JPH0121698B2 - - Google Patents

Description

TECHNICAL FIELD OF THE INVENTION The present invention relates to a multi-output switching power supply having a plurality of output channels including variable load output channels.

[Technical background of the invention and its problems]

Generally, _in this type of switching power supply device, for example, a forward converter type switching power supply device, a switching power transistor _Q1 is connected to a primary winding N1 of a converter transformer T, as shown in FIG. The alternating current waveform voltages generated in the secondary windings _N2 and _N3 of the converter transformer T by periodically turning on and off the transistor _Q1 are converted into direct current voltage and smoothed. , for example 5V,
It can provide 12V output. In other words, above 2
Regarding the AC waveform voltage generated in the next winding _N2 ,
It is converted to DC voltage through diodes D ₁ and D ₂ ,
It is smoothed by a filter consisting of a choke coil L ₁ and a capacitor C _F1 . Similarly, secondary winding N ₃
For the AC waveform voltage generated in the diode
It is converted into a DC voltage through D ₃ and D ₄ and smoothed by a filter consisting of a chiyoter coil L ₂ and a capacitor C _F2 .

In addition, in the example of Fig. 1, especially the secondary winding _N2 side
Efforts have been made to keep the 5V output voltage constant. This feeds back the above 5V output voltage and applies a control pulse to the base of transistor _Q1 via an error amplifier AV that amplifies the difference between the output voltage and the reference voltage, a pulse width modulator PWM, and a drive circuit D. This makes it possible. Furthermore, in the example shown in FIG. 1, measures have been taken to prevent the transistor _Q1 , diodes _D1 to _D4 , etc. from being destroyed in the event of overload. this is,
The collector current of transistor _Q1 is detected by the current transformer (current transformer) CT, and if an overcurrent exceeding a predetermined value is detected, the overcurrent protection circuit AI
This can be achieved by narrowing the pulse width of the control pulse via the pulse width modulator PWM or by stopping the switching operation of the transistor _Q1 by using the overcurrent protection circuit AI.

Here, general overload settings will be explained. For example, in the example shown in Figure 1, the load on the 5V output voltage side is a fixed load and its load current is 5A, whereas the load on the 12V output voltage side is a variable load (pulse load) and its variable load current (pulse load). Current) I ₁₂
As shown in Figure 2, the peak value is 2A and the average value is 0.333A. In this case, (output power on the 5V output voltage side) + (peak value of output power on the 12V output voltage side) = 5 (V) x 5 (A) + 12 (V) x 2 (A) = 49W
It is necessary to set the overload (overload condition) to a value greater than . Therefore, in the switching power supply shown in Figure 1, when there is no load on the 12V output voltage side and overload on the 5V output voltage side, the load current on the 5V output voltage side will be 49 (W) / 5 (V) = 9.8 A or more. The overcurrent detection/protection function by the overcurrent protection circuit AI will not work unless the voltage increases. Therefore, in the switching power supply shown in Fig. 1, the secondary winding N ₂ on the 5V output voltage side
The rated current capacity of the diodes D ₁ , D ₂ , etc. must be approximately twice the rated output current, which has the disadvantage that the ratings of these components become large. This is the same when the 5V output voltage side is unloaded and the 12V output voltage side is overloaded, contrary to the above case.
In this case, it is necessary to increase the ratings of the secondary winding _N3 , diodes _D3 , _D4, etc. on the 12V output voltage side.

As a way to solve this problem, by delaying the output signal of the current transformer CT through an integrating circuit, the pulse load current shown in Figure 2 can be adjusted.
It is possible to avoid responding to the peak value of I ₁₂ (1 ms pulse). However, with this method, it is difficult to detect a sudden increase in the collector current of transistor _Q1 due to saturation of converter transformer T, etc., and the switching loss when turning off transistor _Q1 increases, causing the transistor to turn off.
There is a risk of thermal destruction of _Q1 .

[Purpose of the invention]

The present invention has been made in view of the above circumstances, and its purpose is to provide a multi-output switching power supply device that can bring the overload detection condition as close as possible to the sum of the average power of each output channel including variable load output channels. There is a particular thing.

[Summary of the invention]

The present invention provides a first filter circuit for DC voltage smoothing provided on the fluctuating load output channel side, and a second filter circuit consisting of a current limiting resistor and an energy storage capacitor.
A filter circuit is connected, and the first
The current limiting resistor suppresses large fluctuations in the current flowing through the choke coil of the filter circuit, that is, the current limiting resistor lowers the peak value of the fluctuation. As a result, the secondary current of the converter transformer, that is, the primary current of the converter transformer, becomes flat without being affected by load fluctuations, and it is no longer necessary to consider the peak value of fluctuating load current when setting overcurrent, as in the past. The overcurrent setting value can be brought close to the average current without any problem. In the present invention, even if a small flat current close to the average value of the fluctuating load current is flowing through the current limiting resistor, a part of the energy stored in the capacitor is released, so that the fluctuating load Since a pulsed current flows, there is no problem in driving a variable load.

