JPS6148346B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a control system for a switching power supply using an inverter or a converter.

(Prior Art and Problems) Conventionally, various types of switching power supplies have been used, such as a one-stone flyback type, a one-stone forward converter type, a dropper type, a push-pull type, and a bridge type. By the way, when these switching power supplies are used as large-capacitance load power supplies such as high-output strobe xenon lamp drive power supplies, or when an output terminal short-circuit accident occurs,
The switching element current increases due to DC bias of the output transformer and response delay of the switching element, and there is a risk that the switching element may be damaged due to excessive current, so it is necessary to take some countermeasures. . In other words, in a typical switching power supply, only the primary current of the output transformer (or the collector current of the switching transistor) is detected, and the switching transistor performs a switching operation at a constant frequency. With this configuration, in the event of a short-circuit accident between the output terminals or a large capacitive load, the decrease in the secondary current (reduction in magnetic energy) of the output transformer within a certain period of time is small, so the output transformer large DC bias, which causes a drop in inductance. For this reason, the collector current of a switching transistor that performs a constant frequency switching operation has a large initial value when it is on, so even if the collector current value of the switching transistor is detected and the switching transistor is turned off in response to an excessive current, Even if you do this, it is unavoidable that the collector current will increase due to the storage time of the switching transistor, etc. Furthermore, the decrease in inductance due to the DC bias of the output transformer and the increase in the collector current mutually promote each other, so the output transformer Magnetic saturation is reached and excessive current flows through the switching transistor, leading to its destruction. To avoid this inconvenience, an overcurrent protection circuit is installed.
Although it uses a high-speed switching element and an output transformer with sufficient extra power, it has the disadvantage of being expensive and large.

(Object of the Invention) The present invention eliminates the above-mentioned disadvantages, and detects a decrease in the secondary current to confirm that the magnetic energy of the secondary inductor of the switching power supply has been sufficiently released, and then switches the switching transistor. By turning it on, you can reliably prevent damage to the switching element due to output terminal short-circuit accidents or connection of capacitive loads, improve reliability, stabilize the output voltage, and further reduce the current. By using a current transformer as a detection means, the present invention attempts to provide a control system for a switching power supply that facilitates insulation between a primary side circuit and a secondary side circuit.

(Example) Hereinafter, an example of a control system for a switching power supply according to the present invention will be described with reference to the drawings.

FIG. 1 is a reference example for explaining the basic principle of the present invention, and shows a case where the present invention is applied to a single-stone flyback type switching power supply. In this figure, a transistor 2 is connected in series as a switching element to one end of a primary winding 1A of an output transformer 1.
The other end of the next winding 1A is connected to the positive power supply terminal P of the DC power supply,
The emitters of the transistors 2 are respectively connected to negative power supply terminals N via current detection resistors 3, and the negative power supply terminals N are grounded. Secondary winding 1B of transformer 1
A rectifier circuit consisting of a diode 4 and a capacitor 5 is connected to the terminals 4 and 5, so that DC output is outputted to output terminals A and B. Further, a current detection resistor 6 is inserted between the capacitor 5 and the secondary winding 1B. Note that an input transformer 7 is provided on the base side of the transistor 2.

On the other hand, the control circuit 10 includes a drive circuit 11 that outputs a drive signal to the transistor 2 via the input transformer 7, and a first comparator 1 that receives the voltage e generated in the resistor 3 at its non-inverting input terminal.
2, a second comparator 13 that receives the voltage e ₂ generated in the resistor 6 at its inverting input terminal, an AND gate 14, and a flip-flop 15 that controls activation and deactivation of the drive circuit 11. The other input terminals of the first comparator 12 and the second comparator 13 each have a reference voltage.
Vref ₁ and Vref ₂ are applied, the output of the first comparator 12 is applied to the reset input R of the flip-flop 15, and the output of the second comparator 13 is applied to one input terminal of the AND gate 14, respectively. The other input terminal of the AND gate 14 is connected to the inverted output Q of the flip-flop 15 through a resistor 1 to prevent malfunctions due to time delays in the circuit.
6 and a capacitor 17, and the output of the AND gate 14 is applied to the set input S of the flip-flop 15. The non-inverted output Q of the flip-flop 15 is applied to the drive circuit 11, and the drive circuit 11 applies a drive signal to the transistor 2 to turn on the transistor 2 while the output Q is "1" (high level).

