REGULATION CIRCUIT HAVING OUTPUT CABLE COMPENSATION FOR POWER CONVERTERS AND METHOD THEREOF

Abstract

A regulation circuit with the output cable compensation is developed for a power converter. It includes an error amplifier for generating a feedback signal in accordance with an output of the power converter. A compensation circuit is coupled to a transformer of the power converter for generating a compensation signal in response to a transformer signal generated by the transformer. The feedback signal is applied to generate a switching signal for switching the transformer and regulating the output of the power converter. The compensation signal is coupled to modulate the feedback signal for compensating a voltage drop of the output cable of the power converter.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a power converter, and more particularly, to the regulation circuit of the power converter.

2. Description of Related Art

For an offline power converter, it requires an error amplifier located at the secondary side of the transformer for generating a feedback signal in accordance with the output of the power converter. The feedback signal is utilized to generate a switching signal to switch the transformer and regulate the output of the power converter. FIG. 1 shows a prior art circuit schematic of a power converter. A PWM controller (PWM) 30 generates a switching signal S_PWM to switch a transformer 10 via a power transistor 20 in accordance with a feedback signal V_FB for regulating the output of the power converter. The transformer 10 has a primary winding N_P and a secondary winding N_S. The primary winding N_P of the transformer 10 is coupled to receive an input voltage V_IN. The feedback signal V_FB is generated in accordance with the output of the power converter through an opto-coupler 60.

The opto-coupler 60 is controlled by an error amplifier 50. The error amplifier 50 generates a feedback signal V_F coupled to control the opto-coupler 60. The error amplifier 50 includes a reference signal V_R supplied with a positive input terminal of the error amplifier 50 for regulating the output voltage V_O1 of the power converter. The output voltage V_O1 is coupled to a negative input terminal of the error amplifier 50 via a voltage divider developed by resistors 51 and 52. A capacitor 53 is coupled between the negative input terminal of the error amplifier 50 and an output terminal of the error amplifier 50.

The secondary winding N_S of the transformer 10 is coupled to an output terminal of the power converter to generate the output voltage V_O1. A rectifier 40 is coupled to one terminal of the secondary winding N_S. An output capacitor 45 is coupled to the other terminal of the secondary winding N_S and the output terminal of the power converter to generate the output voltage V_O1. A resister 62 is coupled form the capacitor 45 and the rectifier 40 to the opto-coupler 60.

Normally, the output voltage V_O1 of the power converter is connected to the load through an output cable 70, the output connector etc. The output cable 70, the output connector, etc. will cause the voltage drop that is proportional to its output current. Thus, remote-sense resistors 55, 56 and remote-sense cables 71, 72 are used for the remote sensing of the output voltage V_O at the load. This remote sensing is utilized to regulate the output voltage V_O at the load. Therefore, the output voltage V_O will be no impact to the voltage drop of the output cable 70 and the output connector, etc. However, the remote-sense cables 71 and 72 increase the cost of the power converter, particular when the output cable 70 is a long cable. Therefore, a regulation circuit with output cable compensation is needed for reducing the cost of the power converter and providing an accurate regulation for the output voltage V_O.

Refer to the technology of the output cable compensation for power converters, a prior art can be found in “Primary-side controlled switching regulator”, U.S. Pat. No. 7,352,595. However, this technology can only be applied to the primary side regulation. It means the error amplifier of the power converter must be located in the primary side of the transformer. The present invention provides a method and apparatus of the output cable compensation for an error amplifier located in the secondary side of the transformer.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a regulation circuit and a method with output cable compensation for the power converter. The regulation circuit and method compensate the voltage drop of the output cable of the power converter without the remote-sense cables for reducing the cost of the power converter and providing the accurate regulation for the output voltage.

The present invention provides the regulation circuit with an output cable compensation for the power converter. It includes an error amplifier generating a feedback signal in accordance with an output of the power converter. A compensation circuit is coupled to a transformer of the power converter for generating a compensation signal in response to a transformer signal generated by the transformer. The feedback signal is applied to generate a switching signal for switching the transformer and regulating the output of the power converter. The compensation signal is coupled to modulate the feedback signal for compensating the voltage drop of the output cable, the output connector, etc. of the power converter.

