JPH0785648B2 - Switching regulator power supply circuit - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching regulator power supply circuit that produces a DC voltage from a commercial power supply.

2. Description of the Related Art In recent years, switching regulator power supply circuits have been widely adopted as power supplies for various devices. A large number of power supply circuits including various protection circuits, for example, overcurrent protection circuits or overvoltage protection circuits are also used.

Hereinafter, an example of a conventional power supply circuit will be described with reference to the drawings.

FIG. 7 is a circuit diagram of an example of a separately excited switching regulator power supply circuit including an overcurrent control circuit. That is, (a) and (b) in FIG. 7 are commercial power supply input terminals to which a sinusoidal AC voltage is applied, which AC voltage is rectified by the bridge rectifying diodes 8, 9, 10 and 11, and the capacitor 12
Are smoothed to form a DC voltage. On the other hand, the oscillation control circuit
A signal for switching is propagated from 16 to the drive circuit 15 and further to the output transistor 14, and energy is supplied to the secondary side of the transformer via the switching transformer 13 during the ON period of the transistor 14. In addition,
Reference numeral 19 is a rectifier diode for the secondary output of the transformer, 20 is a smoothing capacitor, and 21 is a load.

As a feedback loop for controlling the secondary side output voltage level constant, the detection circuit 18 detects the transformer secondary side output voltage, and the oscillation control circuit 16 controls the on-duty ratio under a constant oscillation cycle. Reference numeral 5 is an overcurrent detection resistor, which controls the oscillation of the oscillation control circuit 16 by detecting the magnitude of the source current flowing through the resistor 5 by the overcurrent control circuit 17. Incidentally, a bleeder resistor that divides the voltage drop of the overcurrent detection resistor 5 into resistors 1 and 2, and 6 is a noise removing capacitor.

The operation of the switching regulator power supply circuit configured as described above will be described below.

First, an excessive current flows through the output transistor 14 during a transient period at the start of operation of the power supply circuit or during an abnormal state.
This current causes a voltage drop across resistor 5. When the reference potential of the circuit is the source potential of the output transistor 14, a negative voltage is generated in the resistor 5. The voltage waveform is as shown in FIG. The voltage generated in the resistor 5 is divided by the resistors 1 and 2, and the voltage across the resistor 1 is applied to the overcurrent control circuit 17. When the voltage across the resistor 1 exceeds the threshold voltage V _th of the overcurrent control circuit 17, the oscillation control circuit 16 is controlled to perform a so-called intermittent oscillation operation in which oscillation is temporarily stopped. As a result, the element can be protected from accidents such as destruction without overdriving the output transistor 14.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention A drain current waveform flowing through an output transistor in a switching regulator power supply circuit having a constant oscillation period is
The peak current value changes depending on the AC input voltage level.
In particular, when a wide range of input voltage (for example, 80V to 270V) is handled, the peak current value of the drain current changes nearly three times between the minimum value and the maximum value of the input voltage. Since the load of the power supply circuit is constant, the circuit current decreases as the input voltage increases.

Therefore, as shown in FIG. 9, the overcurrent detection voltage E _P0 with respect to the input voltage becomes smaller with a constant slope as the input voltage rises.

On the other hand, since the operation start voltage (threshold voltage V _th ) of the overcurrent control circuit is constant, the higher the AC input voltage,
The overcurrent detection voltage becomes a low level with respect to the operation start voltage. Therefore, during power-on inrush, the overcurrent control circuit works well at low input voltage, but there is no problem at high input voltage, but the overcurrent control circuit does not work at high input voltage and exceeds the safe operating area of the output transistor. There was a problem that the transistor was destroyed.

In view of the above problems, the present invention provides a switching regulator power supply circuit having a circuit that ensures a sufficient overcurrent control operation for a wide range of AC input voltage with a simple circuit configuration.

Means for Solving the Problems To achieve the above object, a switching regulator power supply circuit of the present invention creates a correction current proportional to an input voltage applied to a switching transformer, and the correction current and an ON-state switching element Both of the flowing currents are detected, and the on / off period of the switching element is controlled so that the sum of these two currents does not exceed the predetermined value.

Action With the above configuration, a correction current proportional to the input voltage to the switching transformer is created, the correction current and the current flowing in the switching element are superposed and detected, and the switching element decreases as the input voltage increases. By compensating for the decrease in the current that flows into the circuit, it is possible to reliably detect overcurrent that exceeds the rating of the circuit element when the power is turned on (inrush), etc. even at high input voltage. Accurate protection can be achieved.

Embodiment A switching regulator power supply circuit according to an embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows the configuration of a switching regulator power supply circuit according to the first embodiment of the present invention. The same components as those shown in FIG. 9 are designated by the same reference numerals, and detailed description thereof will be omitted.

In the present embodiment, in addition to the configuration shown in FIG. 9, an auxiliary winding 1 is provided in the switching transformer 13, and one end of the switching transformer auxiliary winding 1 is connected to a series circuit of a resistor 3 and a Zener diode 4, The other end of this series circuit is connected to the input terminal of the overcurrent control circuit, and the other end of the auxiliary winding 1 is connected to the source of the transistor 14.

The operation of the switching regulator power supply circuit configured as described above will be described below with reference to FIGS. 1 to 4.

FIG. 2 is a main part voltage waveform of this embodiment, and FIG. 2B shows the waveform at point (b) in FIG. Since the correction current for overcurrent control obtained through the auxiliary winding 1 is fed back to the overcurrent control circuit 17, the current flows from the switching transformer auxiliary winding 1 even during the OFF period of the output transistor 14 and the positive current is supplied. Voltage is also generated on the side. The peak on the negative side has a voltage −E _P having a larger voltage value than that shown in FIG. 8 because the correction current flows. FIG. 10A shows the output voltage waveform of the auxiliary winding, in which the ON period of the output transistor is voltage −E ₁ and the OFF period is E ₂ .