In the above configuration, in the initial state such as when the power is turned on (however, it is assumed that a general resistive load is connected between output terminals A and B), the primary currents I ₁ and 2 of transformer 1 are Since the next current I ₂ is zero,
The output of the first comparator 12 is "0" (low level), and the output of the second comparator 13 is "1". Therefore, flip-flop 5 is set even in the reset state, and transistor 2 is turned on and started via drive circuit 11. The primary current I ₁ during the on-period of transistor 2 is I ₁ = E/L ₁ t + I ₀ ...(1) (E: power supply voltage, L ₁ : primary inductance of transformer 1, I ₀ : transformer 1's primary inductance). The equivalent primary current of residual energy (t: time after transistor 2 is turned on) increases with time with a slope of E/L ₁ , as shown in FIG. 2A. This primary current I ₁ is connected to resistor 3
is detected as a voltage e ₁ by the first comparator 12 and compared with the reference voltage Vref ₁ . And e ₁
>Vref ₁ , the output of the first comparator becomes "1" and the flip-flop 15 is reset. As a result, the non-inverted output Q of the flip-flop 15 becomes "0" and the transistor 2 is turned off. When the transistor 2 is turned off, the magnetic energy of the transformer 1 stored during the on period of the transistor 2 is released from the secondary winding 1B to the load side through the diode 4 as a secondary current _I2 . Here, the secondary current I ₂ is I ₂ = I′ ₀ −Eout/L ₂ t′ ……(2) (Eout: maximum value of secondary output voltage, L ₂ :
As shown by the secondary inductance of transformer 1, _I'0 : equivalent secondary current of the residual energy of transformer 1, t': time after transistor 2 is turned off, as time passes as magnetic energy is released, Therefore, the fracture shape shown in FIG. 2B is reduced. This secondary current I ₂ is turned into a voltage by resistor 6.
e ₂ and compared with the reference voltage Vref ₂ by the second comparator 13. Then, when e ₂ <Vref ₂ , the output of the second comparator 13 becomes "1". At this time, since the inverted output of flip-flop 15 is "1", the output of AND gate 14 also changes to "1", flip-flop 15 is set again, and transistor 2 is turned on. After that, the primary current I ₁ increases and the detected value e ₁ becomes the reference voltage.
When Vref ₁ is reached, transistor 2 is turned off,
The secondary current I ₂ decreases and the detected value e ₂ becomes the reference voltage Vref ₂
When the voltage decreases below , the switching operation in which transistor 2 is turned on is repeated.

In addition, Fig. 2 A and B show the primary current I ₁ and secondary current I ₂ when the load is a general resistance load.
However, in the case of an output short circuit or a large capacitive load, the primary current I ₁ becomes as shown in FIG. 7A and the secondary current as shown in FIG. 7B. That is, since the transistor 2 is turned on after confirming that the magnetic energy has been sufficiently released, the off period of the transistor 2 becomes longer.

According to the reference example in Fig. 1 above, 1 of transformer 1
The control circuit 10 not only controls the secondary current I ₁ but also controls the secondary current I ₂ .
Since the timing of turning on and off transistor 2 is controlled by detecting the
, or in the case of a large capacitive load, the large DC bias of transformer 1 that tends to occur in a power supply with constant frequency switching operation, the resulting decrease in inductance, and the response delay of transistor 2 when turned off. The increase in the collector current described in No. 2 can be avoided with a simple configuration. That is, since the secondary current I ₂ decreases with a slope of Eout/L ₂ , when the output voltage Eout is small, the decrease in the secondary current within a certain period is small. Therefore, if the above control is not performed, the collector current (=I ₁ ) of transistor 2 will have a large initial value when it is on, so for example, if transistor 2
Even if the collector current value of transformer 1 is detected and transistor 2 is turned off in response to an excessive current, the collector current will inevitably increase due to the storage time of transistor 2, etc. Since the decrease in inductance due to magnetism and the increase in collector current mutually promote each other, the transformer 1 eventually reaches magnetic saturation, and an excessive current flows through the transistor 2, leading to its destruction. According to this reference example, the secondary current _I2 of transformer 1 is the reference voltage
Since the transistor 2 remains off until the voltage decreases to a certain value determined by Vref ₂ , the above-mentioned phenomenon can be reliably prevented.