Further, the error amplifier includes a reference signal for generating the feedback signal. The compensation signal is coupled to compensate the reference signal for modulating the feedback signal. The regulation circuit further includes a resistor coupled to the compensation circuit for programming the level of the compensation signal. The transformer signal is related to an on time of the switching signal and the level of an input voltage of the transformer. The compensation signal is thus generated in accordance with a demagnetizing time of the transformer. Therefore, the compensation signal is increased in response to the increase of an output current of the power converter. The feedback signal is increased in response to the increase of the compensation signal. Because an output voltage of the power converter is increased in response to the increase of the feedback signal, the output voltage can be increased in response to the increase of the output current for the output cable compensation.

A method for compensating the output cable of the power converter according to the present invention comprises generating a feedback signal in accordance with an output of the power converter; generating a compensation signal in response to a transformer signal generated by a transformer of the power converter; and modulating the feedback signal by using the compensation signal. The feedback signal is utilized to generate a switching signal for switching the transformer of the power converter and regulating the output of the power converter. The compensation signal is coupled to compensate a voltage drop of the output cable of the power converter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a prior art circuit schematic of a power converter.

FIG. 2 is a circuit diagram of an embodiment of a power converter according to the present invention.

FIG. 3 is a circuit diagram of an embodiment of a regulation circuit with output cable compensation according to the present invention.

FIG. 4 is a circuit diagram of an embodiment of a compensation circuit for generating a compensation signal according to the present invention.

FIG. 5 shows a reference circuit of a voltage-to-current converter according to the present invention.

FIG. 6 shows signal waveforms of an on-time signal S_ON, a sample signal S₁ and a clear signal S₂ according to the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 2 is a circuit diagram of an embodiment of a power converter according to the present invention. The power converter comprises the transformer 10, the power transistor 20, the PWM controller (PWM) 30, and the opto-coupler 60. The power transistor 20 is coupled from the primary winding N_P of the transformer 10 to the ground for switching the transformer 10. The opto-coupler 60 is coupled to the secondary winding N_S of the transformer 10 through the resistor 62. The secondary winding N_S is coupled to the output terminal of the power converter to provide the output voltage V_O for the load through the output cable 70. The output current I_O of the power converter flows through the output cable 70. The output capacitor 45 is coupled to the secondary winding N_S and the output terminal of the power converter. The power converter has a diode 76 for rectification. The diode 76 is coupled from the output terminal of the power converter to the secondary winding N_S.

The power converter further includes a regulation circuit (REG) 100 for generating a first feedback signal V_F. The regulation circuit 100 is located in the secondary side of the transformer 10. The regulation circuit 100 is coupled to the output terminal of the power converter via the voltage divider developed by the resistors 51 and 52. The voltage divider generates a voltage V_A coupled to the regulation circuit 100. A resistor 115 is coupled to a terminal R_P of the regulation circuit 100 to determine the ratio of signal generation.

The first feedback signal V_F is further coupled to generate the second feedback signal V_FB through the opto-coupler 60. The PWM controller 30 generates the switching signal S_PWM to switch the transformer 10 via the power transistor 20 in accordance with the feedback signal V_FB for regulating the output (the output voltage V_O and/or the output current I_O) of the power converter. Therefore, the feedback signal V_F is applied to generate the switching signal S_PWM for switching the transformer 10 and regulating the output of the power converter. The feedback signals V_F and V_FB are generated in accordance with the output (the output voltage V_O and/or the output current I_O) of the power converter. The feedback signal V_FB is increased in response to the increase of the output current I_O. Because the output voltage V_O is increased in response to the increase of the feedback signal V_FB, the output voltage V_O can be increased in response to the increase of the output current I_O for the output cable compensation.

However, if we use a resistance device to sense the output current I_O, then this resistance device will result a power loss and cause lower power efficiency for the power converter. Therefore, the regulation circuit 100 is developed and coupled to the transformer 10 for detecting a transformer signal through a voltage divider developed by resistors 57 and 58. The transformer signal is generated by the transformer 10. A first terminal of the resistor 57 is coupled to the secondary winding N_S and the diode 76. The resistor 58 is coupled from a second terminal of the resistor 57 to a ground. The ground is located in the secondary side of the transformer 10. The resistors 57 and 58 produce a sense signal V_S. The sense signal V_S is related to the transformer signal. The transformer signal can be utilized to estimate the level of the output current I_O. The transformer signal is related to the level of the input voltage V_IN of the transformer 10 and the on time T_ON of the switching signal S_PWM.

