AN INDIRECT CURRENT MEASURING CIRCUIT

Abstract: The invention relates to an indirect current measuring circuit. The current measuring circuit is comprised of a switch (S1) of the main circuit, a diode (D) and an inductance (L). The measured instantaneous value (u1) of the current of the switch (S1) is proportional to the current (iS) passing through the switch (S1), and when the switch (S1) is conducting, also to the load current (iL). A comparator (COMP1) is fitted to act as an indicator of the current of the diode (D), based on the examination of the polarity of the voltage prevailing over the diode (D), whereby the signal to be obtained from the comparator (COMP1) is (u9 = identically ''1'') when the diode (D) is conducting, whereby a control signal (u8) of the switch (S1) directly indicates the current passing through the switch. The output voltage (u10) of the OR-element (OR1) is during the time (0<T<ts+td) and zero only when the current is switching off during the time interval (ts+td<t<T), on a constant current (ts+td = T), whereby the load current (iL) deviates from zero, if the current of either the diode (D) or the switch (S1) deviates from zero.

Description: An indirect current measuring circuit The invention relates to an indirect current measuring circuit. The inventive current measuring circuit can be adapted e.g. to a con¬ trol method for controlling the output current of a DC/DC inverter, in which method the inverter is supplied e.g. by a solar panel and an accumulator is used as a load. A current corresponding to the maximum effect of the solar panel can then be supplied to the accumulator. A solution is known in prior art, in which the current and the voltage are measured from a solar panel and the maximum effect is calculated by means of a microcomputer or an analog multiplier. A control method is also known in prior art, in which the output cur¬ rent of an inverter is measured separately, and according to the out- put current, a control signal is formed for the control system. A disadvantage in the known solutions of prior art is the fact that the device solutions become relatively complex and thereby expensive. The object of the invention is to provide an improvement on the cur¬ rent measuring circuits known currently. A certain detailed object of the invention is to provide a current measuring circuit, the device solution of which is as simple as poss- ible and the purchase price of which is thereby very low. The objects of the invention are obtained by means of an indirect current measuring circuit, which is mainly characterized in that it is comprised of a switch of the main circuit, a diode and an inductance, whereby the measured instantaneous value of the current of the switch is proportional to the current passing through the switch, and when the switch is conducting, also to the load current, that a signal proportional to the integral part of an equation (3) is obtained from the output of the connection formed by an analog switch and an inte¬ grating RC-circuit - • s that the analog switch and another RC-circuit realize the multiplica¬ tion in the equation (3) + trt that a comparator is fitted to act as an indicator of the current of the diode, based on the examination of the polarity of the voltage prevailing over the diode, whereby the signal to be obtained from the comparator is identically 1 when the diode is conducting, whereby the control signal of the switch directly indicates the current passing through the switch, and that the output voltage of the OR-element Is during the time (0<T<t s + t d ) and zero only when the current is switching off during the time interval (t s + t d <t<T) , on a constant current (t s + t d -= T) , whereby the load current deviates from zero, if the current of either the diode or the switch deviates from zero. The invention is described in detail with reference to a preferred embodiment of the invention shown in Fig. 1-3 of the accompanying drawings, to which embodiment the invention is not intended solely to be limeted. Fig. 1 shows a preferred embodiment of the inventive indirect current measuring circuit as a basic circuit arrangement. Fig. 2 shows the current and voltage signals shown in Fig. 1 on a switched-off current. Fig. 3 shows the current and voltage signals shown in Fig. 1 on a continuos current. The current measuring circuit shown in Fig. 1 is comprised of a switch S-, a diode D and an inductance L, such as in a step-down chopper, however in such a way that the polarities of the components and volt¬ ages are opposite in comparison with a conventional connection. For example the following advantages are obtained by means of the current measuring circuit of Fig. 1. Firstly, the measurement of the current and the control of the switch S x are in the same potential, whereby the control circuit is simplified, which in turn essentially reduces the price of the measuring circuit. Secondly, the internal resistance of the switch S x can be used in the current measurement, owing to which a separate measuring resistance is not needed. As a result, the losses decrease and the efficiency increases. Thirdly, the switch S- can be realized with an N-type FET or an NPN-transistor, which are cheaper than a P-FET or a PNP-transistor at the same current and voltage duration values. The basic idea of the current measuring circuit shown in Fig. 1 is as follows. The average i L of the output current can be calculated by measur-ing the current passing through the switch S--, if the conductivity time t s of the switch S--, the conductivity time t D of the zero diode D and the time T of the entire cycle are known. The method is based on the fact that the current of the inductance L increases linearly when the switch S x is conducting and decreases when the diode D is conducting, whereby the average load current is obtained from the equation (the zero point of the time is the connec¬ tion moment) t s - - J i L (t)dt J i t (t)dt + J i L (t)dt (1) Since the curve form of the current increases and decreases linearly between the maximum value i- j - a x and the minimum value i- j ^-, of the cur- rent, from the equation (1) is obtained 3 -L ( l -_ni-. + iLπ,aχ)/2 + — (i lΛ« +i Lmin )/2 T T ts + C d • (iL+iι_n« /2 (2 ) This can also be expressed in the form t s + t d 1 r- i L • — J i L (t)dt (3) T t, J 0 in which it can be marked t s + t d k- (4) and X, - i L (t)dt ■ ; / ■ (5) The technical realization of the current measuring circuit shown in Fig. 1 is as follows. The signal u-- is proportional to the current i s passing through the switch S-, and when the switch S x is conducting, also to the load cur¬ rent i L . A signal proportional to the integral part X j of the equation (3) is obtained from the output u 3 of the connection formed by the analog switch S 2 and the integrating RC-circuit R j C-- The operational amplifier 0P1 acts as a voltage follower (u- - u 3 ) and prevents the loading of the RC-circuit R 1 C 1 . The analog switch S 3 and the RC-circuit R 2 C 2 realize the multiplication in the equation (3) with (t s + t d )/T. The operational amplifier OP2 acts as a voltage follower. The comparator C0MP1 acts as a current indicator of the zero diode D, based on the examination of the polarity of the voltage prevailing over the diode D (no current passes through the diode D, if the volt¬ age is u d >0) . The signal to be obtained from the comparator C0MP1 is u 9 -= "1", when the diode D is conducting, i.e. t s <t__t s +t d The control signal u 8 of the switch S 1 directly indicates the current passing through the switch (if and when u out <u in ) . The load current i L deviates from zero, if the current of either the diode D or the switch S-- deviates from zero. This is realized with an OR-element ORl, whose output voltage is u 10 = "1", during the time 0<t< ts +t d and zero only when the current is switching off during the time interval ts +t d <t<T (on a constant current t s +t d = T) . Fig. 2 shows the currents and the voltages on a switched-off current and correspondingly, Fig. 3 shows the currents and the voltages on a constant current. In Fig. 2 and 3, the following presumptions have been made and the signals have been marked as follows. Presumptions: - u in and are assumed to be even (u in > out ) - losses are not taken into account except for the threshold voltage *d - the time constants of the RC-circuit are high. Signals: u x : measured instantaneous value of switch S x u 2 : input voltage of RC-circuit RιG x : u 3 : output voltage of RC-circuit R χ C x - u 4 : output voltage of RC-circuit R j C-- when buffered (= u 3 ) u 5 : input voltage of RC-circuit R 2 C 2 u 6 : output voltage of RC-circuit R 2 C 2 u 7 : output voltage of RC-circuit R 2 C 2 when buffered (= u 7 ) - u s : control voltage of switch S x u 9 : current identification (i d >0) of zero diode D u 10 : current identification (i L >0) of inductance L Only one preferred embodiment of the invention has been described above, and it is apparent to those skilled in the art that several changes can be made therein within the scope of the inventive idea defined in the enclosed patent claims.

