JPH05344642A - Protective unit for semiconductor switching element - Google Patents

Abstract

PURPOSE:To realize positive protection against breakdown due to thermal runaway of a switching element without accompanying stoppage of function by limiting the duty of the semiconductor switching element when ON voltage thereof exceeds a predetermined level. CONSTITUTION:Drain-source voltage, i.e., ON voltage, at the time of turn ON of a switching element TR1 comprising a power MOSFET is taken out through a voltage detecting circuit comprising elements D1, C1, R1 and then compared with a voltage reference through an operational amplifier OP1. ON voltage exceeding the voltage reference is fed back to a PWM circuit IC 2 in order to regulate the output voltage. In other words, duty of the switching element TR1 is limited so that the switching element TR1 is not heated over a predetermined level. This realizes a protective unit for protecting a switching element positively without accompanying stoppage of function through simple wiring.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to overload protection for semiconductor switching devices such as MOS-FETs and bipolar transistors.

[0002]

2. Description of the Related Art In recent years, in power supply devices, motor control devices, etc., in order to save power and reduce the size and weight, the input voltage is controlled by a series control method in which the output voltage is directly controlled by a power semiconductor element which is a control element. The power semiconductor device has been switched to a duty ratio control system in which the power semiconductor device is turned on and off at a constant cycle and the voltage is controlled by adjusting the energization time.

In such a device, when the load is abnormal or when the load is heavier than the assumed load condition, the amount of heat radiation is insufficient with respect to the amount of heat generated by the power semiconductor element, and as a result, the device heats up, resulting in the worst case. In some cases, the power semiconductor element used is damaged. Further, even if there is no problem in the load condition, if the operating environment temperature is high, an equivalent failure may occur.

Therefore, in such a device, in order to prevent a failure due to heat generation, a temperature sensitive switch, a thermistor, or the like is attached to the radiation fin of the power semiconductor element, and the operating temperature of the power semiconductor element is measured. 2. Description of the Related Art A temperature detection protection circuit that stops circuit operation and protects a device is widely used.

FIG. 14 is a diagram showing a schematic configuration of a high-voltage power supply device conventionally used in an image forming apparatus using an electrophotographic system. In the figure, reference numeral 1 is a virtual load of the high voltage generating circuit 2, to which a high voltage output is supplied from the high voltage generating circuit 2, and its output voltage and current are controlled by a transistor Q1 which is a semiconductor switching element. 3 is a diode D connected to the collector of the transistor Q1
11 is a smoothing current detection circuit for obtaining the average current of the currents I1 and I2 flowing through the diode 11 and the diode D12 connected in antiparallel thereto.

A resistor R11 is connected in series with the diode D11, and a resistor R12 is also connected in series with the diode D12. A variable voltage source 4 for control is connected to the base of the transistor Q1 via a resistor R13, and a resistor R14 is connected between the base and emitter of the transistor Q1.

FIG. 15 is a block diagram showing the essential parts of the above-mentioned image forming apparatus. In the image forming process of this apparatus, first, the photosensitive drum 5 is rotationally driven in the direction of the arrow in the figure, and the charging device 6 gives a uniform surface potential distribution to the entire surface. Next, the photoconductor drum 5 is irradiated with an optical image corresponding to the original by an exposure device (not shown) to form an electrostatic latent image. Then, toner corresponding to the original image is attached by the developing device 7 to visualize the electrostatic latent image.

Next, the transfer paper 8 sent out in synchronism with the driving of the photosensitive drum 5 by a paper feeding device (not shown) is subjected to corona discharge having a polarity opposite to that of the toner from the non-image forming surface by the transfer device 9. Upon reception, the toner image is transferred from the photosensitive drum 5 to the transfer paper 8. Then, the electrostatic separation force of the transfer paper 8 with respect to the photoconductor drum 5 is reduced by the charge removal separator 10, and the photoconductor drum 5 is separated and discharged in the arrow direction.

Here, the transfer device 9 and the charge-eliminating device 1
The high-voltage control unit shown in FIG. 7 is used for 0. However, for example, the copy paper (transfer paper 8) in the virtual load 1 is jammed between the photoconductor drum 5 and each charging device, so that a short-circuit state occurs. When an overcurrent of a certain level or more flows at point P in FIG. 14, the detection signal is fed back to the high voltage generation circuit 2. As a result, the protection circuit provided in the high voltage generation circuit 2 itself operates, and one end is brought into an intermittent output state. At this time, if the short-circuited state disappears, the normal continuous operation state is immediately restored.

