JP3408502B2 - Switching power supply - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching power supply device configured as a synchronous rectification converter circuit.

[0002]

2. Description of the Related Art A conventional synchronous rectification type switching power supply device is shown in FIG. In the figure, 19 is a DC input power source, 2
Reference numeral 0 is a transformer for voltage conversion, 21 is a switching element composed of an FET (field effect transistor) for switching a DC voltage input to the primary side of the transformer 20, and 24
Is a rectifier circuit including resistors 22 and 23 and FETs 25 and 26, and 28 is a choke coil 2 that smoothes the rectified voltage.
Filter constituted by 9 and capacitor 30, 31
Is an error amplifier, 32 is a PWM control circuit, and these constitute a power supply circuit 33, and a load 34 is connected to the power supply circuit 33.

Next, the operation will be described. Switching element 2
1 performs a switching operation by a control signal output from the PWM control circuit 32, and generates an AC voltage on the primary side and the secondary side of the voltage conversion transformer 20. Of the alternating voltage generated on the secondary side, the positive voltage is generated in the FET 25.
Of the FET 26 and the gate of the FET 26 are controlled by the generated voltage in the negative direction to turn on / off the FETs 25 and 26 to rectify the AC voltage. The rectified voltage is smoothed by the filter 28 including the choke coil 29 and the capacitor 30, and is supplied to the load 34. The error amplifier 31 monitors the smoothed output voltage and sends a feedback signal to the PWM control circuit 32 so that the output voltage is always a constant DC voltage.

FIG. 4 shows a circuit in which a power supply circuit 36 and a power supply circuit 37 similar to the power supply circuit 33 shown in FIG. 3 are connected in parallel. Conventionally, each power supply circuit 36, 3 at the time of parallel connection
In order to prevent the backflow of the load current to the rectifier circuit 24 side as described above, it is necessary to connect a directional backflow prevention element such as a diode 38 or a diode 39 to the output side of 7. .

For example, when the power supply circuits 36 and 37 operate in parallel without connecting the diode 38 and the diode 39, the error amplifier 31 in each of the power supply circuits 36 and 37.
Detects the highest output voltage among the power supply circuits 36 and 37 connected in parallel. The power supply circuit 36 is now the power supply circuit 37.
It is assumed that a higher output voltage is being generated. Power is supplied to the load 40 from the power supply circuit 36, and the error amplifier 31 provided in the power supply circuit 37 monitors the output voltage of the power supply circuit 36 that is not its own output voltage. A feedback signal for lowering the output voltage is sent to the PWM control circuit 32 of 37. If the output voltage difference between the power supply circuit 36 and the power supply circuit 37 is large, the power supply circuit 37 may stop the power conversion control in some cases, and the output voltage may be lowered.

When the power conversion control is stopped in this way, the switching element 21 does not operate, so that no AC voltage is generated on the secondary side of the voltage conversion transformer.
That is, since there is no bias in the gates of the FETs 25 and 26 that are rectifying elements, the FETs 25 and
26 does not turn on. However, since the output voltage of the power supply circuit 36 connected in parallel is applied to the secondary side of the power supply circuit 37 as described above, the gates of the FETs 25 and 26 in the rectifier circuit are biased to perform the ON operation. Will end up. That is, since there is a reverse flow path from the output side of the power supply circuit 36 to the rectifier circuit 24 of the power supply circuit 37, the power supply circuit 37 that is not supplying the load current absorbs the load current, and the other converter output is supplied. As the shunt output cannot be normally supplied to the load 40,
If a current that exceeds the ratings of the FETs 25 and 26 flows, element destruction may occur.

