WO2002021670A1

WO2002021670A1 - Switching mode power supply with high efficiency

Info

Publication number: WO2002021670A1
Application number: PCT/KR2001/001520
Authority: WO
Inventors: Joon-Ho Park
Original assignee: Park Joon Ho
Priority date: 2000-09-08
Filing date: 2001-09-08
Publication date: 2002-03-14
Also published as: KR20020020364A; KR100387380B1; AU2001286306A1

Abstract

A Switching Mode Power Supply (SMPS) that can operate at a high switching frequency, is provided. The SMPS includes a switching unit, a power transformer, a detector, a controller, and a rectifier. The switching unit is turned on or off in response to a pre-defined switching signal. The power transformer has a primary coil connected between an input DC power and the switching unit, and a secondary coil that receives the power flowing through the primary coil. The detector senses the current flowing through the primary coil of the power transformer and then feeds the current signal back to the controller. The controller generates the switching signal depending on an error signal and the detection signal. In the power supply, if the circuit is configured in such a way that power which is provided to the switching frequency oscillator of the PWM control circuit is separate from power which is provided to a main power anplifier that amplifies the switching signal, any oscillation caused by the switching oscillator is not transmitted to the output terminal, and thus efficiency is increased.

Description

SWITCHING MODE POWER SUPPLY WITH HIGH EFFICIENCY

Technical Field

The present invention relates to a Switching Mode Power Supply (SMPS), and more particularly, to an SMPS that can operate at a high switching frequency by preventing any potential oscillation within a circuit.

Background Art

A Switching Mode Power Supply (SMPS) designer has attempted to not only reduce the size of a transformer as much as possible by heightening a switching frequency (fs) but also decrease an energy loss caused by a coil resistance by reducing the number of coil turns. According to an existing circuit configuration, if a switching frequency exceeds 100 KHz, oscillation is generated. Since an oscillation noise distorts a switching waveform, it is hard to obtain a certain regular level of an output voltage.

Disclosure of the Invention

To solve the above-described problems, it is an object of the present invention to provide a highly efficient Switching Mode Power Supply (SMPS) that can operate at a high switching frequency by preventing any potential oscillation within a circuit.

It is another object of the present invention to provide a switching mode power controller used in the SMPS.

To achieve the above object, according to the present invention, the SMPS includes a switching unit, a power transformer, a detector, a controller, and a rectifier. The switching unit is turned on or off in response to a pre-defined switching signal. The power transformer has a primary coil connected between an input DC power and the switching unit, and a secondary coil that receives the power flowing through the pπmary coil depending on the state, i.e., ON or OFF, of the switching unit. The detector senses the current flowing in the primary coil of the power transformer in response to the ON or OFF state of the switching unit and then feeds the current signal back to the controller. The controller generates the switching signal depending on an error signal generated after a comparison between a feedback output DC voltage and a reference signal, and depending on a detection signal output from the detector. The rectifier is connected to the secondary coil of the power transformer and converts AC power into DC power.

To achieve another object, according to the present invention, the switching mode power controller includes a switching unit, a power transformer, a current- coupling transformer, a signal generator, and a controller. The switching unit is turned on or off in response to a pre-defined switching signal. The power transformer has a primary coil connected between an input DC power and the switching unit, and a secondary coil that receives the AC power flowing through the pπmary coil depending on the state, i.e., ON or OFF, of the switching unit. The current-coupling transformer is located between the input DC power ground and the switching unit, receives the power flowing in the pπmary coil of the power transformer, converts the power into a pre-defined ratio, and provides the power to the secondary coil. The signal generator generates a detection signal to be inputted to the controller from the power provided to the secondary coil by the current-coupling transformer. The controller generates the switching signal depending on an error signal generated after comparison between the feedback output DC voltage and a reference signal, and depending on a detection signal.

Brief Description of the Drawings

FIG. 1 is a block diagram showing the overall configuration of a Switching Mode Power Supply (SMPS);

FIG. 2 is a circuit diagram showing the configuration of an input rectifier 11 shown in FIG. 1 ; FIG. 3 is a circuit diagram showing the configuration of DC/DC converters 12 through 14 shown in FIG. 1 ;

FIG. 4 is a circuit diagram showing the configuration of a Pulse Width Modulation (PWM) control circuit 31 shown in FIG. 3;

FIGS. 5A and 5B are circuit diagrams showing an output voltage feedback circuit, which comprises an error amplifier 41 shown in FIG. 4; and

FIG. 6 shows a circuit implemented based on existing technology.

