JPS6355813B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a protection circuit for a switching power supply circuit, and its purpose is to provide a protection circuit that is highly reliable at startup and can safely protect switching transistors in a load short-circuit condition. It's about doing.

First, a typical conventional switching power supply circuit will be explained with reference to FIG. Figure 1 is a circuit diagram of an isolated flyback converter type circuit, in which the output voltage of a commercial AC power supply 1 is rectified via a bridge circuit consisting of diodes D ₁ to _{D 4} and then connected to a capacitor C _{1 .}
The unregulated DC voltage is applied to one end of the winding _L1 of the transformer _T2 , and the other end of the winding _L1 is applied to the secondary side of the transformer _T2 by the switching transistor _Q1 . Switching is repeated under conditions that the output voltage becomes stable. Also, the switching transistor is connected to the winding _L3 of the transformer _T2 .
The square wave pulse generated by Q ₁ turning on and off repeatedly is connected to diode D ₆ and capacitor C _3.
The voltage is rectified and smoothed at the terminal A, and is supplied to the terminal a as the power supply voltage of the control circuit 2. Furthermore, the rectified and smoothed output of diode D ₆ and capacitor C ₃ is divided by resistors R ₃ and R ₄ as a feedback voltage to stabilize the output voltage of this switching power supply circuit, and is applied to terminal b of the control circuit 2 as a detection voltage. is applied as. Further, the rectified and smoothed output of the diode D ₆ and the capacitor C ₃ is applied to a series circuit of a Zener diode D ₇ and a resistor R ₅ , and the terminal voltage of the Zener diode D ₇ is applied to the terminal C of the control circuit 2 as a reference voltage. The control circuit 2 detects the error voltage between terminals b and c and outputs a square wave pulse _P1 controlled by this error voltage from terminal d. The square wave pulse P ₁ drives the transistor Q ₂ after being divided by the resistors R ₇ and R ₈ . The switching transistor Q ₁ is the switching transistor Q ₂
The transformer T 2 is driven by a drive circuit formed by a transformer T ₁ and _a collector resistor R ₂ .
The secondary output voltage is controlled so that it is always constant. In addition, the square wave pulse P ₂ generated in the winding L ₂ B of the transformer T ₂ is connected to the diode D ₅ and the capacitor C ₂
The voltage is rectified and smoothed and applied to the drive circuit as a power supply voltage. When starting up the switching circuit, power is supplied to the control circuit 2 and the drive circuit through resistors _R1 and _R6 , respectively, and the switching circuit is configured to start. The output voltage is determined by the winding ratio of the windings L ₁ and L ₄ of the transformer T ₂ , and a DC stabilized voltage that has been rectified and smoothed by the diode D ₈ and the capacitor C ₄ is applied to the load 3 .

Since the conventional switching power supply circuit is configured in this way, when the load 3 is switched off, there is a risk that an excessive current will flow through the switching transistor _Q1 and damage it. For this reason, various protection circuits have been used in the past, but no one has been proposed that has a simple configuration and is highly reliable and does not malfunction during startup.

FIG. 2 shows an embodiment of the present invention that can avoid the above-mentioned problems, and the following description will be given based on this FIG. 2. Components having the same functions as those in FIG. 1 are given the same reference numerals and their explanations will be omitted.

The part surrounded by broken lines in FIG. 2 is a protection circuit A added to the switching power supply circuit of FIG. 1. _R9 is a detection resistor, which is interposed between one end of the negative side of the capacitor _C1 for smoothing the input DC voltage and a reference level (hereinafter referred to as the ground point). TH is a thyristor connected to the control circuit 2 so as to be able to switch the amplitude of the square wave pulse _P1 generated at the terminal d. D ₉ and R ₁₀ are a diode and a resistor connected in series, and the cathode of the diode D ₉ is connected to the connection point B between the capacitor C ₁ and the detection resistor R ₉ so that the series circuit has the opposite polarity with respect to the input DC voltage. , and one end of the resistor R ₁₀ opposite to the end connected to the diode D ₉ is connected to the ground point. _C5 is a capacitor for charging and discharging,
It is connected in parallel with the resistor _R10 . _R11 is a resistor connected between the gate of the thyristor TH and the ground point. The thyristor TH has an anode connected to the terminal d of the control circuit 2, and a cathode connected to the connection side end X of the resistor _R10 .

With this configuration, when the load is shorted, a discharge current flows out of the capacitor _C1 , causing a high voltage drop across the detection resistor _R9 , turning on the diode _D9 . When diode D ₉ turns on,
Current flows from resistor _R10 to connection point B via diode _D9 , and capacitor _C5 is charged to a negative potential. The charge in the capacitor _C5 flows through the resistor _R11 as a date current for the thyristor TH, and the thyristor TH is turned on. When thyristor TH is turned on, terminal d of control circuit 2 is connected to thyristor TH.
and is connected to the earth point through a resistor R ₁₀ , so
The amplitude of the square wave pulse P ₁ decreases and the transistor
The switching of _Q2 is turned off, and the switching output transistor _Q1 is turned off. Therefore, it can be safely protected even during load shots.

Note that at startup, a current flows in the charging direction to the capacitor _C1 , and at this time, the potential of the detection resistor is on the positive side and the diode _D9 is turned off, so there is very little risk of malfunction.
Also, even if diode _D9 is turned on transiently due to the selection of the time constants of capacitor _C5 and resistor _R10 , capacitor _C5 is charged to a positive potential at this time, so
Thyristor TH never turns on, making it highly reliable.

As described above, according to the present invention, in the switching power supply circuit, a detection resistor is inserted in series with the input current and voltage smoothing capacitor, and the voltage across the detection resistor is detected to control switching of the switching output transistor. Since the switching output transistor is configured to be protected in the event of a load short circuit, it is possible to realize a highly reliable protection circuit that is simple in configuration and does not malfunction at startup.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a conventional switching power supply, and FIG. 2 is a block diagram of a switching power supply equipped with a protection circuit according to an embodiment of the present invention. Q ₁ ...Switching output transistor, A...
...protection circuit, C ₁ ...input DC current smoothing capacitor, R ₉ ...detection resistor, D ₉ ...diode,
_R10 , _R11 ...Resistor, _C5 ...Capacitor, TH...
Thyristor [semiconductor switching device], P ₁ ...
Square wave pulse, 3...Load.

Claims (1)

[Claims] 1. In a switching power supply circuit, a detection resistor is inserted in series with an input DC voltage smoothing capacitor,
A switching power supply protection circuit characterized in that the switching of the switching output transistor is controlled by detecting the voltage across the detection resistor, and the switching output transistor is protected in the event of a load short circuit. 2. A series circuit of a first resistor and a diode is connected in parallel with a detection resistor connected in series to the input DC voltage smoothing capacitor, with the diode having a reverse polarity with respect to the input DC voltage, and the first A switching power supply protection circuit according to claim 1, characterized in that a capacitor is connected in parallel to a resistor, and a semiconductor switching element is controlled by the discharge current of the capacitor to turn off a switching output transistor. .