[0010]

However, in the conventional protection system, a temperature sensor is required to detect the temperature, and the temperature sensor is mounted on the radiation fin to which the power semiconductor element is attached. Since it is necessary, the wiring becomes complicated, and the number of parts also increases, resulting in a high cost. Further, a protection circuit using a temperature sensitive switch or the like has a problem that it takes a long time to recover because the operation is stopped when a certain set temperature is exceeded.

Further, in the power supply device as described above, it is possible to detect an excessive current flowing through the entire circuit when the output is short-circuited, but it is not possible to detect the operation mode of a single transistor which is a switching element. Therefore, for example, even in the normal operation mode, a breakdown current may flow through the transistor alone, but there is a problem in that the abnormal current cannot always be detected by the protection circuit included in the high voltage generation circuit itself. Furthermore, since high voltage continues to be generated even when the transistor is in a breakdown state, there is a great risk of damaging the transistor and there is a problem that the safety of the product cannot be maintained.

The present invention has been made in view of the above problems, and provides a semiconductor switching element protection device which is simple in wiring, can reliably protect the switching element, and does not cause an operation stop. Is provided.

[0013]

According to the present invention, in order to achieve the above object, a protection device for a semiconductor switching element is constructed as follows.

(1) A semiconductor switching element for performing on / off control, an on-voltage detecting means for detecting an on-voltage of the semiconductor switching element, and a semiconductor switching element of the semiconductor switching element when an output of the on-voltage detecting means exceeds a predetermined value. A semiconductor switching element protection device comprising: a duty limiting means for limiting a duty.

(2) In a semiconductor switching element protection device of a high voltage generating circuit, a detecting means for detecting a current flowing in the semiconductor switching element and a voltage applied to the switching element, and the switching element based on an output signal of the detecting means. And a discriminating means for discriminating the operating region of the semiconductor switching device, and controlling the operation of the switching device according to the discriminating signal.

(3) A semiconductor switching element protection device in which the operation mode of the high voltage generating circuit is controlled by an identification signal from an identification means.

[0017]

With the configuration (1), when the ON voltage of the semiconductor switching element exceeds a predetermined value, the duty of the semiconductor switching element is limited and overload is prevented.

With the configuration (2), the current flowing through the switching element and the voltage applied thereto are detected, and the breakdown state of the switching element is detected from the detection signal to identify the operating region. Then, the operation of the switching element is controlled by the identification signal.

[0019]

EXAMPLES The present invention will be described in detail below with reference to examples.
FIG. 1 is a circuit diagram of a "switching regulator" according to the first embodiment. In FIG. 1, TR1 is a semiconductor switching element which is a MOS-FET, and T1 is a converter transformer which performs voltage conversion. D1 is a V _DS (drain-source voltage) detecting diode of the switching element TR1, C1 is a V _DS storage capacitor of the switching element TR1, and R1 is a bias resistor of the capacitor C1. D3 and D4 are rectifying diodes, and the choke coil CH1 and the capacitor C3 form a filter. R8 and R6 are output voltage dividing resistors, and IC1 is a voltage feedback element. C2 is an electrolytic capacitor for absorbing power supply ripple.

PH1 is an insulating photocoupler, and the output thereof controls the PWM circuit by the IC2 to generate a required pulse to drive the gate of the switching element TR1. Operational amplifier OP1 and elements D2, R2 and R3
Performs the feedback by comparing the drain-source voltage V _DS with a predetermined value.

Next, the operation will be described. When an input voltage is applied to the input terminal, the PWM circuit IC2 operates, the pulse width existing in the gate of the power MOS-FET of the switching element TR1 is applied, the MOS-FET turns on and off, and the primary of the converter transformer T1. Chopping current flows in the winding. As a result, a chopping voltage is generated across the secondary winding, is rectified by the rectifying diodes D3 and D4, is smoothed by the LC filters of the elements CH1 and C3 to be a DC voltage, and a DC voltage is generated between the output terminals.
The voltage generated here is input to the voltage detection element of the voltage feedback element IC1 from the voltage dividing circuit of the resistors R8 and R6, and the error signal thereof is passed through the photocoupler PH1 to PW.
The output voltage is stabilized by being fed back to the M circuit IC2 and controlled such that the pulse width is narrowed when the output voltage is high and widened when the output voltage is low.

