JP2506789B2 - Self-oscillation type switching power supply - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a self-oscillation type switching power supply device used in various electronic devices.

2. Description of the Related Art A conventional self-excited oscillation type switching power supply device has a structure as shown in FIG. In FIG. 3, reference numeral 1 is a rectifying / smoothing circuit, which converts the received AC input voltage into a DC input voltage. A switching transformer 2 has a primary winding 21, a bias winding 22, and a secondary winding 23. A main switching element 3 is turned on and off to convert a DC input voltage into a high frequency AC voltage and input it to the primary winding 21. 4 is starting resistance, 5
Is a control circuit, 6 is a drive circuit, 7 is a diode, and 8 is a capacitor. The high-frequency AC voltage generated in the bias winding 22 is rectified and smoothed by the diode 7 and the capacitor 8, and the bias voltage of the control circuit 5 is supplied. 9 is a diode, 10
Is a capacitor for rectifying and smoothing the high frequency AC voltage generated in the secondary winding 23 and supplying a DC output voltage. Reference numeral 11 is a detection circuit for detecting the DC output voltage, and 12 is a photocoupler for feeding back the information of the DC output voltage detected by the detection circuit 11 to the control circuit 5. The operation of the conventional example will be described below.

The AC input voltage input to the rectifying / smoothing circuit 1 is converted into a DC input voltage, and the control circuit 5 is biased via the starting resistor 4. The biased control circuit 5 outputs a pulse, which is amplified by the drive circuit 6 to turn on / off the main switching element 3. Then, the DC input voltage is converted into a high frequency AC voltage and input to the primary winding 21, and the bias winding
A high-frequency AC voltage is generated in the secondary winding 23 and the secondary winding 23 according to the respective turns ratio. The high frequency AC voltage generated in the bias winding 22 is rectified and smoothed by the diode 7 and the capacitor 8 and supplied to the control circuit 5 as a bias voltage.
The high frequency AC voltage generated in the secondary winding 23 is rectified and smoothed by the flyback voltage by the diode 9 and the capacitor 10 and supplied as the DC output voltage V _OUT . DC output voltage
V _OUT is fed back by the detection circuit 11 to the control circuit 5 via the photocoupler 12 with the information. The control circuit 5 detects the high-frequency AC voltage generated in the bias winding 22, and while the main switching device 3 is on (hereinafter, the off period is the same as the on period). When the excitation energy stored in the switching transformer 2 is exhausted and the flyback voltage disappears, the output is set to H and the main switching element 3 is turned on via the drive circuit 6. Then, receiving the information from the photocoupler 12, the ON period is determined in order to stabilize the DC output voltage V _OUT , and when the ON period is reached, the output is set to L and the main switching element 3 is turned off. By the above operation, the stable DC output voltage V _OUT is supplied. The number of turns of the primary winding 21 is N _P , the number of turns of the secondary winding 23 is N _S , the ON period is T _ON , the OFF period is T _OFF , the DC input voltage is V _IN , the DC output voltage is V _OUT ,
Ignoring the forward voltage of diode 9, V _OUT is given by:

On the other hand, assuming that the DC output current I _OUT and the inductance of the primary winding 21 are L _P , I _OUT is expressed by the following equation.

FIG. 4 shows the drain-source open voltage V _DS of the main switching element 3 and the current flowing through the primary winding 21 in the conventional circuit.
A waveform diagram of I _D and the current I _S flowing through the secondary winding 23 is shown.

Problems to be Solved by the Invention In such a conventional configuration, as can be seen from the equations (1) and (2), when the DC output current I _OUT decreases, T _ON and T _OFF decrease in proportion thereto. It will become. However there is a minimum limit value T _{ON MIN} to T _ON by the ability of the main switching device 3 and the drive circuit 6 or the control circuit 5, when you try to less than this T _ON, or causing intermittent oscillation, a main switching element 3 causes Class A operation. When intermittent oscillation occurs, the response to the transient phenomenon that the DC output current changes suddenly becomes poor, when the frequency ripple component of intermittent oscillation occurs in the output voltage, or when class A operation occurs, There is a problem that the main switching element 3 generates heat.

As a means for solving this problem, a circuit configuration as shown in FIG. 5 has been devised. In FIG. 5, 1 to
12 is equivalent to that shown in the above-mentioned conventional example. 13 to 16 are resistors, 17 to 19 are NPN transistors, 30 is a specified voltage source,
Let that voltage be V _TH . 31 is a capacitor, the voltage across which is V _C. 32 is a zener diode, and its zener voltage is V _Z. 33 is a comparator,
V _C is input and V _TH is input to the input terminal. 34 is a constant current source, and its current value is I ₁ . The operation will be described below.

