JPH0557827B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a self-oscillation type switching power supply device used in various electronic devices.

Conventional technology The conventional self-oscillation type switching power supply
The configuration was as shown in the figure. In Figure 3,
1 is a rectifying and smoothing circuit that converts the received AC input voltage into DC input voltage. 2 is a switching transformer, which has a primary winding 21, a bias winding 22,
It has a secondary winding 23. 3 is a main switching element which turns on and off to convert the DC input voltage into a high frequency AC voltage and input it to the primary winding 21. 4 is a starting resistor, 5 is a control circuit, 6 is a drive circuit, 7 is a diode, 8 is a capacitor, and bias winding 2
A diode 7 and a capacitor 8 rectify and smooth the high frequency alternating current voltage generated in the control circuit 5 to supply a bias voltage to the control circuit 5. A diode 9 and a capacitor 10 rectify and smooth the high frequency AC voltage generated in the secondary winding 23, and supply a DC output voltage. 11
Reference numeral 12 indicates a detection circuit for detecting the DC output voltage, and 12 indicates a photocoupler that feeds back information on the DC output voltage detected by the detection circuit 11 to the control circuit 5.
The operation of the conventional example will be explained below.

The alternating current input voltage input to the rectifying and smoothing circuit 1 is converted to a direct current input voltage, and biases the control circuit 5 via the starting resistor 4. The biased control circuit 5 outputs a pulse, which is amplified by the drive circuit b to turn on and off the main switching element 3.
Then, the DC input voltage is converted to high wave AC voltage and 1
The voltage is input to the secondary winding 21, and a high frequency AC voltage is generated in the bias winding 22 and the secondary winding 23 according to their respective turns ratios. The high frequency AC voltage generated in the bias winding 22 is transferred to the diode 7 and capacitor 8.
The voltage is rectified and smoothed by the voltage converter and supplied to the control circuit 5 as a bias voltage. The high frequency AC voltage generated in the secondary winding 23 has its flyback rectified and smoothed by the diode 9 and the capacitor 10, and is supplied as a DC output voltage. Information about the DC output voltage is fed back to the control circuit 5 by the detection circuit 11 via the photocoupler 12. The control circuit 5 detects the high frequency AC voltage generated in the bias winding 22, and when the flyback voltage disappears, that is,
When the excitation energy stored in the switching transformer 2 is exhausted while the main switching element 3 is on (hereinafter referred to as the "on period" and the period in which it is off is also referred to as the "off period"), the output is set to H. , turns on the main switching element 3 via the drive circuit 6. Then, receiving the information from the photocoupler 12, in order to stabilize the DC output voltage,
An on-period is determined, and when the on-period is reached, the output is set to L and the main switching element 3 is turned off. The above operation provides a stable DC output voltage. The number of turns of the primary winding 21 is N _P , and the number of turns of the secondary winding 23 is
The winding is N _S , the on period is T _ON , the off period is T _OFF ,
If the DC input voltage is V _IN , the DC output voltage is V _OFF , and the forward voltage drop of the diode 9 is ignored, the following equation is obtained.

V _OUT =T _ON /T _OFF・N _S /N _P・V _IN =δ/1−δ・N _S /N _P
・V _IN ……(1) δ: Duty ratio On the other hand, if _OUT is the DC output current and L _P is the primary winding inductance, then _OUT = 1/2・N _P /N _S・V _IN /L _P −T _ON・T _OFF /T _ON +
T _OFF =1/2・N _P /N _S・V _IN /L _P・δ(1−δ)・T…
...(2) It is expressed as T=TT _ON +T _OFF (synchronization).

Figure 4 shows the drain-source voltage V _DS of the main switching element 3 and the primary winding 21 in the conventional circuit.
The current _P flowing through the secondary winding 23, the current _S flowing through the secondary winding 23
The waveform diagram is shown.

Problems to be Solved by the Invention In such a conventional configuration, as can be seen from equations 1 and 2 above, as the DC output current _OUT decreases, T _ON and T _OFF decrease in proportion to it. However, depending on the capabilities of the main switching element 3, drive circuit 6, or control circuit 5, there is a minimum limit value T ^MIN _ON for T _ON , and if you try to reduce T _ON below T ^MIN _ON , intermittent oscillation will occur. or the main switching element 3 causes class A operation. When intermittent oscillation occurs, the response to a transient phenomenon such as a sudden change in the DC output current deteriorates, and when class A operation occurs, the main switching element 3 generates heat.

The present invention solves these problems, and even when using the same main switching element, drive circuit, and control circuit capabilities as the conventional configuration, and a switching transformer with the same specifications, it can stabilize down to a lower DC output current. The present invention provides a self-excited oscillation type switching power supply device having a means for operating.

