JPH0636013B2 - Load disconnection detection circuit in PWM control circuit - Google Patents

Description

The present invention relates to a load disconnection detection circuit in a PWM control circuit.

[Problems to be Solved by Prior Art and Invention]

Conventionally, various disconnection detection circuits have been proposed in the disconnection detection circuit of a PWM control circuit for applying a rectangular wave that repeats ON / OFF, that is, “H” and “L” states to a load, but its structure is complicated and designed. Sometimes lacked in ease.

The present invention provides a load disconnection detection circuit in such a PWM control circuit with a simple configuration.

[Means for Solving the Problems]

A PWM control circuit a for feeding back a change in the negative feedback voltage of the negative feedback resistor 3 connected in series with the load 1 to control the duty ratio of the rectangular wave 2 applied to the load 1, and an output voltage based on the negative feedback voltage. And a predetermined reference voltage, and a disconnection detection circuit b having a comparator 5 for detecting disconnection of the load 1 based on the result of the comparison, and an output terminal 5 ′ of the comparator, which is operated by the rectangular wave 2. A load disconnection detection circuit in a PWM control circuit, comprising: a correction circuit 6 for invalidating the output of the comparator 5 when the rectangular wave output is at a low "L" voltage level.

[Work]

When the load 1 is disconnected, no current flows in the negative feedback resistor 3 connected in series with the load 1, and as a result, the negative feedback amplified voltage is not generated and the output of the comparator 8 is set to "H" as in the embodiment. At the same time, the output of the comparator 5 changes from "L" to "H" and outputs a signal notifying that the load 1 is disconnected.

However, even if no disconnection occurs, when the rectangular wave 2 applied to the load 1 is “L”, no current flows in the load 1 and no negative feedback amplified voltage is generated.
Output changes to "H" and outputs the same signal as when the wire is broken even though the wire is not broken.

Therefore, when the rectangular wave 2 applied to the load 1 is “L”, the correction answer 6 is driven to invalidate the signal output from the comparator 5.

Therefore, even though the wire is not broken, the signal to be output is not output when the rectangular wave 2 applied to the load 1, which is the same signal as the signal output when the wire is disconnected, is in the “L” state. It is possible to detect a severe disconnection.

〔Example〕

 First, the illustrated PWM control circuit a will be described.

An input signal that determines the duty ratio of the rectangular wave 2 applied to the load 1 is input to the OP amplifier 7, and the output of the OP amplifier 7 is input to the comparator 8 in which a predetermined comparison reference voltage is set, and is connected in series with the load 1. The OP amplifier 7 is oscillated by the negative feedback voltage generated by the negative feedback resistor 3 connected to
It outputs a triangular wave according to the input signal. Then, the comparator 8 outputs a rectangular wave 2 having a predetermined duty ratio according to the triangular wave, and the rectangular wave 2 turns on and off the transistor T _{ra 2} with respect to the power supply 9, and further turns on and off the transistor T _{ra 2.}
The FF turns on the transistor T _{ra 1} for the power supply 9 '.
N is turned off and the rectangular wave 2 is applied to the load 1.

Next, the load disconnection detection circuit b, which is the main part of the present invention, will be described in detail.

Wherein the negative feedback voltage generated in the negative feedback resistor 3 via a resistor R ₀ is input to the + terminal of the amplifier 4 ', - amplifier through a resistor R ₂ for a negative feedback with grounding the terminal via the resistor R ₁ Connect to the output of 4 '.

That is, assuming that the negative feedback voltage input to the + terminal is V ₂ and the voltage input to the − terminal is V _1. Is amplified to a voltage V ₃ .

Incidentally, this is the amplified voltage V ₃ so that no support is canceled because noise in V ₁ be the noise in the negative feedback voltage V ₂ occurs occurs.

The amplified voltage V ₃ is input to the − terminal of the comparator 5 and compared with the reference voltage V ₄ input to the + terminal, and the result is output to the output terminal 5 ′ of the comparator 5.