The AC input voltage input to the rectifying / smoothing circuit 1 is converted into the DC input voltage V _IN , and the control circuit 5 is biased 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 it is amplified and output so that the main switching element 3 can be turned on and off. The DC input voltage V _IN is converted into a high frequency AC voltage by the main switching element 3 and input to the primary winding 21, and the bias winding 22 and the secondary winding 23 are supplied with the high frequency AC voltage according to the respective turns ratio. Occurs. The high frequency AC voltage generated in the bias winding 22 is rectified and smoothed by the diode 7 and the capacitor 8 and supplied to the control circuit 5 as a bias voltage. The high frequency AC voltage generated in the secondary winding 23 is rectified and smoothed by the flyback voltage by the diode 9 and the capacitor 10 and supplied as the DC output voltage V _OUT . The DC output voltage V _OUT is fed back by the detection circuit 11 to the control circuit 5 via the photocoupler 12 with its information. Control circuit 5
Detects the high-frequency AC voltage generated in the bias winding 22, and when the excitation energy stored in the switching transformer 2 is exhausted during the ON period and the flyback voltage disappears, the output is set to H, the resistor 13, the drive The main switching element 3 is turned on via the circuit 6. Then, receiving the information from the photocoupler 12, the ON period is determined in order to stabilize the DC output voltage V _OUT , and when the ON period is reached, the output is set to L and the main switching element is turned off. By the above operation, the 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, turns ON the transistor 19, and short-circuits both ends of the capacitor 31 and the Zener diode 32. Then, the output of the comparator 33 is H, but the transistor 18
However, since the base current is supplied through the resistor 15 to turn it on, the transistor 17 is turned off, and the input voltage V ₁ of the drive circuit 6 remains H during the on period. 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 transistors 18 and 19 are turned off because the supply of the base current is cut off. Then, the capacitor 31 is charged by the constant current source 34, the comparator 33 outputs H until the voltage V _C across the capacitor 31 becomes the voltage V _TH of the specified voltage source 30, and the transistor 17 is connected to the base via the resistor 14. It is turned on by being supplied with current. Charging of the capacitor 31 progresses, and V _C > V
_When it becomes _TH , the output of the comparator 33 is inverted and becomes L, so that the transistor 17 cuts off the supply of the base current and is turned off. That is, V ₁ is forcibly set to L by the transistor 17 during the charging period until the capacitor 31 changes from 0V to V _TH , and even if the flyback voltage disappears within this period, the output V _{2 of the} control circuit 5 is reduced. H is not transmitted to the drive circuit 6, the main switching element 3 remains off, and V ₁ becomes H after the transistor 17 turns off after the charging period of the capacitor 31 ends and the main switching element 3 Turn on. If V ₂ remains L when the transistor 17 turns off, the off period continues and V ₂
Becomes H and shifts to the ON period. In this case, the capacitor 31 is further charged, and is eventually clamped to the Zener voltage V _Z , and I ₁ flows to the Zener diode 32. V _C
The charging period of the capacitor 31 until _reaching V _TH , that is, the regulation minimum off period ▲ T ^MIN _OFF ▼, ^changes the capacitance of the capacitor 31 to C ₃₁
Then Is represented by Thus, once the main switching element 3 is turned off, the DC output current is reduced by forcibly maintaining the off state for a certain period of time, and even if the on period and the off period are shortened, the off period is reduced. Since the minimum value is provided for the ON period, the ON period is not shortened as compared with the conventional case, and stable operation can be performed up to a smaller DC output current. The minimum limit value of the ON period is ▲ T ^MIN _ON ▼, and ▲
^Assuming that T _OFF ′ is the period during which the flyback voltage is generated with respect to T ^MIN _ON ▼, that is, the period when the excitation energy of the switching transformer 2 is completely released, then T _OFF ′ is _calculated from the equation (1). The minimum output current ▲ I ^MIN _OUT ▼ that can be stably obtained with the conventional configuration is expressed by the above equation (2). Becomes On the other hand, when the above-mentioned means is provided and the minimum regulation off period is ▲ T ^MIN _OFF ▼, Therefore, from equations (5) and (6), (▲ T ^MIN
_ON ▼ + T _OFF ′) / ▲ T ^MIN _ON ▼ + ▲ T ^MIN _OFF ▼) only ▲
It is possible to reduce I ^MIN _OUT ▼. ▲ T ^MIN _OFF ▼ ＞
Clearly, setting it to T _OFF ′.