The drain voltage and drain current waveforms of the MOS-FET, which is a switching element, are as shown in FIG. 2. As shown by the solid line when the load current is large and by the broken line when the load is light. Change.

Further, in the MOS-FET, as shown in FIG. 3, when the drain current increases, the drain-source voltage V _DS rises, and even if the case (junction) temperature of the MOS-FET rises, the drain-source voltage increases. The voltage V _DS rises. Here, when the MOS-FET is switching, the action of the detection diode D1 causes the capacitor C
When the on-voltage of the switching element TR1 is higher than the voltage of 1, the detection diode D1 blocks the voltage, so that the voltage of the capacitor C1 is charged by the resistor R1 and the voltage of the capacitor C1 continues to rise slowly. When the ON voltage of the switching element TR1 becomes lower than the voltage of the capacitor C1, the detection diode D1 is turned on to make the voltage of the capacitor C1 substantially equal to the ON voltage.

Thus, in the V _DS waveform of FIG. 2, MO
Drain-source voltage V _DS when the S-FET is turned on
Can be taken out by using the voltage detection circuit composed of the elements D1, C1 and R1, and the detected voltage waveform is the VC voltage waveform shown in the figure.

The VC voltage detected here is the MOS-F.
Since it is proportional to the drain-source saturation voltage of ET, that is, the ON voltage, this voltage is compared with the reference voltage by the operational amplifier OP1, and when the ON voltage exceeds the reference voltage, it is input to the PWM circuit IC2 to adjust the output power, that is, switching. By limiting the duty of the element TR1, it is possible to prevent the heat generation of the switching element TR1 from increasing above a certain level.

(Second Embodiment) FIG. 4 is a circuit diagram of the second embodiment. In this embodiment, a bipolar transistor TR2 is used as a switching element.

The saturation voltage characteristic of the bipolar transistor as a switching element also has a positive temperature coefficient in the large current region as shown in FIG. 5, so that VCE when the transistor in the switching waveform is turned on as in the MOS-FET _.
The saturation voltage, that is, the ON voltage is detected, and the result of comparing the detected voltage with the reference voltage is fed back to the PWM circuit IC2 to adjust the output power, whereby the heat generation of the bipolar transistor TR2 can be limited.

(Third Embodiment) FIG. 6 is a circuit diagram of the third embodiment. The present embodiment is an example in which a bipolar transistor is used as a switching element and is used in a flyback converter.

The flyback converter has the following features:
Although it has the features that the circuit configuration is simple and the number of parts is small because it is possible to convert the power only by the transformer, there is a drawback that the overcurrent protection operating point changes due to the change of the input voltage. With the same protection characteristics as the type, when the input voltage rises, the output power becomes too large and it becomes difficult to set the protection operating point. In order to solve the problem, a voltage obtained by dividing the input voltage is added to the V _CE detection voltage VC, and a voltage dividing circuit made up of resistors R9 and R10 is added to reduce the protection operating point in inverse proportion to the input voltage. The dependence of the output power adjustment point on the input voltage can be eliminated, and highly accurate protection can be performed.

(Embodiment 4) FIG. 7 shows an example in which an element is added in series to the detection diode D1 in comparison with the embodiment 1.

In this embodiment, when a parasitic LC due to a pattern or wiring is added to the drain terminal of the MOS-FET, the drain-source voltage V _DS waveform oscillates and a voltage different from the actual on-voltage. In order to prevent the occurrence of voltage at the VC terminal, a time constant element L1 is added in series with the detection diode D1 to measure the ON voltage more accurately.

Here, as the series element, a coil, a resistor, or the like is used, and by setting a constant of a time constant longer than the time of parasitic oscillation, V proportional to the accurate ON voltage is obtained.
The C terminal voltage is obtained, and the voltage is hooded back to the PWM circuit IC2 to protect the switching element from overload.

