JPS62123959A - Excess current protection circuit for switching regulator - Google Patents

Abstract

PURPOSE:To prevent a transformer from burning or the like even on short- circuiting a load for a long time, by converting the discharge electromotive voltage of a control coil to DC, and by applying the negative voltage to the base of a control transistor via the series circuit of a Zener diode and a resistance. CONSTITUTION:A switching regulator is organized with a DC power source 1, a transformer 2, a main current circuit 3, a control circuit 4, a control transistor (hereinafter, Tr) 5, an output circuit 6, and the like, and current is fed to a load 19. Then, an excess current bias circuit consisting of the series circuit of a Zener diode 20 and a resistance 21 is connected between a connecting point between a diode 9 and a condenser 10, and the base of the Tr5 so that voltage induced to a control coil 2b may be applied to the base of the Tr5. Then, voltage at the both ends of the condenser 10 is applied to the Tr5 via the Zener diode 20 and the resistance 21, as negative bias voltage. In this state, switching actions are repeated by a main Tr7, and output voltage E0 is made constant.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of the Invention] The present invention relates to an overcurrent protection circuit that protects an overcurrent caused by a short circuit in an output circuit of a switching regulator.

[Prior art and its problems]

A conventional example of a switching regulator is one in which an intermittent blood flow current is passed through the primary coil of a transformer, a capacitor is charged with the voltage obtained at the output coil of the transformer, and this charging voltage is used as a stabilized DC power source. This is shown in Figure 4. Here, the main parts of the regulator (B) path are DC source 1, transformer 2,
．． Main current circuit 3, control circuit 4. Control transistor5. It is composed of an output circuit 6 and the like. The DC tFL source] rectifies and smoothes the AC power source and outputs the DC voltage El. The transformer 2 has a primary coil 2a, a control coil 2b, and an output coil 2C, and a main transistor 7 and a current detection resistor 8 whose collector is connected to the primary coil 2a and the primary coil 2a.
The series circuit of forms a main current circuit 3, and the negative coil 2a is connected to the terminal of the DC source 1.

Control coil 2, with the resistor 8 side connected to one terminal thereof
A separate circuit 4 as shown below is connected to both ends of b. That is, a series circuit of Dimate 9 and capacitor JO is connected to both ends of H1i+1 @coil 2b.

4-1+1 A circuit including a resistor 13 connected in series with a parallel circuit of a diode 1 and a capacitor 12 is connected between one end of the 4-1+1 imco.f circuit 2b and the base of the ±transistor f. A Zener diode 1 is connected between the connection point of the diode 9 and the capacitor 10 and the base of the main transistor 7.
4 is connected, and the base of the main transistor is connected to the DC power supply 10) + g through the starting resistor 15! & continued. The control transistor 5 has its collector connected to the base of the main transistor 7, its emitter connected to the ground terminal of the power supply 1, and its base connected to the connection point between the main transistor 7 and the resistor 8 via a resistor 16. There is. The output circuit 6 includes a diode 1 connected to both ends of the output coil 2C.
In a series circuit of 7 and output capacitor 18, capacitor 18
A load 19 is connected to both ends of the .

! When 12al is applied, a base current flows to the transistor 7 via the starting resistor 15, and the transistor 7 starts to pass. Then # flows through the primary coil 2a of the transformer 2.
begins to flow, pressure is induced in the control coil 2b, and da f
By increasing the °C base current through the ode 11 and the resistor 13, the main transistor 7 momentarily reaches a saturated conduction state. Therefore, the current in the secondary coil 2a increases linearly, and a blade pressure drop proportional to the current (2) is obtained in the resistor 8. When this voltage drop exceeds the weir layer voltage between the base and emitter of the limiting transistor 5, the transistor 5 becomes conductive and the current flows through the main transistor 7 through the resistors 13 and 15.
Base iK 611. Since the main transistor 7 is shunted, the main transistor 7 goes into a non-saturated state. As a result, -order coil 2alC
Since the flowing current decreases, the voltage of the control coil 2b also decreases, and the current flowing through the resistor 13 decreases.

The current ■ of the main transistor 7 is momentarily cut off. As a result of the curse, the base current begins to flow into the main transistor 7 via the resistor 15 again, and the above-described operation is repeated. Each coil 2a of the transformer 2 when the current l is interrupted.