However, this circuit configuration also has the following problems. After the turn-on, if the ON period T _ON is shorter than the time to discharge the potential of the capacitor 31 to 0 _V , the OFF period will start before the capacitor 31 is discharged,
The minimum off period cannot be set normally. FIG. 6 shows this state. The drain-source voltage V _DS of the main switching element 3, the current I _D flowing in the primary winding 21,
7 is a waveform diagram of a current I _S flowing through the next winding 23 and a voltage V _C across the capacitor 31.

Means for Solving the Problems In order to solve this problem, the present invention detects a high frequency AC voltage generated in a bias winding and reduces the flyback voltage generated during the off period of the main switching element. A pulse voltage is generated in synchronization with the control circuit having a function of controlling and outputting the pulse width of the output DC voltage to stabilize the output DC voltage, and the pulse voltage via a resistor to drive the main switching element. In addition to a drive circuit for inputting, amplifying, and outputting the input voltage, a charge / discharge circuit for charging / discharging the capacitor in synchronization with the input pulse voltage of the drive circuit, a voltage across the capacitor, and a first specified voltage value are provided. A comparison circuit for comparison, a first transistor that drives the output of the comparison circuit via a resistor to turn on / off the input of the drive circuit, and an input transistor of the drive circuit. A second transistor that receives a pulse and turns on / off the base / emitter of the first transistor, and once the main switching element is turned off, the main switching element is forcibly kept off for at least a prescribed time, In addition, the charging / discharging circuit is provided with a function of sustaining discharging up to a second specified value provided at least separately when the capacitor is once discharged.

Operation With this configuration, once the main switching element is turned off, it is forcibly kept off for a certain period of time, which allows stable operation up to a smaller DC output current. Even if
The above-mentioned minimum regulation off period can be normally set.

Embodiment FIG. 1 is a circuit configuration diagram showing a self-excited oscillation type switching power supply device according to one embodiment of the present invention. In FIG. 1, 1 to 19 and 30 to 33 are the same as those of the conventional example shown in FIG. It is the same. Reference numeral 40 denotes a discharge circuit, which supplies a base current from the terminal 43 to the transistor 19 when a voltage is applied to the terminal 41 via the resistor 16. The terminal 42 detects the voltage V _C across the capacitor 31, and once a voltage is applied to the terminal 41 and a base current is made to flow from the terminal 43, at least V _C becomes a specified voltage value ▲ V ^′ _TH ▼ set separately. Until then, it has the function of sustaining the base current from the terminal 43.

FIG. 2 shows two specific examples of the discharge circuit 40. The operation of FIG. 2A will be described first. 50 in FIG. 2 (a)
Is a comparator, 51 is an XOR circuit, 52 and 53 are AND circuits, and 54 is an R circuit.
It is an S flip-flop. After turn-on, since terminal 41 is H and terminal 42 has a charging voltage, the output 55 of comparator 50 is L because V _C > V _TH2 . Therefore, output 56 of XOR circuit 51 is H and output 57 of AND circuit 52 is H, the output 58 of the AND circuit 53 is L,
The output of the RS flip-flop 54, that is, the terminal 43 becomes H, and the transistor 19 is operated. When transistor 19 operates, V _C discharges and falls, but it turns on when V _C > V _TH2.
Even if it is turned off and the terminal 41 becomes L, 55 becomes L, 56 becomes L, 57 becomes L, 58 becomes L, and the terminal 43 maintains H. Eventually V _C <V
_When it becomes _TH2 , 55 becomes H, 56 becomes H, 57 becomes L, 58 becomes H, the terminal 43 is inverted and becomes L, the transistor 19 is turned off, and V _C is charged again and rises. V _C rises and V _C ＞ V _TH2
Even if 55 becomes L, since it is an off period, the terminal 41 is L, 56 is L, 57 and 58 are L, L, and the terminal 43 maintains L. Further to turn on again repeating the above operations, the terminal 43 discharges the V _C until V _C <V _TH2 becomes H.

