JP2000069753A - Power supply - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a power supply in which cost and size are reduced while preventing a choke coil from being cut off by dividing a smoothing choke coil in the secondary circuit of a switching power supply into a cored coil and an air-core coil and connecting two coils in series thereby lowering power consumption. SOLUTION: In a DC/DC converter system power supply employing a switching element FET, a smoothing choke coil in the secondary circuit is divided into a cored coil 1 and an air-core coil 2 connected in series. Under light load, inductances of the coils L1, L2 are summed to produce a high inductance for preventing cut-off of a choke coil without relying upon a dummy resistance. Under heavy load, the coil L1 is saturated to produce a low inductance thus reducing power consumption.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power supply device, and more particularly to a rectifier circuit of a DC / DC converter type power supply device using switching.

In recent years, DC / DC converter type power supply devices have been widely used as power supplies for information and communication equipment. Along with this, miniaturization and high efficiency of the power supply device have become a problem.

[0003]

2. Description of the Related Art FIG. 3 is a diagram showing an example of the configuration of a conventional power supply device, showing a forward type. FIG. 4 is a diagram showing operation waveforms of various parts of the conventional device. An on / off signal as shown from the output detection and PWM control circuit 10 turns on a FET (field effect transistor) as a switching element. When the FET is turned on, its drain-source voltage Vds becomes almost 0 as shown in FIG. The input DC voltage Vi is applied to the primary side of the transformer T via the terminals 2 and 3. This DC voltage Vin is smoothed by a smoothing capacitor Cin to remove noise.

Is the drain-source voltage Vd of the FET.
s, and the waveform at the time of OFF is rounded as shown in the figure due to the reset voltage (LC resonance) due to the transformer excitation energy. The drain current Id at this time is as shown below, and the voltage Vt on the primary side of the transformer T at this time is
1 is as shown in FIG.

As a result, a DC voltage Vin is applied between the primary windings of the transformer T, and a voltage corresponding to the turns ratio N2 / N1 is generated between the secondary windings. The voltage Vt2 shown in FIG.
Is a voltage waveform generated on the secondary side of the transformer. The output voltage Vt2 at this time is expressed by the following equation.

Vt2 = (N2 / N1) · Vt1 (1) The generated secondary voltage Vt2 is converted from the diode D1 to the coil L
→ Current is supplied to the load 4 through the smoothing filter of the capacitor Co. On the other hand, when the FET is turned off, the energy stored in the smoothing coil L becomes a voltage source and the coil L
→ Capacitor Co → Supply energy to the load 4 through the smoothing filter of the diode D2. Is a current IL flowing through the coil L, and is a current flowing through the coil L in the discontinuous mode (details will be described later).

The circuit shown in FIG. 3 operates at a constant switching frequency, detects and feedbacks the output voltage and controls the output voltage (controls the output voltage by controlling the ON width of the FET at a constant frequency). The output detection and PWM control circuit 100 shown in the figure detects the voltage, and applies a drive pulse having a pulse width (shown) according to the voltage to the gate of the FET. This keeps the output voltage Vo constant. The output voltage Vo at this time is expressed by the following equation.

[0008]

(Equation 1)

Here, Vo is the output voltage, Vin is the input voltage, N1 is the number of primary windings of the transformer T, and N2 is the number of secondary windings. D is the on / off duty ratio, Tons /
T (constant at T = 1 / f).

In this method, the circuit is very simple,
This is the most commonly used method, but has a problem that the output voltage increases due to the cutoff of the smoothing choke coil L when the output current is small. FIG. 5 is a diagram showing the characteristics of the switching power supply. The vertical axis represents the output voltage, and the horizontal axis represents the output current. Due to the characteristics of the smoothing choke coil L, a discontinuous region and a continuous region are generated by the inductance of L. The region f1 where the current is small is a discontinuous region, and the region f2 where the current is large is a continuous region. A in FIG.
The point is a transition point from the discontinuous area to the continuous area.

