JP5924574B2 - PWM control circuit - Google Patents

Description

  The present invention relates to a PWM control circuit including a switching element that performs PWM control of a load by energizing and interrupting a current passing through a connected load by an input PWM signal.

  The PWM control circuit controls the load (PWM control) by intermittently inputting a current to the load connected to the circuit, and is widely used.

  The PWM control circuit includes a switching element such as a transistor connected to a load, and a control unit such as a CPU connected to the switching element. When a PWM signal is input from the control means to the switching element, the switching element is turned on / off at a duty ratio in the PWM signal, thereby energizing and interrupting the current through the load, An intermittent current corresponding to the duty ratio is input.

  Specifically, in this PWM control circuit, when the PWM signal is at the H level, the terminal on the side connected to the load among the switching elements is energized, and the PWM signal is at the L level (that is, the current of the PWM signal is 0), the switching element is controlled so that the terminal is cut off (see FIG. 3A).

  Here, when the switching element breaks down and a short circuit occurs between the terminals, current may continue to flow to the load regardless of whether the PWM signal is input from the control means to the switching element.

  For example, when a transistor is used as a switching element, the base of the transistor is connected to a control means (CPU or the like), the collector is connected to a load, and the emitter is grounded (see FIG. 1), the transistor fails. During normal PWM control, a current from the collector to the emitter flows when the PWM signal is at the H level. When the PWM signal is at the L level, no current flows from the collector to the emitter.

  However, when the transistor is short-circuited, current may continue to flow from the collector to the emitter regardless of the PWM signal (whether it is H level or L level). If so, the load may be destroyed by overcurrent. In order to protect the load from such breakdown due to overcurrent, it is necessary to detect that a switching element (such as a transistor) is short-circuited.

  In the conventional PWM control circuit, the short-circuit detection of the switching element is performed in a situation where the PWM control is not performed. Specifically, when the PWM control is not performed and the switching element is normal, the switching element should be cut off from the load. Therefore, the circuit is configured to detect the presence / absence of a current passing through the load under this condition and detect a short circuit of the switching element based on the detection result. That is, if a current passing through the load is detected in a situation where the PWM control is not performed, this is due to an “abnormally energized state” and the switching element is short-circuited. That is, it is possible to detect a short circuit of the switching element by detecting the presence or absence of a current passing through the load under this condition.

  Incidentally, as a short-circuit detection technique related to a circuit using a switching element, for example, there is a method disclosed in Patent Document 1. In other words, this is a method for determining a load short-circuit failure when a state in which the pulse width of the load current corresponding to the deviation between the target value and the detected value of the load current is equal to or less than a predetermined value continues for a predetermined time. However, this method detects a short circuit of the load on the premise that the switching element is operating normally, and it is not assumed that the switching element itself is short-circuited.

JP 2002-359919 A

  By the way, even during the PWM control, a short circuit of the switching element occurs. However, conventionally, switching circuit short-circuit detection has not been performed during PWM control. In order to detect a short circuit during the execution of PWM control, if the same idea as the short circuit detection in the situation where the PWM control is not performed as described above is applied, the timing at which the switching element enters the cut-off state. This is because detection must be performed in synchronization.

  However, during the PWM control, the switching element repeats the energization state and the cutoff state in a short time, and the duty ratio also changes, so that the switching element is synchronized with the timing at which the switching element enters the cutoff state with respect to the load. The detection of the “abnormally energized state” means that, for example, when a short circuit is detected by a control unit (CPU or the like), the detection must be performed at a synchronized timing, so that the processing load of the control unit increases. It was not practical.

  Therefore, an object of the present invention is to provide a PWM control circuit capable of detecting a short circuit of a switching element even during execution of PWM control without increasing a processing load.

The present invention includes a PWM signal transmitter for transmitting a PWM signal by the inputted PWM signal, a switching element for PWM controlling a load by performing the energization and interruption of the current through the connected load, the switching element and a short circuit detection means for detecting that a short circuit, the short circuit detection means is configured to detect a shut-off current and (on) between said switching element load (off), the The detection is repeatedly performed in a cycle shorter than the shorter duration of the PWM signal in the duration of the H level and the duration of the same L level of the PWM signal. , When the number of consecutive times of detecting energization (ON) between the switching element and the load is greater than a predetermined number of times, or between the switching element and the load The PWM control circuit determines that the switching element is short-circuited when the number of consecutive times during which no interruption (off) is detected is greater than a predetermined number.

According to the above configuration, the detection of energization (ON) and interruption (OFF) between the switching element and the load by the short-circuit detection means is the same as the duration of the PWM signal during the period of the PWM signal. It is repeatedly performed in a cycle smaller than the shorter duration of the level durations, and the short- circuit detection means detects that energization (ON) is detected more than a predetermined number of times or is shut off (OFF) ) Is detected more than the predetermined number of times, it is determined that the switching element is short-circuited. For this reason, it is not necessary to synchronize the current detection cycle with the PWM cycle and the duty ratio, and short-circuit detection can be easily performed by determining only the number of detections.

And the short circuit detection means of this invention WHEREIN: It is preferable that the said short circuit detection means repeats the detection of electricity supply (on) and interruption | blocking (off) between the said switching element and the said load with a fixed period.

Further, in the present invention, the predetermined number of times, the energization between the time at which current is energized through the load based on the duty ratio of the PWM signal, the said switching element in said short circuit detection unit load ( It is preferable that the number of times is determined based on the detection cycle of ON and OFF (OFF) .

  According to the present invention, it is not necessary to synchronize the current detection cycle with the PWM cycle and the duty ratio, and it is possible to easily detect a short circuit by judging only the number of times of detection. Therefore, it is possible to detect a short circuit of the switching element even during the PWM control without increasing the processing load.

It is a block diagram of the PWM control circuit which concerns on one Embodiment of this invention. It is a block diagram of a PWM control circuit according to the embodiment. It is a conceptual diagram of the current detection and short circuit detection of the PWM control circuit which concerns on the embodiment, (A) is normal, (B) shows the time of short circuit occurrence. It is a flowchart of the PWM control circuit which concerns on the same embodiment.

  The present invention will be described below with reference to the drawings by taking one embodiment.

  The PWM control circuit of this embodiment is provided in a control circuit in an industrial vehicle such as a forklift as an example, and is used to control a load. Examples of the load include a coil provided in a hydraulic valve or a brake, and this coil is excited by an intermittent current input from the PWM control circuit.

  This PWM control circuit is configured as shown in FIG. 1 (schematic diagram) and FIG. 2 (block diagram). As shown in the figure, the PWM control circuit 1 includes a CPU 3 as control means, a switching element 4, and a Vcc (positive voltage power supply) 5, and a load is placed between the switching element 4 and the Vcc (positive voltage power supply) 5. 2 is connected.

  As shown in FIG. 2, the CPU 3 includes a PWM signal transmission unit 31 and a short circuit detection unit 32. The PWM signal transmission means 31 transmits a PWM signal to the switching element 4 with a predetermined duty ratio. The duty ratio of the present embodiment is variable between 0% and 50%. However, since the state where the duty ratio is 0%, that is, the state where there is no energization, the PWM control is performed in this embodiment within a range where the duty ratio exceeds 0% and is 50% or less. However, the present invention does not exclude PWM control in a range where the duty ratio exceeds 50%, and implementation in the range is also possible.

  The switching element 4 receives a PWM signal transmitted from the PWM signal transmitting means 31 and supplies (on) and interrupts (off) current through the load 2 connected to the switching element 4 with a duty ratio corresponding to the input. ) Are repeated alternately. By this operation, an intermittent current corresponding to the duty ratio flows through the wiring between the switching element 4 and the load 2, and the load 2 is PWM-controlled. As shown in FIG. 3A, when the PWM signal is at the H level, the switching element 4 is energized ("ON" in the figure), and when the PWM signal is at the L level, the switching element 4 is in the cut-off state ("OFF" in the figure). It becomes. The cycle of the PWM signal and the on / off cycle of the switching element 4 are the same cycle. However, as shown in the drawing, a slight time lag occurs before the state of the switching element 4 changes according to the change of the PWM signal.