FIG. 3 is a graph showing the dependence of the auxiliary winding output voltage −E _{1 on} the AC input voltage, and the absolute value thereof increases as the AC input voltage increases.

Under the above situation, the correction current i _c for overcurrent control is analyzed.

(1) Output transistor ON period However, E ₁ ... auxiliary winding output voltage, V _Z ... Zener voltage,
−V _C …… Overcurrent control circuit input voltage, R ₃ …… Correction circuit resistance (2) Output transistor OFF period However, E ₂ …… Auxiliary winding output voltage V _F …… Zener diode forward drop voltage In order to properly correct a wide range input voltage, the correction current is calculated by the formula (1) when the AC input voltage is 80V. If it is zero, V _Z = E ₁ (80V) −V _C , and the Zener voltage can be set. Therefore, by selecting E ₁ (80V) = V _Z + V _C , the correction current when the AC input voltage is 80 V can be set to zero with the Zener diode. Then, for the AC input voltage of 80V to 270V, the correction resistance value R ₃ is appropriately selected according to the equation (1), and the correction current is superposed on the bleeder resistance 1 to flow, as shown in FIG. A constant overcurrent control input voltage can be maintained with respect to the AC input voltage.

If it is not necessary to increase the accuracy so much, input voltage 80
The correction current does not necessarily have to be set to zero with V.

As described above, according to the present embodiment, by passing the correction current from the switching transformer auxiliary winding 1 through the resistor 3 and the Zener diode 4 to the overcurrent control circuit 17, it is possible to prevent overcurrent in a wide range of AC input voltage. It is possible to maintain the current control input voltage at a constant level and ensure the overcurrent control operation.

Incidentally, in the embodiment of FIG. 1, a diode 22 may be added to the correction circuit 7 in the direction in which the Zener current flows as shown in FIG. At this time, i _c (OFF) that does not contribute to overcurrent control operation
The current is taken as an example, and the overcurrent control input voltage waveform is also as shown in FIG.

Further, although the overcurrent control input voltage is handled as a negative voltage in the embodiment of FIG. 1, it may be configured as a positive voltage as shown in FIG. In FIG. 6, the overcurrent detection resistor 5 is inserted in the source circuit, and the voltage drop thereof is divided by the bleeder resistors 1 and 2. Therefore, the polarity of the switching transformer auxiliary winding and the polarity of the Zener diode 4 are opposite.

Also, the switching transformer auxiliary winding is not a dedicated winding,
It can also be used in combination with other windings if the polarities match.

Even if the Zener diode 4 is not provided, the correction current obtained from the auxiliary winding is superposed on the detection output of the overcurrent detection resistor 5 to compensate for the decrease in the detection current of the resistor 5 at the time of high input voltage. It is possible to correct the characteristics, though the accuracy is lower than that using the Zener diode.

EFFECTS OF THE INVENTION As described above, according to the present invention, for a wide range of AC input voltage to the switching transformer, a correction current proportional to the input voltage is created, and the correction current and the current flowing through the switching element are detected together. However, by controlling ON / OFF of the switching element so that the sum of the two currents does not exceed the specified value, stable overcurrent control can be performed so that the current does not exceed the rated current even when the input voltage is high. The element can be protected from accidents such as destruction without overdriving the output transistor against a transient state.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a switching regulator power supply circuit according to an embodiment of the present invention, FIG. 2 is a voltage waveform diagram of an essential part of FIG. 1, and FIGS. 3 and 4 are an essential part of FIG. 5 is a characteristic diagram showing the characteristic of the voltage with respect to an AC input voltage, FIG. 5 is a circuit diagram of a main part of another embodiment of the present invention, FIG. 6 is a circuit diagram of another embodiment of the present invention, and FIG. A circuit diagram of the switching regulator power supply circuit, and FIGS. 8 and 9 are waveform diagrams and characteristic diagrams for explaining the operation of the conventional example. 8,9,10,11,19,22 …… Diode, 4 …… Zener diode, 6,12,20 …… Capacitor, 1,2,3,5 …… Resistance, 13
...... Switching transformer, 14 …… Output field effect transistor, 15 …… Drive circuit, 16 …… Oscillation control circuit, 17
...... Overcurrent control circuit, 18 …… Secondary output voltage detection circuit, 21
…… Secondary circuit load.

Claims (1)

[Claims] 1. A DC power supply circuit for converting a commercial power supply input in a wide voltage range into a DC power supply, and a switching in which one end of a primary winding is coupled to an output of the DC power supply circuit and a secondary winding is coupled to a load. One end of the output is joined to the other end of the transformer and the primary winding of the switching transformer, and the other output end is connected to an overcurrent detection resistor whose one end is grounded, and is cycled by a control signal input to the control terminal. A switching element that is controlled to be turned on and off, and a correction current generated by a voltage generated in proportion to the output of the DC power supply circuit from the auxiliary winding of the switching transformer through a resistor and a zener diode series circuit. The correction current creation means to be created and the current flowing through the switching element in the ON state are detected, and the sum of these two currents must be equal to or greater than a predetermined value. As described above, the switching element is provided with a control means for controlling the ON / OFF by supplying the control signal, and even when the input voltage is high, the overcurrent can be stably controlled so that the current does not exceed the rating of the switching element, and the transient current is controlled. Switching regulator power supply circuit characterized by not overdriving the output transistor depending on the state.