Best mode for carrying out the Invention

Preferred embodiments of the present invention will be described in detail with reference to the attached drawings.

FIG. 1 is a block diagram showing the overall configuration of a Switching Mode Power Supply (SMPS). In FIG. 1 , a basic operation principle of the SMPS is described by its function. When AC power is inputted to an input rectifier 11 , the input rectifier 11 generates DC power and outputs it to a switching circuit 12. The switching circuit 12 operates at a pre-defined frequency and chops the input DC voltage signal to a square wave signal having a high frequency. The square wave signal is fed to a power transformer 13, reduced to a pre-defined value depending on a coil winding ratio, and rectified and filtered by the output rectifier 14. Then, a required DC voltage is outputted as an output signal. The output signal from the output rectifier 14 is detected and fed back to the switching circuit 12. The switching circuit 12 compares the voltage of the output signal with a reference voltage and adjusts (PWM control) an ON/OFF time of the switching circuit 12 depending on an error signal. In FIG. 1 , an input AC voltage signal is converted into a DC voltage signal by the input rectifier 11 and the DC voltage signal is fed to the switching circuit 12 and the power transformer 13. The output rectifier 14 functions as a converter that rectifies a square wave signal having a high frequency.

FIG. 2 is a circuit diagram showing the configuration of the input rectifier 11 shown in FIG. 1. A low pass filter comprising a varistor Zι_, a capacitor C, and two conductors l_ι and L₂ is connected to an AC input terminal. When commercial AC power is inputted as an input power, a spark or a high frequency noise is filtered by the low pass filter. Therefore, any thermal loss that may be caused by noise can be prevented. A diode Di determines the flow of a current depending on the phase of the AC power and a switch Si selects any of AC input 110V or 220V. A rectifier D₂ outputs DC voltage depending on a forward direction of the diode and charging of an electric charge of a capacitor. The rectifier rectifies to an average voltage, not a peak value. Therefore, if the input AC voltage is 100 V - 220 V, the rectified DC voltage is 80 V - 150 V.

FIG. 3 is a circuit diagram showing the configuration of the DC/DC converter shown in FIG. 1. An input DC voltage Vj_n is an output voltage of the input rectifier 11 shown in FIG. 1. The circuit including a PWM control circuit 31 , a switching transistor Qi, a current detector 33 and a self-bias circuit 35 corresponds to the switching circuit 12 shown in FIG. 1. A transformer Ti corresponds to the power transformer 13 shown in FIG. 1 while circuits 37a through 37c connected to a secondary coil of the transformer T₁ correspond to the output rectifier 14 shown in FIG. 1. In an embodiment of a flyback converter, a common input power Vj_n is provided to the PWM control circuit 31 , which operates as an oscillator, and the transistor Qi ^• connected to the output terminal. However, because parasitic oscillation is not induced even in a high switching frequency (for example, more than 650 KHz), a required output can be obtained.

According to the embodiment of the present invention shown in FIG. 3, the transistor Qi acts as a switching element that is turned on or off in response to the switching signal outputted from the PWM control circuit 31. The transformer Ti, which is a power transformer, in which the primary coil N_p is connected between an input DC power Vj_n and the switching unit, receives a square wave power of a high frequency which is provided depending on the ON or OFF state of the switching unit, through the primary coil and then supplies the power to the secondary coil. The input current detector 33 detects the current flowing through the primary coil of the power transformer Ti, which is provided depending on the ON or OFF state of the switching element (transistor Qι) and then feeds the detected current signal back to the PWM control circuit 31. The PWM control circuit 31 receives a feedback output DC signal (+FB) and controls the amount of the current flowing through the primary coil by controlling the ON/OFF duration of a switching signal SW_0Ut according to an error signal in which the feedback output DC signal is compared with a reference signal and the current level detected by the input current detector 33. The current level varies as the input DC voltage Vj_n changes. In addition, rectifiers 37a through 37c, connected to the secondary coil of the power transformer Ti, convert AC power into DC power to output one or more constant voltages.

