KR20020036166A - Switching mode power supply - Google Patents

Abstract

PURPOSE: A switching mode power supply is provided to avoid oscillation within a circuit, operate in a high switching frequency, and accomplish high efficiency. CONSTITUTION: A switching mode power supply comprises a transistor(Q1), a transformer(T1), and a PWM(pulse width modulation) control circuit(11). The transistor(Q1) switches on/off depending on a switching signal from the PWM control circuit(11). The first wound wire(Np) of the transformer(T1) is connected between an input DC power supply(V1) and the transistor(Q1) to supply an AC power to the first wound wire based on on/off of the transistor(Q1) so as to supply power to the second wound wire. The PWM control circuit(11) compares a feedback DC output power with a reference to generate the switching signal. The PWM control circuit(11) senses current variation due to output load change to use in generation of the switching signal.

Description

Switching mode power supply

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switched mode power supply, and more particularly, to a switching mode power supply capable of operating at a high switching frequency by preventing oscillation that may occur in a circuit.

In Switching Mode Power Supply (SMPS), there is an attempt to reduce the energy loss caused by the resistance of the coil by reducing the size of the transformer by increasing the switching frequency (fs) as much as possible and reducing the number of turns of the coil. According to the configuration, even if the switching frequency is more than 100 KHz, oscillation occurs, and when the load is connected to the output terminal, the output voltage drops, and power of the power supply side is not properly transmitted to the load of the output terminal. .

The technical problem to be achieved by the present invention is to provide a switching mode power supply that can obtain a high efficiency.

1 is a circuit diagram showing the basic configuration of a power supply operating in a switching mode according to the present invention.

2 is a view showing the configuration of a switched mode power supply according to another embodiment of the present invention.

3 illustrates another example of the output rectifier of the power supply illustrated in FIG. 2.

4 is an example of configuration diagram of the PWM control circuit 11 shown in FIG.

In order to achieve the above object, a switching mode power supply of an embodiment according to the present invention,

A switching unit for operating at a predetermined frequency and outputting the input DC power by chopping the square wave of the frequency; It is provided with an upper output unit and a lower output unit which are alternately turned on or off according to the logic level of the square wave signal output from the switching unit, and the (-) terminal through the output transformer and the upper output unit at the (+) terminal of the input power source An inverter unit configured to generate a square wave signal by setting a current path to the furnace or setting a current path from the positive terminal of the input power supply to the negative terminal through an output transformer and the lower output unit; A current feedback unit for detecting and feeding back a current flowing through the output terminal of the inverter unit; A switching control unit controlling a square wave pulse phase of the switching unit to maintain a constant DC voltage of the final output according to a detection signal output from the current feedback unit; And an output rectifying unit rectifying the square wave signal output from the inverter unit and outputting the rectified square wave signal.

The upper output unit includes at least one module including a transistor, and the drain terminals of the transistors included in each module are connected in common to an up terminal of the primary winding of the output transistor, and the source terminal of the transistor is commonly Connected to a negative terminal of an input power source, and the lower output unit includes at least one module including a transistor, and the drain terminals of the transistors included in each module are connected in common to each other of the primary winding of the output transistor. Connected to the down terminal, the source terminal of the transistor is connected in common to the (-) terminal of the input power, and according to the logic level of the input square wave signal, the transistors included in the upper output is turned on to The current flows from the positive terminal to the negative terminal of the input power source through the upper output part or the lower side. Is turned on to a transistor included in ryeokbu the input power through the lower parts of the output from the (+) terminal of the input power (-) to flow a current in a terminal direction, and generates a square wave to the output terminal.

In order to achieve the above object, a switching mode power supply of another embodiment according to the present invention,

A switching unit which is turned on or off according to a predetermined switching signal; A power conversion unit connected to an input DC power source and the switching unit to supply power to the secondary winding by intermitting power supplied to the primary winding by turning on and off the switching unit; A detector for detecting a current flowing in the primary winding of the power converter according to an on / off operation of the switching unit and feeding it back to the controller; A controller configured to receive an output DC power supply and generate the switching signal according to an error signal comparing the same with a reference signal and a detection signal output from the detection unit; And a rectifying part connected to the secondary winding of the power converter and having at least two diodes, which are alternately turned on or off according to a logic state of a square wave signal transmitted from the inverter part to convert AC power into DC power and output the rectifier. Characterized in that.

