JP2821462B2 - Multi-output switching power supply - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a multi-output power supply used for OA equipment and the like,
In particular, the present invention relates to a multi-output switching power supply suitable for improving the output stability.

[Conventional technology]

Conventionally, in a multi-output switching power supply, an individual stability control circuit such as a series dropper circuit or a chopper circuit has been used to stabilize the voltage on the non-control output side. However, the efficiency is low in the series dropper circuit, and the circuit is complicated in the chopper circuit, which has been an obstacle to downsizing and cost reduction.

Therefore, to solve this problem, for example,
There is a multi-output switching power supply having a configuration as shown in FIG. 4 for winding up a secondary winding.

The multi-output switching power supply shown in FIG. 4 has a primary winding 5 of a transformer 2 and an NPN transistor 3 serving as a main switch between a DC output of a primary rectifier circuit 1 including a bridge diode and a smoothing capacitor. They are connected in series. The output of the main switch control circuit 4 is connected to the base of the transistor 3, one end of the main switch control circuit 4 is connected to the output of the primary side rectifier circuit 1, and the other end is connected to a control output side described later. I have.

The transformer 2 includes, in addition to the primary winding, first and second two secondary windings 6 and 7 that are connected in series. The secondary winding 6 supplies power to the control output side. The secondary winding 7 supplies power to the non-control output side.

An anode of a rectifying diode 8 is connected to one end of the secondary winding 6, and a flywheel diode 9 is connected in the reverse direction between the cathode and the other end of the secondary winding 6. Similarly, one end of the secondary winding 7 is connected to the anode of a diode 10 for rectification, and its cathode and the secondary winding 6 are connected.
And a flywheel diode 11 is reversely connected to the other end of the flywheel.

A choke coil 12 is connected to the cathode of the diode 8, and a choke coil 13 is similarly connected to the cathode of the diode 10. Output terminal of choke coil 12 and 2
A smoothing capacitor 14 is connected between the other end of the secondary winding 6, and a capacitor 15 is similarly connected between the output end of the choke coil 13 and the other end of the secondary winding 6.

The output voltage control circuit 16 and the load 17 are connected in parallel with the capacitor 14.
Are connected, and the output voltage of the output voltage control circuit 16 is applied to the main switch control circuit 4. Meanwhile, the capacitor
A load 18 is connected to 15 in parallel. Further, a reset circuit 19 is connected between the primary windings of the transformer 2.

 Next, the operation of the configuration of FIG. 4 will be described.

When the AC power is supplied, the DC output converted by the primary rectifier circuit 1 is applied to the main switch control circuit 4 and the transformer 2, and the main switch control circuit 4 is activated to supply a current to the base of the transistor 3. Flows. Accordingly, the collector current of the transistor 3 starts to flow, and a voltage is generated in the secondary windings 6 and 7 of the transformer 2.

The rectified output is smoothed by a smoothing circuit including a choke coil and a smoothing capacitor, and supplied to each load and an output voltage control circuit 16. At this time, the voltage of the choke coil is expressed by the following equation.

_{V L = V S - (V} 0 + V F) ...... (1) V L '= V S' - (V 0 '+ V F') ...... (2) where, V _L: voltage of the choke coil 12, V _S : output voltage of the secondary winding 6, V ₀ : load terminal voltage on the control output side, V _F : forward voltage of the diode 8, _VL ': voltage of the choke coil 13, V _S ': secondary winding 7 V ₀ ′: Load terminal voltage on the non-control output side, V _F ′: Forward voltage of the diode 9.

Furthermore, the secondary current i ₂ (control output), i ₂ '(non-control output), when the on-time of the transistor 3 and _{t on, Δi 2 = (V} L / L) · t on = {V _S − (V ₀ + V _F ) / L} · t _on (3) Δi ₂ ′ = (V _L ′ / L ′) · t _on = {V _S ′ − (V ₀ ′ + V _F ′) / L ′｝ · t _on (4) where L is the inductance of the choke coil 12 and L ′ is the inductance of the choke coil 13.

