JP2001292572A - Switching power supply - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent unstable operation resulting from over-current in a switching power supply comprising a synchronous control circuit. SOLUTION: By detecting simultaneous on-state of a main transistor Q1 and a synchronous rectifying transistor circuit 12 with detection of an over current, a secondary side rectifying circuit 12 of the switching power supply 10 turns off a synchronous rectifying transistor SR2 and also stops the operation of the synchronous rectifying transistor SR2 for the constant period even after the current is recovered from the over current condition, in order to prevent unstable operation. Moreover, immediately after the drive, in which the secondary side rectifying voltage does not reach the prescribed value, the synchronous rectifying transistor SR2 is turned off.

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, and more particularly to an improvement in a synchronous rectifier circuit.

[0002]

2. Description of the Related Art A switching power supply using a synchronous rectifier circuit is used as a DC power supply in various devices such as a measuring device. FIG. 8 is a circuit diagram of a Cuk type converter which is one of the conventional switching power supply devices of this type. Q1 is the main switching transistor, S
R1 is a synchronous rectification transistor. When the main transistor Q1 is turned on while the synchronous rectification transistor SR1 is turned off and the DC voltage IN is applied to the primary side circuit 11 connected to the primary winding of the output transformer T1 to flow a current, the output transformer T1 Rectifier circuit 12 connected to the secondary winding of
To supply a load current to the load L1 connected to the output terminal OUT of the secondary side rectifier circuit 12 and charge the main capacitor C1.

When the transistor Q1 is turned off at a predetermined timing, the current direction is reversed in the secondary winding of the output transformer T1, so that the rectification current of the secondary rectifier circuit 12 is detected and monitored. Circuit CS1 and current monitoring circuit C
It is detected by M1 and turns on the synchronous rectification transistor SR1. The output voltage detector VS1 is connected to the output terminal OUT
Is measured, and the voltage value is fed back to the primary side control circuit U1 via the photocoupler PHC. The primary side control circuit U1 controls on / off of the main transistor Q1 so that the output terminal voltage OUT is maintained at a predetermined value.

In the above operation, the synchronous rectification transistor SR1 is turned on when the current (detected value) of the secondary side rectification circuit 12 exceeds an appropriate threshold, and is turned off when the current (detection value) falls below the appropriate threshold. . The primary side control circuit U1 detects that the synchronous rectification transistor SR1 is turned off and the voltage of the output transformer T1 changes with the terminal voltage ZC of the auxiliary winding, and turns on the transistor Q1 at this timing. This converter is a variable frequency control type self-excited converter whose turn-off timing is adjusted so that the output voltage is controlled to a predetermined value. The primary side control circuit U1 and the comparator CMP1 of the current monitoring circuit CM1 operate cooperatively in a hand-jake relationship.

In the above-mentioned conventional converter, a circuit malfunctions due to external noise such as a line transient, and the main transistor Q1 and the synchronous rectification transistor SR1 are simultaneously turned on, so that an abnormal state in which an excessive surge current flows may occur. . Further, once this abnormal state occurs, the synchronous rectification transistor SR1 malfunctions again due to the surge current caused by this, and the output becomes extremely unstable. .

FIG. 9 shows a switching power supply called a flyback converter, which is another example of a switching power supply including a conventional synchronous rectifier circuit. This power supply device is described in JP-A-7-7928. In this converter, when the main transistor Q2 is turned on, the synchronous rectification transistor SR2 is turned off to block the current of the secondary rectifier circuit 14, and when the main transistor Q2 is turned off, the synchronous rectification transistor SR2 is turned on. Thus, a current is supplied to the load L2 by the energy stored in the output transformer T2. Here, in the control of the synchronous rectification transistor SR2, the direction of the current flowing through the rectification circuit 14 is detected by the comparator CMP4,
Synchronous rectification transistor SR2 only when the current is in the forward direction
Turn on.

FIGS. 10 (a) and 10 (b) show waveforms when the converter is operated under fixed frequency control and the main transistor Q2 and the synchronous rectification transistor SR2 are switched in opposite phases. FIGS. 10 (a) and 10 (b) show load currents, respectively. Are large (heavy load) and small (light load). Q2 indicates the on / off state of the main transistor,
SR2 indicates a waveform of a current flowing through the synchronous rectification transistor.

As shown in FIG. 10A, when the converter is operated in the continuous mode, the synchronous rectification transistor SR2 is turned on when the main transistor Q2 is turned on, so that a period occurs in which both transistors are turned on at the same time. I do. In this case, a sharp surge current is generated when the transistors are turned on at the same time, so that the transistor may be broken.

