative impact on the traction battery. Indeed, the succession of surges currents is difficult to support for the battery, especially in very cold conditions.

The present invention aims to overcome the above drawbacks by proposing an architecture of electrical circuit, including electric heater, and its associated driving means to avoid large current calls on the traction battery.

For this purpose, the subject of the present invention is an electrical circuit comprising a set of resistive elements connected in parallel, an electronic switch in series with each resistive element of said assembly capable of allowing selective supply of the resistive elements of said assembly by a DC voltage. , and control means able to determine in said set a final combination of at least one resistive element to supply by closing the associated switch to meet a received target power, characterized in that the resistive elements of the together have different resistance values constituting a geometric progression of reason 2 between a minimum resistance value and a maximum resistance value, and in that the control means are able to control several successive cycles each comprising the switching of one or of several electronic switches of an iron state to an open state or from an open state to a closed state to correspond to a intermediate combination of the resistive elements of said assembly, until said final combination is obtained, in order to reach the nominal power per power stage, each power level corresponding to each cycle being equal to the power value produced by the element resistive said assembly having said maximum resistance value.

According to other possible aspects:

said minimum resistance value is preferably calculated as a function of a maximum power that the electric circuit must be capable of producing for a minimum value of said DC voltage;

said value Ri of minimum resistance is for example calculated according to the relation

wherein U _m i _n represents said minimum value of the DC voltage and P max represents said maximum power.

said set comprises a number N of resistive elements, said number N being determined as a function of a maximum value of said DC voltage, of a predefined maximum inrush current value, and of a predefined value of granularity of power.

- The resistive elements of said assembly are heating elements of an electric heater, adapted to be selectively powered by a DC voltage delivered by a traction battery of an electric or hybrid motor vehicle.

The invention and its advantages will be better understood from the following description, made with reference to the appended figures, in which:

- Figure 1, already described above, schematically shows some elements of a known electric heater;

FIG. 2, also already described, gives the temporal variations of the currents occurring in the elements of the electric heater of FIG. 1, in a known control mode of such a heater; - Figure 3 schematically gives an example of elements involved in an electric heating circuit according to the present invention;

FIGS. 4a and 4b illustrate an example of activation procedure by successive cycles for the electric heating circuit of FIG. 3.

In the rest of the description, the following notations are used to characterize the desired operation of an electric circuit, in particular an electric heater, according to the present invention:

U _m in and Umax correspond to the minimum and maximum continuous voltages delivered by a high voltage supply, in particular a traction battery, and thus define the limits of the voltage range in which the electric circuit will operate;

- Pmax represents the maximum power that the electric circuit must be able to supply, even at the minimum voltage U _m i _n .

- P _∞ ns corresponds to a target power received by the electrical circuit;

P corresponds to the power delivered by the electric circuit as a function of the received target power, and is expressed by the relation:

^- "cons + /" (Pgran / 2)

in which P _gra n represents the permitted power granularity;

- ΔΙ is the maximum allowed current draw on the traction battery.

To fix the ideas but by way of non limiting example, one will use in the following the following values:

Table 1 With reference to FIG. 3, an electric circuit according to the present invention comprises:

a plurality, here eight, heating resistive elements li, l ₂ ... ls, connected in parallel between a high voltage power source 3 and an electrical ground 4;

an electronic switch 2i to 2s, in series with each of the resistive heating elements;

- Control means 5, typically a computer, capable of selectively controlling the different resistive elements via the switching (closing or opening) of their associated electronic switches.

According to the invention, the heating resistive elements are not identical here, as in the prior art, but form a finite set of resistive elements whose resistance values constitute a geometric progression of reason 2. Thus, if the we denote Ri, R ₂ , ... Rs the values of the respective resistances for the elements li, 1 ₂ ... 1 ₈ of figure 3, we have the equation

Ri = 2 ^{(l "1)} Ri for any integer i varying from 1 to 8. The principle of the invention lies in the fact that a total resistance value R can theoretically be obtained by connecting in parallel an infinite number N of resistors whose smallest resistance Ri is twice the total resistance R, and whose resistances Ri to R vary two by two by a coefficient 2. Indeed, for N resistors connected in parallel, there is the relation:

1 1 1 1 1 1 1

- = + +. . . + = + ^ r- +. . . + -

RR ₁ R ₂ R _N ² R 2 ² R 2 R

Let - ¹ = A Λ- ¹

RR Λ

with A = - 1 I-1 = - +. . . H-1- corresponding to the sum of a suite

_{2 2} ² 2 ^N

geometric reason 1/2 whose limit tends to 1 when N tends to infinity.

We will show in the following that we can meet the specifications listed in Table 1 above using a finite number of resistors, here only eight resistors, connected in parallel.

First, we determine the resistance R that would provide the power P _m ax at the voltage U _m in - This resistance is given by the relation

With the values given in Table 1:

R = 10.4 Ω

For simplicity, we choose R equal to 10 Ω to generate a little more power.