At this time, assuming that the maximum values of i ₂ and i ₂ ′ are i _2p and i _2p ′, the energies P _L and P _L ′ of the choke coils 12 and 13 are P _L = (1/2) · L · i _2p ² (5) P _L ′ = (1/2) · L ′ · i _2p ′ ² (6)

Next, when the transistor 3 is turned off, 2
No power is supplied from the primary side to the secondary side, and back electromotive force is generated in the choke coils 12 and 13. Assuming that the current flowing at this time is i ₃ , i ₃ ′, the conduction path on the control output side is the choke coil 12 → the load 17 → the diode 9. In addition, the energization path on the non-control output side is the choke coil 13 → load 18 → diode 11. In this case, each current change rate Δi ₃ ,
Δi ₃ ′ is as follows.

Δi ₃ = ｛(V ₀ + V _F ) / L｝ · t _off (7) Δi ₃ ′ = ｛(V ₀ ′ + V _F ′) / L ′｝ · t _off (8) Normally, a choke coil Since the current flowing through is continuous, the following equation is established.

Δi ₂ = Δi ₃ = {V _S − (V ₀ + V _F ) / L} · t _on = {(V ₀ + V _F ) / L} · t _off (9) Δi ₂ ′ = Δi ₃ ′ = ｛ (V _S ′ − (V ₀ ′ + V _F ′) / L｝ · t _on = ｛(V ₀ ′ + V _F ′) / L ′｝ · t _off (10) [Problems to be Solved by the Invention] However, in the prior art shown in FIG. 4, when the load 17 on the control output side is reduced, the PWM (pulse width modulation)
In the control, t _on is shortened. At this time, as apparent from the expression (5), the information on the non-control output side is not input to the output voltage control circuit 16. Therefore, when the load on the control output side is small and the load on the non-control output side is large, there is a problem that the output voltage on the non-control output side decreases.

Also, the voltage of the secondary winding 6 fluctuates due to the fluctuation of the output current on the control output side, and the voltage of the secondary winding 7 on the non-control output side fluctuates accordingly.

SUMMARY OF THE INVENTION An object of the present invention is to provide a multiple output switching power supply capable of improving the stability of the output voltage, in view of the above-mentioned circumstances of the related art.

[Means for solving the problem]

In order to achieve the above object, according to the present invention, a DC power supply is applied to a primary winding of a transformer via a switch circuit, and an induced voltage associated with turning on / off of the current is connected to a load. In a multi-output switching power supply that generates a voltage in the first and second secondary windings connected in series with each other and uses the voltage generated in the first secondary winding to control the switch circuit. The winding supplies power to the control output side, the second secondary winding supplies power to the non-control output side, and one end of the first secondary winding has an anode of a first diode for rectification. (Cathode), a second diode for a flywheel is connected in the reverse direction between the cathode (anode) of the first diode and the other end of the first secondary line, and a second secondary winding is connected. At one end is the anode (cathode) of the third diode for rectification
And the other end of the second secondary winding is connected to the cathode (anode) of the first diode and the cathode (anode) of the second diode, and to the cathode (anode) of the third diode. A flywheel fourth diode is connected in reverse direction between the other end of the first secondary winding and a flywheel fourth diode is connected between the anode (cathode) side of the second and fourth diodes and the load side ground. And a choke coil for smoothing the rectified output of the first and second secondary windings.

When the output voltage ratio of the first and second secondary windings is large, an intermediate terminal is provided on the choke coil to reduce the output voltage fluctuation of the second secondary winding. It is preferable that the first secondary winding and the second diode are connected to a terminal, and the fourth diode is connected to an end of the choke coil on the non-load connection side.