#

According to the embodiment, the circuit for switching operation includes a PWM control circuit 31 for generating a switching signal, a transistor Qi, a transformer Ti for reducing a voltage, and a transformer T for detecting a current. The transistor Qi is turned on or off in response to the switching signal SW_0Ut generated by the PWM control circuit 31. While the transistor Qi is turned on, a current keeps charged at the primary coil of the transformer Ti_. If the transistor Qi is turned off, the charged current at the primary coil is transmitted to the secondary coil. Depending on the coil winding ratio of the transformer T-i_, the voltage is fed to both ends of the secondary coil. The input current detector 33 located between a source of the transistor Qi and a negative terminal feeds back the signal to the PWM control circuit 31 , which is generated depending on the current when the transistor Qi is ON. The input current detector 33 senses the current change caused by the change of the input DC voltage Vin or the change of the output load. Then, the detector 33 feeds the sensed current signal back to the PWM control circuit 31. To compensate for such current change, the PWM control circuit 31 controls the switching operation depending on a sensed signal.

The input current detector 33 senses a change in the current Ipp caused by a change in the input DC voltage V_in or the output voltage, and converts the sensed current into a voltage signal and feeds it back to the PWM control circuit 31. The PWM control circuit 31 reflects the change in the input DC voltage Vι_n or the output voltage and adjusts the pulse duration of the positive phase of the switching signal SWout so that an output voltage is constant. If the current l_pp increases, the current sensing voltage detected by the input current detector 33 and fed back to the PWM control circuit 31 increases. The PWM control circuit 31 adjusts the pulse duration of the switching signal (SWout) in order to decrease the current l_pp based on the current sensing voltage.

If the transistor Qi is turned on, the current flowing at the primary coil of the current-coupling transformer T₂ is induced to the secondary coil. The resistance R1 converts the current induced in the transformer T₂ to the voltage signal. The variable resistance R₂ adjusts the resistance value to determine the voltage value applied to the sensing terminal SENSE. Capacitors C₂ and C₃ are used to cancel ripples and noises, and a diode D₂ converts and rectifies the detected square wave signal into a DC signal. In sensing the change in the current flowing through the primary coil of the transformer Ti by the current-coupling transformer T₂, the primary coil of the transformer T₂ is separated from the secondary coil thereof and thus (+) pole of the power source for each side is separated, which enables a circuit configuration that supports switching to a high frequency. It is preferable that the winding ratio between the primary coil and the secondary coil of the transformer T₂ is about 1 :50 - 200. For example, for less than 10 Watt, if a continuous current l_PDc flowing through the primary coil of the main transformer Ti is 1.0A or less, the primary coil of the transformer T₂ is wound one time while the secondary coil is wound 100 times. In addition, it is preferable that the material of the core of the transformer T₂ should be the same as that of the main transformer Ti

If the power for supplying power and the power for controlling switching have a common (+) pole, oscillation can occur easily due to a high-frequency clock signal generated in the PWM control circuit 31 or due to switching operation by the switching element Q, In this circuit, since both powers have a common (-) pole and separate (+) poles due to the configuration of the transformer T₂ included in the input current detector 33, high-frequency switching can be performed reliably In particular, the circuit according to the preferred embodiment is useful for a low-power converter (for example, 10 Watts)

The PWM control circuit 31 receives the feedback output voltage signal +FB of the converter and receives the sensing voltage signal generated by the current of the input side and then generates a square wave pulse for turning the switching element Q_ on or off The detailed configuration will be described with reference to FIG 4

The self-bias circuit 35 provides the operational power to the output unit for outputting the switching signal SW_0Ut included in the PWM control circuit 31 The voltage induced by the auxiliary coil NF_B of the power transformer Ti is provided to the Vcc terminal of the PWM control circuit 31 through the diode D1 Here, the capacitor Ci is used to cancel ripples The input power Vι_n provides power to an element included in the PWM control circuit 31 except for the switching output unit

With regard to the selection of the transistor Qi, the following should be considered It is preferable that the internal resistance RDS(OΠ) should be low (for example, 3 ohm or less) and the input gate capacitance of the transistor should be low (for example, 350 pF). Then, power loss can be reduced.