As shown in FIG. 1B, when the load is light, the current flowing backward through the synchronous rectification transistor SR2 increases. In this case, part of the energy of the secondary rectifier circuit 14 And there is a disadvantage that useless rectification loss increases.

[0010]

[Problems to be solved by the invention]

In view of the above-mentioned drawbacks in the prior art, the present invention is directed to a case where an abnormal condition such as an excessive surge occurs when the main transistors Q1 and Q2 and the synchronous rectification transistors SR1 and SR2 are simultaneously turned on and an excessive surge occurs. An object of the present invention is to provide a converter whose operation is stable by detecting this promptly and stopping the operation of the synchronous rectification transistor.

According to the present invention, in order to achieve the above object, especially when an abnormal situation such as an overcurrent occurs, it takes a proper time to return the synchronous control transistor to a soft start when returning from the abnormal state. To break the vicious circle of malfunctions, and
It is another object of the present invention to stably start the switching power supply when starting the switching power supply.

[0013]

To achieve the above object, a switching power supply according to a first aspect of the present invention is connected to a primary winding of an output transformer and supplies power to the output transformer by switching. In a switching power supply device having a main transistor to be supplied and a synchronous rectification transistor for performing synchronous rectification in a secondary rectifier circuit connected to a secondary winding of the output transformer, an overcurrent in the secondary rectifier circuit is provided. An overcurrent detector for detecting a current, and a control circuit for stopping the operation of the synchronous rectification transistor when the overcurrent is detected by the overcurrent detector.

According to the switching power supply of the first aspect of the present invention, the overcurrent flowing through the secondary side rectifier circuit causes
When the abnormal state of the device is detected and the operation of the synchronous rectification transistor is stopped, even if both the main transistor and the synchronous rectification transistor are turned on and an abnormal state occurs, the abnormal state is quickly recovered.

In a preferred configuration of the invention according to the first aspect, while the operation of the synchronous rectification transistor is stopped, rectification is performed by a body diode incorporated in the synchronous rectification transistor. Alternatively, a diode may be connected in parallel to the synchronous rectification transistor and rectification may be performed by the parallel diode.

[0016] It is also a preferable aspect of the present invention that the control device starts the operation of the synchronous rectification transistor when a predetermined time has elapsed after recovery from the overcurrent when the overcurrent is detected. In this case, a particularly stable operation can be obtained.

In a preferred aspect of the first aspect of the present invention, the control device starts the operation of the synchronous rectification transistor after the output voltage of the secondary side rectifier circuit reaches a predetermined value or more. is there. A particularly stable operation can be obtained at startup.

Further, a switching power supply according to a second aspect of the present invention includes a main transistor connected to a primary winding of an output transformer and supplying power to the output transformer by switching; In a switching power supply device having a synchronous rectifier transistor for performing synchronous rectification in a secondary rectifier circuit connected to a secondary winding, a current detector for detecting a current in the secondary rectifier circuit, and the current detector A comparator that compares the current detected by the comparator with a predetermined threshold value, and a control circuit that controls on / off of the synchronous rectification transistor based on an output of the comparator.

According to the switching power supply of the second aspect of the present invention, since the turn-off current of the current flowing through the secondary rectifier circuit can be controlled to a predetermined value, the energy flowing from the secondary side to the primary side is reduced.

In a preferred example of the second aspect of the invention,
A delay circuit for delaying an output of the comparator, wherein the output of the delay circuit controls the synchronous rectification transistor. In this case, the turn-off current can be further controlled as desired.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in more detail with reference to the drawings based on embodiments of the present invention. In addition,
For easy understanding, the same reference numerals are given to the elements having the same operation in each drawing.

First Embodiment FIG. 1 shows a switching power supply according to a first embodiment of the present invention. In the switching power supply (converter) 10 of the present embodiment, the control power generation circuit VG for generating the control power VE from the terminal voltage of the energy transfer capacitor Ce2 has the configuration of the conventional switching power supply shown in FIG. Unlike the above, the overcurrent detection circuit OLP including the comparator CMP2 and the soft start circuit Soft that controls the start of the synchronous rectification transistor SR1 based on the output of the overcurrent detection circuit OLP
And a low-voltage malfunction prevention circuit (under-voltage detection circuit) UVLO including resistors R5 and R6.

The control power generation circuit VG includes a transistor Q
3. Consisting of a capacitor C2 and a Zener diode D1, from an energy transfer capacitor Ce2,
A voltage substantially proportional to the output voltage OUT of the secondary side rectifier circuit 12 is input, and a voltage VE controlled to a constant voltage is output. The low-voltage malfunction prevention circuit UVLO includes resistors R5 and R6.
And a PNP transistor Q4, and monitors a control voltage VE which is an output of the control power generation circuit VG.