Claims: Claims 1. An indirect current measuring circuit, characterized in that it is comprised of a switch (S--) of the main circuit, a diode (D) and an inductance (L) , whereby the measured instantaneous value (u.-) of the current of the switch (S x ) is proportional to the current (i s ) passing through the switch (S x ) , and when the switch (S x ) is conducting, also to the load current (i L ) , that a signal proportional to the integral part (X x ) of an equation (3) is obtained from the output (u 3 ) of the connection formed by an analog switch (S 2 ) and an integrating RC-cir¬ cuit (RiC-) that an analog switch (S 3 ) and another RC-circuit (R 2 C X ) realize the multiplication in the equation (3) t, + d that a comparator (C0MP1) is fitted to act as an indicator of the current of the diode (D) , based on the examination of the polarity of the voltage prevailing over the diode (D) , whereby the signal to be obtained from the comparator (C0MP1) is (u 9 -_ "1") when the diode (D) is conducting, whereby a control signal (u 8 ) of the switch (S-) di¬ rectly indicates the current passing through the switch, and that the output voltage (u 10 ) of the OR-element (ORl) is during the time (0<T<t s +t d ) and zero only when the current is switching off during the time interval (t s +t d <t<T) , on a constant current (t s +t d - T) , whereby the load current (i L ) deviates from zero, if the current of either the diode (D) or the switch (S x ) deviates from zero. 2. A current measuring circuit according to Claim 1, characterized in that an operational amplifier (0P1) is fitted to act as a voltage follower or as a non-inverting amplifier and to prevent the loading of the RC-circuit (R-^) and that an operational amplifier (0P2) is fitted to act as voltage follower or as a non-inverting amplifier preventing the loading of the RC-circuit (R 2 C 2 ) .

Inventors: OLLILA JAAKKO, OLLILA, JAAKKO
Applicants: NESTE OY
Application year: 1991
Main classification: G01R19
IPC codes: G01R19