JP2014086066A - Power supply protection circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve the problem that a circuit scale becomes large and leads to cost increase in order to avoid these problems that a B-CAS card may short-circuit since an insertion slot of the B-CAS card requiring power is open to the outside, and reliability of components for supplying the power is reduced when a certain period of time is required until the short circuit is reached, when the short circuit is a halfway layer short circuit or when short circuiting/open circuiting is repeated.SOLUTION: A voltage for shifting a transistor 2 to saturation is determined by voltages mostly divided by a resistor 1 and a resistor 9 when a high level voltage is applied to an on/off terminal, but a voltage for shifting the transistor 2 to interception can operate a short circuit protection circuit from a high voltage increased by only insertion of a resistor 3.

Description

The present invention relates to an inexpensive control circuit for controlling power supply and a circuit for protecting a power supply from a short circuit.

  In 2011, it was decided that analog television broadcasting would be stopped. It shifts to digital television broadcasting after July 2011. The digital television broadcasting system can efficiently use an efficient band and can transmit a large amount of information. Digital television broadcasting requires a B-CAS (registered trademark) card in order to protect copyrights.

  If the B-CAS card is inserted into the B-CAS card holder of the receiver, the digital broadcast can be viewed. Since the viewer himself / herself has to insert the B-CAS card into the receiver, the receiver is provided with an insertion slot that can be inserted from the outside.

  In order to operate the B-CAS card, it is necessary to supply power to the card from the receiver side. A terminal that contacts the card is provided in the B-CAS card holder, and power for the B-CAS card is supplied to the terminal. Since the B-CAS card insertion slot in the receiver is open to the outside, there is a disadvantage that the power supply is short-circuited and parts are damaged if a coin or the like is inserted by mistake.

JP 2010-287196 A

  A controlled power supply is applied to a power supply terminal in the B-CAS card holder. Recognizing that a B-CAS card is inserted and selecting a broadcast, power is supplied to the B-CAS card.

  An example of the conventional circuit is shown in FIG. The power supply circuit for supplying power has a short-circuit protection function using three transistors, one diode, and five resistors.

  When the on / off terminal becomes high level to supply power, the emitter voltage of the transistor 105 is increased accordingly, and a voltage is applied to the resistor 101 by the current flowing through the resistor 106, and the voltage is divided by the resistor 109 to be A voltage is applied to the base of 102. The voltage is divided by the resistor 109 through the resistors 110 and 101 connected to the emitter of the transistor 105 and the base of the transistor 102 becomes high level, and the transistor 102 is saturated. When the transistor 102 is saturated, the base of the transistor 104 is pulled toward GND through the resistor 103, and the transistor 104 is saturated. Power is supplied from a resistor 107 connected to the collector of the saturated transistor 104.

  When Vout is short-circuited with this power supply, the voltage of Vout decreases. Along with this, the voltage applied to the resistor 101 through the resistor 110 is lowered by the diode 108, the transistor 102 is cut off, and the power supply is stopped. The diode 108 inserted into Vout, the resistor 110, and the resistor 101 is connected such that the load side is a cathode and the resistor 110 and the resistor 101 are anodes.

  If Vout is short-circuited instantaneously, the protection circuit in which the diode is inserted immediately works and no significant problem occurs in terms of reliability. However, to activate the protection circuit, the voltage applied to the anode of the diode 108 must decrease until the load becomes heavy and the Vout voltage decreases and the transistor 102 becomes lower than the saturated voltage.

  Therefore, assuming that the short-circuit state is a rare short, slowly short-circuiting, repeated short-circuiting / opening, and the like, the transistor 104 and the resistor 107 are actually Therefore, it was necessary to adopt a component having a larger power capacity than that required for power.

  In the above conventional circuit example, since the B-CAS card insertion slot is open to the outside, the supply of power is controlled and a protection function against a short circuit is provided. When the state is taken into account, there is a drawback that increases the circuit scale and increases the cost.

  The present invention has been proposed in view of the above-described conventional situation, and an object of the present invention is to provide a control circuit for a power source that supplies a safe and highly reliable B-CAS card.

  In a power supply circuit provided with a short-circuit protection circuit that detects the voltage of the power supply supplied to the B-CAS card and feeds back to the control circuit to stop the supply of power, the threshold value that activates the short-circuit protection can be arbitrarily set. Thus, the protection is activated in a state where the supply current is not excessive, and the load on the transistor and the resistor is reduced.

  By detecting and feeding back the voltage of the power supply supplied to the B-CAS card, and setting the threshold value of the short-circuit voltage that activates the protection arbitrarily, prevent deterioration of parts due to rare shorts, short-circuits, and repeated short-circuits / opens. Therefore, the circuit scale can be reduced, the cost can be reduced, and a highly reliable circuit can be realized.

Shows a circuit diagram of the present invention. Shows a conventional circuit diagram. The current-voltage characteristic diagram when the load current increases with respect to the load of the conventional circuit is shown. The current-voltage characteristic diagram when the load current increases with respect to the load of the present invention is shown. A current-voltage characteristic diagram in which the value of the resistor 3 is made larger than that of the circuit of FIG.

