Primary side controlled switching regulator

[0001] 1. Field of the Invention

[0002] The present invention relates to a regulator, and more specifically relates to a switching regulator.

[0003] 2. Description of Related Art

[0004] Switching regulators are widely used to provide regulated voltage and current. Considerable ongoing research is focused on making regulators more efficient for saving power. A switching regulator typically includes a control circuit, a switch and a transformer. The control circuit is applied to sense the output voltage and/or the output current of the switching regulator, and generate a control signal to control the switch and regulate the output voltage and/or the output current of the switching regulator.

[0005] FIG. 1 shows a circuit diagram of a traditional switching regulator. The traditional switching regulator includes a transformer 10 having a primary winding N_P, a secondary winding N_S and an auxiliary winding N_A. A terminal of the primary winding N_P is coupled to a positive supply rail V_IN. A switch 11 is connected from another terminal of the primary winding N_P to a negative supply rail (a ground) through a resistor 12. A control circuit 25 is coupled to the switch 11 to control the switch 11 for switching the transformer 10 and regulating the output voltage and/or the output current of the switching regulator. A terminal of the secondary winding N_S connects a rectifier 13. A filter capacitor 14 is coupled between the rectifier 13 and another terminal of the secondary winding N_S. Energy is stored into the transformer 10 when the switch 11 is turned on. The energy stored in the transformer 10 is discharged to the output of the switching regulator through the secondary winding N_S once the switch 11 is off. Meanwhile, a reflected voltage V_AUX1 is generated at the auxiliary winding N_A of the transformer 10.

[0006] $\begin{array}{cc}{V}_O+V_F=N_{\mathrm{NS}}\times \frac{d\Phi }{dt}& \left(1\right)\\ V_{\mathrm{AUX}1}=N_{\mathrm{NA}}\times \frac{d\Phi }{dt}& \left(2\right)\end{array}$
In accordance with equations (1) and (2), the reflected voltage V_AUX1 can be expressed as

[0007] $\begin{array}{cc}{V}_{\mathrm{AUX}1}=\frac{N_{\mathrm{NA}}}{N_{\mathrm{NS}}}\times \left(V_O+V_F\right)& \left(3\right)\end{array}$
where N_NA and N_NS are respectively the winding turns of the auxiliary winding N_A and the secondary winding N_S of the transformer 10; V_O is the output voltage of the switching regulator; V_F is a forward voltage drop of the rectifier 13; the φ is magnetic flux, φ=B×A_e (B is flux density, Ae is the core cross-section of the transformer 10).

[0008] The control circuit 25 comprises a supply terminal VDD and a ground terminal GND for receiving power. A voltage divider includes a resistor 15 and a resistor 16 connected between the auxiliary winding N_A of the transformer 10 and the negative supply rail. A voltage detection terminal VS of the control circuit 25 is connected to a joint of the resistor 15 and the resistor 16. A detecting voltage V_DET1 generated at the voltage detection terminal VS is given by,

[0009] $\begin{array}{cc}{V}_{\mathrm{DET}1}=\frac{R_{16}}{R_{15}+R_{16}}\times V_{\mathrm{AUX}1}& \left(4\right)\end{array}$
where R₁₅ and R₁₆ are respectively the resistance of the resistors 15 and 16.