[0007]

In the conventional switching power supply device, when the power supply circuits 36 and 37 are connected in parallel as they are, the rectifier circuit malfunctions and the circuit components in the power supply circuit are damaged. There was a problem. The reason is that when the output voltage of one power supply circuit is lower than the output voltage of the other power supply circuit, current is supplied to the load from the power supply circuit with a high output voltage and current is supplied from the power supply circuit with a low output voltage. Not done On the contrary, the output voltage of the power supply circuit with a high output voltage is applied to the power supply circuit that does not supply current, so connect the gate of the FET of the rectifier circuit to the primary side of the transformer for voltage conversion. A voltage other than the original FET gate control voltage generated on the secondary side of the transformer is applied by the operation of the switching element. As a result, when the FET of the rectifier circuit is turned on by the FET gate control voltage and the externally applied voltage, a backflow phenomenon of the load current occurs, and a current exceeding the use range of the power supply circuit with a low output voltage The current flow of the load current may exceed the current rating of the circuit element.In the worst case, the element may be destroyed and power may not be supplied to the element normally. It was

An object of the present invention is DC / DC using synchronous rectification.
In a DC converter, a rectifying element is prevented from being damaged when a power supply circuit is connected in parallel, and a switching power supply device capable of improving DC / DC conversion performance and reliability during parallel operation is provided. To get.

[0009]

[Means for Solving the Problems] To achieve the above object,
The switching power supply according to the present invention is similar to other power supply circuits.
A switching power supply that is connected in columns, and is a transformer that induces an AC voltage on the secondary side by switching a DC input that is input to the primary side by a switching element, and at least two that full-wave rectifies the AC voltage from the transformer. FE
A rectifier circuit of a synchronous rectifier composed of T, a filter for direct-current smoothing the output voltage of the rectifier circuit and supplying it to a load, and a switching control circuit for monitoring the output voltage of the filter and maintaining the output voltage at a set level. And an error amplifier for controlling the operation of the switching element, wherein the FET in the rectifier circuit is responsive to the output voltage level of the error amplifier.
The rectification operation of the rectification circuit is stopped by being turned off or on.
It is designed to be stopped or restarted .

Also, the switching power supply device according to the present invention is provided with a rectification control circuit for performing cutoff control of the FET in the rectification circuit when the output voltage of the error amplifier exceeds a set level.

Further, the switching power supply device according to the present invention is such that another DC power supply circuit is connected in parallel to the load.

Further, in the switching power supply device according to the present invention, the switching control circuit is a PWM control circuit which receives the output of the error amplifier and pulse-width modulates the control input to the switching element.

Further, in the switching power supply device according to the present invention, the rectification control circuit turns off the FET when the output level of the error amplifier exceeds the level range of the triangular wave which is the reference of the transmission pulse width of the PWM control circuit. It is designed to be controlled.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a circuit diagram showing a switching power supply device of the present invention. In the figure, 1 is a DC input power supply, 2 is a transformer for converting a DC input voltage into a desired AC voltage, 3 is a switching element for switching DC voltage, 4 is FETs 5, 6, resistors 7, 8 and diode 9
A rectifier circuit of a synchronous rectification system composed of 10 is a filter composed of a choke coil 11 and a capacitor 12, 13 is an output voltage Vo generated through the filter 10, and outputs the voltage to the PWM control circuit 15. The function of sending the correction signal and the FE that is the rectifying element of the rectifying circuit 4
These are error amplifiers having a function of sending a control signal to the rectification control circuit 14 for controlling the gates of T5 and T6, and these constitute a power supply circuit 16 to which another power supply circuit 18 is connected. It is connected and supplies power to the load 17.

Next, the operation will be described with reference to the time chart of the signals of the respective parts of the circuit shown in FIG. Here, the PW is supplied by the voltage supplied from the DC input power source 1.
The M control circuit 15 starts its operation, which controls the operation of the switching element 3. Therefore, transformer 2
AC voltage of a predetermined level is generated on the secondary side of the DC voltage, the DC voltage obtained through the rectifier circuit 4 and the filter 10 is output to the load 17, and the DC voltage is output to the error amplifier 13 and the PWM control circuit 15. The desired direct current voltage is obtained by feeding back to the switching element 3 via. This operation is the general switching power supply circuit operation described in the conventional example, and detailed description of the operation is omitted.