The process for the main transformer Ti design is explained below. Peak current lpp and the continuous current I _DC flowing through the primary coil are expressed as follows.

[Mathematical Formula 1]

Inductance Lp and the size of the core of the primary coil are determined based on the following formulae.

[Mathematical Formula 2]

Ippfl

25.32L_PI_pPD^Δ x 10"

A =

B„

Actually, the size of the core should be larger than A_eA_cx 1.5.

The number Np of the primary coil windings is given by the following formula. [Mathematical Formula 3]

n 1 ^{p =} OAπl PP

where l_g is an air gap.

[Mathematical Formula 4]

0ΛπL pJpΪp_P x 10

1 =

A B

^' The number N_sι of coil windings for a 1^st output voltage V_OUM of the secondary coil and the number N_Gι of coil windings for the gate voltage of the transistor Q5 are shown below.

[Mathematical Formula 5]

The number Ν_S2 and N_s3 of the secondary coil windings for the 2 output V_0U[2, and the 3 >rd output V_out3 are given by the following formulae.

[Mathematical Formula 6]

N ' s„x x V out 2

N ' SI v_r out]

Output rectifiers 37a through 37c are connected to the secondary coil of the power transformer Ti. Basically, on the output side of the power transformer T-i_, there are an element for rectification and a capacitor for canceling ripples. When the output current is low (for example, 0.5 A or less), the diode is used for rectification. If the output current is high, it is preferable that an MOFET transistor (preferably, R_DS(_OΠ) = 0.02 ohm or less) is used in order to minimize the power loss. In case the diode and the transistor are used as an element for rectification, each power loss is shown below.

[Mathematical Formula 7]

* L(DIODE) ~ "F 0

p _ D _y T2

¹ L(FET) ~ ⁱ DS(on) ^Λ O

An RC snubber circuit in the output units 37a through 37c shown in FIG. 3 will be described below. A resonance circuit is formed by the leakage inductance of the primary coil Lp of the power transformer Ti for high frequency and the junction capacitance of the diode for rectification. In a transient state, the transient over- voltage ringing is caused by such a resonance circuit. The ringing causes noises during a turn-off period and in some serious cases, may have an amplitude big enough to damage the diode. The RC snubber circuit controls the ringing so that its amplitude can be suppressed to a safe level. The 2^nd output rectifier 37b and the 3^rd output rectifier 37c shown in FIG. 3 include snubber circuits. The snubber circuit can be connected to both ends of the diode for rectification or both ends of the secondary coil of the power transformer Ti .

In the 1^st output rectifier 37a shown in FIG. 3, it is preferable that a transistor Q.₅ used as an element for rectification is a field-effect transistor (FET). The transistor Q5 converts the current induced in the secondary coil of the power transformer Ti due to an on-off operation of the switching element (Qi) into DC. As an element for suppressing ripples, a capacitor C₀ι smoothes ripples so that a regular level of output V_outι is generated. Capacitors C₀ι through C₀3 for filtering used in the power rectifier can prevent ripple noise even if the capacity is reduced to 1/10 when compared with the existing output rectifier.

An RC element including a resistance R₅ and a capacitor C₅ is a 1^st snubber circuit used to prevent any oscillation ringing caused by the leakage inductance LT of the primary coil Lp of the power transformer Ti and the gate input capacitance C_GS of the transistor Q₅. An RC element including a resistance R₇ and a capacitor C₇ is a 2^nd snubber circuit used to prevent any oscillation ringing caused by the leakage inductance of the primary coil Lp of the power transformer Ti and the capacitance C_oss between the drain and the source of the transistor Q5.

In the 1^st output rectifier 37a, a transistor is used as an element for rectification instead of a diode in order to drastically reduce power loss and improve efficiency. Therefore, a snubber circuit suitable for high current and for preventing ringing can resolve the problems of ringing caused by oscillation.

The design process of the snubber circuit shown in FIG. 3 is described. The snubber resistance R₅ and R₇ can be obtained using the following formula.