Hereinafter, with reference to the accompanying drawings will be described in detail a preferred embodiment of the present invention.

1 is a circuit diagram showing a basic configuration of a power supply operating in a switching mode according to the present invention, SMPS for small power by receiving a DC power supply (V ₁ ) of an input rectifier (not shown) for converting AC to DC Can be implemented.

First, an input terminal to which the input power source V1 is input is provided with a pi-type filter composed of an inductor L1 and capacitors C1 and C2 to remove noise of the power source. The transistor Q ₁ operates as a switching element that is turned on or off in accordance with the switching signal output from the PWM control circuit 11. In the transformer T ₁ , the primary winding N _p is connected between the input DC power supply V ₁ and the transistor Q1, and AC power is supplied to the primary winding by turning on and off the transistor Q1. Power the windings. The PWM control circuit 11 receives the output DC power supply (+ FB) and generates a switching signal SW _out by PWM control according to the error signal compared with the reference signal. In addition, the PWM control circuit 11 returns a sensing signal SENSE obtained by sensing a change in current according to a change in the output side load (load), and the PWM control circuit 11 switches in consideration of such a sensing signal. Generate signal SW _out .

The switching element is composed of a transistor Q ₁ , which is turned on / off according to the logic level of the switching signal SW _out of the PWM control circuit 11 and flows to the primary winding N _p of the power transformer T ₁ . Interrupt the current. As the switching transistor Q ₁ is turned on and off, a current flowing in the primary winding N _p of the transformer T ₁ is induced to the secondary winding, and voltage is induced across the secondary winding depending on the turns ratio. The square wave signal generated by the switching transistor Q ₁ is transmitted to the secondary winding side through the transformer T ₁ and input to the gate terminals of the field effect transistors Q3 and Q5. At this time, the polarity of the square wave is applied to each transistor in the opposite direction. Thus, transistors Q3 and Q5 are turned on / off alternately.

In this circuit diagram, the primary winding of the transformer T ₁ includes a basic winding N _p and an auxiliary winding N _T. The auxiliary winding N _T stores energy while the switching transistor Q ₁ is off and returns the stored energy to the output side when the transistor Q ₁ is on, so that the down portion of the square wave, that is, the low level, is stored. Increases the off signal level by transferring the energy of the signal to the output side. Thus, it is possible to supply a gate signal sufficient to turn on the transistor Q ₅ during the down portion of the square wave. On the other hand, it turns on the primary winding (N _p) is a transistor (Q ₁₎ that stores the energy during the turned on transistor (Q ₁₎ in the off is stored in the transistor (Q ₃₎ to transfer energy to the output when the.

The primary winding (N _p ) and the auxiliary winding (N _T ) are wound in opposite polarities, and the diode (D1) uses an ultra fast diode, and the primary winding (N _p ) and the auxiliary winding (N _T) ) Or between the auxiliary winding (N _T ) and the (-) terminal (-V ₁ ) to determine the direction of the current while the switching transistor Q ₁ is off. The thickness and the number of turns of the coil used for the auxiliary winding N _T are configured substantially the same as that of the basic winding N _p .

The PWM control circuit 11 receives the output voltage of the converter (+ FB) and receives the sensing voltage SENSE generated by the inverter output current, and the square wave for the on / off operation of the switching element Q ₁ . Generate a pulse. A detailed configuration thereof will be described with reference to FIG. 4.

The magnetic bias circuit 15 supplies the operating power to the output of the switching signal SW _out in the PWM control circuit 11. The voltage induced by the feedback winding (N _{FB on the} primary side of the power transformer T ₁ , the dot indicating the start point of the winding) is opposite to the polarity indication of the basic winding N _p ). It is applied to the Vcc terminal of the PWM control circuit 11 through D2). Here, the capacitor C is for removing ripple. The input power source V ₁ supplies power to elements in the PWM control circuit 11 other than the switching output unit.

