JPS648310B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a resistor switching circuit, and particularly to a circuit that can suppress spike noise generated in a load circuit when switching resistors.

FIG. 1 is a block diagram of a conventional load current detection circuit with a resistor switching circuit. This circuit consists of a high gain amplifier 3, current sensing resistors 5, 7 connected in parallel,
It is comprised of a load circuit 15, a circuit that negatively feeds back the voltage generated in the load circuit 15 to the amplifier 3 via the voltage follower 11, and a circuit 21 that extracts the voltage generated in the current detection resistor. This circuit is load circuit 15
A desired voltage determined by the input voltage and the resistance values of resistors 17 and 19 is generated (at terminal 13), and the current flowing through load circuit 15 is converted to voltage by current detection resistor 5 or 7 and detected and monitored. It is something. In such a circuit, the load circuit 15 (resistance value
It is common practice to switch the current sensing resistor to change the current sensitivity depending on the magnitude of the current flowing through Rx. This switching can be done, for example, by turning on and off switch (reed relay) 9, and resistor 7 (resistance value
R ₁ ) to resistor 5 (resistance value R ₂ , e.g. R ₂ = 100R ₁ )
This is done depending on whether or not they are connected in parallel. However, considering the moment when the switch 9 is turned on, for example, a certain period of time is necessary for the amplifier 3 to respond to this change, and the output voltage of the amplifier 3 does not change at the moment of switching. Therefore, a voltage change occurs at the terminal 13, which is divided by the new resistor and the load circuit. This voltage change becomes spike noise. This spike noise usually reaches several volts, and when a semiconductor element is connected as the load circuit 15, it becomes particularly harmful as it changes the measurement conditions, and also has an adverse effect on the life of the switch 9.

The present invention has been made in order to eliminate the above-mentioned drawbacks, and an object of the present invention is to provide a circuit that can suppress spike noise generated when switching a current detection resistor, and can control its magnitude and switching time. It is to provide. The present invention will be explained in detail below using the drawings.

FIG. 2 is a block diagram of a load current detection circuit with a resistor switching circuit according to the present invention. The same parts as in FIG. 1 are designated by the same reference numerals. Only the parts different from FIG. 1 will be explained. A semiconductor switch such as a junction FET, MOSFET, bibolar transistor, etc. is connected in series with the resistor 7. One terminal of a sawtooth signal generator 27 is connected to the control terminal (for example, the gate of an FET) of the semiconductor switch 25, and the other terminal of the generator 27 is connected to the common terminal s of the switch 29. The common terminal S is selectively connected to a contact a connected to the output terminal of the amplifier 3, a contact b connected to the output terminal of the voltage follower 11, or a contact c connected to a reference potential point, as described below. The operation will be explained below.

The voltage generated in the load circuit 15 is negatively fed back to the input of the amplifier 3 via the voltage follower 11, and a constant voltage corresponding to the input voltage is generated at the terminal 13.
This is the same circuit as shown in the figure. The operation will be explained below by distinguishing the polarity of the semiconductor switch 25 and the direction of the current flowing through the current detection resistor.

When using N-channel FET - 1 When current flows from A to B and current detection resistor 7 is connected in parallel to current detection resistor 5. Note that resistor 5 is, for example, 100KΩ, and resistor 7 is 1K.
It is Ω. FIG. 3 shows the voltage waveforms at points G, A, and B in this case. switch 29
is connected to sa. First, the output voltage of the sawtooth signal generator (hereinafter simply referred to as a generator) 27 is maximized, and the switch 9 is turned on with the FET 25 turned off. Next, operate the generator 27,
The output voltage is varied at a fairly constant slew rate (slope size, voltage/time). The gate voltage V _G of FET25 increases,
When FET 25 turns on and current begins to flow, V _B becomes lower than before. This voltage change is fed back to the input of the amplifier 3, and V _A changes in the direction.
Since A is connected to the other terminal of the generator 27 (the generator 27 operates in a floating state), the voltage at the gate decreases with respect to the source (s) of the FET 25, turning the FET 25 off. Work in the direction. That is, negative feedback is applied. Amplifier 3 through
Since the rate is sufficiently greater than the slew rate of generator 27, the voltage at A changes such that V _Gs = constant = pinch-off voltage. When FET25 is completely turned on and resistance can no longer change, V _B
will no longer change, and V _A will remain constant. FIG. 3 shows the changes in V _G , V _A , and V _B during the above operation. Therefore, the current sensing resistor is switched slowly and no spike noise is generated in the load circuit 15.

