EP2638406A1

EP2638406A1 - Current measurement for an electric heater

Info

Publication number: EP2638406A1
Application number: EP11781530.8A
Authority: EP
Inventors: Laetitia Gestas; Bertrand Puzenat
Original assignee: Valeo Systemes Thermiques SAS
Current assignee: Valeo Systemes Thermiques SAS
Priority date: 2010-11-09
Filing date: 2011-11-09
Publication date: 2013-09-18
Also published as: US9429596B2; JP2013545429A; CN103492887B; US20130285646A1; FR2967260A1; CN103492887A; WO2012062789A1; FR2967260B1; JP5989654B2

Abstract

The invention relates to a method for measuring a current or voltage output quantity of a switched-mode power supply circuit (H), referred to as a chopper circuit, of period T and of cyclic ratio α, belonging to the interval ]0; 1[, such that, at each period, the supply current is non-zero during an "ON" phase of duration αT, and zero during an "OFF" phase of duration (1-α)T. The measurement method includes the following steps: measuring the value G_on of an image of the output quantity during an "ON" phase; measuring the value G_off of an image of the output quantity during an "OFF" phase; calculating the difference ΔG = G_on - G_off,; and using the difference ΔG for evaluating the output quantity.

Description

CURRENT MEASUREMENT FOR ELECTRIC RADIATOR

The present invention relates to the technical field of power switching power supplies, also called chopper circuits, used for example for the supply of electric heating elements or electric motors DC.

A circuit, called chopper, power switching power supply is designed to cyclically deliver a voltage and / or an electric current during a fraction, variable duration, each cycle or period of its operation. The regime of such a chopper is defined by its period T and its duty cycle which belongs to the interval] 0; 1 [. Thus, at each period or cycle, the supply current is, on the one hand, non-zero during an "on" phase of duration aT and, on the other hand, zero during an "off" phase of duration (1 - a) T. By modulating the value of the duty cycle, it is then possible to vary the average power supply power of the load connected to the chopper circuit.

Such a chopper circuit finds many applications, particularly in electric heating applications where the modulation of the duty cycle makes it possible to regulate the average heating power delivered by an electric heating element.

In order to check the operation of the chopper circuit, either for safety reasons to avoid damaging the connected electrical circuits or its power components, or for regulating purposes to know the average power actually delivered by the circuit chopper, it is necessary to evaluate the current and / or the output voltage of the chopper circuit. To this end, it is known to implement measurement systems involving one or more operational am pliers measuring either directly the output voltage or the voltage across a measurement resistor, also called shunt, in which circulates the output current to derive the value of this output current. However, each operational amplifier has the disadvantage of having an offset voltage between its inverting input and its non-inverting input. This input offset voltage or offset offset voltage at the input of the operational amplifier then generates a measurement error which is amplified by the amplifier itself.

In order to remedy this drawback, an operational amplifier can be chosen whose offset voltage, the lowest possible, is compatible with the voltage values that it is desired to measure. However, operational amplifiers with very low offset voltage are relatively expensive.

In the case of current measurement by means of a measurement resistor, it can be sought to compensate for the error induced by the offset voltage by choosing a value of the measuring resistor sufficiently high that the value of the error has a small relative proportion to the measured value. However, increasing the value of the measurement resistor induces a great heat dissipation and therefore the need to implement heat removal systems which increases the total cost of the power supply. In addition, this dissipation of heat at the resistance level is detrimental to the performance of the power supply.

In order to overcome the above drawbacks, the need has arisen for a method and a device which makes it possible to carry out reliable measurements without having to resort to a low-offset operational amplifier or to a measurement resistor. high value.