It can then be shown that with a limited number of heating resistive elements, it is possible to respect the granularity P _gra n and to avoid currents greater than ΔΙ.

First of all, to respect the granularity, it is necessary that the least powerful heating element (in this case, the one with the highest resistance) generates at the voltage U _max a power lower than P _gra n -

If there are N heating elements, this translates into the relationship

(U _max ) ² / (2 ^N XR) <Pgran

Let 2 ^N > (U _m ax) ² / Pgran XR

Let N> ln [(U _ma x) ² / (Pgran x R)] / ln (2)

Either, using the numerical values in Table 1, N> 7.66.

Moreover, the maximum current demand ΔΙ, consecutive to the addition or the withdrawal of the current in the greatest resistance, is such that

ΔΙ <U _max / (2 ^N x R)

Let N> ln [Umax / (R ΔΙ)] / Ιη (2) Either, using the numerical values in Table 1, N> 6.49.

We can choose

N = 8

i = = 2R = 20 Ω

R ₂ = 2Ri = 40 Ω

Rs = 2R ₂ = 80 Ω

R ₄ = 2R ₃ = 160 Ω

Rs = 2R ₄ = 320 Ω

Re = 2R ₅ = 640 Ω

R ₇ = 2R ₆ = 1280 Ω

Rs = 2R ₇ = 2560 Ω

A circuit has thus been obtained comprising a finite set of resistive elements whose smallest resistance (Ri) is determined as a function of the maximum power P _max that the circuit must be capable of producing and the minimum voltage U _m in of the traction battery, and whose resistance values constitute a geometric progression of reason 2.

We saw above that we chose a value of R slightly lower than that which would produce the desired power P _m ax (10 Ω instead of 10.4 Ω). As a result, by actually using eight resistive elements as described above, a maximum power will finally be obtained that is greater than the power P _max sought initially. It also results that one can opt to use not eight resistive elements in parallel, but only at least the first four resistors Ri to R4. Indeed, by omitting the resistances of higher values R5 to Rs, only a small part of the power is lost. In return, we gain in the optimization of the overall cost of the circuit by avoiding to multiply the resistive elements, their associated electronic switch and connections.

The operation of the electrical circuit of FIG. 3, comprising a set of eight resistive elements li to the, will now be described:

This operation operates in two phases: a first phase during which it is necessary to determine, for a received nominal power P _CO ns, and to the DC voltage value U (any voltage value between U _m in and Umax) actually delivered by the traction battery, what are the resistive elements of said assembly that will have to be activated, by closing their associated switch;

a second phase which consists in effectively activating all the resistive elements selected during the first phase, according to an optimized activation procedure so that the current calls are as low as possible.

The implementation of the first phase, relating to the selection of the resistive elements to be activated by closing their associated electronic switch, is preferably implemented as follows:

When a power setpoint is received, the control means 5 of the electrical circuit will determine, in a first step, the power Pgen generated by the resistive element of all eight elements which has the greatest resistance, here the element lg with a resistance Rs equal to 2560 Ω. This power, which corresponds to the element of the set generating the least power, can be determined by the calculation by applying the relation:

Pgen = U ^_2/8

In a variant, the control means 5 can be searched in a table (not shown) indexed to the voltage value, in which care has been taken to memorize beforehand different power values generated by the element 1 g at different voltages.

Suppose, for example, that the target power is 4356 Watts, and the voltage value delivered by the voltage battery is 382 Volts.

We deduce P = _gen (382) ^2/2560 = 57 W.

The control means 5 then perform the division of the reference power by the value of P _gen , ie:

4356 W / 57 W = 76.4

The approximate whole value of the result is then decomposed into base 2. Here, the approximate value is 77, which is written in base 2 in the form 01001101. The values at "1" correspond to the resistive elements that it It will be necessary to activate (closing the associated electronic switch), and the values at "0" correspond to the resistive elements that will not have to be powered, as summarized in Table 2 below:

Table 2

Table 2 above indicates that, to reach a power close to the desired power, the control means 5 will have to:

- supplying the resistive elements of resistor R _2, R 5, R _O and R ensuring their associated switch is in the closed state;

- Do not power the other resistive elements, ensuring that their associated switch is in the open state.

The previous phase has shown how to determine the set of resistors to activate, that is to say the final combination of resistive elements that should be supplied to achieve the desired power.

In the operation of the electrical circuit, the successive instructions on the one hand, the fact that at startup there is no power on the other hand, cause the heating power to change. In order not to generate too much current surge (not to exceed the specified ΔΙ), the switching (opening or closing) of the resistors switches must be done gradually, without burning step. The second phase of operation therefore consists in applying an optimized activation procedure so that the current calls are as small as possible, in this case for these current calls, due to the removal of resistive elements (by opening of their associated electronic switch) to the addition of resistive elements (by closing their associated electronic switch) do not exceed the maximum current demand ΔΙ allowed. It is immediately apparent, for the combination given in Table 2 that it would not be possible to meet this condition if the control means 5 controlled simultaneously, from an initial situation where all the electronic switches 2i to 2s would be open, the switching, here the closing, of the electronic switches associated with resistive resistance elements R ₂ , R ₅ , R 6 and R 5.