  In the present embodiment, a transistor is used as the switching element 4. Examples of the transistor include a bipolar transistor, a MOS-FET, and an IGBT, but various other transistors can be used. Further, as the switching element 4, an element other than a transistor can be used.

  In the present embodiment, a bipolar transistor is used as the transistor of the switching element 4. As shown in FIG. 1, the base (illustrated “b”) of this transistor is connected to the CPU 3, the collector (“c”) is connected to the load 2, and the emitter (“e”) is grounded.

  Next, the short circuit detection means 32 will be described. As shown in FIG. 2, the short-circuit detecting unit 32 is provided as a part of the CPU 3 and detects that the switching element 4 is short-circuited. The short-circuit detection means 32 includes current detection means (not shown), and detects the presence or absence of current passing through the load 2 connected to the switching element 4 by the current detection means. In this embodiment, the switching element 4 The presence or absence of current flowing through the wiring between the load 2 and the load 2 is detected. The short-circuit detecting means 32 includes a counter 321. The counter 321 counts (accumulates) the continuous number of times that the presence or absence of the current is detected. The count (integration) of the number of times is an accumulation of the number of times that the current detection is continued in a constant cycle, and if the current is not detected even once, the count value (integration value) is reset to zero.

  The short-circuit detection means 32 is the same as the detection means conventionally used for short-circuit detection when PWM control is not performed in terms of the circuit hardware configuration. However, the method of short circuit detection is completely different as will be described later.

  This short-circuit detection means 32 repeatedly detects the presence or absence of the current at a constant period preset in the short-circuit detection means 32 before the PWM control is performed. In the present embodiment, the detection cycle is set at a constant cycle that does not change during PWM control, and is not a cycle synchronized with the PWM cycle and the duty ratio. That is, the detection is performed at a constant period regardless of whether the PWM signal is at the H level or the L level.

  This detection cycle is shorter than the shorter duration of the PWM signal (the PWM signal cycle transmitted from the PWM signal transmitting means 31) between the H signal duration and the L signal duration. Is also a small cycle. The reason for setting the detection cycle in such a cycle is that the detection cycle is not synchronized with the PWM cycle and the duty ratio (because there is a shift in the cycle), so that the energized state (ON) and the cut-off state of the switching element 4 This is because at least one detection is performed in any state of (OFF). By setting the detection cycle to such a cycle, in the range where the duty ratio exceeds 0% and is 50% or less (the duration of the cutoff state is longer than the duration of the energized state), the switching element 4, the short-circuit detecting means 32 detects the current-present state at least once, so that the short-circuit can be determined by how many times the current-present state detection continues as described later. In addition, although outside the range of the present embodiment, in a range where the duty ratio is more than 50% and less than 100% (the duration of the energized state is longer than the duration of the cutoff state), the switching element 4 Since the short-circuit detection means 32 detects the current-free state at least once during the interruption state, as will be described later, the counter 321 is surely reset, and how many times the detection with the current continues. Thus, it is possible to determine a short circuit.

  FIG. 3A shows a case where the duty ratio is 50%, the PWM cycle is 1 / n second (frequency nHz), and the detection cycle is 1 / (4n) second (frequency 4 nHz) according to the present embodiment. . The detection cycle of the short circuit detection means 32 in this case is a cycle shorter than both the time during which the current passing through the load 2 is cut off by the switching element 4 and the time during which the current is applied.

  While the detection by the short-circuit detection means 32 continues, the switching element 4 repeats the energized state (ON) and the interrupted state (OFF) at the duty ratio set by the CPU 3. For this reason, in the short circuit detection means 32, a state with current ("1" on the lower stage in FIG. 3A) is detected, and then a state without current ("0") is detected. In the present embodiment, as shown in FIG. 3A, the frequency of the PWM cycle is nHz (the frequency of the on / off cycle of the switching element 4 is also nHz). On the other hand, the frequency of the detection cycle is set to 4 nHz. For this reason, the energized state (ON) and the cutoff state (OFF) are detected every 2 or 3 times. The “two times or three times” is because the detection cycle is always constant as described above, and therefore the number of detections depends on the overlap (or the shift) with the ON / OFF cycle of the switching element 4. Because it may change. Note that the asterisk symbol shown in the lower part of FIG. 3A indicates that the detection result is either “0” or “1” (indefinite) depending on the timing. When the PWM control is normally performed, the short-circuit detection unit 32 repeats the detection.