  A specific example of the present invention is shown in FIG. When the on / off terminal becomes high level to supply power, the emitter voltage of the transistor 5 is increased accordingly, and a voltage is applied to the resistor 1 by the current flowing through the resistor 6, and the voltage is divided by the resistor 9 and the transistor A voltage is applied to the base of 2. Transistor 2 is saturated and the base of transistor 4 is pulled to GND through resistor 3, transistor 4 is saturated and power is supplied to Vout through resistor 7.

  When Vout is short-circuited with this power supply, the voltage of Vout decreases. Accordingly, the voltage applied to the resistor 1 is lowered by the diode 8 and the transistor 2 is cut off, and the transistor 4 is cut off and the power supply is stopped. The diodes 8 inserted between Vout and the emitter of the transistor 5 are connected such that the load side is a cathode and the emitter side of the transistor 5 is an anode.

  When the on / off terminal is at a low level, the base voltage of the transistor 2 is divided by the resistor 1 and the resistor 9 and is therefore equal to or lower than the base-emitter voltage at the time of saturation, and the transistor 2 is saturated. There is no.

  When the voltage applied to the on / off terminal is sufficiently high and the transistor 2 is saturated, the resistor 3 is connected to the emitter of the transistor 2, so that the emitter current flowing through the emitter of the transistor 2 causes the transistor 2 to The emitter voltage increases and changes according to the value of the resistor 3.

  Therefore, compared with the case where the resistor 3 is not inserted, the threshold value at which the transistor 2 shifts from saturation to cutoff is increased by the increase in the base voltage of the transistor 2, and the transistor 2 is switched from cutoff to saturation. The voltage that shifts from saturation to cutoff is higher than the voltage that shifts.

  Here, as a side effect of protection from a high voltage, the power supply circuit may not start up when the capacitive load is large. It is necessary to charge the capacitive load to some extent and lighten the load. However, since this power supply circuit is a constant voltage power supply and the transistor 4 and the resistor 7 are inserted, if a current flows too much. The voltage drops and the protection works and does not function as a power source.

  However, since the transistor 5 is inserted as an emitter follower, it becomes a constant current source, and the capacitive load is charged through the diode 8. Further, since the transistor 5 is received with high impedance, the high level of the on / off terminal remains preserved, and if charged as much as necessary, the load becomes light and can function as a power supply circuit.

  FIG. 3 shows the current-voltage characteristics when the load current increases with respect to the load in the conventional circuit example, and FIG. 4 shows the characteristics of the present invention in which the resistor 3 is inserted rather than the circuit having the characteristics shown in FIG. FIG. 5 shows a characteristic in which the value of the resistor 3 is larger than that of the circuit having the characteristic shown in FIG.

  In the conventional circuit shown in FIG. 3, when the current flows about 340 mA, the voltage is about 1.2 V and the short circuit protection circuit works to stop the power supply, whereas the resistor 3 is inserted in the main circuit shown in FIG. In the invention, when the current flows about 240 mA, the voltage is about 2.4 V and the short circuit protection circuit is activated to stop the power supply.

  In addition, the characteristic made larger than the value of the resistor 3 of the circuit shown in FIG. 4 is shown in FIG. When the resistor 3 is enlarged, when the current flows about 180 mA, the short circuit protection circuit is activated and the power supply is stopped when the voltage is about 3.0V. Actually, the operation is guaranteed if a current of 20 mA or more flows between 4.5 V and 5.0 V, so there is no problem if a current of about 50 mA flows.

  As described above, the threshold value at which the protection circuit operates is changed according to the resistance value of the resistor 3. However, there is no change in the threshold value at which the power supply circuit shifts to the operation of supplying power.

  When the short-circuit load is removed, the voltage can be automatically restored by inputting the voltage increase due to the current supplied by the resistor 6 and the transistor 5 to the base of the transistor 2.

  Therefore, according to the present invention, by setting a resistance value by inserting a resistor in the emitter of a transistor that controls saturation / cutoff of a transistor that supplies power, the voltage at which the transistor shifts from saturation to cutoff is set arbitrarily high. It is possible to do. Therefore, by increasing the voltage to shift to shut-off, it is possible to reduce the power capacity of the transistors and resistors that supply power to suppress the flow of excess current, and the circuit scale can be reduced. Cost reduction is possible and reliability is maintained.

1, 3, 6, 7, 9, 101, 103, 107, 109, 110 resistors,
2, 4, 5, 102, 104 transistors,
8,108 diode

Claims (2)

A first transistor for controlling the supply of power and a current limiting resistor for limiting the current of the power output from the collector of the first transistor;
The collector of the second transistor that controls the saturation / cutoff of the first transistor is connected to the base of the first transistor,
Inserted between the first resistor connected to the base of the second transistor, the second resistor inserted in the base and emitter of the second transistor, and the emitter and GND of the second transistor. A third resistor,
A third transistor for inputting a signal for controlling the power supply circuit to the base, an emitter resistor connected to the power supply connected to the emitter of the third transistor;
The emitter of the third transistor and the base of the second transistor are connected by a first resistor, and are a diode inserted between the load side of the current limiting resistor and the emitter of the third transistor. Configured,
A power supply protection circuit using the third transistor as an emitter follower.   2. The power protection circuit according to claim 1, wherein a threshold value for shifting the second transistor from cutoff to saturation and a threshold value for shifting from saturation to cutoff are set to different values.

Images