[Action]

According to the above-described means, the choke coil inserted on the ground side acts to flow the output current to both the first and second secondary windings, and the load on the first secondary winding is small. Second
Even when the load on the secondary winding is large, the switch-on time (t _on ) is not shortened more than necessary. Therefore, voltage stability can be improved.

That is, in the multiple output power supply shown in FIG. 4, the pulse width is determined only by the current flowing to the control output side. Since this current flows through the choke coil (12), the pulse width is determined by this current. Therefore, when the load (18) on the non-control output side is constant, the output voltage on the non-control output side is equal to the load (17) on the control output side regardless of the magnitude of the load (18) on the non-control output side. As the load increases, the load increases proportionally, and as the load (17) on the control output side decreases, the load decreases proportionally.

On the other hand, in the present invention, the pulse width is determined by a current flowing through a choke coil (a choke coil 20 in FIG. 1 described later) for smoothing the rectified output of the first and second secondary windings. The current on both the control output side and the non-control output side flows through the choke coil (20). For this reason,
When the load (18) on the non-control output side is constant, the output voltage on the non-control output side is constant even if the load (17) on the control output side is large. The width of the change in width becomes smaller, and the voltage rise is suppressed more than in the conventional example shown in FIG. Also, the load (1
Since the load (18) on the non-control output side is constant even if the value of (7) is reduced, the amount of change in the pulse width is reduced, and the voltage drop is suppressed as compared with the conventional example as shown in FIG. Thus, the fluctuation range of the voltage on the non-control output side with respect to the load fluctuation on the control output side is suppressed, and the stability as a power supply is improved.

Further, by providing a tap (intermediate terminal) in the choke coil, a system in which output current flows through both the first and second secondary windings and a system in which output current flows only through the second secondary winding And the fluctuation of the output voltage of the second secondary winding can be reduced even when the output voltage ratio of the first and second secondary windings is large.

〔Example〕

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 shows a first embodiment of a multi-output switching power supply according to the present invention.
It is a circuit diagram showing an example. In the following embodiments, the same reference numerals are used for the same components as those shown in FIG. 4, and therefore, duplicate description will be omitted below.

In this embodiment, the choke coils 12 and 13 shown in FIG.
Are removed so that they are directly connected to each other, and the choke coil 20 is connected between the anodes of the diodes 9 and 11 and the ground on the load side.

With such a configuration, when the transistor 3 is on, a voltage is induced in the secondary windings 6 and 7, and a current flows on the control output side in a path of the diode 8 → the load 17 → the choke coil 20. Then, when the transistor 3 is turned off, a current flows to the control output side through the path of the choke coil 20 → the diode 9 → the load 17.

On the other hand, on the non-control output side, when the transistor 3 is on, current flows through the path of the diode 8 → the diode 10 → the load 18 → the choke coil 20. When the transistor 3 is off, the path of the choke coil 20 → the diode 11 → the load 18 Flows in

The operation when the transistor 3 is off will be described in detail with reference to FIG. The polarity of the choke coil 20 when the transistor 3 is on is the capacitor shown in FIG.
The side connected to 14 is positive, diodes 9 and 11
Is connected to the minus side. When the transistor 3 is turned off from this state, the polarity of the choke coil 20 is inverted, and the side connected to the capacitor 14 in FIG. 1 becomes negative, and the side connected to the diodes 9 and 11 becomes positive. As a result, current flows from the choke coil 20 to the loads 17 and 18 through the diodes 9 and 11. Here, the load 17 on the control output side and the load on the non-control output side
Since the load 17 on the control output side is larger than the size of 18, the diode 9 has a larger current capacity than the diode 11 as a matter of course. For this reason, the diode 11 has a shorter reverse recovery time, which is a characteristic of the diode. Since there is a difference in the reverse recovery time between the diode 9 and the diode 11, the transistor 3 is turned off, and at the moment when the polarity of the choke coil 20 is reversed, the diode 9 has a slow reverse recovery time and is in a non-conductive state. , The voltage of the choke coil 20 is the voltage of the non-control output side + the diode 11
, The diode 11 conducts, and a current flows through the load 18 on the non-control output side. Then, when the diode 9 is turned on after the reverse recovery time, the current flows to the load 17 on the control output side, and the current stops flowing to the non-control output side.