[Mathematical Formula 8]

L_τ = 10% x L_P, C_j = C_GsorC_oss n N _P: (N _GSorN _S )

Snubber capacitances C₅ and C are shown as follows.

[Mathematical Formula 9]

V..

R C_S ≤ -

²⁰ '/ ^„⁽™^χ)

The output capacitors (electrolytic condenser) C₀ι through C₀₃ used to reduce the ripple of the output voltage can be designed as follows. [Mathematical Formula 10]

Δ F,

C

8 Δ/₀

Actually, the following is preferable

C₀₁ > C₀ x 1000

However, according to existing technology, since the output ripple is larger than the present embodiment, Co should be multiplied by 10,000 Thus, it is possible to reduce the capacity of the output capacitor according to the present embodiment

The RC snubber circuit connected to the diode for rectification can be obtained based on the above formulae Here, Cj is the junction capacitance of the diode

FIG 6 shows a circuit based on existing technology, which is compared with the circuit of FIG 3, according to the embodiment of the present invention The circuits are configured in the same way A PWM control circuit 61 , a transformer Ti, a self-bias circuit 65, and a switching transistor Q-i shown in FIG 6 correspond to a PWM control circuit 31 , a transformer Ti a self-bias circuit 35, and a switching transistor Q_{1 (} respectively, shown in FIG 3, perform substantially the same functions as their counterparts A current feedback unit shown in FIG 6 corresponds to the current feedback unit shown in FIG 3 However, they are configured in different ways. In addition, the output rectifiers 67a and 67b connected to the secondary coil of the transformer Ti include the diodes Di and D₂for rectification and the capacitors Coi and C₀2-

According to FIG. 6, a resistance R_cs detects the current flowing through the transistor Qi and feeds it back to the PWM control circuit 61. That is, the resistance Res senses the change of the current flowing between the drain and the source of the transistor Qi, which is caused by changes of the input voltage or the output load, and feeds the current back to the PWM control circuit. For example, when the input voltage is higher, the PWM control circuit reduces the positive phase amplitude of the switching signal and thus decreases the current, prevents the output voltage from increasing, and maintains regular output voltage.

The embodiment according to the present invention and the embodiment according to existing technology are different from each other in terms of a current sensing method. Due to the difference, the present invention operates at a switching frequency of more than 500 KHz and the power loss caused by the coupler is low enough that it can be ignored. On the contrary, in the embodiment according to existing technology, the voltage reduction (V_Drp = l_PDC x Res) due to the resistance R_Ds results in a large power loss (P_Loss = Res x IPD_C ²), and thus the switching frequency cannot be drastically increased. In the present embodiment, the switching frequency can be increased to 1

MHz in case of the low power (10 Watt) if the 1^st coil and the 2^nd coil of the current sensing transformer T₂ according to the present embodiment are wound as shown in the FIG. 6. According to the present embodiment, since the primary coil of the transformer T₂ is wound only once, coil resistance can be ignored and little power is lost.

FIG. 4 shows one embodiment of the configuration of the PWM control circuit 31 shown in FIG. 3 and illustrates a current-mode control method. The current mode converter uses an internal loop to adjust an error signal with regard to the input peak current. The voltage Vcc fed by the auxiliary coil N_FB of the 1^st side of the power transformer J_. is provided to an amplifier 45 that outputs a switching signal SWou_t. A clock generator 43, a flip-flop 44, an error amplifier 51 , and a comparator 52 receive operational power from the input power V_in fed through a regulator 47.

The error amplifier 41 compares the output signal +FB and the reference voltage V_ref and amplifies the error signal. Then, the error signal is inputted to the comparator 42. When the switching element is turned on, the peak current flowing through the primary coil of the power transformer is sensed and converted into voltage signal, and the sensed signal is inputted to the comparator 42. The comparator 42 compares the sensed signal depending on the peak switching current with the error signal related to the output signal and inputs the compared result to 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. The RS flip-flop 44 receives the output of the comparator 42 and the clock signal, and generates a switching signal SW_0Ut that turns the switching element on or off. The switching signal turns the switching element (in most cases, a transistor) ON/OFF depending on the logic level.