Means for Solving the Problem In order to solve this problem, the present invention converts an input DC voltage into a high-frequency AC voltage by a main switching element, and converts the input DC voltage into a high-frequency AC voltage to connect the primary winding, secondary winding, and bias winding. The high-frequency AC voltage input to the primary winding of a switching transformer equipped with a wire is rectified and smoothed to output from the secondary winding to supply a specified output DC voltage to the load, which is generated in the bias winding. detects the high frequency alternating current voltage that occurs, generates a pulse voltage in synchronization with the drop in flyback voltage that occurs during the off period of the main switching element, controls the pulse width to stabilize the output direct current voltage, and outputs it. a control circuit which has the function of a comparator circuit that outputs a high level when the voltage is lower than the reference voltage; and a comparator circuit that discharges the capacitor during an on period of the main switching element, that is, a period when the input pulse voltage of the drive circuit is at a high level, and outputs a high level signal when the voltage is lower than the reference voltage. a charging/discharging circuit that starts charging the capacitor when the input pulse voltage of the drive circuit becomes low level so as to turn off the input pulse voltage of the drive circuit; a first transistor that is turned on and off and holds the input pulse voltage of the drive circuit at a low level for a period until the voltage of the capacitor reaches the reference voltage; A second transistor turns on and off between the base and emitter of the transistor, and turns off the first transistor during a period when the input pulse voltage of the drive circuit is at a high level.

Effect With this configuration, once the main switching element is turned off, the capacitor is charged (or discharged) in synchronization with it, and the off state is maintained at least for the time required for the voltage across the capacitor to reach the specified voltage. become. Then, even though the DC output current decreases and the on period and off period become shorter, the minimum value of the off period is regulated, so the on period is not shorter than when no regulation is set for the off period, and less DC Stable operation is achieved up to the output current. Let T ^MIN _ON be the minimum value of the on-period, and let T ^OFF _' be the period during which the flyback voltage is generated, that is, the period when the excitation energy of the switching transformer finishes discharging, with respect to T _{MIN ON} , then (1) From the formula, T _OFF ′ is expressed as T _OFF ′=N _S /N _P・V _IN /V _OUT +V _D・T ^MINN _ON ……(4), which is the minimum output current ^MIN _OUT that can be stably obtained with the conventional configuration. From the above equation (2), ^MIN _OUT = 1/2・N _P /N _S・V _IN /L _P・T ^MIN / _TON・T _O
FF ′/T ^MIN / _ON +T _OFF ′……(5). On the other hand, if the means of the present invention is provided and the regulated minimum off period is T ^MIN _ON , then ^MIN _OUT = 1/2・N _P /N _S・V _IN /L _P・T ^MIN / _ON・T _OF
F ′／T ^MIN ／ _ON +T ^MIN ／ _OFF ′...(6), and from equations (5) and (6), according to the present invention, (T ^MIN _ON +T _OFF ′)/(T ^MIN _ON +T ^MIN _OFF ) only ^MIN
_OUT can be made smaller. It is obvious to set T ^MIN _OFF such that T ^MIN _OFF > T _OFF ′.

Embodiment FIG. 1 is a circuit diagram of a self-oscillating switching power supply device according to an embodiment of the present invention. In FIG. 1, 1 to 12 are equivalent to those shown in the conventional example. 13 to 16 are resistors, 17 to 19 are NPN transistors, and 30 is a specified voltage source.
Let V _TH . 31 is a capacitor, and the voltage across it is V _C. 32 is a Zener diode, and its Zener voltage is _VZ . 33 is a comparator, to which V _C is input to the input terminal and V _TH is input to the input terminal. 34 is a constant current source whose current value is ₁
shall be. The operation will be explained below.