[Embodiments of the invention]

Hereinafter, one embodiment of the present invention will be described with reference to the drawings. Note that the same parts as in FIG. 1 are given the same reference numerals and detailed explanations are omitted. In the multi-output switching power supply shown in FIG. 3, R _S is a current limiting resistor connected in series to the 12V output circuit, and REG is a three-terminal regulator for stabilizing the output voltage. The three-terminal regulator REG is, for example, TA78012P manufactured by Tokyo Shibaura Electric Co., Ltd. C _S is a 3-terminal regulator
the regulator to prevent REG oscillation.
The capacitor C _O inserted between the REG input terminal and the common terminal is an energy storage capacitor that supplies pulsed current to a fluctuating load. This capacitor C _O is the output terminal of the regulator REG,
It is provided between common terminals, that is, in parallel with a load (not shown).

Next, the operation of the configuration shown in FIG. 3 will be explained with reference to the signal waveform diagrams shown in FIGS. 4a and 4b. As described in the conventional example, an AC waveform voltage is induced in the secondary windings N ₂ and N ₃ of the converter transformer T by turning on/off the transistor Q ₁ . For example, on the 5V output voltage side, which is a fixed load channel, the AC waveform voltage is converted to a DC voltage by diodes D ₁ and D ₂ , and the DC voltage having a ripple component is converted to a DC voltage by a chain coil L ₁ and a capacitor C. It is smoothed by a filter circuit consisting of _F1 . This causes a relatively flat current to flow through the choke coil _L1 . This current almost matches the average value of the fixed load current, which is 5A as described in the conventional example.

On the other hand, on the 12V output voltage side of a variable load output channel, for example a pulse load channel, the above AC waveform voltage is converted to a DC voltage by diodes D ₃ and D ₄ , and further (corresponding to the on/off period of transistor Q ₁ The _DC voltage with the ripple component (of high frequency)
It is smoothed by a filter circuit consisting of C _F2 .
By the way, in the case of a pulse load channel, as described in the conventional example, the load current (pulse load current) differs between the loaded state and the unloaded state. In other words, a current of 2A flows through the pulse load during the load period (1ms),
No current flows through the pulse load during the no-load period (5 ms). Generally, the cycle of this load fluctuation (6ms) is the on/off cycle of transistor _Q1 (for example, 20μs)
significantly longer than . When the period of variation in the load current is long as described above, the load current cannot be smoothed by the chiyoke coil _L2 . Therefore, in the conventional example shown in FIG.
It flows through L ₂ .

However, in this embodiment, the chiyoke coil L ₂ ,
Since a secondary winding circuit consisting of a current limiting resistor R _S and an energy storage capacitor C _O is connected to the filter circuit consisting of the capacitor C _F2 , the current flowing through the choke coil L ₂ due to the current limiting resistor R _S is is reduced. That is, the current flowing through the secondary winding _N3 of the converter transformer T is reduced compared to the conventional example. The resistance value of this current limiting resistor R _S is determined, for example, as follows. In this example, the current flowing through the resistor R _S , that is, the regulator REG
The input current I ₁₂ (IN) is made to approximately match the average value of the pulse load current on the 12V output voltage side.
As is clear from Fig. 2 and the above explanation, this average value (average current) is (1 (ms)/6 (ms))
×2(A)=0.333A. Therefore, when this average current flows through the resistor R _S , the voltage difference between the input terminal and output terminal of the regulator REG is
The resistance value of the resistor R _S is set to the minimum value (usually around 2V) of the voltage range in which REG can operate. In this embodiment, a filter consisting of a resistor R _S and a capacitor C _O whose resistance values are set in this manner is used to control the current of the choke coil L ₂ according to the peak current of the pulse load current, as shown in FIG. 4a.
In addition to lowering the level of the current flowing through the device, its current smoothing function can flatten the current. Note that the current waveform indicated by a broken line in the same figure shows the conventional example (see FIG. 2).

The energy storage capacitor C _O is connected to the input current
I ₁₂ (IN), which is charged by the current flowing through the choke coil L ₂ . In this embodiment, when a load is applied, part of the charge in the capacitor C _O is discharged to supply a pulse load current of 2A to the pulse load. To do this, the no-load period (second
It is necessary to restore the charge on the capacitor C _O during the T _OFF period in the figure). Therefore, the capacitance of the capacitor C _O is selected so that C _O R _S T _OFF . The voltage across this capacitor _CO , that is, the 12V output voltage waveform of the variable load (pulse load) channel is as shown in FIG. 4b. Further, the voltage waveform indicated by a broken line in the figure is the 12V output voltage waveform in the conventional example shown in FIG. As is clear from the figure, in this embodiment, the output voltage fluctuates due to charging and discharging of the capacitor C _O , so the regulator REG suppresses this fluctuation as much as possible.