The current detection circuit CS1 is composed of a series circuit of LRC connected to the energy transfer capacitor Ce2, and its output voltage terminal Vs is connected to the secondary side rectification circuit 12
And outputs a voltage proportional to the current.

The resistors R1 and R2 and the comparator CMP1
The current monitoring circuit CM1 includes a control signal for the synchronous rectification transistor SR1 based on the output voltage Vs of the current detection circuit CS1, and sends the control signal to the gate of the synchronous rectification transistor SR1 via the buffer BF1. give.

The comparator CM of the overcurrent detection circuit OLP
The inverting terminal of P2 is the output terminal Vs of the current detection circuit CS1.
, And a reference voltage divided by the resistors R3 and R4 is input to the non-inverting terminal. The threshold for overcurrent detection is adjusted by resistors R3 and R4. The threshold set by the comparator CMP2 is set to a value higher than the current value during normal operation. Comparator CM
The output of P2 is supplied to the gate of the synchronous rectification transistor SR1 via the soft start circuit Soft and the buffer transistor BF1.

FIG. 2 shows operation waveforms in a steady operation state in the switching power supply device of the embodiment. In the figure, for example, the operating frequency of the switching power supply is several tens of k.
Hz, and shows a case where switching of the load is performed at several tens of Hz. VD indicates the voltage waveform before smoothing of the secondary side rectifier circuit 12, VS indicates the waveform of the output voltage of the current detection circuit CS1, and Vg indicates the gate voltage waveform of the synchronous rectification transistor SR1.

When an abnormal state occurs in which an excessive surge current flows, as shown in the figure, VS becomes an excessive voltage value. Therefore, this is determined by the comparator CMP2 of the overcurrent detection circuit OLP via the current detection circuit CS1. Is detected. When the comparator CMP2 determines that an abnormal state has occurred, its output is immediately transmitted to the buffer transistor BF1, and the gate voltage V of the synchronous rectification transistor SR1 is output.
By setting g to the L level, the synchronous rectification transistor SR1 is turned off. At this time, the synchronous rectification transistor SR1
Performs a diode rectification operation with a built-in body diode. Note that a parallel diode may be connected to the synchronous rectification transistor SR1 separately from the body diode.

When the abnormal state disappears, the comparator CM
Although the output of P2 is inverted, a resistor R7 and a capacitor C3
Within a fixed period t1 defined by a time constant determined by
The input voltage of the buffer transistor BF1 is clamped by the action of the transistors Q5 and Q6, and the synchronous rectification operation is held. When the predetermined time has passed and the temperature becomes sufficiently stable, the synchronous rectification transistor SR1
The synchronous rectification operation is started again.

FIG. 3 shows operation waveforms at the time of power-on in the converter of the above embodiment. The low voltage malfunction prevention circuit UVLO is connected to the output VE of the control power generation circuit VG.
Is low, the gate voltage Vg is set to the L level to control the synchronous rectification transistor SR1 to maintain the off state, and when the rectified current is stabilized and the voltage VS is sufficiently stabilized, the switching of the synchronous rectification transistor SR1 is performed. Control to start.

As described above, in the converter of the present embodiment, the switching operation of the synchronous rectification transistor SR1 is performed only when the circuit is sufficiently stable, and diode rectification is performed when the control is unstable. The reliability of the operation of the converter is improved. In addition, it can be applied to a wide range of applications.

Further, even if an erroneous operation occurs due to an external noise and the main transistor Q1 and the synchronous rectification transistor SR1 are simultaneously turned on and an abnormal state occurs in which an excessive surge current occurs, a vicious cycle in which the erroneous operation is repeated is brought. And automatically recover from an abnormal state.

Embodiment 2 FIG. 4 shows a switching power supply according to a second embodiment of the present invention. Switching power supply device 10A in FIG.
Differs from the configuration of the first embodiment in that the control circuit portion of the synchronous rectification transistor SR1 is integrated to form an integrated control circuit U3.

