JP2000083376A - Power supply device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a power supply device that can be operated stably by reducing power loss due to a diode for rectification at a secondary side, and at the same time feeding current constantly only in a positive direction to a synchronous rectifying element, regardless of continuous and discontinuous modes. SOLUTION: In a power supply device, a DC input voltage is supplied to a primary coil winding N1 of a transformer T1, an AC voltage Vs that is induced in a secondary coil winding N2 of the transformer is rectified by a diode D1 for rectification and is smoothed by smoothing capacitors C1 and C2 for obtaining a DC output voltage V0, and at the same time a transistor Q2 for synchronous rectification for performing synchronous rectifying operation in parallel with the diode D1 is connected. In the power supply device, a synchronous rectification control circuit is provided to detect the ripple voltage of the smoothing capacitor C1 that is proportional to the rectifying current at the secondary side of the transformer T1, turns on the transistor Q2 when the ripple voltage is equal to or more than a first threshold, and turns off the transistor Q2 when the ripple voltage is equal to or less than a second threshold.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power supply.

[0002]

2. Description of the Related Art Conventionally, in a flyback type power supply device (switching power supply device), in order to reduce power loss due to a rectifying diode on the secondary side of a transformer, a MOS-FET which is a synchronous rectifying element is added to the diode. There is a synchronous rectification system in which transistors are connected in parallel and operated.

FIG. 7 shows a conventional example of such a power supply device.

In this power supply device, a DC input voltage Vi is applied to a primary winding N1 of a transformer T1 as a pulsed voltage by a switching transistor Q1.
AC voltage Vs induced in the secondary winding N2 is rectified by a rectifying diode D1.
A predetermined output voltage Vo is obtained by smoothing with a smoothing circuit comprising C2 and an inductor (choke coil) L1.

The switching power supply shown in FIG. 7 has an auxiliary winding N3 of a transformer T1, a transistor Q2 for synchronous rectification, a resistor R1 for dividing the induced voltage of the auxiliary winding N3 to drive the gate of the transistor Q2, A synchronous rectifier circuit including a resistor R2 is provided.

FIGS. 8 and 9 show waveforms of the AC voltage Vs and the current IQ2 of the transistor Q2 in the continuous mode of the power supply device shown in FIG. 7 and in the discontinuous mode which occurs when the load is lightly loaded, for example. Things.

In the switching power supply shown in FIG. 7, if no synchronous rectifier circuit is provided, a current flows through the diode D1 when the switching transistor Q1 is turned off.

On the other hand, when a synchronous rectifier circuit is provided, when the switching transistor Q1 is turned off, the auxiliary winding N
By the operation of the synchronous rectifier circuit based on the induced voltage of 3, the transistor Q2 is turned on, and the current IQ2 flows.

FIG. 8 shows the case of the continuous mode. When the induced voltage Vs of the secondary winding N2 is positive, the transistor Q2 turns on and the current IQ2 flows. When the induced voltage Vs is negative, the transistor Q2 turns off and the current IQ2 becomes zero.

FIG. 9 shows the case of the discontinuous mode. Similarly, when the induced voltage Vs is positive, the transistor Q2 is turned on,
When the induced voltage Vs is negative, the transistor Q2 turns off.

As described above, in the power supply device shown in FIG. 7, the transistor Q2 is turned on during the period when the current should flow through the diode D1, and the current IQ2 flows through the transistor Q2.
A circuit configuration that reduces power loss is adopted.

For example, when the current to flow through the diode D1 is 5A and the diode drop voltage (VF) is 0.4V
, A power loss of 2 W occurs. On the other hand, when the current IQ2 flows through the transistor Q2 having an on-resistance (Ron) of 10 mΩ, the power loss becomes 0.25 W. As a result, when the synchronous rectification circuit is provided, a power reduction of 1.75 W can be realized.

[0013]

However, in the case of the above-described conventional switching power supply, current may flow not only in the positive direction but also in the negative direction through the transistor Q2, so that the synchronous rectification operation can be performed smoothly. In some cases, abnormal operations such as intermittent oscillation may occur.