(Fifth Embodiment) FIG. 8 is a circuit diagram showing a fifth embodiment of the present invention, and the same reference numerals as those in FIG. 14 indicate the same components. In the figure, reference numeral 1 denotes a virtual load of the high voltage generation circuit 2, to which an AC high voltage power supply 2a of the high voltage generation circuit 2 supplies a high voltage output controlled by a transistor Q1 which is a switching element. Reference numeral 3 denotes a smoothing current detection circuit for obtaining an average current of the current I1 flowing through the diode D11, that is, the transistor Q1 and current I2 flowing through the diode D12, and 4 is a variable voltage source for controlling the transistor Q1. Further, R11 to R15 in the figure are resistors.

FIG. 9 shows an operation detecting circuit for detecting an abnormal state (breakdown state) of the above-mentioned transistor. The voltages Va, Vb and Vc at points a, b and c in FIG. 1 are inputted. This circuit includes resistors R16 to R23 for voltage division, comparators 11 and 12 for comparing each divided voltage, and an AND gate 1.
3 and 3. This circuit also detects a current flowing through the transistor Q1 and a voltage applied to the transistor Q1, and a detecting means 15 for detecting the breakdown state of the transistor Q1 from the detection signal to identify the operating region. And AND
The identification signal output from the gate 13 is input to the CPU. Then, the operation mode of the high voltage generating circuit 2 is controlled by the identification signal, and the high voltage generating circuit 2 is protected.

FIG. 10 is a block diagram showing a schematic structure of an electrophotographic image forming apparatus provided with the high voltage unit 16 having the structure shown in FIGS. In the figure, 17 is the above-mentioned CPU which is the main controller of the main body of the apparatus, and is connected to the high voltage unit 16 by each line of 24V, 5V and GND and each control signal line. Reference numeral 18 is a control system for performing key input and liquid crystal display, 19 is an optical system for performing exposure in the image forming process, 20 is a drive system for operating a motor, and 21 is a peripheral unit such as application equipment.

FIG. 11 is a flow chart showing the outline of the operation in the fifth embodiment. Here, the breakdown state of the transistor Q1 in FIG. 8 is detected, and the high voltage generation is stopped or the power is supplied to the unit. It shows the flow until the cancellation. That is, when the transistor Q1 is in the splay state in step S2 while the high voltage unit 16 is operating in step S1, this is detected by the circuit of FIG. 9, and the high voltage generation of the high voltage generation circuit 2 is stopped in step S3. Alternatively, the power supply of the 24V control power supply is stopped.

Here, the potential V at each point a, b, c in FIG.
a, Vb, and Vc are input to the detection circuit of FIG. 9, divided by resistors connected in series, and then input to the comparators 11 and 12. At this time, the resistors R16, R17, R18, and R
If the division factors of 19, R20, R21, R22, and R23 are appropriately selected and are A, B, C, and D, respectively, the breakdown state of the transistor Q1 can be defined as the time when the following two equations are compatible. AVa-BVb> 0 CVa-DVc> 0 This shows a state in which the emitter current flows while the base-emitter of the transistor Q1 is reverse biased. Then, the above-mentioned identification signal output at this time is output to the CPU 17 through the breakdown detection line. As a result, the CPU 17 outputs a command for stopping the generation of high voltage to the high voltage unit 16, and the transistor Q1
Deterioration and damage are prevented. At this time, the operation mode of the high voltage generation circuit 2 may be controlled by the identification signal.

As described above, when the transistor Q1 is in the breakdown state and deviates from the safe operation area, the protection operation can be immediately performed, the transistor Q1 can be prevented from being damaged, and the safety of the product can be maintained.

In general, a transistor has an effect of accumulating carriers at the base and is in the breakdown state for a minute time when the forward bias is changed to the reverse bias.
A time constant circuit in consideration of this is actually required, but it is omitted here because it is not essential here.

(Sixth Embodiment) FIG. 12 is a circuit diagram of a sixth embodiment of the present invention, in which (a) is a detection circuit similar to that of FIG. 9, and (b) is a detection circuit.
Indicates a switch circuit for turning off the 24V supply switch of the high voltage unit 16 by the signal output from this detection circuit. The rest of the configuration is similar to that of the fifth embodiment.