Next, FIG. 2 (b) will be described. In Figure 2 (b)
Reference numerals 61 to 64 denote current sources, and particularly, the current value of the current source 62 is I ₁ and the current value of the current source 64 is I ₂ . 65 to 68 are PNP transistors which form a comparator. 69 is NPN transistor, 7
0 and 71 are resistors, and the resistance value of the resistor 71 is R _TH .
After turning on, when the terminal 41 becomes H, the transistor 69 is turned on, the resistor 71 is short-circuited, the transistors 67 and 68 operate, and the base current is supplied from the terminal 43 to the transistor 19 to operate. When transistor 19 operates, V _C discharges and drops, but it turns off while V _C does not drop sufficiently and the base current from terminal 41 to transistor 69 is cut off. _Has a potential of V _TH2 = I ₂ · R _TH , and while V _C > V _TH2 , the transistors 67 and 68 continue to operate and the base current from the terminal 43 to the transistor 19 continues to be supplied. When V _C <V _TH2 , the transistor 6
7,68 turns off, and transistors 65,66 operate. Then, the base current from the terminal 43 to the transistor 19 is cut off,
Transistor 19 turns off and V _C charges and rises. On the other hand, a potential of (I ₁ + I ₂ ) · R _TH is generated across the resistor 71. I ₁ , I ₂ , R _TH are set so that (I ₁ + I ₂ ) · R _TH is larger than the Zener potential of the Zener diode 32, and V _TH2 = I ₂ · R _TH is sufficiently small. Then turn again
On the repeat the operation, the terminal 43 supplies the base current to the transistor 19 to discharge the V _C until V _C <V _TH2.

EFFECTS OF THE INVENTION As described above, according to the present invention, once the main switching element is turned off, the main switching element is forcibly kept off for a certain period of time (minimum regulation off period), thereby achieving stable operation up to a smaller DC output current. In addition to the feature that the ON period can be shortened, the regulation minimum OFF period can be normally set.

[Brief description of drawings]

FIG. 1 is a circuit configuration diagram of a self-excited oscillation type switching power supply device according to an embodiment of the present invention, and FIGS. 2a and 2b are electrical diagrams illustrating two specific examples of a discharge circuit which is a main part of FIG. 3 is a schematic circuit diagram, FIG. 3 is a circuit configuration diagram of a conventional self-excited oscillation type switching power supply device, FIG. 4 is an operation waveform diagram of a main part of FIG. 3, FIG. 5 is an improved circuit configuration diagram of a conventional circuit, FIG. 6 is the fifth
It is a main part operation | movement waveform diagram of a figure. 1 ... Rectifying and smoothing circuit, 2 ... Switching transformer, 3
...... Main switching element, 4 ... Starting resistance, 5 ... Control circuit, 6 ... Drive circuit, 7 ... Diode, 8 ...
Capacitor, 9 ... Diode, 10 ... Capacitor, 11
...... Detection circuit, 12 ...... Photo coupler, 13 to 16 ...... Resistance,
17 to 19 …… NPN transistor, 21 …… Primary winding, 22 ……
Bias winding, 23 …… Secondary winding, 30 …… Voltage source, 31 ……
Capacitor, 32 ...... Zener diode, 33 …… Comparator, 40 …… Discharge circuit, 41 to 43 …… Discharge circuit 40 terminals, 50 …… Comparator, 51 …… XOR circuit, 52, 53 …… AN
D circuit, 54 …… RS flip-flop, 61-64 …… current source, 65-68 …… PNP transistor, 69 …… NPN transistor, 70-71 …… resistor.

Claims (1)

(57) [Claims] 1. A switching transformer having at least a primary winding, a secondary winding, and a bias winding, which converts an input DC voltage into a high-frequency AC voltage by a main switching element.
The high frequency AC voltage that is input to the secondary winding and is rectified and smoothed by rectifying and smoothing the high frequency AC voltage that is output from the secondary winding is supplied to the load, and the high frequency AC voltage that is generated in the bias winding is detected. A control having a function of generating a pulse voltage in synchronization with the reduction of the flyback voltage generated during the off period of the main switching element and controlling the pulse width to output the output DC voltage to stabilize it. In addition to a circuit and a drive circuit for inputting, amplifying and outputting the pulse voltage via a resistor to drive the main switch element,
A charge / discharge circuit that charges and discharges a capacitor in synchronization with an input pulse voltage of the drive circuit, a comparison circuit that compares the voltage across the capacitor with a first specified voltage value, and the output of the comparison circuit via a resistor. And a second transistor that is driven by a transistor to turn on / off the input of the drive circuit, and a second transistor that receives an input pulse of the drive circuit to turn on / off the base-emitter of the first transistor. Once the main switching element is turned off, the main switching element is forcibly kept off for at least a specified time, and at least once the capacitor is started to be discharged into the charge / discharge circuit. A self-excited oscillation type switching power supply device provided with a function of sustaining discharge until a separately provided second specified voltage value is reached.