JPH0583932A - Overcurrent protective circuit - Google Patents

Abstract

PURPOSE:To obtain stable protection characteristics even in a power controller, in which the ON-OFF duty ratio of a switching element is varied over a wide range. CONSTITUTION:An overcurrent protective circuit has a current detecting resistor 11 being connected in series with a switching element 8 and detecting currents flowing through the switching element 8, CR filters 12, 13 removing noises induced in the current detecting resistor 11, a main circuit 5 setting the switching element 8 under a non-drive state when the detecting value of the current detecting resistor 11 detected through the CR filters 12, 13 exceeds a fixed value and a discharging circuit 16 discharging the charges of the capacitor 13 for the filter for the non-drive period of the switching element 8.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an overcurrent protection circuit for a switching element used in a switching type power control device.

[0002]

2. Description of the Related Art An example of a conventional overcurrent protection circuit will be described with reference to FIG. The figure shows a part of a switching type power control device having an overcurrent protection circuit. In the figure, 1 is an oscillation circuit, 2 is a duty ratio setting voltage input terminal, 3 is a PWM comparator, 4 is a drive circuit, 5 is an overcurrent protection circuit main body as a main circuit, and 6 is an input line for an overcurrent latch release signal. , 7 is an output line of the output pulse width limiting signal, 8 is a switching element such as MOSFET, 9 is a load such as a transformer or a motor, 10 is a power supply terminal, 11 is a current detection resistor, 12 is a filter resistor, and 13 is a filter. For capacitors. A CR filter is constituted by the filter resistor 12 and the filter capacitor 13. The CR filters 12 and 13 are provided to remove high-frequency noise components induced in the current detection resistor 11 that cause malfunction.

The power control device compares the triangular wave output from the oscillation circuit 1 with the voltage at the duty ratio setting voltage input terminal 2 by the PWM comparator 3 to set the output pulse width, and the drive circuit 4 sets the switching element 8 in place. To drive. By determining the output pulse width, the output voltage and output current are controlled in the case of a switching power supply device, and the rotation speed is controlled in the case of a drive circuit such as a motor.

In such a power control device, the overcurrent protection circuit has a function of protecting the switching element 8 from destruction due to overcurrent. This overcurrent protection circuit is called a pulse-by-pulse overcurrent protection circuit, and the limiter operates every pulse. Then, when an overcurrent flows through the switching element 8, the detection voltage of the current detection resistor 11 increases and enters the overcurrent protection circuit body 5 through the CR filters 12 and 13. Overcurrent protection circuit body 5
When the detected voltage exceeds a predetermined set value, regardless of the output of the PWM comparator 3, the output of the drive circuit 4 is instantly turned off and the latch is applied. Then, at the next turn-on, the latch is released and the output of the drive circuit 4 is turned on again. FIG. 5 shows how the circuit operates. FIG. 5A is a waveform of a current flowing through the current detection resistor 11,
(B) is a detected voltage waveform after passing through the CR filter. When the current of the switching element 8 exceeds the limit value, the output of the drive circuit 4 is turned off as shown in FIG. At this time, the detected voltage waveform after passing through the CR filters 12 and 13 has a blunted waveform as shown in FIG. 7B, but the noise component causing the malfunction is removed. In the case of a normal switching power supply device, the maximum duty ratio of the switching element 8 is set to about 50% or less, so that the filter capacitor 13 is discharged during the off period of the charge detection voltage.

However, as shown in FIG. 6, the duty ratio of the current may reach 100% depending on the type of the load 9. In this case, the detected voltage waveform is irregularly limited, as shown in FIGS. this is,
This is because the off period of the detection voltage is shortened and the filter capacitor 13 cannot be completely discharged each time. This can be dealt with to some extent by adjusting the time constants of the CR filters 12 and 13, but malfunctions are likely to occur.

[0006]

In the conventional overcurrent protection circuit, in the case of the power control device in which the ON / OFF duty ratio of the switching element is large and the conduction period is long, the electric charge of the filter capacitor is completely discharged. However, if the next detection voltage pulse occurs with the electric charge remaining, the set value of the overcurrent protection circuit body will be reached in a faster time than the previous pulse, and the operating time of the overcurrent protection circuit will vary. Occurs. Then, depending on the case, there is a problem that oscillation or the like occurs and the device does not operate normally.

Therefore, an object of the present invention is to provide an overcurrent protection circuit which exhibits stable protection characteristics without malfunction even in a power control device in which the on / off duty ratio of a switching element is variable over a wide range. ..

[0008]

In order to solve the above-mentioned problems, the present invention provides an overcurrent protection circuit for a switching element in an apparatus for controlling output power by driving the switching element, the circuit being in series with the switching element. A current detection resistor connected to detect the current flowing in the switching element, a CR filter for removing noise induced in the current detection resistor, and a detection value of the current detection resistor taken out through the CR filter have a predetermined value. It is characterized in that it has a main circuit for setting the switching element to a non-driving state when exceeding and a discharging circuit for discharging the electric charge of the capacitor forming the CR filter during the non-driving period of the switching element.

[0009]

In the above structure, the charge of the capacitor of the CR filter is immediately discharged during the non-driving (non-conducting) period of the switching element. Therefore, when the next driving of the switching element is started, the detected value from the current detection resistor in the main circuit The potential of the input terminal always starts rising from zero V. Therefore, the main circuit operates in a stable cycle every time, and no malfunction occurs.