That is, in the case of the continuous mode shown in FIG. 8, a current flows through the transistor Q2 only in the positive direction. However, in the case of the discontinuous mode shown in FIG. A pulsating voltage Vp is generated, and a current such as a negative current Ip due to ringing flows through the transistor Q2 in accordance with the pulsating voltage Vp as shown in FIG.
As a result, in the case of the flyback type switching power supply, there is a problem that the synchronous rectification operation is not performed smoothly in the discontinuous mode.

The present invention has been made in order to solve the above-mentioned problems, and a synchronous rectifying element is always used regardless of a continuous mode or a discontinuous mode while reducing power loss by a secondary-side rectifying diode. An object of the present invention is to provide a power supply device capable of flowing current only in a positive direction and performing stable operation.

[0016]

According to the first aspect of the present invention,
The DC input voltage is switched and supplied to the primary winding of the transformer, and the AC voltage induced in the secondary winding of the transformer is rectified by a rectifying diode and smoothed by a smoothing circuit to obtain a DC output voltage. In a power supply device connected with a synchronous rectifying element that performs a synchronous rectification operation in parallel with the transformer, a secondary circuit voltage proportional to a rectified current on the secondary side of the transformer is detected, and when the secondary circuit voltage is equal to or higher than a first threshold value. A synchronous rectification control circuit for turning on the synchronous rectifier and turning off the synchronous rectifier when a secondary circuit voltage is equal to or lower than a second threshold value is provided.

According to the present invention, the synchronous rectification control circuit detects the secondary circuit voltage proportional to the rectified current on the secondary side of the transformer, and performs the synchronous rectification operation when the secondary circuit voltage is equal to or higher than the first threshold value. Since the synchronous rectifying element is turned on and the synchronous rectifying element is turned off when the secondary circuit voltage is equal to or lower than the second threshold value, it is possible to reduce the power loss due to the rectifying diode on the secondary side of the transformer. It is possible to supply a current to the element for synchronous rectification in the positive direction at all times regardless of the continuous mode or the discontinuous mode, thereby providing a power supply device exhibiting a stable synchronous rectification operation.

According to a second aspect of the present invention, a DC input voltage is switched and supplied to a primary winding of a transformer, and an AC voltage induced in a secondary winding of the transformer is rectified by a rectifying diode and smoothed by a smoothing capacitor. In the power supply device connected with a synchronous rectifying element performing a synchronous rectification operation in parallel with the diode, a ripple voltage of the smoothing capacitor proportional to a rectification current on the secondary side of the transformer is detected. A synchronous rectification control circuit that turns on the synchronous rectifier when the ripple voltage is equal to or higher than a first threshold and turns off the synchronous rectifier when the ripple voltage is equal to or lower than a second threshold. is there.

According to the present invention, the synchronous rectification control circuit detects the ripple voltage of the smoothing capacitor proportional to the rectification current on the secondary side of the transformer, and when the ripple voltage is equal to or higher than the first threshold value, the synchronous rectification element Is turned on, and the synchronous rectifying element is turned off when the ripple voltage is equal to or less than a second threshold value, so that it is possible to reduce power loss due to a rectifying diode on the secondary side of the transformer, and
Regardless of the continuous mode or the discontinuous mode, a current can always flow only in the positive direction to the element for synchronous rectification, thereby providing a power supply device exhibiting a stable synchronous rectification operation.

According to a third aspect of the present invention, in the power supply device according to the second aspect, the synchronous rectification control circuit sends an ON drive signal when a ripple voltage of the smoothing capacitor becomes equal to or higher than a first threshold value. A comparator for sending an off-drive signal when the ripple voltage of the smoothing capacitor becomes equal to or less than a second threshold, and a driver for turning on and off the synchronous rectifying element based on the on-drive signal and the off-drive signal from the comparator And characterized in that:

According to the present invention, the same operation as that of the second aspect can be exerted by the configuration using the comparator and the driver.

According to a fourth aspect of the present invention, a DC input voltage is switched and supplied to a primary winding of a transformer, and an AC voltage induced in a secondary winding of the transformer is rectified by a rectifying diode and smoothed by a smoothing circuit. A DC output voltage to obtain a DC output voltage, and a power supply device connected to a synchronous rectifying element that performs a synchronous rectification operation in parallel with the diode, wherein a resistor connected in series to a circuit through which a rectified current flows on the secondary side of the transformer; A comparator that sends an on-drive signal when the both-ends voltage is equal to or greater than a first threshold, and sends an off-drive signal when the both-ends voltage is equal to or less than a second threshold. ON drive signal from the
A synchronous rectification control circuit including a driver for driving the synchronous rectifier on and off based on an off drive signal is provided.