The correction circuit 6 transistor T _r which output end (the aforementioned comparator 8 outputs a square wave 2 of the PWM control circuit a
branched from a ₂ of the previous stage) connected to the base terminal of the transistor _T r _{b 1,} connect the collector terminal of the transistor _T r _{b 1} to the base terminal of the transistor _T r _{b 2} of the second stage, the transistor T The collector terminal of _r b ₂ is connected to the output terminal 5 ′ of the comparator 5.

That is, in the operation of this embodiment, when the rectangular wave 2 applied to the load 1 is in the "H" state, a negative feedback voltage is generated, and the reference voltage V _{4 at the} + terminal is amplified by the amplified voltage V at the − terminal.
₃ becomes larger, the comparator 5 outputs an “L” signal, and when the rectangular wave 2 is in the “L” state or a disconnection occurs, a negative feedback voltage does not occur, and conversely the + terminal Since the amplified voltage V ₃ at the negative terminal becomes smaller than the reference voltage V _{4, the} signal “H” is output.

However, when the comparator 5 outputs the “H” signal, it cannot be determined whether the load 1 is in the normal state in which the rectangular wave 2 in the “L” state is applied or the load 1 is disconnected. .

Therefore, when the rectangular wave 2 is in the “L” state by the correction circuit 6, the first-stage transistor T _r b ₁ is turned off, and the voltage of the power supply 11 is changed to the second-stage transistor T _r b.
_{Since the} transistor _Tr b ₂ is added to the base of ₂ and the output of the comparator 5 is grounded, the disconnection detection circuit b outputs an “L” signal (output from the comparator 5 when the rectangular wave 2 is in the “H” state). (The same signal that is output) is forcibly output. However, when the load is disconnected, the negative feedback voltage input to one terminal of the OP amplifier 7 does not occur, the OP amplifier 7 does not oscillate, and the voltage at the − terminal becomes smaller than the reference voltage at the + terminal. The signal "H" is output.
In addition, the comparator 8 outputs the H output from the OP amplifier 7.
Since the voltage at the + terminal for inputting the signal becomes higher than the reference voltage at the − terminal, the signal “H” is always output, and thus the transistor _Tr b ₁ of the correction circuit 6 is output.
Is on, the transistor _Tr b ₂ is off, and the output of the comparator 5 (the signal H which is output at the time of load disconnection) is directly detected as the output of the load disconnection circuit b.

Therefore, in this embodiment, if the signal "L" is output, it can be detected that the load 1 is not broken.
When the signal "H" is output, the rectangular wave 2 applied to the load 1 is in the "L" state because the correction circuit 6 outputs the signal "L" when the rectangular wave 2 is in the "L" state. It is possible to detect that there is a clear disconnection without misidentifying that it is.

In the figure, the integrating circuit 10 is a delay circuit connected to the preceding stage of the transistor _Tr b ₁ and is for preventing the correction circuit 6 from malfunctioning.

〔The invention's effect〕

Since the present invention is configured as described above, it is of a very simple structure and is extremely practical without confirming that the rectangular wave in the "L" state is applied to the load and that the disconnection occurs. The load disconnection detection circuit in the PWM control circuit is excellent.

[Brief description of drawings]

FIG. 1 is a wiring diagram showing an embodiment of the present invention. a ... PWM control circuit, b ... disconnection detection circuit, 1 ... load, 2 ... rectangular wave, 3 ... negative feedback resistance, 5 ... comparator,
5 '... Output end, 6 ... Correction circuit.

Claims (1)

[Claims] 1. A PWM control circuit for feeding back a change of a negative feedback voltage of a negative feedback resistor connected in series with a load to control a duty ratio of a rectangular wave applied to the load, and an output voltage by the negative feedback voltage. And a predetermined reference voltage, and a disconnection detection circuit having a comparator for detecting a disconnection of the load based on the result of the comparison and a disconnection detection circuit connected to the output end of the comparator and operated by the rectangular wave, and the rectangular wave output is low. A load disconnection detection circuit in a PWM control circuit, comprising: a correction circuit for invalidating the output of the comparator when the voltage level is