[0010] The reflected voltage V_AUX1 further charges a supplied capacitor 17 via a diode 18 to power the control circuit 25. The resistor 12 serves as a current sense device. The resistor 12 is connected from the switch 11 to the negative supply rail for converting the transformer switching current I_P into a current signal V_CS. A current sense terminal VI of the control circuit 25 is connected to the resistor 12 for detecting the current signal V_CS. An output terminal VG of the control circuit 25 generates the switching signal V_PWM to switch the transformer 10. This switching regulator is generally used to regulate output voltage and output current, but it includes several disadvantages. One disadvantage is high power consumption caused by the leakage inductor of the transformer 10. A snubber circuit includes a snubber diode 19, a snubber capacitor 20 and a snubber resistor 21 to consume the stored energy of the leakage inductor of the transformer 10 for protecting the switch 11 from a high voltage spike. Another disadvantage of this switching regulator is a poor load regulation at light load and no load. The power of the control circuit 25 is supplied from the auxiliary winding N_A of the transformer 10. Therefore, the operating current of the control circuit 25 represents the load of the auxiliary winding N_A. If the load at the output voltage V_O of the switching regulator is lower than the load consumed by the auxiliary winding N_A, then the stored energy of the transformer 10 will only be discharged to the supplied capacitor 17 through the diode 18 and the auxiliary winding N_A. The rectifier 13 will remain off when the switch 11 is turned off. Therefore, the output voltage V_O of the switching regulator cannot be feedback through the auxiliary winding N_A. The detecting voltage V_DET1 generated at the voltage detection terminal VS will be only related to the voltage of the supply terminal VDD at light load and no load situations.

[0011] Another prior art is “Primary-side controlled flyback power converter” by Yang, et al; U.S. Pat. No. 6,853,563. One principal drawback of this prior-art invention is high EMI (electric and magnetic interference). The drain terminal of the switch is directly connected to the positive supply rail V_IN. A parasitic capacitor of the switch and a parasitic inductor coupled together develop a high frequency resonant tank, which produces higher EMI.

[0012] The object of the present invention is to provide a switching regulator having high efficiency and low EMI. Besides, the output voltage of the switching regulator can be accurately regulated at light load and no load.

[0013] A switching regulator includes a transformer having a primary winding and an auxiliary winding. The primary winding is coupled to a positive supply rail. The auxiliary winding is coupled to a negative supply rail. A switch is connected in series with the primary winding and the auxiliary winding for switching the transformer. A current sense device is connected from the switch to the auxiliary winding for generating a current signal in accordance with a switching current of the transformer. A control circuit is coupled to the switch and the auxiliary winding of the transformer to generate a switching signal in response to the current signal. The switching signal is used for switching the switch and regulating the output of the switching regulator. A supplied capacitor is connected to the control circuit to supply the power to the control circuit. The auxiliary winding has a leakage inductor to store a stored energy when the switch is on. A diode is coupled from the negative supply rail to the supplied capacitor. The stored energy of the leakage inductor is discharged to the supplied capacitor through the diode once the switch is off. The connection of the transformer and the switch improves the efficiency and reduces the EMI.

[0014] It is to be understood that both the foregoing general descriptions and the following detailed descriptions are exemplary, and are intended to provide further explanation of the invention as claimed. Still further objects and advantages will become apparent from a consideration of the ensuing description and drawings.

[0022] FIG. 2 shows a circuit diagram of a primary-side controlled switching regulator in accordance with the present invention. The switching regulator includes a transformer 30 for transferring a stored energy from a primary side of the transformer 30 to a secondary side of the transformer 30. The primary side of the transformer 30 has a primary winding N_P and a first auxiliary winding N_A1. The secondary side of the transformer 30 has a secondary winding N_S. The primary winding N_P and the first auxiliary winding N_A1 are coupled to the supply rail of the transformer 30. The primary winding N_P is coupled to a positive supply rail V_IN of the supply rail of the transformer 30. The first auxiliary winding N_A1 is coupled to a negative supply rail (ground) of the supply rail of the transformer 30. A switch 35 is connected in series with the primary winding N_P and the first auxiliary winding N_A1 for switching the transformer 30. The switch 35 can be a power transistor or a power MOSFET. Because the switch 35 is connected in series with the primary winding N_P and the first auxiliary winding N_A1, the high frequency resonant tank caused by the parasitic devices is eliminated and also the EMI is reduced.

[0023] A current sense device such as a current sense resistor 37 is connected from the switch 35 to the first auxiliary winding N_A1 for generating a current signal V_CS in accordance with the switching current I_P of the transformer 30. In order to regulate an output voltage V_O of the switching regulator, a control circuit 100 is coupled to the switch 35 and the first auxiliary winding N_A1 of the transformer 30 to generate a switching signal V_PWM. The switching signal V_PWM is used for switching the switch 35 and regulating the output voltage V_O of the switching regulator. A supplied capacitor 70 is connected to the control circuit 100 to supply the power to the control circuit 100. A first diode 60 is coupled between the supplied capacitor 70 and the negative supply rail of the transformer 30.

[0024] A snubber circuit 45 is coupled between the primary winding N_P and the positive supply rail V_IN. The snubber circuit 45 includes a snubber diode 40, a snubber capacitor 41 and a snubber resistor 42. A terminal of the snubber diode 40 is coupled to the primary winding N_P and the switch 35. The snubber capacitor 41 is coupled between another terminal of the snubber diode 40 and the positive supply rail V_IN. The snubber resistor 42 is coupled in parallel with the snubber capacitor 41. A voltage divider 50 is coupled between the first auxiliary winding N_A1 and the negative supply rail. The voltage divider 50 includes resistors 52 and 55. The resistor 52 is coupled between the control circuit 100 and the negative supply rail. The resistor 55 is coupled between the resistor 52 and the first auxiliary winding N_A1. A rectifier 80 is coupled to the secondary winding N_S. A filter capacitor 90 is coupled between the secondary winding N_S and the rectifier 80.

[0025] FIG. 3 shows an equivalent circuit diagram of the switching regulator shown in FIG. 2. The primary winding N_P and the first auxiliary winding N_A1 include leakage inductors L₁₁ and L₁₂ respectively. Due to the geometrical structure of the transformer, the stored energy of the primary side winding of the transformer cannot be fully transferred to other windings of the transformer. The leakage inductors L₁₁ and L₁₂ stand for stored energy that cannot be transferred. The switching current I_P is flowed into the transformer 30 when the switch 35 is turned on. The energy is thus stored into the transformer 30 and leakage inductors L_l1 and L_l2. When the switch 35 is turned off, the stored energy of the transformer 30 is discharged to the secondary winding N_S. Meanwhile the stored energy of the leakage inductors L_l1 and L_l2 will be circulated within the loop. If the loop is locked, a voltage spike will be produced.

[0026] $\begin{array}{cc}V=L\times \frac{di}{dt}& \left(5\right)\end{array}$

[0027] The snubber circuit 45 is used to consume the stored energy of the leakage inductor L_l1 for protecting the switch 35 from a high voltage spike. The power consumed by the snubber resistor 42 of the snubber circuit 45 can be shown as,

[0028] $\begin{array}{cc}{P}_R=\frac{V_{R42}^2}{R_{42}}=\frac{1}{2}\times L_l\times I_P^2\times f_{\mathrm{SW}}& \left(6\right)\end{array}$
where R₄₂ is the resistance of the snubber resistor 42; V_R42 is the voltage across the snubber resistor 42; L_l is the inductance of the leakage inductor L_l1; f_SW is the switching frequency of the switch 35.

[0029] Therefore, reducing the inductance of the leakage inductor of the transformer 30 will increase the efficiency of the switching regulator. However, in order to meet the safety requirement, the winding of the transformer 30 always produces a significant leakage inductance. A simple way to reduce the leakage inductance is to reduce the winding turns.

[0030] $\begin{array}{cc}L=\mu \times \frac{0.4\pi \times \mathrm{Ae}}{\mathrm{li}}\times N^2& \left(7\right)\end{array}$
where L is the inductance; μ is core permeability; li is magnetic path length; N is the number of winding turns; Ae is the core cross-section of the transformer 30.

[0031] Connecting the primary winding N_P in series with the first auxiliary winding N_A1 can reduce the winding turns so that the leakage inductance in the primary winding N_P is reduced. The stored energy of the leakage inductor L_l2 is discharged to the supplied capacitor 70 through the first diode 60 once the switch 35 is off. Therefore, the stored energy of the leakage inductor L_l2 is supplied to the control circuit 100. The voltage V_DD generated in the supplied capacitor 70 can be shown as

[0032] $\begin{array}{cc}{V}_{\mathrm{DD}}=\left[\frac{N_{\mathrm{NA}1}}{N_{\mathrm{NS}}}\times \left(V_O+V_F\right)\right]+V_{L12}& \left(8\right)\end{array}$
where N_NA1 and N_NS are respectively the winding turns of the first auxiliary winding N_A1 and the secondary winding N_S of the transformer 30.
The V_Ll2 is the voltage generated by the leakage inductor L_l2. It is given by,

[0033] $\begin{array}{cc}\frac{1}{2}\times C_{70}\times V_{L12}^2=\frac{1}{2}\times L_2\times I_P^2& \left(9\right)\\ V_{L12}=\sqrt{\frac{L_{12}}{C_{70}}}\times I_P& \left(10\right)\end{array}$
where C₇₀ is the capacitance of the supplied capacitor 70; L₂ is the inductance of the leakage inductor L_l2.

[0034] Because the voltage V_Ll2 generated by the leakage inductor L_l2 causes the voltage V_DD on the supplied capacitor 70 is higher than the voltage reflected from the secondary winding N_S of the transformer 30. The rectifier 80 is thus switched on once the switch 35 is switched off. Therefore, the output voltage V_O of the switching regulator can be fed to the control circuit 100 through the first auxiliary winding N_A1. By properly developing the leakage inductor L_l2 of the first auxiliary winding N_A1 will improve the load regulation at light load and no load circumstances.

[0035] FIG. 4 shows the circuit diagram of the control circuit 100 that includes a supply terminal VDD and a ground terminal GND parallel connected to the supplied capacitor 70 for receiving power. The supply terminal VDD is connected to the first diode 60. The ground terminal GND is connected to the first auxiliary winding N_A1. A voltage detection terminal VS is coupled to the first auxiliary winding N_A1 through the voltage divider 50 for detecting a detecting voltage V_DET2 from the first auxiliary winding N_A1 of the transformer 30. The detecting voltage V_DET2 can be expressed as,

[0036] $\begin{array}{cc}{V}_{\mathrm{DET}2}=\frac{R_{52}}{R_{52}+R_{55}}& \left(11\right)\end{array}$
where R₅₂ and R₅₅ are respectively the resistance of the resistors 52 and 55; V_AUX2 is the voltage of the first auxiliary winding N_A1.

[0037] A current sense terminal VI is coupled to the switch 35 and the current sense resistor 37 for receiving the current signal V_CS. An output terminal VG is coupled to an output terminal of a flip-flip 160 to generate the switching signal V_PWM for switching the transformer 30 via the switch 35. An oscillator 150 generates a periodic pulse signal transmitted to a set terminal of the flip-flop 160. The periodic pulse signal is utilized to start the switching signal V_PWM. A comparator 125 is used to turn off the switching signal V_PWM. A negative input terminal of the comparator 125 is connected to the current sense terminal VI to receive the current signal V_CS. A positive input terminal of the comparator 125 is connected to an output terminal of an error amplifier 120 to receive a feedback signal V_FB.

[0038] Once the current signal V_CS is higher than the feedback signal V_FB, the switching signal V_PWM will be turned off. An output terminal of the comparator 125 is connected to a reset terminal of the flip-flip 160 to generate a reset signal V_RST transmitted to the reset terminal to turn off the switching signal V_PWM. The error amplifier 120 is utilized to generate the feedback signal V_FB. A positive input terminal of the error amplifier 120 receives a reference voltage V_R. A negative input terminal of the error amplifier 120 is connected to an output terminal of a sample-hold circuit 110 to receive a sample signal V_S. An input terminal of the sample-hold circuit 110 is coupled to the voltage detection terminal VS to detect the detecting voltage V_DET2 from the transformer 30 via the voltage divider 50 for generating the sample signal V_S. The output voltage V_O of the switching regulator is therefore regulated.