FIG. 5 shows the configuration of the integrated control circuit U3.
The integrated control circuit U3 converts a voltage Vcc obtained from the transfer capacitor Ce2 to a predetermined reference voltage Vref1,
Reference voltage generator Vref for generating divided voltage Vref2
A voltage dividing unit (not shown) for dividing the reference voltage Vref1 to generate predetermined threshold voltages ZC and S / S, and a comparator for comparing the voltage Vref2 with a voltage Vcc proportional to the output voltage of the secondary side rectifier circuit 12. CMP5 and a comparator CMP6 that compares a voltage CS proportional to the rectified current of the secondary side rectifier circuit 12 with a predetermined threshold ZC.
And a voltage CS proportional to the rectified current of the secondary side rectifier circuit 12
Is compared with a predetermined threshold Cth to detect an overcurrent, a timer TIMER for providing a delay for soft start, an AND gate AND for ANDing the output of each comparator, and an AND gate A
A buffer BF2 for amplifying the output of the ND.

The switching power supply of this embodiment is
Compared with the first embodiment, the function is the same, but it can be composed of the reference voltage source Vref, three comparators, a voltage divider, a timer, and a buffer, and thus has an advantage of excellent integration. In particular, when a low-cost general-purpose 8-pin package is used as a package, high added value can be obtained. In addition, since the threshold can be set freely for each of the operating current, the overcurrent, and the undervoltage, the applicable range is wide, and the size and cost can be reduced.

Embodiment 3 FIG. 6 shows an example in which the present invention is applied to a flyback converter. The secondary side of a current transformer (current transformer) CT for detecting the current of the secondary side rectifier circuit 14 is a comparator C
MP3 is connected to the non-inverting terminal and its output voltage V
1 is input to the delay circuit D1. Comparator CM
The threshold voltage of P3 is Vth, the comparator CMP3 compares the output voltage V1 of the transformer CT with the threshold Vth, and gives the comparison result to the gate of the synchronous rectification transistor SR2 via the delay circuit D1. Drive SR2. The primary side control circuit U2 detects the off state of the synchronous rectification transistor SR2 at the terminal ZC of the auxiliary winding of the output transformer T2, and turns on the main transistor Q2. This constitutes a variable frequency type self-excited converter.

FIGS. 7A and 7B are timing charts showing operation waveforms of the converter of FIG. 6, wherein FIG. 7A shows a heavy load, and FIG. 7B shows a light load. The switching power supply device 20 of the present embodiment operates close to the boundary mode by the variable frequency control. As shown in FIG. 3B, the operating frequency increases at light load compared to heavy load. The figure shows a case where the control circuit U2 of the main transistor Q2 is configured to limit the upper limit of the operating frequency, in which case the device is in the discontinuous mode.

When the voltage V1 proportional to the current flowing through the synchronous rectification transistor SR2 exceeds the threshold voltage Vth of the comparator CMP3, the comparator CMP3
Of the rectifier circuit 1
4 is detected. This period is given to the gate of the synchronous rectification transistor SR2 as a high-level period of the voltage SR, which is extended by the delay of the delay circuit D1.

Here, the current peak ip when the synchronous rectification transistor SR2 is turned off is expressed as follows. ip∝Vth-k * Vo * D1 / Lsk is a constant determined by the current transformer CT, Vo is the output voltage of the converter CMP3, and Ls is the inductance of the secondary winding of the output transformer T2.

As is apparent from the above equation, when the output voltage Vo is controlled to be constant, ip always has a constant value regardless of the load current. That is, the current peak ip can be controlled to a constant value.

When the primary transistor Q2 on the primary side is turned off, the energy stored in the output transformer T2 biases the body diode of the synchronous rectification transistor SR2 in the forward direction, and the body diode turns on.
The forward current of the synchronous rectification transistor SR2 is detected and monitored by the current transformer CT and the comparator CMP3, and turns on the channel of the synchronous rectification transistor SR2. When the current of the synchronous rectification transistor SR2 gradually decreases and the secondary output of the current transformer CT reaches the threshold voltage Vth of the comparator CMP3, the synchronous rectification transistor Q2 is turned off.

The threshold voltage Vth and the delay circuit D
The delay by 1 is set such that the current peak value ip when the synchronous rectification transistor Q2 is turned off becomes an appropriate value in the reverse direction. When the synchronous rectifier transistor Q2 is turned off, no current flows through the body diode of the synchronous rectifier transistor Q2. Thereby, the transformer T
The problem of power recovery via the two windings is avoided.

The current ip promotes discharge of the output parasitic capacitance of the main transistor Q2, and reduces the turn-on loss of the main transistor Q2. When the synchronous rectification transistor SR2 is turned off, the back electromotive voltage of the output transformer T2 changes, and the terminal voltage ZC of the bias winding changes. Primary side control circuit U2
Detects the voltage ZC and turns on the main transistor Q2. Therefore, there is no period during which the main transistor Q2 and the synchronous rectification transistor SR2 are simultaneously turned on. When the current of the main transistor Q2 reaches a certain peak value, the primary side control circuit U2 turns off the main transistor Q2. The peak value is controlled to stabilize the output.