It should be noted that even a switching power supply using a self-oscillating blocking oscillator is inferior in ease of control to achieve the same operation mode as in the above-mentioned reference example. That is, in the above reference example, a three-input AND gate is used instead of the two-input AND gate 14, and the advantage is that operation or stop can be controlled by setting one of the inputs to "1" or "0". be. However, the reference example shown in FIG. 1 does not have an output stabilization function.

FIG. 3 shows a first embodiment of the present invention in which the primary current and secondary current of the output transformer are detected via current transformers, and the output voltage is also detected to stabilize the output. . In this figure, a switching transistor 2 is connected in series to one end of a primary winding 1A of an output transformer 1, and an AC input terminal C is connected between the other end of the primary winding 1A and the emitter of the transistor 2. , D is rectified by a rectifier 20 and smoothed by a capacitor 21, and a DC voltage is applied thereto. transformer 1 of 2
A rectifier circuit consisting of a diode 4, an inductor 22, and capacitors 5A and 5B is connected to the next winding 1B, so that a DC output is output between output terminals A and B. A secondary winding 7B of an input transformer 7 is connected to the base circuit of the switching transistor 2, and a drive transistor 23 is provided on the primary winding 7A side of the transformer 7.

On the other hand, the control circuit 10A includes a current transformer 30 for detecting the primary current _I1 of the transformer 1, and a current transformer ₃ for detecting the secondary current I2 of the transformer 1.
1, an operational amplifier 32 as an error amplifier that outputs the error between the output voltage value between output terminals A and B and the desired voltage value, and the detected value of the primary current I ₁ and the reference voltage.
Vref ₁ (set equal to the voltage of the second winding 30B corresponding to the turn-off threshold of the primary current I ₁ )
A first comparator 12 that compares the secondary current
A second comparator 13 that compares the detected value of I ₂ with a reference voltage Vref ₂ (defined as being equal to the voltage of the second winding 31B corresponding to the turn-on threshold of the secondary current I ₂ ), and an AND gate 14A. , and a flip-flop 15. The first winding 30A of the current transformer 30 is inserted into the collector side of the transistor 2, the induced voltage e _CT1 of the second winding 30B is applied to the first comparator 12, and the third winding 30C
is connected to the output terminal of the operational amplifier 32 via a diode 33 and a resistor 34. Operational amplifier 3
A resistor 50 and a variable resistor 5 are connected to one input of 2.
1. A voltage obtained by dividing the output voltage by a resistor 52 is applied, and a voltage obtained by dividing the output voltage by a resistor 53 and a voltage regulating diode 54 is applied via a resistor 55 to the other input. In addition, the first winding 3 of the current transformer 31
1A is inserted in series with the secondary winding 1B of the transformer 1, and the induced voltage e _CT2 of the second winding 31B is applied to the second comparator 13. The inverted output of the flip-flop 15 is applied to the first input of the AND gate 14A via an amplifier circuit consisting of a resistor 16 and a capacitor 17, and the output of the second comparator 13 is applied to the second input. A synchronization signal supplied between synchronization input terminals F and G is applied to the input of No. 3 via a capacitor 35. The non-inverting output Q of the flip-flop 15 is applied to the drive transistor 23 via the resistor 36, and the transistor 23 is activated while the output Q is "1" (high level). is now turned on. Note that the voltage across a constant voltage diode 38 connected to the output side of the rectifier 20 via a resistor 37 is supplied as a power source to the control circuit 10A, and the voltage is connected to the resistor 39,
The reference voltage is divided by 40, 41, and 42, respectively.
First and second comparators 1 as Vref ₁ and Vref ₂
2 and 13, and is also applied to the third input of the AND gate 14A via a parallel circuit of a resistor 43 and a diode 44.