FIG. 4 illustrates a current-mode control method as an example of the configuration diagram of the PWM control circuit 11 shown in FIG. The power supply Vcc induced by the feedback winding NFB on the primary side of the power transformer T ₁ supplies power to the amplifier 45 that outputs the switching signal SW _out , and the clock generator 43 and the flip line. Circuits such as the flop 44, the error amplifier 41, the comparator 42, and the like are supplied with operating power from an input power V ₁ applied through the regulator 47.

The error amplifier 41 compares and amplifies the output signal + FB and the reference voltage V _ref , and the error signal is input to the comparator 42. Then, the output current of the inverter is sensed and converted into a voltage, and then the sensing signal SENSE is input to the comparator 42. The comparator 42 compares the detection signal SENSE according to the peak switch current with an error signal associated with the output signal and inputs the RS flip-flop (latch) 44. The clock generator 43 generates a clock signal which is a square wave signal corresponding to the switching frequency fs, and the RS flip-flop 44 receives the output of the comparator 42 and the clock signal to drive on / off of the switching element. To generate a switching signal SW _out . The switching signal turns on / off a switching element (transistor) connected at the rear end according to its logic level.

The main output voltage (+ FB) fed back from the final output stage is compared with the reference voltage (V _ref ) in the error amplifier 41, and the current feedback signal (SENSE) is compared with the reference voltage (1.2V) in the comparator 42 The result is input to the flip flop 44. The flip-flop 44 increases or decreases the phase (or width) of the clock signal generated and input from the oscillator 43 according to the output signal of the comparator 42 (that is, the pulse width modulation in which the duty cycle of the clock signal is changed ( By generating a PWM) signal to generate a switching signal SW _out , the current flowing through the transformer T ₁ may be increased or decreased according to the change of the input voltage and the load, thereby keeping the output voltage of the final output terminal constant.

The configurations of the push-pull inverter unit 17 and the output rectifying unit 19 connected to the secondary winding side of the transformer T1 of FIG. 1 are as follows. The push-pull inverter unit 17 includes forward type upper and lower output units 171 and 173, and receives a square wave signal transmitted from the primary winding side of the transformer T ₁ and transmits to the turns ratio. Accordingly, the transistors Q _{3 and} Q ₅ are alternately turned on and off by the square wave signal whose level is changed so that each forward type output unit supplies power to the load in half cycle units. The upper output unit 171 and the lower output unit 173 are substantially the same except that the position of the polarity dot of the secondary winding is reversed.

A tuning circuit is formed at turn-off by the leakage inductance of the primary coil of the power transformer T _{1 for} high frequency and the capacitance C _GS between the gate and the source of the transistors Q _{3 and} Q ₅ . The tuning circuit causes transient overvoltage ringing in the transient state. Ringing can have amplitudes large enough to destroy diodes or transistors in the turn-off period. RC element composed of a resistor (R) and a capacitor (C) is a snubber (snubber) circuit, and serves to suppress such ringing phenomenon, is connected in parallel to the secondary winding of the power transformer (T ₁ ).

The gate resistor R connected to the gate terminals of the transistors Q _{3 and} Q ₅ has a rising time of the square wave transferred from the power transformer T ₁ and a rising time of the transistors Q _{3 and} Q ₅ . Is added to match.

On the other hand, each output unit 171, 173 may be configured by connecting at least two modules in parallel as shown in the figure. That is, each output unit includes two or more modules having substantially the same configuration to minimize the loss power generated at the output unit. As the number of modules increases, the power loss due to the internal resistance of the transistor decreases in proportion to the increased number.

Each module included in the upper output unit 171 and the lower output unit 173 has substantially the same internal configuration, except that the gate terminal of the transistor Q ₃ included in the upper output unit 171 is a transformer ( Current is applied in the same direction as the polarity of the primary winding of T ₁ ), and the gate terminal of the transistor Q5 included in the lower output unit 171 is opposite to the polarity of the primary winding of the transformer T ₁ . Current is applied. Accordingly, it can be seen that the transistors included in the upper output unit 171 and the lower output unit 173 are alternately turned on / off according to the level of the square wave transmitted through the transformer T ₁ .