When the smoothing choke coil L is cut off due to the discontinuous region, there is a problem that the peak voltage of the capacitor Co is applied to the load 4. Therefore, in order to prevent such a problem, conventionally, a dummy resistor is connected to the output so that the choke coil L is not cut off. Here, when the current flowing through the choke coil L is continuous, equation (2) is erected, but when the current is discontinuous due to cutoff, the output voltage Vo is expressed by the following equation.

[0012]

(Equation 2)

Here, L is the inductance of the choke coil, T is the switching cycle, Ton is the ON time at T, Vt2 is the voltage generated on the secondary side, and Io is the output current.

[0014]

It is necessary to reduce the control width due to the voltage rise due to the cutoff of the coil when the output is lightly loaded, but the control width cannot be reduced due to the limitation of the control circuit. Also,
As shown in FIG. 6, in a two-output-side output circuit, only one side is controlled and the other side is a simple circuit configured by a dropper circuit (serial control method).
The problem that the dropper 10 does not operate normally due to a drop in input voltage is prevented by using the above-described dummy resistor.

In FIG. 6, D3 and D4 are rectifier diodes, L is a choke coil, C1 is a capacitor, and L and the capacitor C1 constitute a smoothing circuit. Reference numeral 10 denotes a dropper that receives the output of the smoothing circuit, and C2 denotes a smoothing capacitor that is connected to the output of the dropper 10.

Therefore, there is a problem that the efficiency is deteriorated due to a power loss consumed by the dummy resistor. In addition, since the power loss due to the resistance is relatively large, there is a problem that two or three parallel connections must be used. For this reason, the cost is increased and the size is increased.

The present invention has been made in view of the above problems, and has as its object to provide a power supply device that can achieve low power consumption, cost reduction, and downsizing. I have.

[0018]

FIG. 1 is a block diagram showing the principle of the present invention. The same components as those in FIG. 3 are denoted by the same reference numerals. In the figure, Vin is an input DC voltage, Cin is an input smoothing capacitor, and FET is a field effect transistor as a switching element. Vds is a drain-source voltage of the FET, and Vt1 is a voltage applied to the primary side of the transformer. T is a transformer, and the number of turns on the primary side is N1.

Vt2 is the secondary voltage of the transformer T, N2
Is the number of turns on two sides. D1 and D2 are rectifying diodes, L
1 and L2 are obtained by dividing the smoothing choke into two parts.
1 is a coil containing a core, and L2 is an air-core coil.
Co is a secondary-side smoothing capacitor, and 10 is an output detection and PWM control circuit that detects the secondary-side voltage and controls the on-time of a pulse applied to the gate of the FET according to the detected voltage.

The operation of the configuration shown in FIG. 1 will now be described. This circuit uses the smoothing choke coil L shown in FIG.
Is divided into L1 and L2, which are connected in series. And
At light load, a coil having an inductance of L1 + L2 is formed. In this case, since L1 includes a core, the inductance value is L1 >> L2. In addition, L1 has an inductance value considerably larger than L in FIG.

First, to prevent the coil L from being cut off in the case of FIG. 3, a dummy resistance Rd is obtained from L = Rd × Toff (max) / 2 (4). Here, Toff is a time when the FET is off in one switching cycle T. In order to reduce the size of the coil, it is necessary to reduce the number of turns and the inductance. Therefore, the dummy resistance Rd becomes small. Therefore, the dummy current increases, and the power loss of the dummy resistor Rd increases.

Therefore, in the present invention, the inductance is increased, the dummy current is reduced, and the loss of the resistance Rd is reduced by dividing the smoothing choke coil L into two parts. First, when the output current Io is lightly loaded, L1 + L2
Of the coil determines the inductance of the coil. Here, L1 is much larger than L2,
The dummy current Id is represented by the following equation.

