CN1052541C - Circuit arrangement for current measurement via a switching transistor - Google Patents

Abstract

A circuit arrangement in which instead of a current-measuring resistor a load circuit of a switching transistor is used to determine a current in a load circuit of a pulsed switching transistor. No additional measuring resistors are required. Such a circuit arrangement is particularly suitable for use as a switching power supply or charger, in which case pulses of different lengths are applied to the switching transistors. The pulse length is controlled by the current in the load circuit, which can be a function of temperature. A commercially available integrated circuit can be used for the control circuit.

Description

Circuit arrangement for measuring current via switching transistors

The invention relates to current measuring circuits and in particular to a circuit arrangement in which the load circuit of a switching transistor has a resistor for measuring current. It is known, for example in the case of switched-mode power supplies, to measure the current flowing through a switching transistor load circuit. In this case, a resistor is connected in the load circuit, and the voltage drop across the resistor is proportional to the load current. It is not appropriate to connect another resistor (shunt resistor) to measure the current. Because the addition of the resistor not only increases the power dissipation in the load circuit, but also increases the cost of the installation, especially since the resistor must be dimensioned for the highest possible load.

The circuit arrangement of the main item of the present invention is characterized in that the load circuit of the switching transistor is connected with a measuring circuit, which measures the voltage drop when the switching transistor is turned on, and thus measures the current, which has the advantage that due to the use of the transistor Intrinsic internal resistance ("ON" resistance), so no additional measuring resistor is required. Consequently, the efficiency of the circuit arrangement is further increased due to the reduced power consumption. In addition, the production cost of the equipment is also reduced.

The circuit arrangement described in the sovereign claim can be advantageously further improved by taking the measures described in the dependent claims. It is particularly advantageous to design the switching transistors as metal-oxide field-effect transistors (MOSFETs). The "on" resistance of this transistor in the on state is determined, that is, around several milliohms. In the off state, this resistance is essentially infinite, so that a definite current measurement can be obtained through this different resistance. Another benefit is that MOSFET transistors are widely used, so the purchase price is economical.

However, due to the pulse skipping of switching transistors during switching, it is beneficial to use a delay element so that the current is not measured until after the turn-on oscillation has decayed.

In the conduction state, the drain/source resistance RDS(ON) of the MOSFET transistor has a large positive temperature coefficient. This effect has the advantage that the control loop can be controlled accordingly. The current in the load circuit can also be automatically limited by means of a positive temperature coefficient, which current-limiting effect corresponds to the temperature of the MOSFET. The switching threshold can be set using a voltage divider in the load circuit connected in parallel, so that the switching threshold at which the switching transistor switches to the off state can be preset as required. In addition, it is advantageous to delay the current measurement signal by means of a delay element, so that the current is only measured when the conducting switching transistor has reached a steady state, thus advantageously suppressing the occurrence of pulse jumps that occur during the measurement of the current signal. Incorrect measured values are obtained.

The circuit arrangement according to the invention is more efficient due to the reduced power consumption and is therefore preferably used as a switching power supply or as a charger for charging a stored current.

An embodiment of the invention is shown in the drawings and will now be described in more detail below. FIG. 1 shows this embodiment, and FIG. 2 shows a waveform diagram of the voltage.

It can be seen from the circuit arrangement of FIG. 1 that the input terminal b of the measuring circuit 1 is connected in parallel with the power path of the switching transistor 2 via the first resistor 7 and the diode 8 . There is an excitation circuit in the measurement circuit 1 , which is not shown in the figure, and its output terminal is connected to the control input terminal of the switching transistor 2 via the third resistor 6 . The switching transistor 2 can be, for example, an n-channel field effect transistor (MOSFET).

Such switching transistors are commercially available, for example under the trade name SIPMOS. The drain connection D of the FET transistor together with the cathode of the diode 8 is connected via an inductance L to the DC voltage + outside 9 . The power connection S is grounded together with the grounding line of the measuring circuit 1 . The parallel circuit, formed by a capacitor 4 and a second resistor 3 , is connected in parallel to the terminal 5 of the measuring circuit 1 to ground. Said parallel circuit together with the feedback resistor 5 forming a node 10 between the first resistor 7 and the anode of the diode 8 constitutes a timer. By means of this timer, the determination of the "on" resistance of FET transistor 2 is delayed until after the decay of the transition duration, the second connection of resistor 5 is connected to gate G and to third resistor 6 .