The drain terminal of the transistor Q3 in the upper output section 171 is connected to the up side of the primary winding of the output transistor T3, and the source terminal of the transistor Q3 is connected to the lower output section 173. Commonly connected to the source terminal of the transistor Q5 is connected to the negative terminal (-V ₁ ) of the input power source (V ₁ ) through the primary winding of the transformer T2. The drain terminal of the transistor Q5 in the lower output unit 173 is connected to the DOWN side of the primary winding of the output transistor T3, and the source terminal of the transistor Q5 is connected to the upper output unit 171 of the output terminal T1. It is connected in common with the source terminal and is connected to the negative terminal (-V ₁ ) of the input power source V ₁ through the primary winding of the transformer T2.

The output terminal of the inverter is connected to the primary winding of the output transformer T3, and the output terminal thereof is a drain terminal of the transistor Q3 in the upper output unit 171, a positive terminal of the input power V ₁ , and a lower side of the output transformer T3. The drain terminals of the transistor Q5 in the output portion 173 form terminals on the up side, the middle tap, and the down side, respectively.

When the square wave signal is transmitted to the secondary winding of the transformer T ₁ , the transistor included in the upper output unit 171 is turned on or the transistor included in the lower output unit 173 is turned on according to its logic level. Square wave is generated. The output signal becomes the input of the output rectifier to be connected next. In other words, when the switching transistor Q ₁ connected to the PWM control circuit 11 is turned on, the transistors of the upper output unit 171 are turned on and the transistors of the lower output unit 173 are turned off to turn off the high voltage power supply V ₁ . In the + terminal, current flows in the direction of the negative terminal (-V ₁ ) of the input power source V ₁ through the upper output unit 171 and the primary winding Na1 of the transformer T3. Next, when the switching transistor Q ₁ is turned off, the transistors of the upper output unit 171 are turned off and the transistors of the lower output unit 173 are turned on so that the lower output unit (at the positive terminal of the input power source V ₁ ) is turned on. 173) and a current flows toward the negative terminal (-V ₁ ) of the input power source V ₁ through the primary winding Na2 of the transformer T3. As described above, the square wave having the voltage level amplified is formed in the primary coil of the output transformer T3 by the on / off operation of the switching transistor Q ₁ , and the square wave is transmitted to the secondary coil side.

The current feedback unit 13 senses a current flowing from the inverter unit 17 to the negative terminal (-V ₁ ) of the input power source V _{1 and} returns it to the PWM control circuit 11. The current feedback unit 13 includes a current coupling transformer T2, and the polarity of the winding is as shown in the figure. The resistor R2 converts the current induced in the secondary winding of the transformer T2 into a voltage signal. Regardless of the on or off state of the switching transistor Q ₁ , the direction of current flows from the positive terminal of the high voltage power supply V ₁ through the primary winding of the transformer T3 to the negative terminal (-V ₁ ). The flow causes the diode D4 to conduct a forward bias. The capacitor C3 is used to remove noise, the variable resistor VR is used to adjust the potential level of the output signal SENSE, and the voltage signal SENSE adjusted by the variable resistor VR is the PWM control circuit 11. Is fed back.

The current feedback unit 13 senses an output current to generate a detection signal SENSE for controlling the switching transistor Q ₁ , and electrically disconnects the switching circuit unit from the inverter output unit by the transformer T2. Therefore, oscillation and noise appearing at the output of the inverter can be prevented by the high frequency operation in the switching transistor Q ₁ .

Next, the output rectifying unit 19 of the switching power supply will be described. The output terminals of the inverter unit 17 are connected to the primary winding of the output transformer T3, and the output rectifying unit 19 is connected to the secondary winding side. The output transformer T3 receives the square wave output from the inverter unit 17 and lowers it to a constant voltage according to the turns ratio, and rectifies the rectifier to provide a DC power.

On the output side of the transformer T3, a rectifying element and a filter capacitor are provided. The transformer adjusts the output voltage according to the turns ratio. The transistors are rectifying elements and the capacitors are filter elements.

The output rectifier 19 may include an output module including an upper module 191 and a lower module 193 and a filter capacitor Co connected to both ends of the output terminal. The upper module 191 and the lower module 193 are connected to the secondary winding of the output transformer T3 to perform a rectifying action. If the switching transistor Q ₁ is turned on, the transistor Q ₇ of the upper module 191 is turned on. If the switching transistor Q ₁ is turned off, the transistor Q ₉ of the lower module 193 is turned on. The output capacitor Co filters the small ripple smoothly to provide a DC power output.

The upper module 191 includes a snubber circuit in addition to the transistor Q ₇ . The RC element composed of the resistor R8 and the capacitor C8 is a first snubber circuit for preventing the overvoltage ringing caused by the leakage inductance of the primary coil of the transformer T3 and the gate capacitance of the transistor Q ₇ . The RC element composed of resistor R and capacitor C prevents overvoltage ringing due to leakage inductance of primary coil of output transformer T3 and capacitance Coss between drain and source of transistor Q ₇ . To this end, a second snubber circuit composed of an RC series circuit may be further provided between the drain and the source terminal of the transistor Q ₇ . A gate resistor (R7) connected to the gate terminal of the transistor (Q ₇₎ is added to ensure that the rise time of the square wave rise time (rising time) and the transistor (Q ₇₎ of the transfer from the transformer (T3) matched.

The lower module 193 is substantially the same as the internal configuration of the upper module 191, and the functions and parameters of the snubber circuit and the gate resistor are also the same. However, in the upper module 191, the gate of the transistor Q ₇ is used. The terminal is connected to the winding with the polarity dot of the transformer T3, while in the lower module 193 the gate terminal of the transistor Q ₉ is connected to the winding without the polarity dot of the transformer T3. There is a difference in that. Therefore, the square wave power signal transmitted from the primary winding of the transformer T3 can be conducted alternately according to its logic level.

The power supply shown in FIG. 1 is particularly useful as a power supply device having a relatively low input voltage and a low output voltage but a relatively high output current, such as a mobile phone, and can supply stable power with high efficiency. That is, the desired output power can be obtained by using the battery voltage (DC 11-15V) for the mobile phone or the power of the vehicle as the input power.

2 is a view showing the configuration of a switched mode power supply according to another embodiment of the present invention. The transistor Q ₁ operates as a switching element that is turned on or off in accordance with the switching signal output from the PWM control circuit 11. Transformer (T ₁ ) is connected to the primary winding (N _p ) between the input DC power supply (V ₁ ) and the switching unit, the high-frequency square wave power is supplied to the primary winding by the on-off of the switching unit to power the secondary winding To supply. The input current detector 23 detects a current flowing in the primary winding of the transformer T ₁ according to the on / off operation of the switching element (transistor Q ₁ ) and returns it to the PWM control circuit 21. The PWM control circuit 31 receives the output direct current power supply (+ FB) and detects an input signal that is changed according to an error signal and a change in the input voltage V ₁ compared with the reference signal through the input current detector 23. The amount of current flowing in the primary winding is controlled by controlling the on / off duration of the switching signal SW _out according to the voltage level. In addition, an output rectifying unit 27 connected to the secondary winding of the power transformer T ₁ to convert AC power into DC power may be provided to obtain one or more constant voltage outputs.

The output rectifier 27 may obtain a plurality of output voltages by the plurality of rectifiers 271 to 274. In each rectifier, two diodes are connected to the secondary winding of the transistor T1 to determine the direction of the current. That is, if the square wave signal transmitted from the transformer T1 is positive, the diode D11 is turned on. If the square wave signal is negative, the diode D12 is turned on. The inductors Lo1 to Lo4 of each rectifying part are ripple cancellation choke coils for each output voltage, and the auxiliary winding NT of the transformer T1 functions to supply magnetization power to each coil, and the transistor Q1 is turned on. When current flows, these coils are magnetized. At this time, the diodes D11, D21, D31, and D41 of the output terminal are turned on. Next, when the transistor Q1 is turned off, energy stored in the basic coil Np is transferred to the output through the diodes D12, D22, D32, and D42. In particular, it can be seen that the third and fourth rectifiers 273 and 274 can obtain an output voltage of negative polarity.

