JPH0783589B2 - Switching regulator overcurrent protection circuit - Google Patents

Description

The present invention relates to an overcurrent protection circuit for protecting an output circuit of a switching regulator from an overcurrent due to a short circuit or the like.

[Conventional technology]

Fig. 4 shows a conventional example of a switching regulator in which an intermittent DC current is passed through the primary coil of a transformer, a capacitor is charged with the voltage obtained at the output coil of this transformer, and this charging voltage is used as a DC stabilized power supply. .

In FIG. 4, the main part of the regulator circuit is composed of a DC power supply 1, a transformer 2, a main current circuit 3, a control circuit 4, a control transistor 5, an output circuit 6, and the like. The DC power supply 1 rectifies and smoothes the AC power supply and outputs a DC voltage E _i .
The transformer 2 has a primary coil 2a, a control coil 2b, and an output coil 2c.
The series circuit of the primary coil 2a, the main transistor 7 whose collector is connected to the primary coil 2a, and the current detection resistor 8 forms the main current circuit 3, and the primary coil 2a side is connected to the + terminal of the DC power supply 1. The resistor 8 side is connected to its negative terminal. The following control circuit 4 is connected to both ends of the control coil 2b. That is, a circuit in which the series circuit of the diode 9 and the capacitor 10 is connected to both ends of the control coil 2b, and the resistor 13 is connected to the parallel circuit of the diode 11 and the capacitor 12 between one end of the control coil 2b and the base of the main transistor 7. Are connected. A Zener diode 14 is connected between the connection point of the diode 9 and the capacitor 10 and the base of the main transistor 7, and the base of the main transistor 7 is connected to the + terminal of the DC power supply 1 via the starting resistor 15. There is. The control transistor 5 has a collector at the base of the main transistor 7 and an emitter at the DC power supply 1
, And the base thereof is connected to the connection point between the main transistor 7 and the current detection resistor 8 via the resistor 16. The output circuit 6 is composed of a series circuit of a diode 17 connected to both ends of the output coil 2c and an output capacitor 18, and a load 19 is connected to both ends of the capacitor 18.

When a voltage is output from the DC power supply 1, a base current flows through the starting resistor 15 to the base of the main transistor 7, and the main transistor 7 starts to conduct. Then, the current I begins to flow in the primary coil 2a of the transformer 2 and the voltage is induced in the control coil 2b, increasing the base current via the diode 11 and the resistor 13, so that the main transistor 7 is momentarily saturated and conductive. Reach the state. Therefore, the current in the primary coil 2a linearly increases, and a voltage drop proportional to the current I is obtained in the current detection resistor 8. This voltage drop exceeds the weir layer voltage between the base and emitter of the control transistor 5, and the control transistor 5 conducts and shunts the base current of the main transistor 7 flowing through both resistors 13 and 15, so that the main transistor 7
Goes into the unsaturated state. As a result, the current flowing through the primary coil 2a decreases, so the voltage of the control coil 2b also decreases, and the resistance
The current flowing through 13 decreases and the main transistor 7
Current I is cut off instantaneously. Then, the base current starts to flow again through the resistor 15 into the main transistor 7, and the above operation is repeated. When the current I is cut off, the transformer 2
A voltage in the direction opposite to the current I is induced in each of the coils 2a, 2b, 2c. The output coil 2c is the main transistor 7
Releases the energy stored in the transformer 2 when
It is rectified by the diode 17 and charged in the capacitor 18, and the current I _o is supplied to the load 19. Here, the voltage of the control coil 2b and the voltage of the output coil 2c are proportional to the turns ratio because the coils 2b and 2c are wound on the same iron core, and both capacitors charged by rectifying these voltages are charged. The same applies to the voltages of 10 and 18. Therefore, the voltage between the terminals of the capacitor 10 is monitored as the DC reference voltage E _c , and if the voltage E _c becomes high, the Zener diode 14 absorbs the base current of the main transistor 7 to reduce the current I, and the voltage E _c becomes the output voltage E _o if stopped absorbing voltage E _c to the constant base current becomes lower can also be used as a stabilized power supply apparatus becomes constant.