According to the present invention, the comparator of the synchronous rectification control circuit compares the voltage across the resistor connected in series to the circuit on the secondary side of the transformer, through which the rectified current flows. When the voltage between both ends is less than or equal to a second threshold, and sends an off drive signal when the both ends voltage is less than or equal to a second threshold value. Is turned on and off, power loss due to a rectifying diode on the secondary side of the transformer can be reduced with a configuration using the resistor, the comparator and the driver. Regardless of the continuous mode, current can always flow only in the positive direction to the element for synchronous rectification,
This makes it possible to provide a power supply device that exhibits a stable synchronous rectification operation.

[0024]

Embodiments of the present invention will be described below in detail.

(Embodiment 1) FIG. 1 shows a flyback type power supply device according to Embodiment 1 of the present invention. The power supply device having the same functions as those of the conventional power supply device shown in FIG. The same reference numerals are given.

In this power supply, a DC input voltage Vi is applied to the primary winding N1 of the transformer T1 as a pulsed voltage by the switching transistor Q1.
AC voltage Vs induced in the secondary winding N2 is rectified by a rectifying diode D1.
A predetermined output voltage Vo is obtained by smoothing with a smoothing circuit including C2 and an inductor (choke coil) L1.

Further, the power supply device includes the diode D
1 is provided with a transistor Q2 which is a synchronous rectifier having a source and a drain connected in parallel.

The voltage dividing resistors R3 and R4 are connected in parallel with the smoothing capacitor C1, and the voltage dividing resistors R5 and R6 are connected in parallel with the smoothing capacitor C2.

Further, this power supply device comprises a switching circuit comprising a synchronous rectification control circuit 1 for controlling the transistor Q2 to be turned on and off, an output voltage detection circuit 3 for controlling the switching operation of the switching transistor Q1, and a PWM circuit 4. And a control circuit 2.

The synchronous rectification control circuit 1 comprises a comparator IC2 having a hysteresis characteristic,
A driver IC1 for driving the transistor Q2 on and off based on the output signal of C2.

The comparator IC2 has a non-inverting terminal connected to a connection point of the voltage dividing resistors R3 and R4, and an inverting terminal connected to a connection point of the voltage dividing resistors R5 and R6 to connect with the voltage Vc1 of the smoothing capacitor C1. DC output voltage Vo
Are compared with each other, and the ripple voltage (V
c1−Vo) is higher than the first threshold value VH, an on drive signal is sent out, and when the ripple voltage (Vc1−Vo) of the smoothing capacitor C1 is lower than the second threshold value VL, the off drive signal is output. It is designed to be sent.

The driver IC1 is provided with a comparator IC2
The transistor Q2 is turned on and off based on the on-drive signal and the off-drive signal from the CPU.

FIGS. 2 and 3 show the AC voltage Vs, the ripple voltage (Vc1−Vo), and the transistor in the continuous mode of the power supply device shown in FIG. 1 and in the discontinuous mode which occurs when the load is lightly loaded. Current I flowing through Q2
Q2 shows the waveform of the current ID1 flowing through the diode D1.

Next, the operation of the power supply according to the first embodiment will be described.

In this power supply device, when the switching transistor Q1 is turned off, the secondary side diode D
1 is turned on, the current ID1 flows through the diode D1, the secondary current IS flows, and a predetermined DC output voltage Vo is output.
A ripple current flows through the secondary-side smoothing capacitor C1.

The comparator IC2 of the synchronous rectification control circuit 1
Is a voltage difference between the DC output voltage Vo and the voltage VC1 of the smoothing capacitor C1 which is divided and inputted by the voltage dividing resistors R3 and R4 and the voltage dividing resistors R5 and R6, respectively, and a ripple which is a difference voltage between the voltage VC1 and the DC output voltage Vo. When the voltage (VC1-Vo) reaches a preset first threshold VH level, an on-drive signal is sent to the driver IC1. As a result, the driver IC1 turns on the transistor Q2.

As a result, the current IQ2
Flows, and the current ID1 of the diode D1 becomes 0 while the current IQ2 flows.