[0010]

Embodiments of the present invention will be described below with reference to the drawings.

1 and 2 are views showing a first embodiment of the present invention. In addition, in FIG. 1 and a diagram showing a second embodiment described later, the same or equivalent elements as the devices and elements in FIG. 4 are denoted by the same reference numerals as those described above, and a duplicate description will be omitted.

This embodiment is applied to a driving device for controlling the speed of a DC motor.

In FIG. 1, 14 is a DC motor, 15 is a freewheel diode, and 16 is a discharge transistor. The discharge transistor 16 discharges the electric charge of the filter capacitor 13 during the non-driving period of the switching element 8. A discharge circuit is configured. The output signal of the PWM comparator 3 is directly applied to the base of the discharging transistor 16 and is driven by a signal having a phase opposite to that of the gate of the switching element 8.

The driving device is such that the switching element 8 is turned on.
By changing the off duty ratio, the DC motor 1
4 speed control. By changing the on / off duty ratio from zero to 100%, the DC motor 14
It is possible to control from the stop to the maximum rotation speed.

In such a driving device, the overcurrent protection circuit includes the discharge transistor 16 and the switching element 8.
When the switching element 8 is turned off, the discharge transistor 16 is turned on by driving with a signal having a phase opposite to that of the drive signal, and the electric charge of the filter capacitor 13 can be immediately discharged. Therefore, the potential of the filter capacitor 13 starts from zero V for each pulse of the detection voltage of the current detection resistor 11, and an irregular operation occurs for each pulse of the detection voltage as in the conventional overcurrent protection circuit. Disappear. It should be noted that, although it depends on the time constant of the CR filter, the peak of the discharge current flowing in the discharging transistor 16 is suppressed,
The value of the filter capacitor 13 may be reduced, and in some cases, a resistor of about several Ω may be inserted in series with the collector of the discharging transistor 16 to protect it from the discharge current.

FIG. 2 shows operation waveforms at this time. 2A is the current waveform at point c in FIG. 1, and FIG. 2B is the detected voltage waveform at point d. Even if the duty ratio of the current flowing through the switching element 8 reaches close to 100%, as shown in FIG. 2B, when the switching element 8 is turned off, the potential at the point d instantly becomes zero V and becomes zero at the next turn-on. Rise from V. In particular, when driving a motor load or the like, the current waveform becomes close to a square wave, and the potential of the filter capacitor 13 rises quickly. Therefore, the electric charge is discharged at each pulse of the detection voltage, and the filter capacitor 1 is discharged.
Setting the potentials at both ends to zero V is extremely effective in ensuring normal operation.

Next, FIG. 3 shows a second embodiment of the present invention. The present embodiment is applied to a drive device that controls the speed of an AC motor.

In FIG. 3, 17 is a bridge diode, 18 is an AC motor, 19 is a bidirectional freewheel circuit, and 20a and 20b are AC power input terminals. Also in the drive device for driving the AC motor 18 as shown in the figure, the overcurrent protection circuit of the switching element 8 is configured similarly to the first embodiment.

Operation as a driving device for controlling the speed of the AC motor 18 by varying the ON / OFF duty ratio of the switching element 8 and the discharging transistor 16
The operation of the overcurrent protection circuit provided with is almost the same as the operation of the first embodiment.

Although each of the above-described embodiments has been described with respect to the motor drive device, in addition to this, in a switching type power control device such as a power supply device or an inverter, a discharge transistor is used as in the above embodiment. The overcurrent protection circuit of the present invention having is effective.

[0021]

As described above, according to the present invention,
Since the discharge circuit that discharges the electric charge of the capacitor that forms the CR filter is discharged during the non-driving period of the switching element, the potential of the detection value input terminal from the current detection resistor in the main circuit must be zero at the start of the next driving of the switching element. Start rising from V. Therefore, even in the power control device in which the on / off duty ratio of the switching element can be varied over a wide range, stable protection characteristics can be obtained without malfunction.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a partial block diagram of a first embodiment of an overcurrent protection circuit according to the present invention.

FIG. 2 is a timing chart showing detected voltage waveforms and the like for explaining the operation of the first embodiment.

FIG. 3 is a circuit diagram showing a partial block of a second embodiment of the present invention.

FIG. 4 is a circuit diagram showing a partial block of a conventional overcurrent protection circuit.

FIG. 5 is a timing chart showing detected voltage waveforms and the like for explaining the operation of the conventional example.

FIG. 6 is a timing chart showing detected voltage waveforms and the like for explaining the problems of the conventional example.

[Explanation of symbols]

5 Overcurrent protection circuit main body (main circuit) 8 Switching element 11 Current detection resistor 12 Filter resistor 13 Filter capacitor that constitutes a CR filter together with filter resistor 16 Discharge transistor that constitutes a discharge circuit

Claims (1)

[Claims] 1. An overcurrent protection circuit for a switching element in an apparatus for controlling output power by driving a switching element, comprising: a current detection resistor connected in series to the switching element to detect a current flowing through the switching element. A CR filter for removing noise induced in the current detection resistor, and a main circuit for setting the switching element in a non-driving state when a detection value of the current detection resistor taken out through the CR filter exceeds a predetermined value And an electric discharge circuit for discharging the electric charge of the capacitor forming the CR filter during the non-driving period of the switching element.