At this time, the DC input power source 1 is turned on and t0
When the secondary voltage of the transformer 2 rises from the time point and the output voltage Vo of the filter 10 rises further, the error amplifier 13 operates to send the pulse of the maximum level to the PWM control circuit 15. At this time, the output voltage Vo is in a rising state as shown in FIG. 2A, and the rectification control circuit 14 has FET5, FET unless the output signal VFB from the error amplifier 13 becomes equal to or higher than the set level VFB ″.
Since the gate stop control of No. 6 is not performed, the output voltage VD of the rectification control circuit 14 is set to each FET5, FET6.
A high-level signal that does not perform stop control is sent to each gate. Further, when the power supply circuit 16 is activated, the output voltage Vo is set to the set output voltage Va due to the delay in transmission of the feedback signal from the error amplifier 13 to the PWM control circuit 15.
Overshoot phenomenon is going to occur, but
As shown in FIG. 2B, at time t1, the error amplifier 1
Since the output VFB of 3 exceeds the level range of the triangular wave which is the transmission pulse width reference source of the PWM control circuit 15 and shifts to the VFB ″ area, the rectification control circuit 14 to which the output of the error amplifier 13 is input is The output VD is set to the L level as shown in FIG. 2C, the gates of the FETs 5 and 6 are controlled, and these are cut off.

By doing so, the rectifying circuit 4 does not perform the rectifying function, and the output voltage Vo drops rapidly. After this,
At time t2, the output voltage Vo becomes the set output voltage Va, so that the output of the error amplifier 13 shifts to within the range of the triangular wave, and the level signal within the VFB ′ area is output. Therefore, the rectification control circuit 14 detects that the output of the error amplifier 13 is input, and changes the level of the output VD from L level to H level.
Move to a level. As a result, the above-mentioned cutoff state of the FET 5 and the FET 6 is eliminated, the rectification circuit 4 restarts the synchronous rectification operation, and supplies the DC voltage stabilized by the error amplifier 13. Therefore, even if the output voltage overshoots at the time of start-up and the output level of the error amplifier 13 reaches the excessive level, the FET in the rectifier circuit 4 is
Since the response speed is increased by stopping 5 and 6, it is possible to suppress the overshoot of the output voltage exceeding the set output voltage Va.

As described above, the error amplifier 13 has the filter 1
The DC / DC converter supplies a stable output voltage by detecting the output voltage Vo of 0 and feeding back the output correction signal to the PWM control circuit 15 so that the output voltage Vo becomes the set output voltage. ing. The output signal VFB of the error amplifier 13 at this time is the level VF of the triangular wave.
It is controlled within the range of B ', and the output VD of the rectification control circuit 14 becomes H level by inputting the output signal in the VFB' area, and the FET 5 and the FET 6 are AC generated on the secondary side of the transformer 2. Rectify by voltage. Here, when the output voltage Vo becomes high at time t3 and exceeds the area of VFB ′ which is the range of the triangular wave at time t4, the output VFB of the error amplifier 13 is output to the PWM control circuit 15 to the secondary side of the transformer 2. Sends a signal to reduce the amount of power. At this time, the rectification control circuit 14 identifies the VFB ″ signal, inverts the level of the output VD to the low level, and
Controls the gate of ET5, FET6, and FET5, FET
Cut off 6 Therefore, the rectifier circuit 4 is an FET
When the FET 5 and the FET 5 are turned off, the rectifying function is lost and the output voltage Vo decreases.

If the rise in the output voltage Vo is caused by the internal circuit of the power supply circuit 16, the error amplifier 13 detects that the output voltage of the filter circuit 10 has risen, and sets the output signal level to VFB. '' Area to VF
The PWM control circuit 1 functions so as to shift to the area B '.
5 operates to reduce the power level to the secondary side of the transformer 2, and the rectification control circuit 14 controls the error amplifier 13 to VF.
Since the output signal in the B'region is input, the output VD becomes H level, the cutoff state of each gate of the FET5 and FET6 is canceled, and the synchronous rectification operation is restarted.
The operation in the direction in which the output voltage is controlled and stabilized at the set output voltage is as described above.

On the other hand, if the rise of the output voltage Vo is caused by the output voltage of the other power supply circuit 18 connected in parallel to the power supply circuit 16, that is, the power supply circuit 1 is more than the power supply circuit 16.
When the output voltage of 8 is high, as described above, the rectifier circuit 4
However, since the detection voltage of the error amplifier 13 becomes the output voltage V1 of the other power supply circuit 18 (where V1> Vo), unless the output voltage V1 of the other power supply circuit 18 decreases, The output VFB ″ of VFB ″ remains unchanged. That is, even if the output of the other power supply circuit 18 is applied to the gate of the FET 6 and the gate-on threshold voltage of the FET is exceeded and an on operation is attempted, the FE
Since T6 is still under cutoff control, it does not turn on.