In the converter of the above embodiment, regardless of the input voltage, load current and operating frequency of the converter,
Since the constant reverse current peak value at the time of turning off the synchronous rectification transistor SR2 is always constant, there is no problem of power recovery from the rectification transistor Q2 and the turn-on loss of the main transistor is small. Therefore, the noise characteristics are good and low. A lossy converter can be realized.

As described above, the present invention has been described based on the preferred embodiment. However, the switching power supply having the synchronous rectifier circuit of the present invention is not limited to the configuration of the above-described embodiment, but is described above. Various modifications and changes from the configuration of the embodiment are also included in the scope of the present invention.

[0046]

As described above, according to the switching power supply of the present invention, when an abnormal state caused by the simultaneous turning on of the main transistor and the synchronous rectification transistor is detected, the synchronous rectification transistor is turned off.
By employing the rectification by the diode, no overcurrent occurs in the converter.

Also, by adopting soft start and not operating the synchronous rectification transistor for a predetermined period after the abnormal situation such as overcurrent is not detected, unstable operation such as repeated turning on / off of the synchronous rectification transistor. Can be prevented.

Further, when a shortage of the output voltage of the secondary side rectifier circuit is detected, an unstable operation which tends to occur at the time of starting the converter can be prevented by adopting a configuration in which the synchronous rectifier transistor is not operated during that time.

[Brief description of the drawings]

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

FIG. 2 is an operation waveform diagram at the time of load response in the switching power supply device of FIG. 1;

FIG. 3 is an operation waveform diagram at the time of startup in the switching power supply device of FIG. 1;

FIG. 4 is a circuit diagram of a switching power supply device according to a second embodiment of the present invention.

5 is a logic circuit diagram showing a configuration of the integrated control device shown in FIG.

FIG. 6 is a circuit diagram of a switching power supply according to a third embodiment of the present invention.

7 is an operation waveform diagram of the switching power supply device of FIG.

FIG. 8 is a circuit diagram of a first conventional switching power supply device.

FIG. 9 is a circuit diagram of a second conventional switching power supply device.

FIG. 10 is an operation waveform diagram of the switching power supply device of FIG. 9;

[Explanation of symbols]

 10: Switching power supply device 11: Primary side circuit 12: Secondary side rectifier circuit CS1: Current detection circuit CM1: Current monitoring circuit VG: Control power generation circuit UVLO: Low voltage malfunction prevention circuit OLP: Overcurrent detection circuit Soft: Soft start Circuits Q1 to Q6: Transistor SR1: Synchronous rectification transistor U1, U2: Primary control circuit U3: Integrated control circuit VS1: Secondary voltage detection circuit PHC: Photocoupler C1, C2, C3, Ce2: Capacitors R1 to R8: Resistance

Claims (6)

[Claims] 1. A main transistor connected to a primary winding of an output transformer and supplying power to the output transformer by switching, and a secondary rectifier circuit connected to a secondary winding of the output transformer. In a switching power supply device having a synchronous rectification transistor that performs synchronous rectification within, an overcurrent detector that detects an overcurrent in the secondary-side rectifier circuit, and an overcurrent is detected when the overcurrent detector detects the overcurrent. And a control circuit for stopping the operation of the synchronous rectification transistor. 2. The switching power supply according to claim 1, wherein a body diode included in the synchronous rectification transistor performs rectification while the operation of the synchronous rectification transistor is stopped. 3. The control device according to claim 1, wherein when an overcurrent is detected, the control device starts the operation of the synchronous rectification transistor after a lapse of a predetermined time from the recovery from the overcurrent. 3. The switching power supply device according to 2. 4. The switching power supply device according to claim 1, wherein said control device starts operation of said synchronous rectification transistor after an output voltage of a secondary side rectifier circuit reaches a predetermined value or more. . 5. A main transistor connected to a primary winding of an output transformer and supplying power to the output transformer by switching, and a secondary rectifier circuit connected to a secondary winding of the output transformer. A switching power supply device having a synchronous rectification transistor that performs synchronous rectification within the current source, a current detector that detects a current in the secondary rectifier circuit, and a current detected by the current detector and a predetermined threshold value. A switching power supply device comprising: a comparator to be compared; and a control circuit that controls on / off of the synchronous rectification transistor based on an output of the comparator. 6. The switching power supply device according to claim 5, further comprising a delay circuit for delaying an output of the comparator, wherein the output of the delay circuit controls on / off of the synchronous rectification transistor.