The AC input voltage input to the rectifying and smoothing circuit 1 is converted to a DC input voltage V _IN , which biases the control circuit 5 via the starting resistor 4. Then, the control circuit 5 outputs a pulse, which is input to the drive circuit 6 via the resistor 13, where the main switching element 3
It is amplified and output so that it can be turned on and off. The main switching element 3 converts the DC input voltage V _IN into a high frequency AC voltage and inputs it to the primary winding 21. Voltage is generated. The high frequency AC voltage generated in the bias winding 22 is rectified and smoothed by the diode 7 and the capacitor 8, and is supplied to the control circuit 5 as a bias voltage. The flyback voltage of the high frequency AC voltage generated in the secondary winding 23 is rectified and smoothed by the diode 9 and the capacitor 10, resulting in a DC output voltage.
Supplied as V _OUT . The DC output voltage V _OUT is sent to the photocoupler 12 by the detection circuit 11.
It is fed back to the control circuit 5 via. The control circuit 5 detects the high frequency AC voltage generated in the bias winding 22, and the flyback voltage disappears. In other words, when the excitation energy stored in the switching transformer 2 during the on period is exhausted, the output becomes H.
and turns on the main switching element 3 via the resistor 13 and the drive circuit 6. Then, upon receiving information from the photocoupler 12, the DC output voltage is
In order to stabilize V _OUT , an on-period is determined, and when the on-period is reached, the output is set to L and the main switching element 3 is turned off. Through the above operation, a stable DC output voltage V _OUT is supplied. On the other hand, during the on period, the base current flows through the transistor 19 through the resistor 16, and the transistor 19
is turned on, and both ends of the capacitor 31 and the Zener diode 32 are short-circuited. Then comparator 33
The output of is H, but since the transistor 18 is turned on by being supplied with the base current through the resistor 15, the transistor 17 is turned off, and the input voltage _V1 of the drive circuit 6 remains H during the on period. becomes. Next, when the output V ₂ of the control circuit 5 becomes L, the main switching element 3 is turned off, and at the same time, the base currents of the transistors 18 and 19 are cut off, and the transistors 18 and 19 are turned off. Then, the constant current source 34 causes the capacitor 3 to
1 is charged, and the comparator 33 outputs H until the voltage V _C across the capacitor 31 reaches the voltage V _TH of the specified voltage source 30, and the transistor 17 is turned on by being supplied with base current through the resistor 14. . As charging of the capacitor 31 progresses and V _C >V _TH , the output of the comparator 33 is inverted and becomes L, so that the base current supply to the transistor 17 is cut off and the transistor 17 is turned off. That is, during the charging period when the capacitor 31 is charged from O _V to V _TH , V ₁ is forcibly set to L by the transistor 17, and even if the flyback voltage disappears within this period, the output of the control circuit 5 Even if V ₂ becomes H, it is not transmitted to the drive circuit 6 and the main switching element 3 remains off, and after the charging period of the capacitor 31 ends and the transistor 17 turns off, V _{1 becomes H and the main switching element 3} remains off. This turns on element 3. Also, when the transistor 17 is turned off, V ₂ becomes low.
If it remains the same, the off period continues, and V ₂ becomes H and transitions to the on period. In this case, the charging of the capacitor 31 further progresses, and eventually the Zener voltage
Clamped to V ₂ , ₁ is Zener diode 3
Flows to 2. Capacitor 3 until V _C becomes V _TH
1 charging period, that is, the regulated minimum off period T ^MIN _OFF is expressed as T ^MIN _OFF = C ₃₁ ·V _TH / ₁ A, where the capacitance of the capacitor 31 is C ₃₁ .

FIG. 2 shows the drain-source voltage V _DS of the main switching element 3, the drain current _D , the current _S flowing through the secondary winding 23, the voltage V _C across the capacitor 31, and the output V of the control circuit 5 during the above operation. ₂ ,
A waveform diagram of the input _V1 of the drive circuit 6 is shown.

Effects of the Invention As described above, according to the present invention, it is possible to regulate the minimum value of the off period of a self-excited oscillation type switching power supply device, in which the on period and off period become shorter as the DC output current becomes smaller. With the main switching element, control circuit, and drive circuit having the same capabilities as the conventional one, it is possible to operate stably with less DC output current than the conventional one, which has a great practical effect.

[Brief explanation of drawings]

Fig. 1 is a circuit configuration diagram showing a self-excited oscillation type switching power supply device according to an embodiment of the present invention, Fig. 2 is a waveform diagram of its main parts, and Fig. 3 is a circuit configuration diagram of a conventional self-excited oscillation type switching power supply device. 4 are waveform diagrams of the main parts thereof. DESCRIPTION OF SYMBOLS 1... Rectifier smoothing circuit, 2... Switching transformer, 3... Main switching element, 4... Starting resistor, 5... Control circuit, 6... Drive circuit, 7...
...Diode, 8...Capacitor, 9...Diode, 10...Capacitor, 11...Detection circuit,
12...Photocoupler, 13-15...Resistance, 1
7 to 18...NPN transistor, 30...Specified voltage source, 31...Capacitor, 32...Zener diode, 33...Comparator, 16...Resistor, 19...NPN transistor, 34...Constant current source.

Claims (1)

[Claims] 1. Converting an input DC voltage into a high-frequency AC voltage by a main switching element and applying it to the primary winding of a switching transformer equipped with a primary winding, a secondary winding, and a bias winding. The high-frequency AC voltage input and output from the secondary winding is rectified and smoothed to supply a specified output DC voltage to the load, and the high-frequency AC voltage generated in the bias winding is detected to control the main switching element. A control circuit having a function of generating a pulse voltage in synchronization with a drop in flyback voltage occurring during an off period and controlling and outputting the pulse width in order to stabilize the output DC voltage, and the main switching element. a drive circuit that inputs the pulse voltage through a resistor, amplifies and outputs the pulse voltage to drive the capacitor, and compares the voltage of the capacitor with a reference voltage source, and outputs a high level when the voltage of the capacitor is lower than the reference voltage. and a comparison circuit, the capacitor is discharged during the ON period of the main switching element, that is, the period when the input pulse voltage of the drive circuit is at a high level, and the input pulse voltage of the drive circuit is set so as to turn off the main switching element. a charging/discharging circuit that starts charging the capacitor when the level becomes low; and an output of the comparison circuit that is driven through a resistor to turn on and off the input pulse voltage of the drive circuit, so that the voltage of the capacitor reaches the reference voltage. a first transistor that maintains the input pulse voltage of the drive circuit at a low level for a period of time; and a first transistor that is driven by the input pulse voltage of the drive circuit to turn on and off between the base and emitter of the first transistor, and and a second transistor that turns off the first transistor during a period when the input pulse voltage is at a high level.