The activation procedure carried out by the control means 5 thus advantageously consists in reaching the target power per successive power level, each power level being equivalent to the power generated by the resistive element which has the highest value resistance. , here the element of resistance equal to 2560 Ω.

The control means 5 will thus control several successive cycles each comprising switching one or more electronic switches from a closed state to an open state or from an open state to a closed state to correspond to an intermediate combination of the elements. resistive of said assembly, until said final combination is obtained, to achieve the power of instruction per step power.

FIGS. 4a and 4b illustrate the succession of cycles that must be made in the context of the invention to obtain, from an initial state in which none of the heating resistive elements is powered (inactive electric heater), the final combination determined previously (supply of resistive resistance elements R ₂ , R5, Re and Rs). In cycle 1, only the resistive element that has the largest resistance Rs of 2560 Ω among all eight resistors is powered by closing the associated electronic switch 2s. As a result, the transition from the initial state to the intermediate combination of cycle 1 has incremented the power of a step equal to 57 Watts and generated an Icycii current draw equal to

Icydi = U / R ₈ , ie Icycii = 149 mA for U = 382 Volts

In the intermediate combination corresponding to cycle 2, only the resistive element 1 ₇ , which has a resistance R7 of 2560 Ω, is energized by closing the associated electronic switch ₂₇ , while the supply of the resistive element is stopped (by opening the associated switch 2s). The load variation therefore remains equal to 2560 Ω. As a result, the transition from the intermediate combination of cycle 1 to the intermediate combination of cycle 2 has generated a current draw Ic _y ci2 equal to Icycii. The power generated is now 114 Watts, which corresponds to a new level of 57 Watts compared to the previous cycle. The same reasoning can be applied for each succession of cycles until cycle 77 which corresponds to the final combination of resistive elements to be activated.

It is appropriate that at each switching cycle, all loads switch at the same time.

Nevertheless, even if the microcontroller activates its outputs at the same time (in the same microsecond), there may be a slight time difference (for example between an opening and a closing transistor).

In order to make these discrepancies imperceptible, the presence of an input capacitance of the electric circuit will make it possible to supply the transient charges without impacting the traction battery. In the case of the electrical circuit which has just been described, with eight resistive elements, the number of switching cycles can be significant.

At worst, it takes ²⁸ , or 256 cycles, to go from a powerless state to a full power state in which the eight resistive elements are powered simultaneously. Cycling times of the order of 50 ms are preferably provided to obtain a maximum acceptable duration of 12 seconds.

In practice, it appears that the use of eight resistive elements is a maximum beyond which the number of cycles becomes too complicated to manage. Solutions with six or even four resistive elements may be more appropriate if greater tolerance is accepted for current ripple and power accuracy.

Claims

Electrical circuit comprising a set of resistive elements (li, 1 ₂ , ... le) connected in parallel, an electronic switch (2i, 2 ₂ , ... 2s) in series with each resistive element (li, 1 ₂ , ... le) of said assembly adapted to allow a selective supply of the resistive elements of said assembly by a DC voltage, and control means (5) able to determine in said set a final combination of at least one resistive element to be powered by closing the associated switch to respond to a received target power, characterized in that the resistive elements of the assembly have different resistance values constituting a geometric progression of reason 2 between a minimum resistance value and a resistance value maximum, and in that the control means (5) are able to control several successive cycles each comprising the switching of one or more electronic switches (2 i, 2 ₂ , ... 2s) from a closed state to an open state or from an open state to a closed state to correspond to an intermediate combination of the resistive elements (li, 1 ₂ , ... le) of said together, until said final combination is obtained, in order to reach the nominal power per power stage, each power level corresponding to each cycle being equal to the power value produced by the resistive element of said assembly having said value maximum resistance.

Electrical circuit according to claim 1, characterized in that said minimum resistance value is calculated as a function of a maximum power that the electric circuit must be capable of producing for a minimum value of said DC voltage.

Electrical circuit according to claim 2, characterized in that said minimum resistance value Ri is calculated according to the relation

Ri = 2 (U _min ) ² / P max wherein U _m i _n represents said minimum value of the DC voltage and P _max represents said maximum power.

Electrical circuit according to any one of the preceding claims, characterized in that said assembly comprises a number N of resistive elements, said number N being determined as a function of a maximum value of said DC voltage, a predefined current value maximum call, and a predefined value of power granularity.

Electrical circuit according to any one of the preceding claims, characterized in that the resistive elements (li, 1 ₂ , ... ls) of said assembly are heating elements of an electric heater, able to be selectively powered by a DC voltage delivered by a traction battery of an electric or hybrid motor vehicle.