  Here, when the switching element 4 fails and is short-circuited, the switching element 4 continues to be energized as shown in the middle part of FIG. 3B regardless of the input of the PWM signal from the CPU 3 to the switching element 4. Will continue to do. From this, the short-circuit detecting means 32 that is continuously detecting continuously detects a current-present state ("1" in the figure).

  When the switching element 4 is normal, the duration of the energized state becomes a predetermined value corresponding to the duty ratio. For this reason, the normal number of times that the short circuit detection means 32 detects a state with current is also a predetermined number of times. The predetermined number of times is a number determined based on the time during which the current passing through the load 2 is energized based on the duty ratio of the PWM signal and the detection period of the presence / absence of the current in the short-circuit detection means 32. That is, the number of continuous detections that should be repeated in the detection period during the duration of the energized state.

Specifically, the detection result becomes indefinite to a value (integer) obtained by dividing the duration value of the energized state by the detection period of the short-circuit detection means 32 and rounding off the decimal point (FIG. 3 ( A) is a number obtained by adding 1 in consideration of the detection result of the asterisk symbol shown in the lower part. In the present embodiment, the duty cycle is 50%, the PWM cycle is set to 1 / n second, and the detection cycle is set to 1 / (4n) second.
(1 / n) × 0.5 = 1 / (2n) seconds, and the predetermined number of times is
(1 / (2n)) / (1 / (4n)) + 1 = 3 times.

  Since the calculation of the predetermined number of times may be performed only at the beginning of changing the duty ratio, short-circuit detection is performed at a period synchronized with the timing at which the switching element 4 is turned off corresponding to the PWM period and the duty ratio. As compared with the above, it is possible to remarkably suppress an increase in the processing load on the CPU 3. The predetermined number of times may be determined not by calculation each time but by referring to a table in which the relationship between the duty ratio and the number of times is determined in advance.

  The counter 321 in the short-circuit detection means 32 counts the number of times that the state with current is continuously detected. Judgment means provided in the short-circuit detection means 32 on condition that the number of times counted by the counter 321 exceeds the predetermined number of times of detection (that is, the number of times one more than the predetermined number of times of detection). (Not shown) determines that a short circuit has occurred in the switching element 4. The number of times exceeding the predetermined number is counted because the state with current continues for the number of times exceeding the predetermined number of detections, that is, the switching element 4 is short-circuited. This is because it means that the blocking state no longer appears.

  Taking the case shown in FIG. 3A as an example, the continuous number of times that the state with current is detected is two or three times. For this reason, when the state with current is detected four or more times continuously, the short-circuit detecting means 32 can determine that a short circuit has occurred in the switching element 4. The “four times” is one more than the predetermined number of “three times” obtained by the above calculation.

  As described so far, the short-circuit detection means 32 of the present embodiment performs short-circuit detection with a specific detection cycle that is not in synchronization with the PWM cycle and the duty ratio. For this reason, the same idea as short-circuit detection in a situation where PWM control is not performed, which has been conventionally performed, is applied to increase the processing load as in the case of performing short-circuit detection in synchronization with the PWM cycle. In addition, the circuit configuration is not complicated by providing a means for synchronizing with the PWM cycle, and the short circuit of the switching element 4 can be detected even during the PWM control.

  Moreover, the short circuit detection means 32 of this embodiment is the same as the detection means conventionally used for short circuit detection in the situation where PWM control is not implemented on the hardware structure of a circuit. For this reason, it is also possible to improve the existing PWM control circuit so that the short circuit can always be detected without modifying the hardware by updating the program of the control means (CPU or the like).

  In addition to the short-circuit detection means 32, an overcurrent interruption means may be provided on the PWM control circuit 1. However, in the present embodiment, this overcurrent cutoff means is not necessarily required. Therefore, when the overcurrent cutoff means is not provided, the manufacturing cost of the PWM control circuit can be reduced accordingly.