Next, the ripple voltage (VC1-Vo) becomes the first voltage.
When the threshold VL level is reached, the comparator IC2
An off drive signal is sent to the driver IC1. As a result, the driver IC1 turns off the transistor Q2.

As a result, the transistor Q2 is turned off, and the current ID1 flows through the diode D1 again.

As described above, in the flyback type power supply, regardless of the continuous mode or the discontinuous mode, only the current IQ2 in the positive direction can always flow through the transistor Q2, and the synchronous rectification operation by the transistor Q2 is stabilized. Can be run with

Further, it is possible to reduce the power loss of the diode D1 on the secondary side. That is, the current to flow through the diode D1 is 5A, and the diode drop (VF) is 0.4
V, a power loss of 2 W occurs, but the current IQ2 is applied to the transistor Q2 having an on-resistance (Ron) of 10 mΩ.
When (5A) flows, the power loss becomes 0.25 W.

As a result, also in the first embodiment, a power reduction of 1.75 W can be realized as in the conventional example.

FIG. 4 shows a modification of the power supply device of the first embodiment. The basic configuration is the same as that of the power supply device shown in FIG. 1, but the circuit configuration on the input side of the comparator IC2 is changed. It is. That is, the inverting terminal of the comparator IC2 is connected to the connection point of the voltage dividing resistors R3 and R4, and the non-inverting terminal is connected to the connection point of the voltage dividing resistors R5 and R6, so that the voltage Vc1 of the smoothing capacitor C1 and the DC output voltage The difference is that Vo is compared. FIG.
The same operation as that of the power supply device shown in FIG. 1 can be exerted by the power supply device having the circuit configuration shown in FIG.

FIG. 5 shows another modification of the power supply device of the first embodiment. In the case of the power supply device shown in FIG. 1, an N-channel FET is used as the transistor Q2. The power supply device shown in FIG. 5 is different from the power supply device in that a P-channel FET is used as the transistor Q2 and a driver IC1 composed of an inverter is used.

The power supply device having the circuit configuration shown in FIG. 5 can exhibit the same operation as that of the power supply device shown in FIG.

(Embodiment 2) FIG. 6 shows a power supply device according to a second embodiment. This power supply device has the same basic circuit configuration as the power supply device according to the first embodiment. A resistor R7 connected in series to a circuit through which a rectified current Is flows on the secondary side of the transformer T1 is compared with the voltage across the resistor R7. When the voltage across the resistor R7 exceeds a first threshold value VH, an ON drive signal is transmitted. A comparator IC2 for sending an off drive signal when the voltage between both ends becomes equal to or less than a second threshold value VL.
And a driver IC1 for driving the transistor Q2 on and off based on the on-drive signal and off-drive signal from the comparator IC2. The voltage dividing resistors R3 to R3
6 is omitted.

According to the power supply device of the second embodiment, the comparator IC2 of the synchronous rectification control circuit includes the transformer T1
Of the resistor R7 connected in series to the circuit through which the secondary-side rectified current Is flows, and the voltage across the resistor R7 is equal to the first threshold VH.
An on-drive signal is sent when the above is reached, and an off-drive signal is sent when the voltage between both ends falls below the second threshold value VL.

The driver IC1 includes a comparator I
The transistor Q2 is turned on and off based on the on drive signal and the off drive signal from C2.

As a result, the resistor R7, the comparator I
With the circuit configuration using C2 and the driver IC1, the power loss due to the rectifying diode D1 on the secondary side of the transformer T1 can be reduced as in the case of the first embodiment. Regardless of the mode, a current can always flow only in the positive direction through the transistor Q2 for synchronous rectification, whereby a stable synchronous rectification operation can be exhibited.

[0050]

According to the first aspect of the present invention, it is possible to reduce the power loss due to the rectifying diode on the secondary side of the transformer, and to perform synchronous rectification regardless of the continuous mode or the discontinuous mode. A current can always flow only in the positive direction to the element for power supply, and a power supply device that always exhibits a stable synchronous rectification operation can be provided.

According to the second aspect of the present invention, the power loss due to the rectifying diode on the secondary side of the transformer can be reduced by the synchronous rectification control using the ripple voltage of the smoothing capacitor, and the continuous mode can be achieved. Regardless of the discontinuous mode, a current can always flow only in the positive direction to the element for synchronous rectification, and a power supply device that always exhibits a stable synchronous rectification operation can be provided.