In this way, according to this embodiment, the chiyoke coil
The current value of the current flowing through L ₂ , that is, the input current I ₁₂ (IN), can always be made to almost match the average value of the pulse load current regardless of whether there is a load or no load, so when setting overload detection conditions, as before
There is no need to consider the peak output power (24W) on the 12V output voltage side. Therefore, the overload standard can be set to slightly exceed 25W of output power on the 5V output voltage side + 4W of average output power on the 12V output voltage side = 29W (49W in the conventional example).

In the above embodiment, a case was explained in which a 3-terminal regulator REG was used to suppress output voltage fluctuations due to charging and discharging of the capacitor _CO , that is, output voltage fluctuations under load and no load.However, it is also possible to use a combination of discrete components. A regulator may also be used. FIG. 5 shows an example of such a regulator. In the figure, Q ₂ is an NPN power transistor, and D ₅ is inserted between the base of transistor Q ₂ and OV (Zener voltage is 12.6V).
It is a Zener diode. Further, R _B is a resistor inserted between the collector and base of the transistor _Q2 . The resistor R _B is for supplying the bias current of the Zener diode _D5 . In addition,
According to the subject matter of the invention, a regulator is not necessarily required.

Further, in the above embodiment, the regulator
Further effects can be obtained by providing an overcurrent drooping circuit on the output side of the REG.

Further, in the above embodiment, the case of a switching power supply device with two outputs has been described, but the present invention can be similarly implemented in a switching power supply device with three or more outputs.
Further, in the above embodiment, the case of a forward converter type switching power supply device has been described, but the present invention can be similarly implemented in a flyback converter type, a half bridge converter type, a full bridge converter type, etc. Furthermore, it is not limited to the control method with a constant switching frequency (duty cycle control method) as in the above embodiment, but can also be applied to various control methods such as a variable frequency type, a constant on time type, a constant off time type, and a self-excited switching power supply. can also be implemented.

[Effects of the Invention] As detailed above, according to the present invention, the following effects can be achieved.

(1) Even when the load is pulsed, the current flowing to the primary side of the converter transformer can be flattened, so the overcurrent setting value can be brought closer to the average current.

(2) Therefore, it is possible to use a switching power transistor in the primary circuit with a low current rating.

(3) Since the peak current no longer flows through the switching power transistor as in the conventional case, the drive base current can be reduced, the drive circuit can be made smaller, and its power consumption can be reduced.

(4) Since the detection level when the secondary side is overloaded can be lowered, the current rating of each component of the secondary side circuit can be lowered.

(5) Therefore, it can be constructed using inexpensive parts and the power supply can be downsized.

[Brief explanation of drawings]

FIG. 1 is a circuit configuration diagram of a conventional switching power supply device, FIG. 2 is a diagram showing a pulse load current waveform in the above conventional example, and FIG. 3 is a circuit diagram showing an embodiment of the switching power supply device of the present invention. FIGS. 4a and 4b show signal waveforms of each part in the above embodiment, and are diagrams showing input current waveforms and output voltage waveforms, respectively. FIG. 5 is a circuit configuration diagram showing another embodiment of the regulator. Q ₁ ...Switching power transistor,
T...Converter transformer, _L1 , _L2 ...Chiyoke coil, CT...Current transformer, AI...Overcurrent protection circuit, R _S ...Current limiting resistor, _CO ...Energy storage capacitor.

Claims (1)

[Claims] 1. In a multi-output switching power supply device that has a plurality of output channels including a pulse load channel that repeats loaded and unloaded states on the secondary side of a converter transformer and is equipped with an overload detection function, A second filter circuit including a current limiting resistor and an energy storage capacitor and a regulator for stabilizing the output voltage are connected to the first filter circuit for DC voltage smoothing including a choke coil. When the side pulse load is loaded, part of the charge in the energy storage capacitor is discharged to supply a load current to the pulse load, and the energy storage capacitor is charged during the no-load period of the pulse load. The output voltage stabilizing regulator suppresses fluctuations in the output voltage due to charging and discharging of this capacitor, and the current is adjusted such that the current flowing through the current limiting resistor approximately matches the average value of the load current flowing through the pulse load. The value of the limiting resistor is determined, and the value of the energy storage capacitor is determined such that the product of the value of the energy storage capacitor and the value of the current limiting resistor is approximately equal to the no-load period of the pulse load. A multi-output switching power supply device characterized by: