JPH06351244A - Switching regulator - Google Patents

Abstract

PURPOSE:To realize a switching regulator having duty ratio control function through a simple circuitry. CONSTITUTION:A duty ratio control voltage VD is produced from an input voltage Vin received through resistors R2, R3, and a reference voltage Vref received through resistors R1, R2. The voltage VD is fed to a duty ratio control circuit 3 which controls the duty ratio of switching pulse thus stabilizing the output voltage Vout. This circuitry realizes a switching regulator having simple constitution while reducing the size and the number of components.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power supply circuit for industrial electronic equipment, and more particularly to a power supply circuit using a switching regulator system.

[0002]

2. Description of the Related Art AC100V of an AC power supply and DC24V of a DC power supply are generally used as a power supply of a process measurement control device (hereinafter referred to as a meter). A switching regulator system is often used as a power supply circuit for obtaining a stable internal circuit drive voltage even if the voltage of the power supply changes.

There are roughly two types of switching regulator systems used in power supply circuits of measuring instruments. One is a one-step method such as a forward method using one switching element such as a transistor and a flyback method, and the other is a two-step method represented by a push-pull method using two switching elements such as a transistor. .

The two-stone type switching system has good switching efficiency, but has a drawback that the number of parts is increased. On the other hand, the one-stone type switching system has an advantage that the switching efficiency is deteriorated but the number of constituent parts is small.

Generally, the power required for the instrument power supply is
Since it is as small as several watts and switching loss does not pose a problem, a single-stone switching regulator with a small number of parts is often used.

The single-stone switching regulator will be described below.

First, the flyback method will be described with reference to FIG. In FIG. 2, Vin is an input voltage, T is a power transformer, E1 is a voltage applied to the primary side of the power transformer, E2 is a voltage transmitted to the secondary side of the power transformer, Vout is an output voltage, and Tr1 is switching. A transistor, D is a rectifying diode, and C is a smoothing capacitor.

The flyback type power transformer T has one
The secondary side and the secondary side are connected in opposite polarities, and the energy stored in the power transformer when the switching transistor Tr1 is ON is 2 when the transistor Tr1 is OFF.
The power is transmitted to the next side, rectified by the diode D, smoothed by the capacitor C, and output as the output voltage Vout.

When the switching transistor Tr1 is ON (Ton), the primary winding of the power transformer T is applied with the power transformer primary side applied voltage E1. Next, when the switching transistor Tr1 is OFF (Toff),
The secondary winding of the power transformer T supplies the voltage E2 to the load. Further, as a result, a counter electromotive voltage Ef proportional to the number of turns is also generated in the primary winding of the power transformer T.

[0010]

(Equation 1) Ef = (n1 / n2) × E2 (Equation 1) On the other hand, since the transformers T must have the same amount of change in magnetic flux, (Equation 2) is established.

[0011]

[Equation 2] E1 × Ton = Ef × Toff (Equation 2) From (Equation 1) and (Equation 2), input / output E1 and E of the power transformer
The relationship of 2 is as in (Equation 3).

[0012]

## EQU00003 ## E2 = (n1 / n2) .times. (Ton / Toff) .times.E1 (Equation 3) where E1 is the power transformer primary side voltage, E2 is the power transformer secondary side supply voltage, and n1 is the power transformer. The number of primary windings, n2 is the number of secondary windings of the power transformer, Ton is the ON time of the switching transistor, and Toff is the OFF time of the switching transistor.

Generally, Ton / Toff is called a duty ratio. In (Equation 3), in order to obtain a constant power supply transformer secondary side supply voltage E2 when the power supply transformer primary side applied voltage E1 is changed, the duty ratio (Ton
/ Toff) or the transformer winding ratio (n1 /
It is understood that it is sufficient to change n2).

When control is performed by changing the duty ratio, the control width is Ton / Toff from (Equation 3), and thus theoretically becomes infinite. However, as the duty ratio increases, the OFF time Toff of the switching transistor decreases, and the time Ts required to supply the energy stored in the power transformer T to the load Ts.
And the off time Toff of the switching transistor becomes Ts> Toff, the switching transistor Tr1 is turned on again before all the energy stored in the power transformer T is supplied to the load on the secondary side of the power transformer.
Then, the power transformer T is excited again.
If the above condition continues, the maximum value of the magnetic flux change of the power transformer will increase until it finally exceeds the maximum magnetic flux density of the iron core,
The power transformer causes magnetic saturation. Therefore, it is necessary to control the duty ratio so that Ts <Toff is always maintained.

Next, the other forward method will be described with reference to FIG. In FIG. 3, Vin is an input voltage, T is a power transformer, E1 is a voltage applied to the primary side of the power transformer, E2 is a voltage transmitted to the secondary side of the power transformer, Vout is an output voltage, and Tr1 is switching. Transistors, D and Db are rectifying diodes, L is a smoothing choke coil, and C is a smoothing capacitor.

In the forward system, the power transformer T
Is a system in which the primary side and the secondary side are connected to the same polarity, and when the switching transistor Tr1 is ON, power is transmitted to the secondary side of the transformer. The relationship between the input and output of the power transformer at this time is shown in (Equation 4).

[0017]

[Equation 4] E2 = (n2 / n1) × (Ton / T) × E1 (Equation 4) Here, Ton is the ON time of the switching transistor Tr1, T is the switching period, and n1 is 1 of the power transformer T.
The number of secondary windings, n2, is the number of secondary windings of the power transformer.

In order to obtain a constant output voltage Vout when the input voltage Vin is changed in (Equation 4), the duty ratio is changed as in the flyback method and the winding ratio (n2 / n1) of the power transformer. ) Can be changed. In the control by the duty ratio, the flyback method is controlled by Ton / Toff as shown in (Equation 3).
In the forward method, it is Ton / T as shown in (Equation 4). Since T = Ton + Toff, it can be seen that the forward method has a narrower control width based on the duty ratio than the flyback method.

As described above, regardless of whether the flyback method or the forward method is used, in order to obtain the stable output voltage Vout with respect to the fluctuation of the power supply voltage Vin, the duty ratio or the winding ratio of the power transformer is obtained. I understand that it is better to change.

Here, since it is impossible to change the winding ratio with respect to the fluctuation of the input voltage, it is considered most common to change the duty ratio.

Hereinafter, a conventional example in which the duty ratio is controlled by using the forward method will be described with reference to FIGS. 5, 6 and 7.
Will be explained.

In FIG. 5, Vin is an input voltage, Vout is an output voltage, T is a power transformer, n1 is the number of primary windings of the power transformer, n2 is the number of output secondary windings of the power transformer, nr is the number of secondary side feedback windings for duty ratio control, Tr1 is a switching transistor, Vr is a feedback voltage for duty ratio control, V _DTC is a dead time control voltage, V _TR is a triangular wave signal voltage, and V _O is a switching pulse signal. Represents The relational expression of the transformer T in the forward method is (Equation 5) from (Equation 4).

[0023]

Vout = (n2 / n1) × (Ton / T) × Vin (Equation 5) The duty ratio (Ton / T) is determined by the duty ratio control circuit 3 inside the switching control IC. . FIG. 6 shows a detailed diagram of the duty ratio control circuit 3. As shown in FIG. 6, + of the comparator C _omp 1 of the duty ratio control circuit
The triangular wave from the triangular wave generating circuit is applied to the (positive) input terminal, and V _DTC is applied to the _two- (negative) input terminals of the C _omp 1.
And Vr by the secondary side feedback winding nr of the power transformer T are applied. FIG. 7 shows the relationship between the duty ratio control circuit input and the pulse output Vo of the switching pulse generation circuit.

The output V _O of the comparator of the duty ratio control circuit is H, V when Vr, V _DTC <V _TR.
When r, V _DTC > V _TR , L is obtained and the pulse output Vo shown in FIG. 7 is obtained. Here, the dead time control voltage V _DTC is always given a constant voltage to limit the maximum value of the duty ratio so that the transformer does not cause magnetic saturation.

Vr is a voltage obtained from the secondary winding nr of the transformer. If the input voltage Vin rises, Vr naturally rises in proportion. However, as shown in FIG. 7, Vr rises. If it rises, Ton becomes shorter and switching O
It shortens N time and suppresses the output on the secondary side of the transformer. Conversely, if Vin decreases, Vr decreases, so Ton becomes longer, and the switching ON time is lengthened to increase the output voltage Vout on the secondary side of the transformer.

As described above, V of the secondary winding of the transformer
Since r is kept constant, Vout, which is the same secondary winding of the transformer, can also be kept constant.

[0027]

The structure shown in the conventional example has the following problems.

Generally, a meter is often used by isolating input / output signals for the purpose of noise resistance, separation of system power supply system, etc. Therefore, each internal power supply supplied from the secondary winding of the transformer is It is necessary to insulate the internal power source used in the input circuit from the internal power source used in the output circuit. The transformer is provided with a plurality of secondary windings to supply each internal power source. For this reason, the secondary side of the power transformer is often multi-point output, but in the method shown in the conventional example, the duty ratio for stable output even when the primary side (supply side) voltage of the power transformer fluctuates. Since it is necessary to provide the secondary winding for the control transformer, there is a problem that the number of parts is large and the circuit is complicated and large. An object of the present invention is to solve the above problems and realize a switching regulator having a duty ratio control function with a simple configuration.

[0029]

The above object can be realized by resistance-adding a power supply voltage and a separately prepared reference voltage and inputting them to a switching control IC to control a duty ratio.

[0030]

The above-mentioned structure is shown in FIG.

In FIG. 4, V _D is a duty ratio control voltage, Vin is a power supply voltage, Vref is a reference voltage, and R1, R2 and R3 are resistors.

The duty ratio control according to the present invention will be described below with reference to FIG. Vref is connected to the duty ratio control section via resistors R1 and R2, and the input voltage Vin is connected to resistors R2 and R3.

By connecting in this way, V _D becomes as shown in (Equation 6).

[0034]

[Equation 6]

Here, V _D when the duty ratio is 0%
Is V _D0 (for example, DC 1.5V) and the duty ratio is 100
If V _D at the time of% is V _D100 (for example, DC3.5 V), the optimum duty ratio can be obtained by appropriately selecting the values of R1, R2, R3 and Vref in (Equation 5). .

[0036]

EXAMPLE An example of the present invention will be described below with reference to FIG.

FIG. 1 shows DC 24 as the instrument power supply Vin.
It is an example of the present invention when a V system power supply is used.

In FIG. 1, the duty ratio control voltage V _D is the input voltage Vin through the resistors R2 and R3.
And Vref via resistors R1 and R2. The V _D is connected to the duty ratio control circuit 3 of the switching control IC 1 together with the triangular wave generation circuit 4. The output of the duty ratio control circuit 3 is connected to the switching pulse generation circuit 5, and its output is connected to the base pole of the switching transistor Q1.

The operation of the switching regulator according to the present invention of FIG. 1 configured as above will be described below.

The relational expression of the input and output of the forward type transformer is as described above.

[0041]

## EQU00007 ## Vout = (n1 / n2) .times. (Ton / T) .times.Vin (Equation 7).

It is generally known that the maximum duty ratio in the forward system is 0.5.
Look at the margin and set it to 0.45. Also generally DC24
What is used as the permissible range of power fluctuation of V system power is ±
Since it is 10%, DC20V is also set as the minimum voltage Vinmin of Vin in view of the margin. That is, the duty ratio may be maximized to 0.45 at Vinmin.

Here, the maximum triangular wave voltage V _H of the triangular wave generator 2 of the switching control IC shown in FIG.
Is 3.5 V and the minimum voltage V _L is 1.5 V, the V _D required to set the duty ratio to 0.45 is

[0044]

V _D = V _H − ((V _H −V _L ) × 0.45) = 2.6 (v) (Equation 8) If 5 V is used as the reference voltage Vref and the above V _D is applied to (Equation 6),

[0045]

[Equation 9]

Holds.

If the duty ratio is 0.45 when Vin = 20 (V), V _D for keeping Vout constant when Vin = 24 (V) is 20 from (Equation 5). (V)
× 0.45 = 24 (V) × (Ton / T), Ton / T = 0.3
75, V _D = 2.75 when V _D is calculated in the same manner as (Equation 8).
(V), and if V _D is applied to (Equation 6) as in (Equation 9),

[0048]

[Equation 10]

Holds. From (Equation 9) and (Equation 10),

[0050]

[Equation 11]

From (Equation 9), (Equation 10), (Equation 11), R2 =
0.625 × R1, R3 = 9.87 × R1, where R1
= 10 (kΩ), R2 = 6.25 (kΩ), R
3 = 98.7 (kΩ). In reality, there is no resistance having the above-mentioned resistance value, and since a resistance value close to this is used, R2 = 6.2 (kΩ) and R3 = 100 (kΩ).

As described above, R1, R2 and R shown in FIG.
If you decide 3, the instrument supply voltage Vin is DC24V ± 1
A constant output voltage Vout can be obtained in the range of 0%.

In the embodiment of the present invention, the DC 24
Although the V-type power supply is used for the description, the type of the power supply voltage is not limited to this, and any value may be used for the power supply as long as the constants of the resistors R1 to R3 according to the present invention are determined according to the power supply voltage.

As addition resistors R1, R2, R
Although the explanation has been given for three of three, it is not limited to this. Any method can be applied as long as it is a method of resistance addition between the power supply and the reference voltage (for example, it can be applied even when there is no R2).

Further, although the DC power supply is used in the embodiment of the present invention, the present invention is not limited to this, and by providing a rectifying / smoothing circuit at the power supply input stage also for the AC power supply,
The present invention can be applied.

Further, in the present invention, the duty ratio control circuit 3, the triangular wave generating circuit 4, and the switching pulse signal generating circuit 5 are provided as the internal structure of the switching control IC 1, but the present invention is not limited to this. It can also be realized by configuring as a circuit and a part.

[0057]

According to the present invention configured as described above, it is not necessary to provide a transformer secondary winding for duty ratio control as in the conventional example, and the duty ratio control is performed. The circuit can be realized with a very simple structure, and the number of parts can be reduced and the size can be reduced.

As described above, according to the present invention, it is possible to realize a switching regulator capable of solving all the drawbacks of the conventional example, and it is possible to achieve all the initial objectives.

[Brief description of drawings]

FIG. 1 is a diagram showing an embodiment of a switching regulator of the present invention.

FIG. 2 is a configuration diagram of a flyback switching regulator.

FIG. 3 is a configuration diagram of a forward type switching regulator.

FIG. 4 is a configuration diagram of a voltage detection circuit according to the present invention.

FIG. 5 is a configuration diagram of a conventional switching regulator.

FIG. 6 is a configuration diagram of a duty ratio control circuit of a switching regulator.

FIG. 7 is a diagram showing voltage signals of respective parts of the duty ratio control circuit of the switching regulator.

[Explanation of symbols]

1 ... Switching control IC, 2 ... Dead time control (DTC) circuit, 3 ... Duty ratio control circuit, 4 ... Triangular wave signal generation circuit, 5 ... Switching pulse signal generation circuit.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Tomohiro Taguchi 500 Mt. Mt. Sanyu, Uchihara-cho, Higashi-Ibaraki-gun, Ibaraki Prefecture Hitachi Naka Electronics Co., Ltd.

Claims (1)

[Claims] 1. A switching regulator having a switching pulse generation circuit, a duty ratio control circuit, a switching element, and a power transformer, wherein a reference voltage is provided, and the input voltage of the switching regulator and the reference voltage are resistance-added. A switching regulator, wherein a voltage is input to the duty ratio control circuit to control the duty ratio.