As described above, the respective output currents on the control output side and the non-control output side flow through the diode 8 and the choke coil 20. By this operation, when the load 17 on the control output side becomes small, the following two states occur.

Normally, one choke coil is required for one system, but one choke coil can be used for two systems. That is, choke coil on the ground side
By inserting 20, a current flows to both the control output side and the non-control output side. As a result, even when the load 17 on the control output side is small and the load 18 on the non-control output side is large, t _on
Is not shorter than necessary. This is in t _on period, the energy stored in the choke coil 20, due to the fact that determined by both the output current of the control output and the non-controlled output.

Assuming that the energy stored in the choke coil 20 is P _L ″, the following equation is derived from Equation (5), where L ″ is the inductance of the choke coil 20.

P _L ″ = (1/2) · L ″ · (i _lp + i _2p ) ² (11) Since current flows from the choke coil 20 to the non-control output side via the diode 11, the current passes through the diode 9. The flowing current decreases, and the output voltage on the control output side decreases. In response to this operation, the output voltage control circuit 16 operates to expand the t _on. For this reason, the load 17
Is smaller, the fluctuation of the output voltage on the non-control output side is reduced.

Note that in the equations (5) and (6), P _L ≫P _L ′
In this case, the choke coil 12 having the specification of FIG. 4 can be used as it is by replacing it with the choke coil 20 of FIG. This makes it possible to reduce the cost and simplify the circuit.

Since the reference of the secondary winding on the non-control output side is based on the rectified output voltage on the control output side, the reference voltage is always kept almost constant, and the voltage stability is higher than when the winding of the transformer 2 is wound up. Is improved.

As described above, a stable output voltage with little voltage fluctuation can be obtained. The inventors of the present invention carried out the configuration shown in FIG. 1 by setting the output voltage on the control output side to +24 V and the output voltage on the non-control output side to +30 V. It was confirmed that the fluctuation was suppressed.

 FIG. 2 is a circuit diagram showing a second embodiment of the present invention.

In the present embodiment, a smoothing capacitor 21 is connected between the cathode of the diode 10 and the cathode of the diode 8 while the smoothing capacitor on the non-control output side is connected in parallel to the load 18 in the previous embodiment. There is a characteristic in that the configuration is changed.

With such a configuration, the withstand voltage (V ₀ −
V ₀ ′) can be reduced, a standard having a small withstand voltage can be used, and the cost can be reduced. In this case, the energy stored in the capacitor 21 is represented by the following equation.

E = (1/2) · CV ² (12) In the equation (12), V is represented by (V ₀ ′ −V ₀ ) / V ₀ ′.
Where (V ₀ ′ −V ₀ ) / V ₀ is 1 / n, it is expressed by the following equation.

E = (1/2) · C ｛(1 / n) · V｝ ² = (1/2) · (1 / n ² ) · C · V ² (13) It is clear from equation (13). As described above, not only the withstand voltage can be reduced, but also the capacity can be reduced to 1 / n ² , and the cost can be reduced.

 FIG. 3 is a circuit diagram showing a third embodiment of the present invention.

This embodiment is characterized in that, based on the circuit of FIG. 1, a choke coil 22 with a tap is used in place of the choke coil 20, and the anode of the diode 9 is connected to this tap. When the tap is provided in the choke coil 22, a portion for flowing the output current to each of the control output side and the non-control output side and a portion for flowing the output current only to the non-control output side are formed.

With such a configuration, there is an advantage when the ratio between the output voltage on the control output side and the output voltage on the non-control output side is large.

In the configuration of FIG. 1 described above, when the transistor 3 is turned off, the terminal voltage of the choke coil 22 is set to V ₀ and the polarity is inverted different from that on. In this state, the time during which the current flows from the choke coil 22 to the non-control output side becomes extremely short.

Therefore, in the present embodiment, the tap is provided to divide the choke coil, the terminal voltage on the non-control output side is increased by the choke coil portion flowing only to the non-control output side, and the fluctuation characteristics at the time of switching are smoothed. The portion through which the output currents on both the control output side and the non-control output side flow
The same effect as that of the embodiment can be obtained.

On the other hand, in the part where only the output current on the non-control output side flows,
It has the effect of increasing the terminal voltage of the choke coil on the non-control output side when the transistor 3 is off, and flowing current also to the non-control output side.

Therefore, even when the ratio between the output voltage on the control output side and the output voltage on the non-control output side is large, the fluctuation of the output voltage on the non-control output side is reduced by using the choke coil 22 having the winding divided according to the ratio. be able to.

In the above embodiment, the example of the main switch using one transistor has been described. However, the present invention can be similarly applied to a half bridge circuit, a push-pull circuit, and the like.

〔The invention's effect〕

Since the present invention is configured as described above, even when the load on the control output side decreases, the fluctuation of the output voltage of the non-control output decreases, and a stable output voltage can be obtained. Also, since the reference of the secondary winding on the non-control output side is based on the rectified output voltage on the control output side, almost constant voltage is always maintained, and the output voltage is more stable than when the winding of the transformer is wound up. Can be obtained.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a first embodiment of a multi-output switching power supply according to the present invention, FIG. 2 is a circuit diagram showing a second embodiment of the present invention, and FIG. 3 is a third embodiment of the present invention. FIG. 4 is a circuit diagram showing an example of a conventional multi-output switching power supply. 1 Primary rectifier circuit 2 Transformer 3 Transistor 4 Main switch control circuit 5 Primary winding 6,7 Secondary winding 8,9, 10,11 …… Diode, 14,1
5,21 …… Capacitor, 16 …… Output voltage control circuit, 17,18
…… Load, 20,22 …… Choke coil.

Continued on the front page. (72) Inventor, Hisahiro Kamada, Nakamei, Shinmei-do, Shimada-cho, Shibata-gun, Miyagi Prefecture, No. 3 Tohoku Ricoh Co., Ltd. No. 3 in Tohoku Ricoh Co., Ltd. (56) References JP-A-61-73574 (JP, A) JP-A-61-49660 (JP, A) JP-A-3-239158 (JP, A)

Claims (2)

(57) [Claims] 1. A DC power supply is applied to a primary winding of a transformer via a switch circuit, and an induced voltage accompanying ON / OFF of the current supply is supplied to a first load connected to a load and connected in series with each other. , A voltage generated in the second secondary winding, and a voltage generated in the first secondary winding used for controlling the switch circuit, wherein a rectifier is provided at one end of the first secondary winding. One end of a first diode for a flywheel is connected, a second diode for a flywheel is connected in the reverse direction between the other end of the first diode and the other end of the first secondary winding, One end of a secondary diode is connected to one end of a third diode for rectification, and the other end of the second secondary winding is connected to the other end of the first diode and the anti-first diode of the second diode. The other end of the secondary winding is connected, and a flywheel is connected between the other end of the third diode and the other end of the first secondary winding. And a second diode between the connection side of the second and fourth diodes with the anti-first and third diodes, respectively, and the ground on the load side. A choke coil for smoothing the rectified output of the secondary winding is connected, and power is supplied from the first secondary winding to the load on the control side and from the second secondary winding to the load on the non-control side. A multi-output switching power supply characterized in that: 2. The choke coil has an intermediate terminal, the first secondary winding and the second diode are connected to the intermediate terminal, and the fourth diode is connected to the choke coil in an anti-load connection. The multi-output switching power supply according to claim 1, wherein the multi-output switching power supply is connected to an end on the side.