  Finally, the processing flow of the PWM control circuit 1 of the present embodiment will be described with reference to FIG. 4 which is a flowchart. Note that FIG. 4 shows only processing related to short circuit detection, and omits description of other processing (processing before detection, etc.).

  After the start of the PWM control, the short circuit detection means 32 confirms whether or not the detection timing is in accordance with a predetermined constant period (step S1). If it is not the detection timing, the process returns to step S1.

  When detecting in step S1, the short circuit detection means 32 detects the presence or absence of the electric current which flows through the wiring between the switching element 4 and the load 2 (step S2). When the current is detected, the counter 321 of the short-circuit detection means 32 counts (integrates) the number of continuous detections (step S3). If no current is detected, the counter 321 resets the count value (integrated value) (step S4) and returns to the step before step S1.

  Next, it is determined whether the count value (integrated value) of the counter 321 in step S3 is greater than a predetermined number corresponding to the duty ratio (step S5). If the count value (integrated value) is less than or equal to the predetermined number, the process returns to step S1.

  If the count value (integrated value) is greater than the predetermined number of times, there is a high possibility that the switching element 4 has failed and is in a short-circuited state. Therefore, for the sake of safety, the energization of the load 2 is cut off (step S6). . Then, an abnormality display is issued by a display means (not shown), and the user of the PWM control circuit 1 is notified of the occurrence of a short circuit (step S7).

  As mentioned above, although one embodiment was taken up and explained about the present invention, the present invention is not limited to the above-mentioned embodiment, and various changes are possible in the range which does not deviate from the gist of the present invention.

  For example, although a CPU is used as the control unit 3 in the present embodiment, an FPGA may be used instead of the CPU.

  In this embodiment, the PWM signal transmitting means 31 is provided in the CPU 3, but the PWM signal transmitting means 31 is connected to the CPU as indicated by a “PWM output element (in the case of external attachment)” indicated by a broken line in FIG. It may be provided separately.

  In the present embodiment, when the number of continuous times that the energized state is detected is greater than the predetermined number, the short-circuit detecting unit 32 determines that a short-circuit has occurred, but conversely, it does not detect the cut-off state (non-detection). When the number of continuous times is greater than the predetermined number, the short-circuit detection means may determine that a short-circuit has occurred.

  In addition, in the present embodiment, the detection cycle for detecting the presence / absence of current of the short-circuit detection means 32 is always constant during the execution of the PWM control, but can be changed. For example, it can be changed according to the fluctuation of the PWM cycle or duty ratio, or can be changed completely independently of these. However, even when the detection cycle is not constant as described above, it is essential that the detection cycle be shorter than the time during which the switching element is in the cutoff state.

1 PWM control circuit 2 Load 3 Control means, CPU
31 PWM signal transmission means 32 Short-circuit detection means 4 Switching element, transistor 5 Vcc (positive voltage power supply)

Claims (3)

PWM signal transmitting means for transmitting a PWM signal;
By the inputted PWM signal, a switching element for PWM controlling a load by performing the energization and interruption of the current through the connected load,
Short-circuit detecting means for detecting that the switching element is short-circuited,
The short circuit detection means is configured to detect energization (on) and interruption (off) between the switching element and the load ,
The detection is repeatedly performed in a cycle smaller than the shorter duration of the PWM signal in which the PWM signal is at the H level and the L level .
The short-circuit detecting means performs the interruption (off) between the switching element and the load when the number of consecutive times of detecting energization (on) between the switching element and the load is greater than a predetermined number of times. A PWM control circuit that determines that the switching element is short-circuited when the number of consecutive times not detected is greater than a predetermined number. The PWM control circuit according to claim 1, wherein the short-circuit detection unit repeatedly detects energization (ON) and interruption (OFF) between the switching element and the load at a constant period. Wherein the predetermined number of times, the time at which current is energized through the load based on the duty ratio of the PWM signal, blocking (off current and (on) between the switching element and the load in the short circuit detection means PWM control circuit according to claim 1 or 2 and the detection period, the number of times determined on the basis of the).

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