In order to achieve this objective, the invention relates to a method for measuring an output quantity, current or voltage, of a circuit, called chopper, power switching power supply, of period T and duty cycle ratio belonging to at the interval] 0; 1, so that at each period the supply current is, on the one hand, non-zero during an "on" phase of duration aT and, on the other hand, zero during an off phase of duration (1 -a) T. According to the invention this measuring method comprises the following steps:

measuring the value G _on of an image of the output quantity during an "on" phase,

measuring the value G ₀ ff of an image of the output quantity during an off phase,

- calculation of the difference AG = G _on - G ₀ ff

- use of the difference AG for the evaluation of the output quantity. Thus, since the amplification-induced offset voltage or measurement errors are the same during the "off" phase and during the "on" phase, an evaluation of the output quantity that is unaffected by the "on" phase is obtained. the offset voltage or by measurement errors. The invention thus makes it possible to obtain a permanent calibration of the measurement.

According to one form of implementation of the invention, the measurements G _on and G ₀ ff are carried out during the "on" and "off" phases of the same period.

According to a variant of this embodiment, the measurements G _on and G ₀ ff are performed at a frequency greater than the switching frequency of the power supply.

According to one form of implementation of the invention, the measurements G _on and G ₀ ff are each carried out during the phase respectively "on" and "off" of two consecutive periods.

According to the invention the measured or measured magnitude may be the output voltage or the output current of the chopper supply circuit. Thus, according to one characteristic of the invention, the evaluated quantity is the output current and the measured images correspond to an amplified value of the voltage across a measurement resistor in which the output current flows. According to one characteristic of the invention, a correction component is added to the evaluated quantity. Thus, in the context of the evaluation of the output current, the measured images then correspond to an amplified value of the sum, on the one hand, of the voltage at the terminal of a measurement resistor in which the output current flows. and, on the other hand, a constant value correction voltage. The implementation of such a correction component makes it possible to compensate the offset due to amplification defects.

According to a variant of the invention, the correction voltage is a positive DC voltage, which makes it possible to compensate for the negative value shifts.

The invention also relates to a device for measuring a voltage or current output quantity, a circuit, said chopper, power switching power supply, period T and duty cycle a belonging to the interval] 0 ; 1, so that at each period the supply current is normally, on the one hand, non-zero during an "on" phase of duration aT and, on the other hand, zero during an "off" phase of duration (1-a) T. In order to allow the implementation of the method according to the invention, this measuring device comprises:

a circuit for amplifying a voltage representative of the quantity to be measured,

a unit of measurement, of the amplified voltage at the output of the amplification circuit, which is adapted to:

measuring the value G _on of the amplified voltage during an "on" phase,

measuring the value G ₀ ff of the amplified voltage during an off phase,

- calculate the difference AG = G _on - G ₀ ff

- use value of the difference AG to evaluate the output quantity.

According to one embodiment of the invention, the measuring device comprises a circuit for adding a correction voltage to the voltage representative of the quantity measured at the input of the amplification circuit. According to a feature of this embodiment, the correction voltage is a positive DC voltage. According to an alternative embodiment of the invention, the measurement unit is adapted to perform the measurements G _on and G ₀ en during the "on" and "off" phases of the same period.

According to a characteristic of this variant, the measurement unit is adapted to perform measurements G _on and G ₀ ff at a frequency greater than the switching frequency of the power supply.

According to another characteristic of this variant embodiment, the measurement unit is adapted to calculate the difference Δ at a frequency lower than the switching frequency of the power supply.

According to an alternative embodiment of the invention, the measurement unit is adapted to perform measurements G _on and G ₀ ff during "on" and "off" phases of two different periods.

According to one embodiment of the invention the device is adapted to measure the output current of the chopper circuit and comprises a measurement resistor in which the output current flows, the amplification circuit being adapted to amplify the voltage across the terminals. the measuring resistance.

Of course, the various features, variants and embodiments of the invention may be associated with each other in various combinations to the extent that they are not incompatible or exclusive of each other.