The error amplifier 41 compares the main output voltage +FB fed back by the final output terminal with the reference voltage V_ref. The comparator 42 compares the feedback signal in which an input current is sensed with the reference voltage (1.2 V) and outputs the result to the flip-flop 44. The flip-flop 44 increases or decreases the phase (or width) of the clock signal generated and provided by the oscillator 43 in response to the output signal of the comparator 42, to generate a switching signal SW_0Ut. Then, the current flowing through the transformer Ti is adjusted depending on the change in the input voltage and the load so that the output voltage at the final output terminal can be maintained to be constant. That is, the switching signal SW_0Ut is the PWM signal whose duty cycle is changed depending on the output voltage and the input current.

If there are several output voltages, other output voltages are adjusted based on the highest output voltage. That is, even though the input voltage or load is changed within a certain range, the output voltage should be maintained to be constant. Therefore, besides feedback by the input current detector 33, the main output voltage +FB should be fed back to the PWM control circuit 31. Then, the converter circuit becomes more stable, and an SMPS with a high efficiency operating at a high frequency can be implemented.

FIGS. 5A and 5B are circuit diagrams showing an output voltage feedback circuit, which comprises an error amplifier 41 shown in FIG. 4.

FIG. 5A illustrates a circuit having a regular gain between frequencies f1 and f2. A bias resistance Rw_as, capacitances Ci and C₂, a gain (AV), the cutoff frequencies fi and f₂, and a switching frequency f_s are indicated by the following formulae.

[Mathematical Formula 11]

^ΛV - R, ' 2ffR₂C₁ -,'Λ^{J 2}

FIG. 5B shows a circuit having enhanced transient response characteristics compared to the circuit of FIG. 5A. A bias resistance Rbias, capacitances Ci through C , gains AVi and AV₂, cutoff frequencies fi through , and a switching frequency f_s are indicated by the following formulae.

[Mathematical Formula 12]

R λ 1 AV = — - ,/j = ,f₂ ~

R, * 2πR₂C, ² 2πR_λC₃

A n >J

^3 2πR₃C₃ -^,Λ 2πR C,

/, = -f_s

The efficiency (Mathematical Formula 1) of the converter circuit diagram shown in FIG.3 is given below.

[Mathematical Formula 13]

Efficiency ( η

It is assumed that output powers P_outι, Pout2, and P_out3are given below.

[Mathematical Formula 14]

P_oω. = V₀ xI_ou = 15Vx0.25A=3.75W P_out2 = V₂ I_out2 = 12V 0.lA=1.2W P_O!ll3 = V₀₃xI_out3 = 22V 0.12A=2.64W

Thus, the total output power is given below.

[Mathematical Formula 15] P out = P out\ + ^τ P ^M out! + ^τ P "* oκ(3 = 7 ' - 5^J9^W"

The continuous current I D_C flowing through a primary coil of the power transformer Ti is given below. Here, the switching frequency f_s is 650 KHz, the minimum input voltage V/_n(m/n) is 160 Vdc, the maximum input voltage Vj_n(_maχ) is 240 Vdc, and the maximum pulse width Dmax is 0.45.

[Mathematical Formula 16]

I_PDC = 0.215 x 0.4 = 0.086,4

Total power loss can be calculated as follows. Here, the copper wire resistance of the primary coil of the power transformer is 0.4 ohms. The resistance RD_S(_OΠ) between the drain and the source while the switching transistor Q, is turned on is 1.5 ohms. The resistance RD_S(O_Π) between the drain and the source while the transistor Q5 for rectification is turned on is 0.02. The forward voltage V_F of a diode D8 or D9 for rectification is 0.3 V.

[Mathematical Formula 17] P_TL = A x I_P ² _DL = 0.4 x 0.086² = 0.003(PF)

P_Q1L = 1.5 x I_P ² _DL = 1.5 x 0.086² = 0.01 (W)

P_Q2L = 0.02 x 7^, = 0.02 x 0.25² = 0.00 l(W)

P_DIL = V_F I_ml2 = .3 0.1 = 0. 3(W) ^_D2i = V_F x I_mt3 = 0.3 x 0.12 = 0.036(W)

[Mathematical Formula 18]

Pτt = 0.08 W Therefore, the efficiency of the converter is given below.