[0023]

(Equation 3)

Here, L1 and L2 are the inductances of the respective coils, and L1 is a value such that the inductance can be ensured only under a light load, and usually the inductance becomes extremely small due to magnetic saturation. This is because the magnetic flux density of the core is increased by increasing the number of turns to increase the number of turns in order to increase the inductance to increase the inductance in order to reduce the size of the coil. I have. That is, according to the following equation, if the current is not long, it can be used without reaching saturation.

Bdc = LI / (NS) (6) where Bdc is a magnetic flux, N is the number of turns, I is a current flowing through the coil L, L is an inductance of the coil, and S is an effective area of the core. It is. Normally, the magnetic flux Bdc saturates at 3000 G (Gauss) or more.

Therefore, at the time of heavy load, it operates with almost the inductance value of only L2. If the output current increases, it will replace the dummy current, and will not cut off even with low inductance. As a result, the current flowing through the dummy resistor can be extremely reduced, and the resistance loss can be reduced.

As described above, according to the present invention, the transformer 2
The secondary side choke coil is divided into a core-containing choke coil and an air-core choke coil, and the inductance is set to L1 >> L2.
By operating with the inductance of L2, an inductance of L1 + L2 is provided until the current at the cutoff point, and when the output current becomes large, L1 is saturated, the other is reduced in inductance, and the ripple of the smoothing capacitor is reduced. By setting the current to a tolerable level and using it as a coil even when the output current increases, the power consumption is reduced, the cutoff of the choke coil is prevented, and the cost and size are reduced. be able to.

According to a second aspect of the present invention, the smoothing choke coil of the secondary side circuit of the switching power supply is divided into two parts: one having a core and no air gap, and the other having a core and having an air gap. It is characterized in that these two coils are connected in series.

According to the structure of the present invention, at the time of light load, the choke coil operates by adding the inductance of the choke coil, so that the cut-off of the choke coil can be prevented. Saturates and operates with only the other coil, so that power consumption can be reduced, cutoff of the choke coil can be prevented, and cost and size can be reduced.

[0030]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 2 is a configuration diagram showing an embodiment of the present invention. 1 are denoted by the same reference numerals. In this embodiment, the secondary side circuit has a two-circuit configuration. The DC voltage Vin connected to the terminals 2 and 3 is applied to the primary side of the transformer and the FET
Is turned on / off by an on / off signal provided from the output detection and PWM control circuit 100.

On the other hand, a feedback loop for the constant voltage control includes an output detection and a PW output from one of the two lines.
M is supplied to the M control circuit 100 to turn on / off the FET. During this time, a high-frequency AC is generated on the secondary side of the transformer T. A voltage corresponding to the number of turns is generated in the first winding (number of turns N2) on the secondary side, and a voltage corresponding to the number of turns is similarly generated in the second winding (number of turns N3) on the secondary side.

On the secondary side, the voltage rectified by the diodes D1 and D2 enters the following choke coils L1 and L2. Here, it is assumed that L1 and L2 have a core and are wound around the core by a predetermined number of turns. The coil L1 having no core air gap and having a large inductance value is used. On the other hand, for the other coil L2, a coil having an air gap to increase the magnetic resistance is used.

As a result, when the output current is small, the output of the secondary side first winding side secures the inductance of L1 + L2 so that the coil is not cut off even if the dummy current is small. On the other hand, when the load current increases, the coil L1 side without an air gap saturates immediately and becomes the same as the air-core coil, and hardly contributes as inductance. Therefore, when the load current increases, the coil L2 functions as an inductance, so that a ripple current larger than the allowable ripple of the smoothing capacitor Co can be prevented from flowing, and the dummy resistance Rd can be extremely reduced.

On the other hand, a high-frequency AC is also generated between the secondary windings on the secondary side, and rectified by the diodes D3 and D4 to obtain a DC voltage. This DC voltage is input to the dropper 10, and a predetermined DC voltage is obtained from the output of the dropper 10. No feedback is applied to the power supply circuit formed using the secondary winding on the secondary side, but feedback is applied to the power supply circuit using the first winding. An AC voltage corresponding to the ON / OFF of the FET is generated in the secondary winding, and a power supply circuit having substantially the same stabilization characteristics as the first power supply circuit can be obtained.

According to the configuration of this embodiment, the cut-off of the choke coil can be prevented at the time of light load because the choke coil operates by adding the inductance of the choke coil. The coil saturates and operates with only the other coil, so that low power consumption can be achieved, cut-off of the choke coil can be prevented, and cost and size can be reduced.

As described above, according to this embodiment,
Since higher efficiency can be achieved as compared with the conventional power supply, low power consumption can be achieved (reduction of operating cost). In addition, the number of dummy resistors conventionally used can be reduced or eliminated, so that cost reduction and size reduction can be achieved.

[0037]

(1) As described above in detail, according to the first aspect, the smoothing choke coil of the secondary circuit of the switching power supply is divided into a cored coil and an air-core coil. By dividing these two coils in series, the transformer secondary-side smoothing choke coil is divided into a core-containing one and an air-core one, and the inductance is set to L1 >> L2. By doing so, while the output current is small, the operation is performed with the inductance of L1 + L2, the inductance of L1 is provided up to the current at the cutoff point, and when the output current becomes large, L1 is saturated, and the other is inductance. And make it small enough to allow the ripple current of the smoothing capacitor, and use it as a coil even if the output current increases. Achieving reduction, to prevent the cutoff of the choke coil, it is possible to reduce the cost and size.

(2) According to the second aspect of the invention, the smoothing choke coil of the secondary circuit of the switching power supply is divided into two parts: one having a core and no air gap, and the other having a core and having an air gap. By connecting these two coils in series, it is possible to prevent cut-off of the choke coil at the time of a light load, since the coil operates at the time of addition of the inductance of the choke coil. The choke coil is saturated and operates with only the other coil, so that power consumption can be reduced, cutoff of the choke coil can be prevented, and cost and size can be reduced.

[Brief description of the drawings]

FIG. 1 is a principle configuration diagram of the present invention.

FIG. 2 is a configuration diagram showing an embodiment of the present invention.

FIG. 3 is a diagram illustrating a configuration example of a conventional device.

FIG. 4 is a diagram showing operation waveforms of each unit of the conventional device.

FIG. 5 is a diagram showing characteristics of a switching power supply.

FIG. 6 is a diagram showing another configuration example of the conventional device.

[Explanation of symbols]

 Reference Signs List 10 output detection and PWM control circuit FET switching element Cin capacitor T transformer N1 primary winding N2 secondary winding D1 diode D2 diode L1 choke coil L2 choke coil Co capacitor

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Tamio Shimizu 4-1-1 Kamikadanaka, Nakahara-ku, Kawasaki City, Kanagawa Prefecture Inside Fujitsu Limited (72) Inventor Takahiro Miyazaki 4-1-1 Kamiodanaka, Nakahara-ku, Kawasaki City, Kanagawa Prefecture No. 1 Fujitsu Limited (72) Inventor Yasunari Mizoguchi 4-1-1, Kamiodanaka, Nakahara-ku, Kawasaki-shi, Kanagawa F-term within Fujitsu Limited (Reference) 5H730 AA14 BB23 BB57 DD04 EE02 EE08 EE10 EE43 EE51 EE73 FD01 FG05 ZZ17

Claims (2)

[Claims] 1. A power supply device, wherein a smoothing choke coil of a secondary circuit of a switching power supply is divided into a core-containing coil and an air-core coil, and these two coils are connected in series. . 2. A smoothing choke coil of a secondary circuit of a switching power supply is divided into a choke coil having a core and no air gap and a choke coil having a core and an air gap.
A power supply device comprising these two coils connected in series.