Furthermore, various other features of the invention will emerge from the following detailed description with reference to the single appended figure which schematically illustrates a non-limiting form of embodiment of a measuring device implementing the method according to the invention for measuring the output current of a power switching power supply circuit.

According to the illustrated example, the invention is implemented in the context of controlling and regulating the operation of an electric heating element 1 used for heating a passenger compartment of the vehicle. The heating element 1 is supplied by a power switching power supply circuit also called chopper H. This power supply circuit H has an operating period T and a duty cycle can vary in the interval] 0; 1 [. The supply circuit H is controlled by a control unit UC which modulates the value of the duty cycle a as a function of the desired heating power at the level of the heating element 1. At each period T, both in the time interval [nT; ηΤ + αΤ], said phase "on", the output current Is has a non-zero value while in the time interval [ηΤ + αΤ; (n + 1) T], said phase off, the output current Is is zero.

In order to know the heating power actually provided by the heating element 1 or to verify that the output current delivered by the supply circuit H is admissible by the electric cables connecting the supply circuit H to the heating element 1 it is proposed to measure the output current Is. To this end, a measurement resistor S, also called a "shunt", is used which is connected in series with the heating element 1. In known manner, the measurement of the voltage U across the latter makes it possible to evaluate the value of the output current Is delivered to the heating element 1.

The invention therefore proposes to implement a measurement device 2 which will amplify the voltage U by means of an amplification circuit 3 and then evaluate this voltage by means of a measurement unit UM. According to the illustrated example, the amplification circuit 3 comprises an operational amplifier 4 whose inverting input (-) is connected via a resistor R1 to a first terminal of the measurement resistor S. The circuit of FIG. amplification 3 also includes a counteraction line 5 which connects, by a resistor R2, the inverting input (-) to the output 6 of the operational amplifier 4. The non-inverting input (+) of the operational amplifier is further connected to the second terminal of the resistor measurement S through a resistor R3 and to the ground by a resistor R4. Finally, the amplification circuit 3 comprises a circuit 7 for adding a correction voltage which is formed of a resistor R5 of which one terminal is connected to the non-inverting input (+) while the other is fed by a constant correction voltage Vc of constant value. According to the invention, the supply circuit, the measurement resistor, the amplification circuit and the measurement unit can be made in the form of a single integrated circuit or in the form of discrete components associated with each other. others or in the form of a combination of integrated circuits and discrete components.

The voltage Vs at the output of the amplification circuit is given by the following relation:

Or :

- R345 = R3 // R4 // R5 is the resistance equivalent to the three resistors R3, R4, R5 in parallel,

- and Offset is the offset voltage, called offset, between the inverting input (-) and the non-inverting input (+) of the operational amplifier 4.

Moreover, not to suffer saturation effects will be

preferably chosen to be greater than the maximum value likely to be met for the absolute value of the Offset offset voltage.

The measurement unit UM is then adapted to measure, on the one hand, the value G _on of the output voltage Vs during each "on" phase and the value G ₀ ff of the output voltage Vs during each "off" phase. ". The unit of measure UM is preferably adapted to have a sampling period less than the period T hash of the supply circuit. The sampling period may for example be less than or equal to 1 / 10th of the hash period T. Thus, several measurements of the value G _on and the value G ₀ ff will be carried out during each hash period.

As during the "off" phases the output current of the supply circuit H is zero, the value G ₀ ff corresponds to the error induced by the Offset offset attention.

By calculating the difference AG = G _on - G ₀ fr, the unit of measurement will cancel the error induced by the offset offset voltage so that AG will be perfectly proportional to the output current during the "on" phase. This calculation of the difference AG can, for example, be performed every two periods T per unit of measurement.