Referring now to FIG. 2, the method of operation of the circuit arrangement of FIG. 1 will be explained in more detail. When this circuit arrangement is used, for example, in a switching power supply or in a charger to charge a NiCd storage battery or similar device, the measuring circuit 1 applies a pulse to the control input G of the switching transistor 2 via the third resistor 6, so that the pulse Current flows from the voltage source through the inductor L through the switching transistor 2 . The power supply + is, for example, a power supply device for supplying a load current i _D via an inductance L. In practice it is used, for example, as a pulse charging device for recharging batteries, which is powered from a 12 volt or 24 volt source.

The measuring circuit 1 can adopt commercially available integrated circuits, such as SG 384× (Philips) or UC384× (SGS-Thomson) integrated circuits. A commercially available circuit generates a pulse width modulated signal at output a. In this case, the length of the pulse train is controlled by the voltage signal at input b. The feedback resistor 5 in this case synchronizes the excitation process of the grid G with the determination process at the node 10 or at the input b, and at the same time captures the energy for the measurement. Figure 2 illustrates the relationship between them in more detail. The ordinate in FIG. 2 shows the voltage V10 which is present at the input b of the measuring circuit 1 corresponding to the voltage divider ratio over time t. It can be seen from the curve in the figure that the beginning part of the curve presents a pulse on the rising edge, which produces a serious overshoot phenomenon when the pulse reaches the input voltage Ub=1.2 volts (time t=0). The switching transistor 2 does not switch until this time, so that the switching transistor 2 is turned off, the potential at the connection D of the switching transistor 2 rises, so that the input voltage Ub falls towards zero value (time t≈9 microseconds). After this, switching transistor 2 receives a control pulse via output a of the integrated circuit, which in turn causes the input voltage at terminal 10 or b to rise. At this point, the switching transistor 2 is switched on again, to be precise until the input b reaches the switching threshold of 1.2 volts again. Afterwards, the switching transistor 2 is turned off again, and the above process is repeated. The length of the pulse is thus regulated or controlled in a control loop as a function of the voltage at input b. The timer consists of a capacitor 4 and resistors 3, 5 and possibly 7, which keep the line from being measured during severe overshoots.

A comparator is installed inside the integrated circuit to compare the input voltage Ub with a predetermined reference value. Resistors 3 and 7 are provided for changing the reference value, and these resistors form a voltage divider through which the switching threshold is selected by the center tap.

Although a diode with a threshold voltage of Us produces a voltage drop, this has no disturbing effect in the application concerned. On the other hand, the diode 8 protects the integrated circuit from overvoltages which occur when the switching transistor 2 is in the off state. But the threshold voltage Us should be considered when determining the current value of the current i _D , because when the switching transistor 2 is turned on, the input voltage at the terminal b is:

Ub=Us+i _D ·R _DS The resistor 4 between the output terminal of the integrated circuit and the control connector G acts as a signal match.

Claims (9)

1. current measurement circuit device, have a metering circuit in order on the measurement resistor in the detector switch transistor load circuit with the electric current correspondent voltage, it is characterized in that, metering circuit (1) is connected with the load circuit of switching transistor (2), measure the pressure drop of the switching transistor (2) of conducting, and measure current value thus.

2. circuit arrangement according to claim 1 is characterized in that, switching transistor (2) is a field effect transistor (MOSFET).

3. circuit arrangement according to claim 1 and 2 is characterized in that, the temperature coefficient of switching transistor (2) can be used as the control variable of load circuit.

4. circuit arrangement according to claim 1 and 2 is characterized in that, the electric current in the temperature coefficient volitional check load circuit of switching transistor (2).

5. circuit arrangement according to claim 1 and 2 is characterized in that, metering circuit (1) has the usefulness of a control circuit for energizing switch transistor (2).

6. circuit arrangement according to claim 1 and 2 is characterized in that, is provided with voltage divider (7,4) in the current measurement circuit, can set the switching threshold of pulse width controller by means of this voltage divider (7,4).

7. circuit arrangement according to claim 1 and 2 is characterized in that, can establish delay element (3,4 between switching transistor (2) and metering circuit (1), 5,7), this delay element (3,4,5,7) current measurement signal being deferred to switching transistor (2) reaches till the steady state (SS).

8. circuit arrangement according to claim 1 and 2 is characterized in that, the formula of the changing power supply that this circuit arrangement can be used as charger or cuts, and its current impulse is that pulsewidth is in check.

9. circuit arrangement according to claim 1 and 2 is characterized in that, metering circuit (1) has the pulse width modulating chip of selling in the city, and the control output end (a) of its switching transistor (2) connects current measurement circuit through feedback resistor (5).

Images