In the above configuration, first, considering the case where the synchronization signal is not input to the synchronization input terminals F and G, the third input of the AND gate 14A is connected to the resistor 4.
3, it becomes "1", and the primary current I ₁ and 2
Since the secondary current I ₂ is detected by the current transformers 30 and 31, the basic operation is the same as in the reference example described above, and the primary current I ₁ increases to reach the turn-off threshold. When the secondary current I 2 reaches the turn-off threshold, the transistor 2 is turned off, and when the secondary current I ₂ decreases below the turn-off threshold, the transistor 2 is turned on. The switching operation is repeated, and the primary current I ₁ becomes as shown in FIG. 9A. The secondary current I ₂ has the waveform shown in FIG. 9B. Here, the dotted line H shows the turn-off threshold, and the dotted line J shows the turn-on threshold.

Now, since the current transformer 30 has the third winding 30C, the voltage e _CT1 induced in the second winding 30B is determined by the equivalent load impedance (at both ends of the winding) as seen from the third winding 30C. It also changes depending on the voltage/current flowing into the circuit from the windings). The relationship between the primary current I ₁ and the voltage e _CT1 is shown in FIG. Here, straight line A is when the equivalent load impedance is small, and straight line B is when the equivalent load impedance is large. That is, as the output voltage increases, the output voltage of the operational amplifier 32 increases, and the equivalent load impedance (voltage at both ends of the winding/current flowing into the circuit from the winding) increases. As a result,
The voltage e _CT1 increases and at a smaller collector current the output of the converter 12 changes to "1", resetting the flip-flop 15 and turning off the transistor 2. In other words, the turn-off threshold in FIG. 9A has fallen to the dotted line H', and the primary current I ₁ becomes as shown in FIG. 9C in accordance with the lowered turn-off threshold H'. Accordingly, the secondary current I ₂ has a waveform as shown in FIG. 9D. In this case, the turn-on threshold J of the secondary current I ₂ does not change. Therefore, the on period of transistor 2 is shortened and the output voltage is reduced. The output voltage is kept constant by such a negative feedback effect that controls the output voltage.

When adding a synchronizing signal from, for example, the flyback transformer 60 of a television receiver to the synchronizing input terminals F and G, the natural operating frequency in steady state when no synchronizing signal is input is set to be slightly higher than the synchronizing signal frequency. If the flip-flop 15 is set and the output of the second comparator 13 is "1" and the synchronization signal becomes "1" (high level), the output of the AND gate 14A is "1". Then, flip-flop 15 is set and transistor 2 is turned on. Therefore, the switching power supply operates in synchronization with the synchronizing signal as shown in Fig. 4C, and the primary current _I1 changes as shown in the waveform of Fig. 4A, and the secondary current _I2 changes as shown in the waveform of Fig. 4B. become. Here, the dotted line H shows the turn-off threshold, and the dotted line J shows the turn-off threshold.

According to the first embodiment, it is possible to reliably prevent damage caused by excessive current flowing through the transistor 2 with a simple circuit configuration in the case of a short circuit between the output terminals A and B or in the case of a large capacitive load. At the same time, stabilization control of the output voltage is possible. Furthermore, since it operates naturally at startup and can operate synchronously during normal operation, it is ideal for power sources for television receivers, etc. Furthermore, since the primary current I ₁ and the secondary current I ₂ are detected using the current transformers 30 and 31, there is an advantage that the primary side circuit and the secondary side circuit can be isolated.

Note that in the first embodiment, three
The turn-off threshold of the collector current of the transistor 2 is made variable according to the output voltage by using a current transformer 30 having several windings, but it is possible to reduce fluctuations in the natural operating frequency when load fluctuations are large. For this purpose, the turn-on threshold of the secondary current _I2 of the transformer 1 may also be changed depending on the output voltage.

FIG. 5 shows a second embodiment of the present invention, which is applied to a one-stone forward converter type switching power supply. In this figure, a switching transistor 2 is connected in series to one end of a primary winding 70A of an output transformer 70.
The other end of 0A is connected to the positive power supply terminal P of the DC power supply, and the emitter of the transistor 2 is connected to the negative power supply terminal N. A diode 71, an inductance 2, and a capacitor 73 are connected to the secondary winding 70B of the transformer 70.
A diode 74 is provided as a current path when the inductance 72 releases energy, and a DC output is output to the output terminals A and B.

