JPS5857074B2 - transistor protection circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a transistor protection circuit that protects a switching transistor from thermal breakdown in a ringing choke type switching regulator type power supply circuit.

FIG. 1 shows a conventional ringing choke type switching regulator type power supply circuit.

An alternating current voltage from an AC power source 1 is converted into direct current by a diode 2 and a capacitor 3.

This DC voltage is supplied to the base of the transistor 6 via the starting resistor 4.

When the base current of the transistor 6 flows through the resistor 4 and the transistor 6 becomes conductive, a current flows to the primary winding 7 of the transformer 30, and the voltage applied to the primary winding 7 is applied to the tertiary winding 9 of the transformer 30. A proportional positive voltage is induced, which is supplied to the base of transistor 6 via resistor 10 and capacitor 11, bringing transistor 6 into saturation.

At this time, since a negative voltage is induced in the secondary winding 8 of the transformer 30, the diode 12 is in a cut-off state.

The collector current Ic of the transistor 6 in the saturated state increases with time and is expressed by the following equation (1).

Here, Vi: Transformer input voltage (rectified voltage)
L1 Nidorance primary winding inductance t1: Transistor saturation state elapsed time When the expression (2) is satisfied due to an increase in the collector current Ic, the transistor 6 suddenly changes from the saturation state to the cut-off state.

Ic>hpB・Ib・・・・・・・・・・・・・・・
... (2) Here ■b: Current flowing into the base hFE: Current amplification factor of the transistor When the transistor 6 is cut off, a positive voltage is induced in the secondary winding 8, the diode 12 becomes conductive, and the smoothing capacitor 13
and supplies current and voltage to the load resistor 17.

At this time, the current ID flowing through the diode 12 is expressed by equation (3).

Here, the ratio of the number of turns between the primary winding 7 and the secondary winding 8 is 2. ■ Peak collector current of the Op Nidrangistaro ■ o: Output voltage L2: Flows through the diode 12 on the inductance of the secondary winding 8. Current elapsed time (transistor 6 cutoff time) While current flows through the diode 12, the tertiary winding 9 has an output voltage ■.

A negative voltage proportional to is induced.

When the current in the diode 12 becomes zero, the voltage induced in the tertiary winding 9 also becomes zero, and the current flowing in from the resistor 4 makes the transistor 6 conductive again.

The above operation is repeated to supply the output voltage to the load 17.

The error detection circuit 15 compares the output voltage with the reference voltage of the reference voltage source 14, and outputs a detected voltage according to the difference to the control circuit 16.
supply to.

The control circuit 16 controls the conduction time of the transistor 6 according to the detected voltage, thereby making the output voltage constant.

Further, a horizontal flyback pulse is supplied to the terminal 16', and the transistor 6 is turned on and off in horizontal cycles.

When the load 17 becomes short-circuited in the above power supply circuit (
3) If l0pt is constant as shown in equation 3), since ■o becomes almost zero, the cut-off time of the transistor 6 becomes longer.

FIG. 2 shows the base voltage waveform of the transistor 6 in a short-circuited state on the load side.

tl is the conduction period of the transistor 6, t2 is the cutoff period of the transistor 6, t3 is the period when the reverse bias voltage is applied to the base of the transistor 6, and t4 is the period when the base voltage is zero during the cutoff period t2 of the transistor 6. indicates a period of time.

The reverse bias time t3 is the capacitor 11 and the starting resistor 4.
determined by the time constant of

When the load side is short-circuited, the period t3 is shorter than the cut-off time t2 of the transistor 6, and a time t4 during which the transistor 6 is not reverse biased occurs during the cut-off time t2.

This period t4 is a period in which the transistor 6 is in an unstable state, and a small collector current flows during this period t4 to the extent that the transistor does not conduct due to oscillation or noise.

This period t4 extends to several milliseconds, which is much longer than the horizontal period (1), and causes thermal breakdown of the transistor 6.

SUMMARY OF THE INVENTION An object of the present invention is to provide a transistor protection circuit which eliminates the above-mentioned drawbacks of the prior art and prevents a switching transistor from being thermally destroyed when a load is short-circuited in a switching regulator type power supply circuit.

The present invention bypasses the inflow current of the starting resistor when the load side of the power supply circuit is short-circuited, lengthens the reverse bias time between the base and emitter of the switching transistor, and also doubles the bypass path as the short-circuit detection circuit. , the period during which the switching transistor generates heat is shortened.

FIG. 3 is a circuit diagram showing one embodiment of the present invention.

In FIG. 3, the same parts as in FIG. 1 are designated by the same reference numerals.

In Fig. 3, a resistor 1 is connected between resistor 4 and transistor 6.
Further, a resistor 19 and a diode 20 are provided between the connection point A of the resistor 4 and the resistor 18 and the capacitor 13.
are connected in series.

The values of the resistors 4 and 18 are such that the potential at the connection point A between the resistor 4 and the resistor 18 is the output voltage ■ that is applied to the load resistor 17 in the normal operating state.

is selected so that it is approximately equal to .

Therefore, almost no voltage is applied to the resistor 19, and in the normal operating state, the circuit of FIG. 3 performs the same operation as that of FIG. 1.

When the power supply circuit load side is short-circuited, the output voltage is ■.

becomes almost zero, current flows to the load side through the resistor 19, the potential at the connection point A decreases, and the voltage across the resistor 18 also decreases.

That is, the current flowing through the resistor 18 decreases.

Therefore, as shown by the broken line in FIG. 2, the period 1/ during which the base voltage of the transistor 6 returns from negative to zero becomes longer, and the period 14/ during which the transistor 6 is in an unstable operating state becomes shorter.

The diode 20 is for preventing overcurrent from flowing through the resistor 19 when the resistor 18 is destructively short-circuited.
It does not affect basic operation.

As described above, heat generation in the transistor 6 can be prevented by making the time during which the reverse bias is applied between the base and emitter of the transistor 6 longer than in the conventional circuit using the resistors 18 and 19.

According to the present invention, it is possible to prevent destruction of the expensive switching transistor 6, prevent performance deterioration and destruction of other components due to heat generated by the transistor 6, and improve safety.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing a conventional ringing choke type switching regulator system power supply circuit, Fig. 2 is a waveform diagram showing the base voltage waveform of the switching transistor when the load side of the power supply circuit is short-circuited, and Fig. 3 is a transistor according to the present invention. FIG. 2 is a circuit diagram showing an example of a protection circuit. 4...Starting resistor, 6...Switching transistor, 7...Primary winding, 8...
...Secondary winding, 9...Tertiary winding, 11...
... Capacitor, 18, 19... Resistor.

Claims (1)

[Claims] 1. A transformer that has an unstable DC power supply, an input winding whose one end is connected to one end of the DC power supply, a load winding and a feedback winding, and a transformer that has an input winding connected to the other end of the DC power supply. A switching transistor with a collector emic path connected to the other end, a starting resistor connected between the base of the switching transistor and a DC power source, and a feedback voltage generated in the feedback winding of the switching transistor. a feedback means for repeatedly turning the switching transistor on and off by feedback to the base of the switching transistor; a direct current converting means for converting the voltage generated in the load winding to direct current again; a load to which the output of the direct current converting means is supplied; stabilizing means for controlling on/off of the switching transistor so as to suppress voltage fluctuations in the output of the converting means; and a resistor and a diode connected between the resistance dividing point of the starting resistor and the output of the direct current converting means. A transistor protection circuit comprising a series circuit consisting of: