JPH1056777A - Flyback type multioutput dc-dc converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance safety against serious accident due to overcurrent through a simple arrangement. SOLUTION: The transformer 14 in a DC-DC converter 10 comprises flyback windings, i.e., secondary winding N1-N3, including the secondary winding N1 of a feedback system output circuit 1. A forward winding wound in the same direction as the primary winding Np is employed for the secondary N3 having lower power capacity between the secondary windings N2, N3, for example. Consequently, the effect of a current I3 on the core of the transformer 14 is enhanced as compared with a flyback winding and the core is saturated easily with an overcurrent. When the core of the transformer 14 is saturated, the current flowing the series circuit of the primary winding Np and a transistor Q is increased abruptly. Consequently, the transistor Q is broken down to interrupt switching thus preventing a serious accident, e.g. electric shock or fire.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multi-output DC-DC converter using a flyback transformer, and more particularly to a flyback type multi-output DC having means for preventing an accident due to overheating of the transformer due to overload. A DC converter;

[0002]

2. Description of the Related Art As a power supply device for various electronic devices, a DC device which is inexpensive, small and light in weight compared to a power capacity, has a high power conversion efficiency, reduces power loss and accompanying heat generation, and has a constant voltage control function. -DC converters are widely used. Here, there is no problem when the primary DC power input to the DC-DC converter is supplied from a power source independent of a commercial AC power source, such as a solar cell, a fuel cell, or a storage battery.

As a practical problem, since primary DC power obtained by rectifying and smoothing AC power input from an AC power supply is often supplied, a DC-DC converter is connected to a primary side to which primary DC power is input and a load. Since the transformer is insulated from the secondary side to which the DC-DC converter is connected, a connection cable that connects the DC-DC converter and the load by mistake,
Care is taken so that accidents such as shocks and electric shocks do not occur even when touching uninsulated bare parts of the internal circuit of the load.

FIG. 4 shows a conventional flyback type multi-output DC.
FIG. 3 is a circuit diagram illustrating an example of a basic configuration of a DC converter. The DC-DC converter 30 shown in FIG. 4 converts the primary DC power obtained by rectifying and smoothing the AC power input from the AC power supply 7 by the diode bridge 8 and the capacitor C0,
The current is input to a series circuit of a primary winding Np of a flyback transformer (hereinafter, also simply referred to as a “transformer”) 34 and a transistor Q, and the current flowing through the primary winding Np is switched by the transistor Q.

The transformer 34 is excited when the transistor Q is turned on and stores magnetic energy.
Are turned off, the current in which the magnetic energy induced in the secondary windings N1 to N3 is reconverted is output to the output circuits 31 to 3, respectively.
Supply 3 Output circuits 31 to 33 each including a diode for flowing a current when the transistor Q is off and a smoothing capacitor supply secondary DC power obtained by smoothing the supplied current and charging the capacitor via respective positive and negative output terminals. Output to the load.

The switching control circuit (SWC) 35
By detecting the output voltage of the output circuit 31 of the feedback system and controlling the switching of the transistor Q so that the output voltage becomes a preset voltage, the output circuit 31 is controlled at a constant voltage, and the output circuit 32 is controlled. , 3
3 also have a substantially constant voltage. Normal forward type D
The basic configuration of the C-DC converter is such that the secondary windings N1 to N
3 is the same as the primary winding Np, except that the power is supplied to the output circuits 31 to 33 when the transistor Q is on.

However, a DC-DC converter having only a basic configuration, whether it is a forward type or a flyback type, erroneously short-circuits between output terminals or connects without knowing the internally shorted load. When the overload condition occurs, the constant voltage control is performed, so an excessive current flows inside the DC-DC converter, and there is a danger that the insulation between the primary and the secondary will be destroyed due to overheating of the transformer, resulting in electric shock. There was a danger that serious accidents such as smoke and fire would occur.

Therefore, in order to prevent such an accident caused by an excessive current, an AC line between the AC power supply 7 and the diode bridge 8 shown in FIG. 4 or a DC line connecting the diode bridge 8 and both terminals of the capacitor C0. A fuse on the power supply side having a current capacity corresponding to the input current at the time of rated output is provided in one of the lines, or a plurality of output circuits 3
A method has been used in which a fuse on the output side having a current capacity corresponding to the rated output current is provided for every 1 to 33 to cut off the current when an excessive current flows.

Alternatively, a thermal fuse or a temperature sensing element is provided inside the transformer 34 to detect an abnormal rise in the temperature of the transformer due to an excessive current and cut off the current, or the switching control circuit 35 outputs to the transistor Q A method of suppressing the pulse and stopping the switching of the transistor Q to cut off the power supply to the secondary side of the transformer 34 has also been used.

In order to further enhance the sensitivity of temperature detection, for example, Japanese Patent Application Laid-Open No. 52-40764 (JP-B-59-28976).
No.) As shown in the official gazette, a temperature detection interrupting section is provided in series with a winding using a low-temperature softened insulated copper wire, and when the internal temperature of the winding rises due to overcurrent, insulation is broken and rare short-circuit occurs. However, there has been a proposal that a large amount of heat generated due to a rare short circuit causes a temperature detection interrupting unit to interrupt an electric current.

[0011]

However, the method of detecting the rise in the temperature of the transformer, including the proposal disclosed in Japanese Patent Application Laid-Open No. 52-40764, has a problem that the cost is likely to increase. Further, like the fuse provided on the power supply side, it is easy to respond to an overload in a large-capacity output circuit among a plurality of output circuits. Since there is no response even if an overload occurs, the insulation of the transformer may be destroyed at that portion, or smoke or ignition may occur.

Therefore, if an output side fuse corresponding to the rated output current is provided for each of the plurality of output circuits, the current is cut off in response to an overload regardless of the capacity of the output circuit. There is no problem in point. However, it is difficult to reduce the size of the device due to the increase in the number of fuses, and it is necessary to prepare various types of fuses from the viewpoint of maintenance.If a large-capacity fuse is mistakenly used for a small-capacity output circuit, There is a problem that the effect of providing a fuse for each output circuit becomes zero.

As described above, since each of the overcurrent preventing means has its own advantages and disadvantages, there is a problem in safety if only one of them is used. Therefore, in a DC-DC converter actually used, a plurality of excessive current preventing means are used. It is the fact that safety is enhanced by using a combination of the two. For this reason, there is a problem that a sharp increase in cost is inevitable as the safety is enhanced.

The present invention has been made in view of the above points, and has as its object to further improve safety without incurring any cost increase.

[0015]

In order to achieve the above object, the present invention provides a flyback transformer having a primary winding and a secondary winding comprising a plurality of flyback windings which are insulated from each other. A plurality of output circuits respectively connected to the plurality of secondary windings of the flyback transformer, and a switching element connected in series with the primary winding of the flyback transformer, and a series of the primary winding and the switching element. The primary DC power is input to the circuit, and the secondary AC power induced in the plurality of secondary windings of the flyback transformer by switching the switching elements is rectified and smoothed by the respective output circuits, and the secondary DC power is Is output to a load, and one of the plurality of output circuits is used as a feedback output circuit to detect the output voltage, and the output voltage becomes a preset voltage. In the flyback type multi-output DC-DC converter for controlling the switching of the switching element, one of the secondary windings other than the secondary winding connected to the feedback system output circuit is switched to the primary winding. It is the same as the winding direction of the wire.

In the flyback type multi-output DC-DC converter described above, the secondary winding whose winding direction is the same as that of the primary winding is replaced by another secondary winding other than the secondary winding connected to the feedback system output circuit. The secondary winding having the smallest power capacity among the secondary windings may be used.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be specifically described below with reference to the drawings and embodiments. FIG.
Flyback type multi-output DC according to one embodiment of the present invention
FIG. 3 is a circuit diagram illustrating an example of a basic configuration of a DC converter.

The difference between the DC-DC converter 10 shown in FIG. 1 and the DC-DC converter 30 shown in FIG.
While the secondary windings N1 to N3 of the (flyback) transformer 34 used in the DC-DC converter 30 are all flyback windings, the secondary winding of the transformer 14 used in the DC-DC converter 10 is used. Only N3 is a forward winding whose winding direction is the same as the winding direction of the primary winding Np, and the other parts (except for the difference in sign)
It is exactly the same.

The DC-DC converter 10 shown in FIG.
Is a flyback transformer 14 having a primary winding Np, secondary windings N1 and N2 formed of a flyback winding, and a secondary winding N3 formed of a forward winding, and the primary winding N
The transistor Q (which may be a FET), which is a switching element connected in series to p, includes diodes R1 to D3 for rectification and capacitors C1 to C3 for smoothing, and outputs connected to the secondary windings N1 to N3. Circuits 1-3
And a switching control circuit (SWC) 5 that detects an output voltage of the output circuit 1 that is a feedback output circuit and outputs a drive pulse to the transistor Q.

A primary DC power obtained by full-wave rectifying and smoothing the AC power input from the AC power supply 7 by the diode bridge 8 and the capacitor C0 is input to a series circuit of the primary winding Np of the transformer 14 and the transistor Q. . Primary winding Np
Is turned on / off by a transistor Q that switches according to a drive pulse input from the switching control circuit 5.

When the transistor Q is on, the primary winding Np
A part of the electric power becomes an electromotive force whose upper end induced in the secondary winding N3 is positive, and the electric current I3 charges the capacitor C3 via the diode D3 and is smoothed by the current flowing through the output circuit 3. Of secondary DC power. Secondary winding N1,
Since the electromotive force induced in N2 becomes negative at the upper end, the current I
Since 1,1 is blocked by diodes D1 and D2, the remaining power excites transformer 14, ie, is stored as magnetic energy.

When the transistor Q is turned off, the polarity of the electromotive force induced in the secondary windings N1 to N3 is inverted, so that the current I due to the electromotive force induced in the secondary winding N3 is changed.
3 is blocked by the diode D3 and does not flow through the capacitor C3, and the current obtained by reconverting the magnetic energy stored in the transformer 14 flows from the secondary windings N1 and N2 to the current I
1, I2, flow to the capacitors C1 and C2 via the diodes D1 and D2, are smoothed, and
It is output to the load as the next DC power.

The switching control circuit (SWC) 5 detects an output voltage (voltage between terminals of the capacitor C1) of the output circuit 1 which is a feedback system output circuit, and detects the detected output voltage from a preset set voltage. By outputting drive pulses to increase the on-time of the transistor Q if it is higher and to increase the on-time if it is lower than the set voltage, the output circuit can be operated even if there is a voltage fluctuation or load fluctuation of the AC power supply 7. The constant voltage control is performed so that the output voltage of No. 1 keeps the set voltage, but at the same time, the output voltages of the output circuits 2 and 3 are also maintained at substantially constant voltages.

FIG. 2 shows the magnetomotive force Em of the transformer 14 of the DC-DC converter 10 (the same applies to the conventional transformer 34).
FIG. 4 is a diagram illustrating an example of a relationship between the inductance and an inductance L.
Needless to say, the unit of the inductance L is Henry, but since the value differs depending on the size of the transformer core, it is shown here as a relative%. The unit of the magnetomotive force Em is AT (ampere turn), which is the product of the current I flowing through the winding and the winding N as shown in Expression 1, and if there are a plurality of windings, the product of each winding is Become sum.

[0025]

(Equation 1)

As shown in FIG. 2, while the magnetomotive force Em is small, the inductance L shows a constant value, that is, 100%, but when the magnetomotive force Em exceeds a certain value, the inductance L suddenly decreases. A trend appears. Generally, the supporting force Em when the inductance L becomes 90% is set to a limit value E.
As for ms, the region below that is regarded as an unsaturated region, and the region exceeding the limit value Ems is regarded as a saturated region.

FIG. 3 is a diagram showing an example of the saturation of the core shown in FIG. 2 in the relationship between the magnetic field strength H and the magnetic flux density B. The magnetic field strength H is a value obtained by dividing the magnetomotive force Em by the length of the magnetic path, that is, the magnetomotive force per unit length of the magnetic path, and its unit is AT / m. The magnetic flux density B is the product of the strength H of the magnetic field and the magnetic permeability μ _{0 of the} vacuum, and is also a value obtained by dividing the magnetic flux Φ by the sectional area of the magnetic path.

As shown in FIG. 3, while the magnetic field strength H is small, the magnetic flux density B increases in proportion to the magnetic field strength H, but when the magnetic field strength H reaches a certain value Hs Is exceeded, the magnetic flux density B becomes the maximum magnetic flux density Bm which does not increase any more. That is, by entering the saturation region, the inductance L of the transformer using this core starts to rapidly decrease as shown in FIG.

A DC-DC converter 10 according to an embodiment of the present invention shown in FIG. 1 and a conventional DC-DC converter shown in FIG.
The operation of each part of the DC converter 30 is almost the same as that of the DC converter 30.
4 except that the winding directions of the respective secondary windings N3 are opposite to each other.

However, when the transistor Q is on, the current I3 flowing through the secondary winding N3 of the transformer 14 is changed to If
w, and when the transistor Q is off, 2
If the current I3 flowing in the next winding N3 is Ifb, the peak current of the current Ifw flowing in the forward winding is several times larger than the current Ifb flowing in the flyback winding. In order to make the output voltage of the output circuit 33 and the output current the same, the number of turns of each secondary winding N3 of the transformer 14 and the transformer 34 must be changed.

That is, as an embodiment of the DC-DC converter 10 shown in FIG. 1, the output capacitance of each of the output circuits 1 and 2 is 24V and the current 4A, and the output capacitance of the output circuit 3 is 15V and the current If it is 0.1 A, the number of turns N1 and N2 of the secondary windings N1 and N2 of the transformer 14 is 12T (turn), and the number of turns N of the forward secondary winding N3 is N
fw becomes 3T.

On the other hand, the output capacities of the output circuits 31 to 33 of the DC-DC converter 30 of the conventional example shown in FIG. 4 are made the same as the output capacities of the output circuits 1 to 3 of the DC-DC converter 10, respectively. For this, the number of turns N1 and N2 of the secondary windings N1 and N2 of the transformer 34 may be the same as 12T, but the number of turns Nfb of the flyback secondary winding N3 is 9T and must be three times the number of turns Nfw. Must. Conversely, the number of turns of the transformer 14 used in the embodiment is reduced to 1/3 since the secondary winding N3 is a forward winding.

In the equation of the magnetomotive force Em shown in Equation 1, the currents I1 and I2 flowing through the secondary windings N1 and N2 of the transformer 14 and the transformer 34 and the numbers of turns N1 and N2 of the transformers 14 and 34 are I1 = I2 = 4 A, respectively. , N1 = N2 = 12T, the portion of the entire magnetomotive force Em due to the secondary windings N1, N2, that is, (4A × 12T) × 2 = 96AT is the same, and the portion due to the respective secondary windings N3. Only different.

The core of the transformer (as well as the inductor)
Each shape, size, and gap, if any, has a unique induction coefficient K determined by the shape and size. However, it is assumed that the cores of the transformer 14 and the transformer 34 have the same shape and size. Are the same. Generally, the inductance L is a product of the induction coefficient K and the number of turns N as shown in Expression 2.

[0035]

(Equation 2)

The inductance L of each of the secondary windings N3 in the transformers 14 and 34 is represented by Lf
Assuming that w and Lfb, Nfw = 3 and Nfb = 9 may be substituted for the winding N shown in Equation 2, respectively, so that each inductance L becomes Lfw = 9K and Lfb = 81K as shown in Equation 3. Become. Therefore, the current I flowing through each of the secondary windings N3 of the transformer 14 and the transformer 34 is
3, the current I is the inductance L
Is inversely proportional to the current Ifw in the transformer 34, as shown in Expression 3.
9 times fb.

[0037]

(Equation 3)

The saturation of the core of the transformer 14 and the transformer 34 having the same shape and size is determined by the secondary winding N
Since it is necessary to consider the peak values of the currents flowing through N1, N2, and N3, Ifb shown in Expression 3 is calculated as I3p
And the peak values of the currents I1 and I2 are I1p and I1p, respectively.
If 2p, the magnetomotive force Em (f
w) p and the magnetomotive force Em (fb) p in the transformer 34
Can be obtained by the equations shown in Equation 4, respectively.

[0039]

(Equation 4)

As is apparent from Equation 4, the magnetomotive force Em (fw) p in the transformer 14 and the magnetomotive force Em (fb) p in the transformer 34 are exactly the same as the magnetomotive force Em due to the currents I1p and I2p. Only the magnetomotive force Em by I3p is different. The peak currents I1p to I1p
3p are currents I1 to I1 flowing through the respective windings N1 to N3.
Since it can be considered to be proportional to I3, I1p
When I3p are replaced with I1 to I3, respectively, as shown in Expression 5, not the peak current value but the ordinary current value I1
Expressions for obtaining the magnetomotive forces Em (fw) and Em (fb) based on I3 to I3 are obtained.

[0041]

(Equation 5)

The magnetomotive force Em when a rated current is applied to each of the secondary windings N1 to N3 is given by I1 = I2 = 4 (A) and I3 = 0.1 (A) in the equation shown in Expression 5. Is substituted in the transformer 14 of the embodiment, Em (fw) =
98.7AT, Em (fb)
= 96.9AT.

If the limit values Ems of the magnetomotive force of the cores of the transformers 14 and 34 are set to the above values, respectively, when each of the output circuits 1 to 3 and 31 to 33 outputs the rated current, If any output circuit becomes overloaded and an excessive current larger than the rated current flows, the core enters a saturation region (FIG. 2) and the inductance L decreases.
In the series circuit of the primary winding Np and the transistor Q, a current much larger than the primary current corresponding to the total output current including the excessive current flows.

Therefore, the transformer 14 or the transformer 34
Before the temperature rises abnormally due to excessive current,
Since the current flowing in the secondary circuit, that is, the series circuit of the primary winding Np and the transistor Q far exceeds the maximum rated current of the transistor Q, the transistor Q is instantaneously destroyed. Serious accidents such as electric shock and fire due to dielectric breakdown can be prevented.

[0045]

[Table 1]

As a practical problem, the limit value Ems of the magnetomotive force of the core is changed to the magnetomotive force Em (f
w) or Em (fb) is set higher with a margin. Table 1 shows that, for example, the limit value of the magnetomotive force is Ems = 120A.
FIG. 9 is a table showing comparisons between the numerical values of the embodiment and the conventional example when T is set, wherein the magnetomotive force Em when the rated current is applied is 98.7 AT or 96.9 AT, respectively. The margin of the magnetomotive force with respect to the limit value Ems is 21.3AT or 23.1AT as shown in Table 1.

Therefore, either the output current I1 or I2 or the sum thereof is equal to the coefficient 1
2 (T), 1/12 of the margin of the magnetomotive force, that is, the rated current (4A or 8
When it exceeds 1.75A or 1.925A from A), it reaches 120AT which is the set limit value Ems.
In terms of the ratio to the rated current (4 A), each is 44
% Or 48% over.

On the other hand, when the output current I3 is calculated similarly, in the embodiment or the conventional example, 1/27 or 1/27 of the margin of the magnetomotive force is obtained by the coefficient 27 (T) or 9 (T) shown in Expression 5 respectively. 9, that is, when the rated current exceeds the rated current (0.1) by 0.789A or 2.567A,
ms. This means that the ratio is over 790% or 2570% respectively, that is, 8.9 of the rated current.
If a double or 26.7 times excess current flows, the core will saturate.

As described above, an output circuit having a smaller rated output current (or electric power) tends to have a lower detection sensitivity for an excessive current, because a fuse having a capacity corresponding to the rated current is provided for each output circuit. In addition to the means, even if a fuse is provided on the input side of the AC power supply, or if an abnormal rise in the temperature of the transformer is detected and the circuit is cut off, there is an inevitable problem.

However, in the DC-DC converter 10 according to the embodiment of the present invention shown in FIG. 1, the winding direction of the secondary winding N3 of the transformer 14 is changed to that of the conventional DC-DC converter 30 (FIG. 4). Contrary to the secondary winding N3, which is a flyback winding of the transformer 34, the forward winding is formed in the same direction as the primary winding Np, so that the output circuit 3 of the embodiment and the output circuit 33 of the conventional example are formed. And the same output capacity (15
V, 0.1 A), the number of turns of the secondary winding N3 is 3T and 9T, respectively.

That is, the number of turns in the embodiment is 1/3 of that in the conventional example. As a result, as shown in Equation 1, in the conventional example, when the output current I3 exceeds 27 times the rated current (0.1 A), the transistor Q is destroyed and switching stops, causing a serious accident. On the other hand, in the embodiment, the switching is stopped when the excessive current exceeds 9 times, so that the excessive current detection sensitivity is improved to about 3 times.