[Problems to be Solved by the Invention]

By the way, this switching regulator interrupts and regulates the primary current I of the transformer as already described, and the purpose thereof is to transfer constant power to its output side through the transformer. Since the current I is interrupted so that the product of the output voltage E ₀ and the load current I ₀ is substantially constant, the current I is permanently suppressed when, for example, a short circuit occurs on the load side,
Or you cannot cut off. Therefore, the output voltage E _o
And when the relationship between the load current I _o is gradually increased load current I _o, as shown in FIG. 5, the voltage E _o is substantially constant course in the following rated current with increasing current I _o even more rated current Gradually decreases. Moreover, for the same output voltage E _o by the level of the supply voltage E _i, different load current I _o, the curve a when the power supply voltage E _i is high, the curve b shows the case the power supply voltage E _i is low. Therefore, when the load is short-circuited, the load current
I _o is more than 200% of the rated current, which has the drawback of causing deterioration and damage of transformers and diodes.

Therefore, an object of the present invention is to eliminate the drawbacks of the conventional device described above, cut off the current in the primary coil of the transformer when the load current exceeds the rated value, and prevent the power from being transmitted to the load side. It is an object of the present invention to provide a switching regulator in which a current blocked so as not to be transmitted to a power supply is not influenced by a power supply voltage.

[Means for Solving the Problems]

In order to achieve the above-mentioned object, the present invention provides a main current circuit in which a primary coil of a transformer, a main transistor, and a current detection resistor are connected in series, a control coil wound around the transformer, and the main transistor while conducting. A control circuit for controlling a base current of the main transistor with a voltage induced in the control coil, and an overcurrent protection circuit for a switching regulator including a control transistor that suppresses the current of the current detection resistor when the current exceeds a predetermined value. There is a negative voltage obtained by converting the voltage induced in the control coil into a direct current through a series circuit of a Zener diode and a resistor, and a positive voltage induced in the control coil is a Zener diode and a resistor different from the above. Is applied to the base of the control transistor via a series circuit of And wherein the changing in accordance with the voltage induced in the control coil.

[Action]

When the load is short-circuited and the load current rapidly increases, the output voltage decreases and the voltage induced in the control coil decreases.
As a result, the negative voltage obtained by converting the voltage induced in the control coil into a direct current is reduced, so the current that shunts the base current of the control transistor via the series circuit of the Zener diode and the resistor is reduced, and the overcurrent is detected. The level is lowered, and the base current of the main transistor is drawn by the control transistor so that the main transistor becomes non-conductive. Also,
When the input voltage rises, the voltage induced in the control coil also rises, and the positive voltage induced in the control coil is exceeded by the base current of the control transistor that is applied via the series circuit of the Zener diode and the resistor. The current detection level decreases and the main transistor becomes non-conductive.

〔Example〕

An embodiment of the present invention will be described in detail with reference to FIGS. Here, parts and circuits that have the same functions as those shown in FIG. 4 are assigned the same reference numerals as those in FIG. 4 and their explanations are omitted.

In FIG. 1, the connection of the DC power supply 1, the transformer 2, the main current circuit 3, the control circuit 4, the control transistor 5, the output circuit 6 and the like as a switching regulator is exactly the same as the conventional one, so this description will be omitted. Omit it. The difference between this switching regulator and the conventional one is that the voltage induced in the control coil 2b is applied to the base of the control transistor 5, and the connection point between the diode 9 and the capacitor 10 and the control described in FIG. An overcurrent bias circuit including a series circuit of a Zener diode 20 and a resistor 21 is connected between the base of the transistor 5 and the Zener diode 20. With this configuration, the voltage across the capacitor 10 is applied as a negative bias voltage to the base of the control transistor 5 via the Zener diode 20 and the resistor 21, whereby the overcurrent detection level of the control transistor 5 becomes a negative bias voltage. It is increased by the voltage. In this state, the main transistor 7 repeats the switching operation to keep the output voltage E _o or the voltage E _c constant. Here, if the load current I _{o of the} load 19 _suddenly increases, the output voltage E _o drops, the voltage E _c also drops, and the negative bias voltage applied to the base of the control transistor 5 drops, so that the control transistor 5, the base current increases, that is, the overcurrent detection level decreases. When the voltage E _c drops to the Zener voltage of the Zener diode 20, the Zener diode 20 becomes non-conductive, and the negative bias voltage of the control transistor 5 disappears. Therefore, the current of the control transistor 5 increases and the base current of the main transistor 7 increases. It stops flowing and the current I of the main transistor 7 is cut off. In this case, the current I of the main transistor 7 is cut off because the current of the control transistor 5 is increased, so that even if the current of the starting resistor 15 flows, this current is
To the base of the main transistor 7,
The main transistor 7 does not become conductive again, power is no longer transferred to the control coil 2b and the output coil 2c, and the output voltage E _o is the curve a shown in FIG. 2 (when the power supply voltage E _i is high). Alternatively, as in the curve b (when the power supply voltage E _i is low), the current I _o decreases sharply with a slight increase.

