FI88966B - INDIREKT STROEMMAETNINGSKRETS - Google Patents

Description

88966

Indirect current measuring circuit Indirekt strömmätningskrets 5

The invention relates to an indirect current measuring circuit for a step-down switch whose main circuit consists of a switch, a diode and an inductance.

The current measuring circuit according to the invention can be applied e.g.

to a control method for controlling the output current of a DC / DC converter, where the converter is supplied by, for example, a solar panel and the load is a battery. In this case, the battery is supplied with a current corresponding to the maximum power of the solar panel.

15 A solution is known from the prior art in which current and voltage are measured from a solar panel and the maximum power is calculated by a microcomputer or an analog multiplier.

A control method is also known from the prior art, in which the output current of the converter is measured separately and a control signal is generated for the control system on the basis of this.

A disadvantage of the solutions known from the prior art is that the hardware solutions become relatively complex and thus expensive: 25 expensive.

The object of the invention is to provide an improvement on the currently known current measuring circuits.

. . It is a detailed object of the invention to provide a current measuring circuit which is as simple as possible in terms of equipment solution and thus very cheap in terms of purchase price.

The objects of the invention are achieved by an indirect current measuring circuit, which is mainly characterized in that the measured instantaneous value of the switch is proportional to the current through the switch and also to the load current f / (3) T c · ΐ 5 where ts - conduction time of switch Si td - conduction time of diode D T - time iL (t) “current as a function of time 10 that the analog switch and the second RC circuit perform the multiplication in the equation

ts + td T

15 that the comparator is adapted to act as an indicator of the diode current based on examining the polarity of the voltage across the diode, the signal from the comparator being ug - "1" when the diode is conducting, the switch control signal directly indicating the current 20 through the switch and the output voltage u10 - "1 "is for time 0 <t <ts + td and zero only when the current is open in the time interval ts + td <t <T, for a continuous current ts + td - T, where the load current is non - zero if either the diode or the switch current is non - zero.

The invention will be explained in detail with reference to a preferred embodiment of the invention shown in Figures 1-3 of the accompanying drawings, to which, however, the invention is not intended to be exclusively limited.

Figure 1 shows a preferred embodiment of an indirect current measuring circuit according to the invention 3 88966 in principle.

Figure 2 shows the current and voltage signals shown in Figure 1 with an overcurrent.

5

Figure 3 shows the current and voltage signals shown in Figure 1 at a continuous current.

The current measuring circuit shown in Fig. 1 consists of a main circuit switch S :, a diode D and an inductance L, as in a step-down switch, however, so that the polarities of the components and voltages are opposite to those of a conventional circuit.

With the current measuring circuit according to Fig. 1, e.g. the following 15 benefits. First, the current measurement and the switch control are at the same potential, thus simplifying the control circuit, which in turn substantially lowers the cost of the measurement circuit. Second, the internal resistance of the switch Si can be used to measure the current, as a result of which a separate measuring resistor is not required. As a result, losses are reduced and the efficiency-20 ratio is increased. Third, the switch Si can be implemented with an N-type fet or NPN transistor, which are cheaper than a P-Fet or a PNP transistor with the same current and voltage resistance values.

The basic idea of the current measuring circuit shown in Fig. 1 is as follows.

: 25

The average output current iL can be calculated by measuring the current flowing through the switch Sx if the conduction time of the switch Sx, the conduction time tD of the zero diode D and the time T of the whole period are known.

30 The method is based on the fact that the current of the inductance L increases linearly under the conduction of the switch Sx and decreases linearly under the conduction of the diode D, whereby the average load current is obtained from the equation (zero point of time is the switching moment) 35 4 88966 τ t, t, * td * 1 = \ f iL (t) dt = 1 J iL (t) dt + J, f iL (t) dt (l) oo e.

Since the shape of the current curve increases or decreases linearly between the maximum value of the current itm „and the minimum value, the equation (1) * -L ~ rp (i-Lmln + ^ Lmax ^ + ~ ψ ^ Lmax * ^ Lmin ^ 5 • e '* Ttd» E * U / i «> This can also be written in the form H. ± 1β £ · ± 'ί lLit) dt (3) * 0 10 where icx = -¾½ U) 1 Γ c can be denoted.

xx = / i4 (t) dt (5) C * o

The technical implementation of the current measuring circuit shown in Fig. 1 is as follows.