According to the third aspect of the present invention, it is possible to provide a power supply device having a configuration using a comparator and a driver and capable of exhibiting the same operation as the second aspect of the present invention.

According to the fourth aspect of the present invention, the rectifying diode on the secondary side of the transformer is provided by synchronous rectification control using the voltage between both ends of the resistor connected in series to the circuit through which the rectified current flows on the secondary side of the transformer. A power supply that can reduce power loss, and can always supply current in the positive direction to the synchronous rectification element regardless of continuous mode or discontinuous mode, and always exhibit stable synchronous rectification operation Equipment can be provided.

[Brief description of the drawings]

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

FIG. 2 is a waveform diagram of the switching power supply device of the first embodiment in a continuous mode.

FIG. 3 is a waveform chart in a discontinuous mode of the switching power supply device of the first embodiment.

FIG. 4 is a circuit diagram showing a switching power supply according to Embodiment 2 of the present invention.

FIG. 5 is a circuit diagram showing a switching power supply according to Embodiment 3 of the present invention.

FIG. 6 is a circuit diagram showing a switching power supply device according to Embodiment 4 of the present invention.

FIG. 7 is a circuit diagram showing a conventional switching power supply device.

FIG. 8 is a waveform diagram in a continuous mode of a conventional switching power supply device.

FIG. 9 is a waveform diagram of a conventional switching power supply device in a discontinuous mode.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Synchronous rectification control circuit 2 Switching control circuit 3 Output voltage detection circuit 4 PWM circuit IC1 Driver IC2 Comparator C1 Capacitor C2 Capacitor T1 Transformer D1 Diode Q1 Switching transistor Q2 Transistor

Claims (4)

[Claims] 1. A DC input voltage is switched and supplied to a primary winding of a transformer, and an AC voltage induced in a secondary winding of the transformer is rectified by a rectifying diode and smoothed by a smoothing circuit to obtain a DC output voltage. A power supply device connected to a synchronous rectifying element for performing a synchronous rectification operation in parallel with the diode, wherein a secondary circuit voltage proportional to a rectified current on the secondary side of the transformer is detected, and the secondary circuit voltage A power supply unit, comprising: a synchronous rectification control circuit that turns on the synchronous rectifier element at a threshold value or more and turns off the synchronous rectifier element at a secondary circuit voltage equal to or less than a second threshold value. 2. A DC input voltage is switched and supplied to a primary winding of a transformer, and an AC voltage induced in a secondary winding of the transformer is rectified by a rectifying diode and smoothed by a smoothing capacitor to obtain a DC output voltage. A power supply device connected to a synchronous rectifying element performing a synchronous rectification operation in parallel with the diode, wherein a ripple voltage of the smoothing capacitor, which is proportional to a rectification current on the secondary side of the transformer, is detected, and A power supply device, comprising: a synchronous rectification control circuit that turns on the synchronous rectifier element when the threshold voltage is equal to or more than one threshold value and turns off the synchronous rectifier element when the ripple voltage is equal to or less than a second threshold value. 3. The synchronous rectification control circuit sends an on-drive signal when the ripple voltage of the smoothing capacitor is equal to or higher than a first threshold, and the ripple voltage of the smoothing capacitor is equal to or lower than a second threshold. 3. The power supply according to claim 2, further comprising: a comparator for transmitting a time-off drive signal; and a driver for turning on and off the synchronous rectifying element based on the on-drive signal and the off-drive signal from the comparator. apparatus. 4. A DC input voltage is switched and supplied to a primary winding of a transformer, and an AC voltage induced in a secondary winding of the transformer is rectified by a rectifying diode and smoothed by a smoothing circuit to obtain a DC output voltage. In the power supply device connected with a synchronous rectifying element performing a synchronous rectification operation in parallel with the diode, a resistor connected in series to a circuit through which a rectified current flows on the secondary side of the transformer is compared with a voltage across the resistor. A comparator that sends an on-drive signal when the voltage at both ends is equal to or greater than a first threshold, and sends an off-drive signal when the voltage at both ends is equal to or less than a second threshold; an on-drive signal from this comparator; A power supply device, comprising: a synchronous rectification control circuit including a driver that drives the synchronous rectifier on and off based on an off drive signal.