Next, consider the magnitude of the voltage change in _VB and the switching time. The change in V _A is △A, the change in V _B is △B,
Let the switching time be Δt. Assuming that the amplifier 3 is a primary system amplifier with a gain of 1 at frequency o, the following equation holds true.

2πo・△t・△B=△A △B=△A/2πo・△t For example, △A=10V, △t=2mS, o=200KHz
Then, △B≒4mV. and generator 2
The slew rate of 7 is 10V/2mS = 5V/mS. From the slew rate of the generator 27, the amplifier 3,
Since the slew rate of the loop including voltage follower 11 etc. is sufficiently large, the change in V _A will approximately match the slew rate of generator 27. Therefore, if the generator 27 is operated at 5V/mS, △B≒4mV,
Only very small voltage changes occur in the load circuit 15. By appropriately selecting the slew rate of the generator 27, ΔB and Δt can be controlled and their optimum values can be selected.

-2 When the current is flowing from A to B and the current detection resistor is only set to 5.

FIG. 3 shows the voltage waveforms of V _G , V _A , and V _B. Switch 29 remains connected to sa. The output voltage of generator 27 is varied from maximum to maximum at a constant slew rate.
When FET 25 begins to turn off, V _B changes, and this change is negatively fed back to generator 27 via amplifier 3 . Therefore, as in the case of -1, when V _B changes for a certain period of time and the FET 25 is completely turned off, ΔB becomes zero. Then turn off switch 9. The switch 9 is not an essential requirement, and is provided because in the case of minute current measurement, it may be considered that the FET 25 is not turned off sufficiently. Of course, in the above case as well, ΔB and Δt can be controlled by appropriately selecting the slew rate of the generator 27.

-3 When current is flowing from B to A and the current detection resistor 7 is connected in parallel to the current detection resistor 5 The voltage waveforms of V _G , V _A , and V _B are shown in d of FIG. Connect switch 9 to sb. FET2
It is the same as in the case of -1 until 5 starts to turn on, V _B changes to , and V _A changes to . Here, since s-b, than the change in V _B
The change in V _A is large, and the drain becomes larger rapidly in the direction relative to the gate. Therefore, negative feedback is applied and the FET 25 operates in the off direction. The variation in V _A is controlled to be approximately the same as the slew rate of generator 27. -1, △B and △t can be controlled. Note that although the description has been made using s-b, it goes without saying that similar operations and effects can be obtained using s-c.
s-b or s-c when turning off FET25
The operation is the same as in the case of -2.

FIG. 4 summarizes the above operation and shows the voltage waveforms of V _G , V _A , and V _B corresponding to the polarity of FET 25, the current direction, and the set position of switch 29. As is clear from the figure, since spike noise occurs in the load circuit (V _B ) in cases b, c, e, and h, it is understood that configurations a, d, f, and g are used. This can be summarized as follows.

A When using an N channel and the current is s
-a, then s-b or s-c (a, d).

(b) When using the P channel and the current is s
- When b or sc, use s-a (g, f) C. If the current is connected to s-a, use the N channel, and if , use the P channel (a, f) D. If the current is connected to s-b or s-c, use the P channel, and if the current is, use the N channel (g, d). In the case of c) and d), use the N and P channels.
FETs may be connected in parallel and connected in series with the resistor 7, and only one FET may be operated depending on the current direction.

As is clear from the above description, according to the present invention, it is possible to suppress the spike noise that occurs when switching resistors, and the magnitude of the spike and the switching time can be appropriately selected to desired values. Therefore, even if a semiconductor element is connected as a load circuit, the measurement is not adversely affected, and a great effect can be obtained.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a conventional load current detection circuit with a resistor switching circuit, and FIG. 2 is a block diagram of a load current detection circuit with a resistor switching circuit according to the present invention.
FIG. 3 is a voltage waveform diagram of each part in one operating mode of the circuit of FIG. 2, and FIG. 4 is a voltage waveform diagram of each part of the circuit of FIG. 2 in various operating modes. 15...Load circuit, 25...Semiconductor switch, 27
...sawtooth signal generator, 29...switch.

Claims (1)

[Claims] 1. A current detection resistor is provided between the output terminal of the amplifier and the load circuit, and the voltage generated in the load circuit is fed back to the input of the amplifier to provide the load circuit with an output voltage corresponding to the input voltage of the amplifier. The generating circuit includes a series circuit of a semiconductor switch and a resistor connected between the output terminal of the amplifier and the load circuit, and a sawtooth waveform having one terminal connected to the control input terminal of the semiconductor switch. a signal generator;
and means for supplying at least one of the output voltage of the amplifier, the voltage of the load circuit, or the reference voltage to the other terminal of the sawtooth signal generator. Current detection circuit.