The same applies to the FET 5, and even if an AC voltage is generated on the secondary side of the transformer 2 by the PWM control circuit 15, the FET 5 remains in the cutoff state. When the output voltage V1 of the other power supply circuit 18 drops, the error amplifier 13 in the power supply circuit 16 detects that the output voltage V1 has dropped, and sets the output signal level to VFB ″ so as to reach the set output voltage Va. To VFB '. The PWM control circuit 15 performs an operation of increasing the power transmission to the secondary side of the transformer 2, and the rectification control circuit 14 receives the output in the VFB 'region of the error amplifier 13 so that the output VD becomes H level. Become, FET5, F
Since the cutoff state of each gate of ET6 is canceled and the synchronous rectification operation is restarted, the power supply circuit 16 operates to control and stabilize the output voltage Vo that has been set.

Therefore, the load current sinking phenomenon due to the ON malfunction of the synchronous rectification FET due to the application of the output voltage of the other power supply circuit 18 which occurs when the power supply devices are operated in parallel by the conventional synchronous rectification method is eliminated, and thus the rectification circuit is provided. It is possible to prevent a current more than the rating of the used component from flowing into the FETs 5 and 6 in 4 and to prevent the power regeneration to the primary side, thereby damaging the rectifying component or the power conversion switching element. Therefore, even the power supply device using the synchronous rectification method can perform the parallel connection operation without any problem.

[0023]

As described above, according to the present invention, the output voltage of the power supply circuit is detected, and if the output voltage is abnormal, the operation of the rectifier circuit is stopped. Even if the output voltage of the power supply of is higher,
Detecting that the output voltage is high in the same way as in-circuit control,
In the worst case, even if the converter operation is stopped, it is possible to prevent the rectifier circuit from malfunctioning due to the application of a high voltage external voltage. That is, when the converter is stopped, the rectifier circuit does not absorb the load current, circuit components can be prevented from being damaged, and the reliability of the power supply circuits connected in parallel can be improved. Further, since the rectifier circuit control of the converter when the output voltage rises is performed based on the level detection of the error amplifier, there is an effect that the circuit configuration can be made up of a small number of parts.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a switching power supply device according to an embodiment of the present invention.

FIG. 2 is a time chart of signals at various parts of the circuit in FIG.

FIG. 3 is a circuit diagram showing a conventional switching power supply circuit.

FIG. 4 is a block diagram showing an example in which conventional switching power supply circuits are connected in parallel.

[Explanation of symbols]

1 DC input power supply 2 transformers 3 switching elements 4 Rectifier circuit 5, 6 FET 10 filters 13 Error amplifier 14 Rectification control circuit 15 PWM control circuit 16 power supply circuit 17 load 18 Other power circuits

Claims (5)

(57) [Claims] 1. A switch connected in parallel with another power supply circuit.
Is a switching power supply, and is composed of a transformer that induces an AC voltage on the secondary side by switching a DC input input to the primary side by a switching element, and at least two FETs that full-wave rectify the AC voltage from the transformer. A synchronous rectification type rectification circuit, a filter for direct-current smoothing the output voltage of the rectification circuit and supplying it to a load, and a switching control circuit for monitoring the output voltage of the filter and maintaining the output voltage at a set level. An error amplifier for controlling the operation of the switching element is provided, and the FET in the rectifier circuit is turned off or turned on according to the output voltage level of the error amplifier.
Stop the rectification operation of the rectifier circuit or restart it.
Switching power supply device is characterized in that the so that. 2. A rectification control circuit for controlling cutoff of the FET in the rectification circuit when the output voltage of the error amplifier exceeds a set level.
The switching power supply device according to. 3. The switching power supply device according to claim 1, wherein another DC power supply circuit is connected in parallel to the load. 4. The switching power supply device according to claim 1, wherein the switching control circuit is a PWM control circuit that receives an output of an error amplifier and pulse-width modulates a control input to the switching element. 5. The rectification control circuit controls the FET to turn off when the output level of the error amplifier exceeds a level range of a triangular wave which is a reference of a transmission pulse width of the PWM control circuit. 4. The switching power supply device according to item 4.