A voltage in the opposite direction to the current (2) is induced in Qb and 2c, respectively. Then, the output coil 2C releases the energy stored in the voltage generator 2 (when the main transistor 7 is conductive), is rectified by the diode 17, and is deflected to the capacitor 18, supplying tI#i and IO to the load 19. .Here ill (
Since the coils 2b and 2c are wound around the same iron core, the voltage of the m coil 2b and the voltage of the output coil 2C are proportional to their turns ratio, and these voltages are rectified and photoelectrically connected to both capacitors 10, The same applies to voltage 18. Therefore, the voltage between the terminals of the capacitor lO is monitored as the DC reference voltage Ec, and if the voltage Ec becomes high, the Zener diode 14 absorbs the base current of the main transistor 7 to reduce the current ■, and if the voltage -Eα becomes low, then If the absorption of the base current is stopped and the voltage EC is kept constant, the output voltage Eo will also be constant and it can be used as a stabilized power supply.

By the way, as already mentioned, this switching regulator cuts the primary current l of the transformer on and off, and also switches it on and off.
The purpose of this is to transmit constant power to the output side of the transformer, and the current (2) is intermittent so that the output voltage (1) and the load current (1) are approximately constant. For example, when a short circuit occurs on the negative side, the flow of # cannot be suppressed like a water pillow or interrupted.For this reason, the output voltage E.

As shown in Figure 5, when the current ■o is gradually increased, the relationship between the load current IO and the load current IO is as shown in Fig.
It decreases gradually as o increases.

Moreover, the load current Io differs for the same output voltage Eo depending on the level of the power supply voltage E1, and the curve a corresponds to the case where the voltage l;i is high and the curve 1 corresponds to the case where the voltage E is low. Therefore, when the load is short-circuited, the current o is 200c10 or more than the rated 'drift'), f[6 or
The drawback is that it overheats, leading to burnout of the transformer and deterioration or damage of the diode.

[Purpose of redundancy]

Mokkomei interrupts the current flow in the transformer's single arrow coil when the negative current exceeds the rated value, preventing power from being transmitted to the load side, and further blocking the current from being transmitted to the load side. The purpose of the present invention is to provide a switching regulator that is not affected by power supply voltage.

[Rest of invention]

The present invention is a main current circuit in which the primary coil of a transformer, a soil transistor, and a current detection resistor are connected in series.

Did you get caught up in the transformer? I! lJ control coil, a control circuit that passes through the main transistor 24 and controls the base current of the main transistor with a voltage induced in the control coil, and when the '+'m current of the current detection resistor exceeds a predetermined value, this 11 : This is an overcurrent protection circuit for a switching regulator that has I+1a control transistors to suppress 61u. A voltage induced in the tllJ control coil is applied to the base of the control transistor to make the control transistor conductive, and the base current of the main transistor is cut off to cut off the current of the main transistor, and the output t
The idea is to cut off IT. It is recommended that the voltage applied to the filJ transistor be a negative voltage obtained by DC converting the voltage induced in the control coil and always applied through a series circuit of a Zener diode and a resistor. When applied through a series circuit of a resistor and a resistor, the load for changes in power supply voltage is effective in narrowing the difference in current.

[Embodiments of the invention]

Embodiments of the present invention will be described in detail with reference to FIGS. 1 to 8143. Here, parts and circuits that play the same role as the fourth factor are given the same reference numerals as in FIG. 4, and their explanations are omitted.

In Fig. 1, a DC 1rJt source 1° transformer 2, a main current circuit 3, ! IJ subcircuit 4. city 1j gun transistor 5
, the output circuit 6, and other connections as a switching regulator are exactly the same as those of its cousin, so a description thereof will be omitted. The difference between this regulator and the previous one is that the four voltages induced in the control coil 2b are divided once by the transistor 5.
At the point on the [4] side of the base of
An overcurrent bias circuit consisting of one series circuit is connected.

In this way, the four voltages at the 1st and 1st ends of the capacitor 10 are applied as a negative bias voltage to the control transistor 5 via the Zener diode and the resistor 21, and in this state, the main transistor 7 performs the switching operation. of?
In return, the output voltage Eo or voltage EC is kept constant. Here, if the current 10 of the load 19 suddenly increases (11), the voltage E drops by 1, and the voltage Ec also drops. Therefore, the Zener diode becomes non-conductive, and the control transistor 5
Since the bias voltage of the shell disappears, the braking transistor 5
The current increases, the base current of the main transistor 7 stops flowing, and the current 2 of the main transistor 7 is cut off.

In this case, since the current of the control transistor 5 has increased, l
Since the current 1 of the main transistor 7 is interrupted, even if the current of the starting resistor 15 flows, this current flows to the control transistor 5 and does not flow to the base of the main transistor 7, so the main transistor 7 becomes conductive again. There's nothing to do,
Shimaka is no longer Ill mJ coil 2b, output coil 2C
7 is not transmitted, and the voltage Eo is a slight increase in the current lo as shown in Figure 2J2 (when the power supply voltage is 4 voltage Ei) or curve b (when the power supply voltage E1 is low). It decreases rapidly. Here, if a voltage adjustment circuit consisting of a series circuit of a Zener diode and a resistor is connected between the control coil 21) and the control transistor 5,
When the electric TM voltage Ei is high, the voltage E decreases relatively quickly due to the ability of the Zener diode a to pass through the Zener diode a more quickly than when it is low, causing the base current of the control transistor 5 to flow quickly.
As shown in Fig. 3, the interval between song+liM a and song ima is reduced, the influence I of the height of the power supply voltage Ei becomes smaller, and the power supply' pressure becomes almost constant within song a, which is at the rated value. effective.

〔Effect of the invention〕

As described above, the present invention (1) short-circuits the load by applying a negative voltage obtained by converting the voltage induced in the IIJ control coil to the base of the control transistor via a series circuit of a Zener diode and a resistor. On the other hand, C does not cause a large overcurrent to flow, and even if it is short-circuited for a long time, the transformer will not burn out or the output circuit diode will be damaged.Furthermore, the voltage induced in the control coil is directly passed through the Zener diode and resistor. When the series circuit is blown and applied to the base of the control transistor, there is an effect that the relationship between the output current and the cutting voltage is hardly influenced by the level of the power supply voltage.

[Brief explanation of drawings]

FIG. 1 is an assembly diagram showing an embodiment of the overcurrent protection circuit for switching legs and regulators according to the present invention, and FIGS. 2 and 3 show the relationship between the output current and output voltage of the overcurrent protection circuit according to the present invention. Figure 2 shows the curves when only the overcurrent bias circuit is inserted, Figure 3 shows the curves when the overcurrent bias circuit and power supply voltage compensation circuit are inserted, and Figures 4 and 5 respectively show the curves for the conventional device. FIG. 2 is a wiring diagram and a load characteristic diagram shown in FIG. 2...Pressure device, 2a...-secondary coil, 2b...control coil, 3...main current circuit, 4...tttlJ control circuit, 5...! IJIJ control transistor, 7... Main transistor, 8... Redundant current detection resistor, Δ), 23...
・Zena Daio)', 21. S! 13...Resistance. Figure 3 Figure 4 □IO Figure 5

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 main current circuit that conducts the main transistor and connects the control coil to the control coil. An overcurrent protection circuit for a switching regulator, comprising: a control circuit that controls the base current of the main transistor using an induced voltage; and a control transistor that suppresses the current when the current of the current detection resistor exceeds a predetermined value. An overcurrent protection circuit for a switching regulator, characterized in that a voltage induced in a control coil is applied to the base of the control transistor to make the control transistor conductive. 2) In the overcurrent protection circuit for a switching regulator according to claim 1, the voltage applied to the base of the control transistor converts the voltage induced in the control coil into DC and passes through a series circuit of a Zener diode and a resistor. An overcurrent protection circuit for a switching regulator characterized by applying a negative voltage. 3) In the overcurrent protection circuit for a switching regulator according to claim 1, the voltage applied to the base of the control transistor is applied via a series circuit of a Zener diode and a resistor. Current protection circuit. 4) In the overcurrent protection circuit for a switching regulator as set forth in claim 1, the voltage applied to the base of the control transistor is a voltage applied to the base of the control transistor. protection circuit.