On the other hand, the control circuit 10B includes a current transformer 80 that detects the current I ₃ of the inductor 72 when the transistor 2 is on, that is, the current of the diode 71.
and the inductor 72 during the off period of the transistor 2.
The current transformer 31 detects the current I ₄ , that is, the current of the diode 74, and the detected value of the current I ₃ and the reference voltage Vref ₃ (the voltage of the second winding 80B corresponding to the turn-off threshold of the current I ₃ A first comparator 12 compares the detected value of the current I ₄ with a reference voltage Vref ₄ (defined equal to the voltage of the second winding 81B corresponding to the turn-on threshold of the current I ₄ ). a second comparator 13 for comparison;
AND gate 14A and flip-flop 15
and a drive circuit 11 controlled by the flip-flop 15. Furthermore, output terminal A,
It is equipped with an error amplifier consisting of a series circuit of a constant voltage diode 82 and a resistor 83 connected between B, a transistor 84 which receives the voltage at the connection point thereof at its base, and a low resistor 85 on the collector side. . The first winding 80A of the current transformer 80 is inserted in series with the secondary winding 70B of the transformer 70, the induced voltage e _CT3 of the second winding 80B is applied to the first comparator 12, and the third winding 80C is connected to resistor 85 via diode 86. The first winding 81A of the current transformer 81 is connected to the diode 7.
The induced voltage e _CT4 of the second winding 81B is applied to the second comparator 13, and the third
Winding 81C is connected to resistor 85 via diode 87.
connected to. Note that a DC voltage +V is supplied as a power source for the control circuit 10B, and a synchronization signal can be applied to the AND gate 14A from a synchronization input terminal K.

In the above configuration, in the initial state, diode 7
Since both the current I ₃ of the transistor 1 and the current I ₄ of the diode 74 are zero, the flip-flop 15 is set as in the case of FIG. 1, and the transistor 2 is turned on via the drive circuit 11 and started. As a result, during the ON period of the transistor 2, the input power is supplied to the load connected to the output terminals A and B through the transformer 70, the diode 71, and the inductor 72. And transistor 2
When the current _I3 , which is approximately proportional to the collector current of the current I3, increases and reaches the turn-off threshold, _eCT3 > _Vref3 , the flip-flop 15 is reset and the transistor 2 is turned off. During this off period, the energy stored in the inductor 72 is released to the load via a path passing through the diode 74. Then, when the current I ₄ falls to the turn-on threshold and e _CT4 <Vref ₄ , the flip-flop 15 is set and the transistor 2 is turned on.

The switching operation of the transistor 2 is performed as described above, and synchronization can be achieved as in the first embodiment by adding the synchronizing signal K as necessary. In addition, a resistor 85 is used as a load for the third windings 80C and 81C of the current transformers 80 and 81.
Since the error amplifier including the transistor 84 is connected, stabilization control of the output voltage is also possible.

In the second embodiment, the collector current of the transistor 2 may be measured instead of detecting the current _I3 of the inductor 72 when the transistor 2 is in the on state.

FIG. 6 shows a third embodiment of the present invention, which is applied to a push-pull type switching power supply. In this figure, a switching transistor 9 is connected to the primary winding 90A of the output transformer 90.
1 and 92 are push-pull connected, and the base circuits of these transistors 91 and 92 are provided with input transformers 93 and 64, respectively. The center tap of the primary winding 90A is connected to the positive power supply terminal P, and the emitters of the transistors 91 and 92 and the input transformer 9
One end of the secondary side of terminals 3 and 94 is connected to the negative power supply terminal N, respectively. A double-wave rectifier circuit consisting of diodes 95, 96, an inductor 97, and a capacitor 98 is provided in the secondary winding 90B of the output transformer 90, so that a DC output is output between output terminals A and B.

On the other hand, the control circuit 10C includes transistors 91,
Current transformer 100 for detecting emitter current of 92
and a current transformer 101 that detects the alternating current component of the total current of the inductor 97, and the other configurations are the same as the control circuit 10 in the first embodiment. However, the drive circuit 11A includes a logic circuit such as a well-known T flip-flop and a gate circuit, and inputs a push-pull drive signal to each transistor 91 and 92 through the transformer 9.
3,94.

The operation of the third embodiment is similar to that of the reference example described above, except that the switching transistors 91 and 92 are turned on alternately, and the effects are also the same.