[0039] $\begin{array}{cc}{V}_O+V_F=\frac{N_{\mathrm{NS}}}{N_{\mathrm{NA}}}\times V_{\mathrm{AUX}2}& \left(12\right)\end{array}$

[0040] In accordance with equations (11) and (12), the output voltage V_O can be expressed as

[0041] $\begin{array}{cc}{V}_O=\left(\frac{R_{52}+R_{55}}{R_{52}}\times \frac{N_{\mathrm{NS}}}{N_{\mathrm{NA}}}\times V_{\mathrm{DET}2}\right)-V_F& \left(13\right)\end{array}$

[0042] FIG. 5 shows a circuit diagram of another preferred embodiment of the switching regulator according to the present invention. The transformer 38 includes the primary winding N_P, the first auxiliary winding N_A1 and a second auxiliary winding N_A2. The second auxiliary winding N_A2 of the transformer 38 is connected to the first auxiliary winding N_A1. The second auxiliary winding N_A2 is further connected to the supplied capacitor 70 through a second diode 65. The ground terminal GND is connected to the auxiliary windings N_A1 and N_A2. The supply terminal VDD is connected to the supplied capacitor 70, the first diode 60 and the second diode 65. Because the first auxiliary winding N_A1 is used for providing the power to the control circuit 100 when the switch 35 is off, the voltage V_DD generated on the supplied capacitor 70 is correlated to the output voltage V_O of the switching regulator. The voltage V_AUX2 of the first auxiliary winding N_A1 would be too low to supply power to the control circuit 100 if the output voltage V_O of the switching regulator is under the overcurrent and/or short circuit circumstances. The second auxiliary winding N_A2 is thus developed to charge the supplied capacitor 70 when the switch 35 is switched on. Therefore, the second auxiliary winding N_A2 provides a second source to supply power to the control circuit 100, which ensures a proper operation of the control circuit 100 during the fault conditions.

[0043] FIG. 6 shows a circuit diagram of another preferred embodiment of the switching regulator according to the present invention. The transformer 39 includes the primary winding N_P, an inductor 32 and a third auxiliary winding N_A3. The inductor 32 is connected in between the current sense resistor 37 and the negative supply rail. The third auxiliary winding N_A3 of the transformer 39 is connected to the supplied capacitor 70 through the second diode 65. The third auxiliary winding N_A3 furthermore is coupled to the inductor 32. A voltage divider 56 having a resistor 58 and a resistor 59 is coupled between the third auxiliary winding N_A3 and the inductor 32. The resistor 58 is coupled to the third auxiliary winding N_A3. The resistor 59 is connected in series with the resistor 58 and the inductor 32. The voltage detection terminal VS of the control circuit 100 is coupled to the joint of the resistor 58 and the resistor 59. The ground terminal GND of the control circuit 100 is connected to the third auxiliary windings N_A3 and the inductor 32. Furthermore, the inductor 32 is used for providing the power to the control circuit 100 through the first diode 60 when the switch 35 is turned off. Therefore, the power source of the control circuit 100 is provided from the third auxiliary winding N_A3 and the inductor 32, which will improve the load regulation at light load and no load conditions.

[0044] According to present invention, the transformer windings minimize the inductance of the leakage inductor. Besides, the stored energy of the leakage inductor of the auxiliary winding or inductor is used to provide power to the control circuit, which achieves better efficiency and improves the load regulation at light load and no load. Furthermore, the connection of the transformer and the switch results a lower EMI.

[0045] It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention covers modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.