Next, the present embodiment is also different from the conventional device in that a power supply voltage compensating circuit including a Zener diode 22 and a resistor 23 is connected between the control coil 2b and the base of the control transistor 5. In this power supply voltage compensating circuit, when the power supply voltage E _i is high, the Zener diode 22 conducts earlier than when the power supply voltage E _i is low, and the base current of the control transistor 5 flows earlier. Therefore, the output voltage E _o drops relatively quickly. As shown in FIG. 3, the distance between the curve a and the curve b is reduced, the influence of the level of the power supply voltage E _i is reduced, and the power supply voltage is substantially constant within the rated value range. .

〔The invention's effect〕

As described above, according to the present invention, the primary coil of the transformer, the main transistor, and the main current circuit in which the current detection resistor is connected in series, the control coil wound around the transformer, and the main transistor are electrically connected. A control circuit for controlling a base current of the main transistor with a voltage induced in the control coil, and an overcurrent protection circuit for a switching regulator including a control transistor that suppresses the current of the current detection resistor when the current exceeds a predetermined value. There is a negative voltage obtained by converting the voltage induced in the control coil into a direct current through a series circuit of a Zener diode and a resistor, and a positive voltage induced in the control coil is a Zener diode and a resistor different from the above. Is applied to the base of the control transistor via a series circuit of It is configured to change according to the voltage induced in the control coil, and in particular, the negative voltage obtained by converting the voltage induced in the control coil to DC is applied to the base of the control transistor through the series circuit of Zener diode and resistor. Then, by changing the overcurrent detection level of the control transistor according to the voltage induced in the control coil, a large overcurrent does not flow even if the load is short-circuited, and even if the short-circuit for a long time, the transformer is protected. It will not burn out or damage the diode of the output circuit. Moreover, by directly applying the voltage induced in the control coil to the base of the control transistor through the series circuit of the Zener diode and the resistor, the effect that the relationship between the output current and the output voltage is almost independent of the level of the power supply voltage. Have.

[Brief description of drawings]

FIG. 1 is a connection diagram showing an embodiment of an overcurrent protection circuit for a switching regulator according to the present invention, and FIGS. 2 and 3 show relationship curves between output current and output voltage of the overcurrent protection circuit according to the present invention. 2 is a curve diagram when only an overcurrent bias circuit is inserted, FIG. 3 is a curve diagram when an overcurrent bias circuit is inserted in a power supply voltage compensating circuit, and FIGS. 4 and 5 are connection diagrams showing conventional devices, respectively. It is a load characteristic diagram. 2: Transformer, 2a: Primary coil, 2b: Control coil, 3: Main current circuit, 4: Control circuit, 5: Control transistor, 7: Main transistor, 8: Current detection resistor, 20, 22: Zener diode, 21 ,twenty three:
resistance.

 ─────────────────────────────────────────────────── ─── Continuation of front page (56) References JP-A-60-216770 (JP, A) JP-A-60-47834 (JP, A) JP-B-52-56671 (JP, B2)

Claims (1)

[Claims] 1. A main current circuit in which a primary coil of a transformer, a main transistor, and a current detection resistor are connected in series, a control coil wound around the transformer, and a voltage which conducts the main transistor and induces in the control coil. A control circuit for controlling the base current of the main transistor, and an overcurrent protection circuit for a switching regulator having a control transistor that suppresses the current of the current detection resistor when the current of the current detection resistor exceeds a predetermined value. The negative voltage obtained by converting the induced voltage into a direct current is passed through a series circuit of a Zener diode and a resistor, and the positive voltage induced in the control coil is passed through a Zener diode and a resistor series circuit different from the above. A voltage applied to the base of the control transistor to induce the overcurrent detection level of the control transistor in the control coil. Overcurrent protection circuit of the switching regulator and changing accordingly.