In the present embodiment, the circuit for the switching operation is composed of a switching signal generation PWM control circuit 31, a transistor Q ₁ , a voltage step-down transformer T ₁ and a current sensing transformer T ₂ . The transistor Q ₁ is turned on / off by the square wave SW _out generated from the PWM control circuit 31. When the transistor Q ₁ is turned on, the current is charged in the primary winding of the transformer T ₁ , and when the transistor Q ₁ is turned off, the current charged in the primary winding is transferred to the secondary winding, so that the transformer ( Voltage is induced across the secondary winding according to the turns ratio of T ₁ ). The operation description of such a configuration is basically the same as described with reference to FIG. 1.

On the other hand, the transistor source (source) terminal and the minus (Q ₁₎ (-) is the input current detecting section 23 between the terminal connected to the transistor a signal generated according to the current time (Q ₁₎ is turned on the PWM control circuit ( 21). The input current detector 23 senses a change in the input side current due to a change in current or a change in output load (load) that appears due to a change in potential of the input side voltage V ₁ , and returns it to the PWM control circuit 21. In order to compensate for the phenomenon caused by the same current variation, the PWM control circuit 21 controls the switching operation according to the sensing signal.

The input current detector 23 detects a change in the current Ipp according to a change in the input voltage V ₁ or a change in the output voltage (or load), converts it into a voltage signal, and then converts it into a voltage signal, followed by a PWM control circuit 21. The PWM control circuit 21 adjusts the pulse period of the positive phase of the switching signal SW _out by reflecting the variation of the input voltage V ₁ or the variation of the output voltage. Control the output voltage to be constant. If the current I _pp is increased, the current sensing voltage detected by the input current detector 23 and fed back to the PWM control circuit 31 is increased, and the PWM control circuit 21 is based on the current sensing voltage. _pp ) adjusts the pulse interval of the switching signal SW _out to be controlled in the decreasing direction.

When the transistor Q ₁ is turned on, current is induced in the secondary winding by the current flowing in the primary winding of the current-coupling transformer T ₂ . The resistor R1 converts the current induced in the transformer T ₂ into a voltage signal, and the voltage applied to the sensing terminal SENSE is determined by adjusting the resistance value of the variable resistor R ₂ . The capacitors C ₄ and C ₅ are used for ripple and noise cancellation, and the diode D ₃ serves to convert and rectify the detected square wave signal into a DC signal.

The ratio of the primary winding to the secondary winding of the transformer T ₂ is preferably about 1:50 to 200. For example, for low power consumption of less than 10 watts, the primary winding of transformer T ₂ is once when the continuius current (I _PDC ) flowing in the primary coil of main transformer T ₁ is 1.0 A or less. The secondary winding is preferably 100 times. The core of the transformer T ₂ is preferably made of the same material as the core of the main transformer T ₁ .

The PWM control circuit 21 receives the output voltage (+ FB) of the converter and receives the sensing voltage SENSE generated by the input side current, and the square wave pulse for the on / off operation of the switching element Q ₁ . Generates.

The magnetic bias circuit 25 supplies the operating power to the output of the switching signal SW _out in the PWM control circuit 21. The voltage induced by the auxiliary winding N _FB on the primary side of the power transformer T ₁ is applied to the Vcc terminal of the PWM control circuit 31 via the diode D2.

2 is particularly useful when the input voltage is relatively low, such as a mobile phone, it is possible to obtain an output voltage of different polarities. In addition, the capacity of the output electrolytic capacitors can be used very low, which is particularly advantageous for integrated circuits.

3 illustrates another example of the output rectifier of the power supply illustrated in FIG. 2, except that there is no auxiliary winding NT, the configuration connected to the primary winding side of the transformer T1 is the same as that described with reference to FIG. 2. Substantially the same. In the case of the output rectifier 37, two diodes D4 and D5 having the same rating can be connected in series to double the breakdown voltage, and the breakdown voltage is doubled by using two or more output capacitors having the same rating. Each dose can be cut in half.

Therefore, the circuit can operate at a high switching frequency and obtain a high output voltage with a low input voltage. In particular, a very high voltage (about 550 V) applied to a fluorescent lamp providing a backlight of an LCD display screen such as a notebook computer can be obtained. ) Can be obtained.