Note that in the third embodiment, the inductor 9
The alternating current component of the total current of 7 is detected by the current transformer 101 and used as it is as a detection signal for the inductor current when the transistors 91 and 92 are off. , the output of the current transformer 101 may be used by regenerating the DC component. Furthermore, output voltage control, external signal synchronization, etc. can be performed by the means shown in the first and second embodiments.

In each of the above embodiments, a resistor or a current transformer is used as the current detection circuit, and the detection signal waveform is used as is. However, various modifications such as various waveform corrections and shaping to make timing easier are possible. It is also clear that it can be done.

(Effects of the Invention) As described above, according to the present invention, it is possible to reliably prevent damage to the switching element due to an output terminal short-circuit accident or connection of a capacitive load, etc., improve reliability, and improve output voltage. Furthermore, by using a current transformer as a current detection means, it is possible to obtain a switching power supply control system that facilitates insulation between the primary side circuit and the secondary side circuit.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram of a reference example to explain the basic principle of the switching power supply control method of the present invention, Fig. 2 is a waveform diagram to explain its operation, and Fig. 3 is a control example of the switching power supply according to the present invention. A circuit diagram showing a first embodiment of the method, FIG. 4 is a waveform diagram for explaining the operation when synchronized with a synchronization signal in the first embodiment, and FIG. 5 is a circuit diagram showing a second embodiment. 6th
The figure is a circuit diagram showing the third embodiment, Figure 7 is a waveform diagram supplementing the operation explanation of the reference example in Figure 1, Figure 8 is a graph for explaining the operation of the first embodiment, and Figure 9 is FIG. 3 is a waveform diagram for explaining a stabilizing operation in the first embodiment. 1, 70, 90...output transformer, 2, 23, 8
4, 91, 92...transistor, 3, 6, 16,
36, 37, 39 to 43, 50, 52, 53,
55, 83, 85...Resistor, 4, 33, 44, 7
1, 74, 86, 87...diode, 5, 5A,
5B, 17, 21, 35, 73, 98... Capacitor, 7, 93, 94... Input transformer, 10, 10
A to 10C...control circuit, 11...drive circuit,
12... First comparator, 13... Second comparator, 14, 14A... AND gate, 15... Flip-flop, 22, 72... Inductor, 30, 3
1, 80, 81, 100, 101... current transformer,
32...Operation amplifier.

Claims (1)

[Scope of Claims] 1. An output transformer having a primary winding and a secondary winding, a switching element that switches an input DC voltage supplied to the primary winding side, and a drive circuit that drives the switching element. Detecting one or both of the currents flowing in the primary winding and the secondary winding of the output transformer via a current transformer, and detecting the detected value of the current in the secondary winding as a first winding. A comparing means compares the switching element with a first reference value to turn on the switching element via the drive circuit, and a second comparing means compares the detected value of the current of the primary winding to a second reference value. In a switching power supply that repeats the operation of turning off the switching element by comparing it with a reference value, an error in the output voltage of the output transformer secondary winding side from a desired value is detected by an error amplifier, and the output voltage of the error amplifier is detected. By changing the equivalent load impedance of a circuit connected to the winding of the current transformer, one or both of the detected values compared by the first and second comparing means is changed. A switching power supply control method characterized by stabilizing the output voltage on the secondary winding side. 2. An output transformer having a primary winding and a secondary winding, a switching element that switches the input DC voltage supplied to the primary winding side, a drive circuit that drives the switching element, and an output transformer that switches the input DC voltage supplied to the primary winding. a rectifier circuit having a series circuit of a first diode and an inductor connected to a secondary winding, and a second diode serving as a current path when the inductor releases energy; detecting the current of the first current through the first current transformer;
A detected value of the current transformer is compared with a first reference value by a first comparing means, and the switching element is turned on via the drive circuit, and the current of the primary winding of the output transformer is detecting a substantially proportional current in the secondary winding via a second current transformer, and comparing the detected value of the second current transformer with a second reference value by a second comparing means; In a switching power supply that repeatedly turns off the switching element, an error amplifier detects an error in the output voltage of the rectifier circuit from a desired value, and the output of the error amplifier is used to control the first and second current transformers. By changing the equivalent load impedance of a circuit connected to one or both of the windings, one or both of the detected values compared by the first and second comparing means is changed. A control method for a switching power supply, characterized in that the output voltage on the secondary winding side is stabilized.