The unit of measurement calculates or evaluates from AG the value of the output current. This evaluation of the output current can then be used by the control unit UC to control the duty cycle a of the supply circuit H so as to regulate the available power level of the heating element 1, to ensure that the output current does not exceed a value the permissible value for the electrical wiring or to identify any malfunction of the supply circuit. According to the previous example, the output current of the supply circuit is evaluated; however, the invention could also be used to evaluate the output voltage of the supply circuit during each "on" phase. Naturally, various modifications, in particular with regard to the constituent components of the supply circuit and / or the amplification circuit, may be made to the invention within the scope of the appended claims.

Claims

1. A method of measuring a current output quantity or voltage of a circuit, said chopper, power switching power supply (H) of period T and duty cycle a, belonging to the interval] 0; 1, so that at each period the supply current is, on the one hand, non-zero during an "on" phase of duration aT and, on the other hand, zero during an "off" phase of duration ( 1 - a) T, measurement method characterized in that it comprises the following steps:

- calculation of the difference AG = G _on - G ₀ ff

- use of the difference AG for the evaluation of the output quantity.

2. Measuring method according to claim 1, characterized in that the measurements G _on and G ₀ ff are performed during the "on" and "off" phases of the same period.

3. Measuring method according to claim 2, characterized in that the Gon and Goff measurements are performed at a frequency greater than the switching frequency of the power supply.

4. Measuring method according to claim 1, characterized in that the measurements G _on and G ₀ ff are each carried out during the phase respectively

"On" and "off" two consecutive periods.

5. Measuring method according to one of claims 1 to 4, characterized in that a correction component is added to the measured quantity before measurement.

6. Measuring method according to one of claims 1 to 4, characterized in that the measured quantity is the output current and in that the measured images correspond to an amplified value of the voltage across a measuring resistor in which the output current flows.

7. Measuring method according to one of claims 1 to 4, characterized in that the evaluated quantity is the output current and in that the measured images correspond to an amplified value of the sum, on the one hand, of the voltage to the terminal of a measuring resistor in which the output current flows and, on the other hand, a constant value correction voltage.

8. Measuring method according to claim 7, characterized in that the correction voltage is a positive DC voltage.

9. A device for measuring an output quantity of a circuit, said chopper, power switching power supply (H) of period T and duty cycle a, belonging to the interval] 0; 1, so that at each period the supply current is normally, on the one hand, non-zero during an "on" phase of duration aT and, on the other hand, zero during an "off" phase of duration (1 - a) T, device comprising:

an amplification circuit (3) of a representative voltage (U) of the quantity to be measured,

a unit (UM) for measuring the amplified voltage (Vs) at the output of the amplification circuit, which is adapted to:

measuring the value G _on of the amplified voltage (Vs) during an "on" phase,

measuring the value G ₀ ff of the amplified voltage (Vs) during an off phase,

- calculate the difference AG = G _on - G ₀ ff

- use value of the difference AG to evaluate the output quantity.

10. Measuring device according to claim 9, characterized in that it comprises a circuit (7) for adding, at the input of the amplification circuit, a correction voltage (Vc) to the representative voltage (U). of the measured quantity.

1 1. Measuring device according to claim 10, characterized in that the correction voltage is a positive DC voltage.

12. Measuring device according to one of claims 9 to 11, characterized in that the measuring unit is adapted to perform the measurements G _on and G ₀ ff during the "on" and "off" phases of a same period.

13. Measuring device according to claim 13, characterized in that the measuring unit is adapted to perform measurements G _on and G ₀ ff at a frequency higher than the switching frequency of the power supply.

Measuring device according to claim 13, characterized in that the measuring unit is adapted to calculate the difference AG at a frequency lower than the switching frequency of the power supply.

15. Measuring device according to one of claims 9 to 1 1, characterized in that the measuring unit is adapted to perform measurements G _on and G ₀ ff during phases "on" and "off" of two periods different.

16. Measuring device according to one of claims 9 to 15, characterized in that it is adapted to measure the output current of the chopper circuit (H) and comprises a measurement resistor (S) in which circulates the current of output, the amplification circuit (3) being adapted to amplify the voltage across the measuring resistor.