In this embodiment, when the transistor Q1 is in the breakdown state as described above, the AND gate 13 in FIG. 12A outputs "H" as an identification signal.
A (high level) signal is output, and this signal is input to the switch circuit in (b). As a result, the transistor Q
2 is turned off, the 24V control power supply of the high voltage unit 16 is cut off, and the operation of the high voltage generation circuit 2 is stopped. Therefore, the deterioration and damage of the transistor Q1 can be prevented as in the above-described embodiment.

When the transistor Q1 is in the normal state, the output signal of the circuit of FIG. 12 (a) is "L" (low level), and the transistor Q2 of FIG. 12 (b) is on.
The high voltage generation circuit 2 operates normally. In the circuit of FIG. 12B, R24 to R26 are dividing resistors.

(Embodiment 7) FIG. 13 is a circuit diagram of Embodiment 7 of the present invention, in which the transistor Q in each of the aforementioned embodiments is used.
1 shows the configuration of the operation detection circuit No. 1. In the figure, T11 is a transformer having a ratio of the number of windings of 1: 2, and 23 is a rectifying circuit having a certain time constant, which is composed of a diode D13, a resistor 27 and a capacitor C11. 24 is each resistance R
28, R29, R30, and R31 are comparators that compare the divided voltages.

The variable voltage source 4 of FIG. 13 has the CPU of FIG.
When the control value from 17 is input and a current of a certain level or more flows through the transistor Q1, the comparator 24 outputs a signal of "H". This signal is input to the CPU 17 or the switch circuit of FIG. 12B similarly to the above, whereby the operation of the high voltage generation circuit 2 is stopped, and similarly, the deterioration and damage of the transistor Q1 are prevented.

In the power supply system of this embodiment, it is assumed that the CPU outputs a control signal to the transistor Q1 for constant current, so that the CPU itself recognizes the bias state of the transistor Q1. If so, the CPU uses the transistor Q
It is possible to identify one breakdown state.

[0046]

As described above, according to the present invention,
By limiting the duty of the switching element when the on-voltage of the switching element exceeds a predetermined value, it is possible to reliably protect the switching element against damage due to thermal runaway without stopping the operation.

Further, since the current detection resistor for detecting the overcurrent is not necessary, the power efficiency is improved.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a first embodiment.

[Figure 2] Voltage and current waveform diagram of switching element

FIG. 3 is an operation explanatory diagram of the first embodiment.

FIG. 4 is a circuit diagram of a second embodiment.

FIG. 5 is an operation explanatory diagram of the second embodiment.

FIG. 6 is a circuit diagram of a third embodiment.

FIG. 7 is a circuit diagram of a fourth embodiment.

FIG. 8 is a circuit diagram of a fifth embodiment.

FIG. 9 is a circuit diagram of the operation detection circuit of the transistor of FIG.

FIG. 10 is a schematic configuration diagram of an image forming apparatus.

FIG. 11 is a flowchart showing the operation of the fifth embodiment.

FIG. 12 is a circuit diagram of the sixth embodiment.

FIG. 13 is a circuit diagram of Example 7.

FIG. 14 is a circuit diagram of a conventional example.

FIG. 15 is a configuration diagram of main parts of an image forming apparatus using an electrophotographic method.

[Explanation of symbols]

 2 high voltage generation circuit 14 detection means 15 identification means TR1 switching element Q1 transistor (switching element)

Claims (3)

[Claims] 1. A semiconductor switching element for performing on / off control, an on-voltage detection means for detecting an on-voltage of the semiconductor switching element, and a duty of the semiconductor switching element when an output of the on-voltage detection means exceeds a predetermined value. And a duty limiting means for limiting the duty ratio of the semiconductor switching element. 2. A protection device for a semiconductor switching element of a high voltage generating circuit, wherein a detection means for detecting a current flowing through the semiconductor switching element and a voltage applied to the switching element, and an output signal of the detection means are used to detect the switching element. A protection device for a semiconductor switching element, comprising: an identification unit that detects a breakdown state and identifies an operation region thereof, and controls the operation of the switching element by the identification signal. 3. The protection device for a semiconductor switching element according to claim 2, wherein the operation mode of the high voltage generating circuit is controlled by an identification signal from an identification means.