15 5 88966

The signal Uj is proportional to the current isis passing through the switch Sj ^ and also to the load current iL when the switch Sj.

From the output u3 of the connection 5 formed by the analog switch S2 and the integrating RC circuit R1C1, a signal c * u3 a f Uldt C8 0 10 proportional to the integral part xx of equation (3) is obtained.

The analog switch S3 and the RC circuit R2C2 perform the multiplication in equation (3) with (ts + td) / T.

15 U6 “\ f U * dt” - j. ^

The op2 operational amplifier acts as a voltage follower. The comparator compl acts as an indicator of the current of the zero diode D based on the examination of the polarity of the voltage above the diode D (no current flows through the diode D:: if the voltage ud> 0). The signal from the comparator compl is ug - "1" when:; : diode D conducts, i.e. ts <t <ts + td. The control signal u8 of the power switch Sj directly indicates the current flowing through the switch (if and when Uout <uin).

25 The load current iL is non-zero if the current of either diode D or switch Sj is non-zero. This is accomplished by a or-member stallion with an output voltage u10 to "1" for time 0 <t <t1 + td and zero only when the current is open in the time interval t, + td <t <T (with continuous current ta + td - T).

Figure 2 shows currents and voltages with an open current and Figure 6 88966 shows currents and voltages with a continuous current, respectively. The following assumptions have been made in Figures 2 and 3 and the signals are labeled as follows.

5 Assumptions: - uin and Uout are assumed to be constant (uln> Uout) - losses are not taken into account except for the diode threshold voltage ud - the time constants of the RC circuit are large.

10

Signals: - ux: measured instantaneous current value of switch Sr - u2: input voltage of RC circuit R ^ 15 - u3: output voltage of RC circuit - u *: output voltage of RC circuit RjCi buffered (-u3) - us: input voltage of RC circuit R2C2 - u6: RCC R2C2 output voltage - u7: RC circuit R2C2 output voltage buffered (-u7) 20 - u8: power switch control voltage and signal at the same time (is> 0) - u9: zero diode D current detection (id> 0) - u10: inductance L current detection (iL> 0)

Only one preferred embodiment of the invention has been described above and it will be apparent to those skilled in the art that numerous modifications may be made thereto within the scope of the inventive idea set out in the appended claims.

Claims (2)

7 88966 An indirect current measuring circuit for a counting switch whose main circuit consists of a switch (S1), a diode (D) and an inductance (L), characterized in that the measured instantaneous current value (ux) of the switch (S ^) is proportional to the current flowing through the switch (Sx) (is) and the switch (Sx) also lead to the load current (iL) that the output of the connection (u3) formed by the analog switch (S2) and the integrating RC circuit (R ^ j) gives a signal proportional to the integral part of Equation (3) (X ^ 10 -¾½ · -§- / <3> 1 0 where t, - conduction time of switch Si 15 td - conduction time of diode D T - time iL (t) - current as a function of time that the analog switch (S3) and the second RC circuit (R2C2) implement the multiplication ts * td T in Equation 20 (3) that the comparator (compl) is adapted to act as an indicator of the current 25 of the diode (D) based on examining the polarity of the voltage across the diode (D), wherein the signal from the comparator (compl) is u9 - "1" with the diode (D) conducting, whereby the switch (S ^ the control signal (u8) indicates the current flowing directly through the switch And that the output voltage u10 - "1" of the ori (stallion) is for the time 0 <t <t, + td And zero only when the current 30 opens in the time interval t, + td <t <T , with a continuous current t, + td - T, for which the load current (iL) is non-zero if the current of either the diode (D) or the switch (Sx) is non-zero. Current measuring circuit according to Claim 1, characterized in that the operational amplifier (opi) is adapted to act as a voltage follower or a non-inverting amplifier and to prevent loading of the RC circuit (R 1), and in that the operational amplifier (op2) is adapted to act as a voltage follower or not. as an inverting amplifier, preventing the RC circuit (R2C2) from being loaded. 10 9 88966