[Mathematical Formula 19]

7.59 - 0.081 η = = 0.989 * 99(%)

¹ 7.59 ^J

The converter as described above can be used as a driving circuit (drive amplifier) of a power converter that requires high power and is especially useful for various inverters or converters used in a battery charger and a driving device of a DC electric motor. In addition, the converter according to the present invention is suitable as a driving circuit of a power supply for low voltage/high power like the notebook computer. In particular, the converter according to the present invention can be applied as a basic circuit in not only a wireless mobile phone and a wireless video phone but also a battery charger or a UPS (Unmanned Power Supply) built in a portable radio, TV, or a computer. Industrial Applicability

As described above, in the power supply according to the present invention, if the circuit is configured in such a way that power which is provided to the switching frequency oscillator of the PWM control circuit is separate from power which is provided to a main power amplifier that amplifies the switching signal, any oscillation caused by the switching oscillator is not transmitted to the output terminal, and thus efficiency is increased. If the circuit is configured as shown in the figures and the claims even if the present invention uses a common power instead of two separate power sources, a power supply with a high efficiency can be implemented. The efficiency of existing SMPS is just about 65 % - 75 %. However, the present invention can operate in a high-frequency switching mode at higher 90% efficiency. In particular, in case the present invention is applied to a DC motor that uses an air compressor, the motor size can be reduced and the motor torque (rotation power) can be increased. Therefore, the electrical energy can be saved drastically.

Claims

What is claimed is:

1. A power supply comprising: a switching unit which is turned on or off in response to a pre-defined switching signal; a power transformer which has a primary coil connected between an input DC power and the switching unit, and the secondary coil that receives the power flowing through the primary coil depending on the ON or OFF state of the switching unit; a detector for detecting a current flowing through the pπmary coil of the power transformer in response to the ON or OFF state of the switching unit and for feeding the current signal back to a controller; the controller for generating a switching signal depending on an error signal generated after comparison between a feedback output DC power and a reference signal, and a detection signal output from the detector; and a rectifier connected to the secondary coil of the power transformer to convert AC power to DC power.

2. The power supply of claim 1 , wherein the detector comprises: a current-coupling transformer, which is located between a negative pole of an input DC power and a switching unit, for receiving the power flowing through the primary coil of the power transformer depending on the on or off state of the switching unit, converting the power at a pre-defined ratio, and providing the power to the secondary coil; and a signal generator for generating a detection signal to be inputted to the controller, from the power induced in the secondary coil by the current-coupling transformer.

3. The power supply of claim 1 , wherein the rectifier comprises: a transistor having a gate and a source connected to both ends of the secondary coil of the power transformer and a drain as an output terminal; and a snubber unit having any of a 1^st snubber circuit used to prevent any ringing caused by a leakage inductance of the power transformer between the gate and the source and the gate capacitance, and a 2^nd snubber circuit used to prevent any ringing caused by the leakage inductance of the power transformer between the drain and the source and the capacitance.

4. The power supply of claim 1 , wherein the rectifier comprises: a diode for rectification, which is connected to the secondary coil of the power transformer; and a snubber circuit connected to both ends of the diode or both ends of the secondary coil of the power transformer to prevent any ringing caused by the leakage inductance of the power transformer and the junction capacitance of the diode.

5. The power supply of claim 1 , further comprising an input rectifier which receives AC power, converts the AC power into DC power and provides the

DC power to a power transformer.

6. A switching mode power controller comprising: a switching unit which is turned on or off in response to a switching signal; a power transformer for receiving AC power flowing through the primary coil connected between an input DC power and the switching unit, which is provided depending on the ON or OFF state of the switching unit, and then supplying the AC power to a secondary coil; a current-coupling transformer which is located between the input DC power ground and the switching unit, for receiving the power flowing through the primary coil of the power transformer, converting the power at a pre-defined ratio, and providing the power to the secondary coil; a signal generator for generating a detection signal to be inputted to a controller, from the power provided to the secondary coil by the current-coupling transformer; and the controller for generating the switching signal depending on an error signal generated after comparison between a feedback output DC power and a reference signal, and a detection signal output from the detector.