In general, assuming that the number of turns becomes 1 / M when the secondary winding is changed from the flyback winding to the forward winding, the inductance L becomes 1 / M ² as shown in Expression 3. And the current I, which is inversely proportional to the inductance L, becomes M ² times. Magnetomotive force E shown in Equations 4 and 5
Since the coefficient of the peak current I3p or the current I3 in the equation of m is the product of the current and the number of turns, the coefficient in the case of the forward winding is M times larger than that of the flyback winding.

Therefore, ignoring the slight difference (1.8 AT) of the magnetomotive force Em at the rated time shown in Table 1, the magnetomotive force E
Assuming that the margin of m is the same, the core of the transformer 14 of the DC-DC converter 10 has a 1 / M excess of the current I3 with respect to the rated current compared to the core of the transformer 34 of the DC-DC converter 30. , The detection sensitivity is increased by a factor of M, and the safety against a serious accident due to excessive current is improved accordingly.

The effect described above is obtained by using the present invention in any other output circuit (2) other than the feedback system output circuit 1.
Or 3) can be obtained in the same way by applying to the secondary winding (N2 or N3) connected to 3), but has the smallest power capacity of other secondary windings (usually DC-DC converter 10) (usually Obviously, a greater effect can be obtained by applying the secondary winding N3 having the smallest current capacity.

If the present invention is applied to the secondary winding N2 having a power capacity much larger than that of the secondary winding N3, as can be easily estimated from Table 1, the detection sensitivity of the excessive current is too high.
The transistor Q is destroyed even by a slight excessive current,
This is a practical problem. However, in the case of a DC-DC converter having a larger number of output circuits, the number of secondary windings to be converted to the forward winding is not limited to one, and two or more secondary windings are used in order from the output circuit having the smaller power capacity. Needless to say, the secondary winding may be a forward winding.

Alternatively, a DC having a larger number of output circuits
-If the power capacity (or current capacity) of each output circuit of the DC converter is the same, 1
It is also possible to convert one or more secondary windings to forward windings.

[0057]

As described above, the flyback type multi-output DC-DC converter according to the present invention can improve the safety without any cost increase.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an example of a configuration of a flyback type multi-output DC-DC converter according to an embodiment of the present invention.

FIG. 2 is a diagram illustrating an example of a relationship between a magnetomotive force and an inductance of a flyback transformer used in a DC-DC converter.

FIG. 3 is a diagram illustrating an example of a relationship between a magnetic field strength and a magnetic flux density of a flyback transformer used in a DC-DC converter.

FIG. 4 is a circuit diagram showing an example of a configuration of a conventional flyback type multi-output DC-DC converter.

[Description of Signs] 1: Output circuit (feedback output circuit) 2, 3: Output circuit 5: Switching control circuit 10: DC-DC converter 14: Transformer (flyback transformer) N1 to N3: Secondary winding Np: Primary winding Q: Transistor (switching element)

Claims (2)

[Claims] 1. A flyback transformer having a primary winding and a secondary winding composed of a plurality of flyback windings insulated from each other, and respectively connected to the plurality of secondary windings of the flyback transformer. A plurality of output circuits, and a switching element connected in series with a primary winding of the flyback transformer, wherein primary DC power is input to a series circuit of the primary winding and the switching element, and the switching element The plurality of output circuits rectify and smooth secondary AC power induced in the plurality of secondary windings of the flyback transformer by switching the output, and output secondary DC power to the load. One of the output circuits is used as a feedback system output circuit to detect the output voltage, and the switching element is turned on so that the output voltage becomes a preset voltage. Flyback multiple-output D to control the switching of
In the C-DC converter, one of the secondary windings other than the secondary winding connected to the feedback system output circuit may have the same winding direction as the winding direction of the primary winding. Flyback type multi-output DC-DC converter. 2. A flyback type multi-output D according to claim 1.
In the C-DC converter, the secondary winding whose winding direction is the same as that of the primary winding is the power capacity of the other secondary windings other than the secondary winding connected to the feedback system output circuit. A flyback-type multi-output DC-DC converter characterized by a secondary winding having a small size.