As described above, according to the switching mode power supply according to the present invention, it is possible to operate at a high switching frequency and obtain high efficiency. It is especially useful for obtaining the required output voltage and power even at low input voltages.

Claims (7)

A switching unit for operating at a predetermined frequency and outputting the input DC power by chopping the square wave of the frequency; It is provided with an upper output unit and a lower output unit which are alternately turned on or off according to the logic level of the square wave signal output from the switching unit, and the (-) terminal through the output transformer and the upper output unit at the (+) terminal of the input power source An inverter unit configured to generate a square wave signal by setting a current path to the furnace or setting a current path from the positive terminal of the input power supply to the negative terminal through an output transformer and the lower output unit; A current feedback unit for detecting and feeding back a current flowing through the output terminal of the inverter unit; A switching control unit controlling a square wave pulse phase of the switching unit to maintain a constant DC voltage of the final output according to a detection signal output from the current feedback unit; And a rectifier for rectifying the square wave signal output from the inverter and outputting the rectified square wave signal. The method of claim 1, The upper output unit includes at least one module including a transistor, and the drain terminals of the transistors included in each module are connected in common to the up terminal of the primary winding of the output transistor, and the source terminal of the transistor is commonly Connected to the negative terminal of the input power source. The lower output part includes at least one module including a transistor, and the drain terminals of the transistors included in each module are connected in common to the down terminal of the primary winding of the output transistor, and the source terminal of the transistor is commonly Connected to the negative terminal of the input power source. According to the logic level of the input square wave signal, the transistors included in the upper output part are turned on to allow a current to flow from the positive terminal of the input power source through the upper output part to the negative terminal direction of the input power source, or the lower output part. Switching power supply characterized in that the transistors included in the portion is turned on to flow a current from the (+) terminal of the input power through the lower output portion toward the (-) terminal of the input power, generating a square wave at the output terminal . The method of claim 1, wherein the current feedback unit And a current-coupling transformer placed on a current path flowing through the output terminal of the inverter unit, converting a current induced in the secondary winding of the transformer into a voltage signal, and feeding it back to the switching unit. According to claim 1, wherein the output rectifying unit The square wave signal output from the inverter unit is delivered by placing a transformer, the upper module and the lower module alternately turned on or off according to the phase of the transmitted square wave signal, Each of the upper module and the lower module A transistor having a gate terminal and a source terminal connected to both ends of the secondary winding of the transformer and having an output terminal drawn from a drain terminal; And The first snubber circuit for preventing ringing due to leakage inductance and gate capacitance of the transformer between the gate terminal and the source terminal and the leakage inductance of the transformer between the drain terminal and the source terminal and the capacitance between the drain and the source terminal And a snubber unit including at least one of second snubber circuits for preventing a ringing phenomenon. The switching mode power supply of claim 1, further comprising a pi filter formed of a capacitor and an inductor to remove power noise at a terminal to which the input power is input. A switching unit which is turned on or off according to a predetermined switching signal; A power conversion unit connected to an input DC power source and the switching unit to supply power to the secondary winding by intermitting power supplied to the primary winding by turning on and off the switching unit; A detector for detecting a current flowing in the primary winding of the power converter according to an on / off operation of the switching unit and feeding it back to the controller; A controller configured to receive an output DC power supply and generate the switching signal according to an error signal comparing the same with a reference signal and a detection signal output from the detection unit; And It is connected to the secondary winding of the power conversion unit, and provided with at least two diodes are alternately turned on or off in accordance with the logic state of the square wave signal transmitted from the inverter unit includes a rectifying unit for converting the AC power to DC power and outputting it. Power supply characterized in that. The method of claim 6, wherein the detection unit It is connected between the negative pole of the input DC power supply and the switching unit, the power flowing in the primary winding of the power conversion unit according to the on-off of the switching unit is supplied to the primary winding and converted to a predetermined ratio to the secondary winding A current-coupled converter for supplying power; And And a signal generator for generating a sensing signal input to the controller from power induced in the secondary winding of the current-coupling converter.