$\begin{array}{cc}{T}_{\mathrm{discharge}}=\frac{V_M}{V_O}\times T_{\mathrm{charge}}& \left(1\right)\end{array}$

where the T-charge is the magnetizing time of the transformer 10; the T-charge is thus equal to the on time T_ON of the switching signal S_PWM; T-discharge is the demagnetizing time of the transformer 10. The V_M is the magnetizing voltage that is correlated to the input voltage V_IN of the transformer 10.

Thus, the equation (1) can be rewritten as equation (2),

$\begin{array}{cc}{T}_{\mathrm{discharge}}=\frac{K\times V_{\mathrm{IN}}}{V_O}\times T_{\mathrm{ON}}& \left(2\right)\end{array}$

where K is a constant.

Refer to an output power P_O of the flyback power converter, it can be expressed as,

$\begin{array}{cc}{P}_O=V_{O\times }I_O=\frac{V_{\mathrm{IN}}^2\times T_{\mathrm{ON}}^2}{2\times L_P\times T}& \left(3\right)\end{array}$

where L_P is the inductance of the primary winding N_P of the transformer 10; T is the switching period of the switching signal S_PWM.

In accordance with the equations (2) and (3), if the output voltage V_O is a fixed value, then the demagnetizing time T-discharge and the output current I_O are correlated to “the input voltage V_IN of the transformer 10” and “the on time T_ON of the switching signal S_PWM”. Therefore, the input voltage V_IN of the transformer 10 and the on time T_ON of the switching signal S_PWM can be used instead of the output current I_O to control the output voltage V_O for the output cable compensation.

FIG. 3 is a circuit diagram of an embodiment of the regulation circuit 100 according to the present invention. It includes a compensation circuit (S/I) 200 that is coupled to the secondary winding N_S of the transformer 10 (as shown in FIG. 2) to receive the sense signal V_S for generating a compensation signal I_COMP in accordance with the sense signal V_S. The compensation signal I_COMP is generated in the secondary side of the transformer 10. The sense signal V_S is correlated to the input voltage V_IN of the transformer 10 and the on time of the switching signal S_PWM (as shown in FIG. 2). As mentioned above, the demagnetizing time of the transformer 10 and the output current I_O are correlated to the input voltage V_IN of the transformer 10 and the on time of the switching signal S_PWM, and therefore the compensation signal I_COMP is generated in accordance with the demagnetizing time of the transformer 10.

The resistor 115 is coupled to the compensation circuit 200 via the terminal R_P to determine the ratio of signal generation. The resistor 115 is used for programming the level of the compensation signal I_COMP. The compensation signal I_COMP generates a compensation voltage at a resistor 117. A reference voltage V_R1 is connected in serial with the resistor 117 via a buffer amplifier 110. The reference voltage V_R1 is supplied with a positive input terminal of the buffer amplifier 110. The resistor 117 is coupled from an output terminal of the compensation circuit 200 to an output terminal of the buffer amplifier 110. A negative input terminal of the buffer amplifier 110 is coupled to the output terminal of the buffer amplifier 110 and the resistor 117. The reference voltage V_R1 associates with the compensation voltage at the resistor 117 to generate a reference signal V_REF for the error amplifier 170. The reference signal V_REF can be expressed as,

V _REF =V _R1+(I_COMP×R₁₁₇)  (4)

According to equation (4), the reference signal V_REF is correlated to the compensation signal I_COMP. Therefore, the reference signal V_REF can be programmed and compensated by the compensation signal I_COMP. The reference signal V_REF can be programmed in response to the input voltage V_IN of the transformer 10 and the on time of the switching signal S_PWM (as shown in FIG. 2) due to the compensation signal I_COMP is correlated to the sense signal V_S, and the sense signal V_S is correlated to the input voltage V_IN of the transformer 10 and the on time of the switching signal S_PWM. Further, according to equation (4), the reference signal V_REF is further correlated to the reference voltage V_R1 of the buffer amplifier 110. Therefore, the buffer amplifier 110 is coupled to the compensation signal I_COMP for generating the reference signal V_REF.

A capacitor 175 is used for the frequency compensation of the feedback loop of the power converter. The voltage V_A is generated from the output voltage V_O through the resistors 51 and 52 (as shown in FIG. 2). The error amplifier 170 is coupled to receive the reference signal V_REF and the voltage V_A to generate the feedback signal V_F. That is, the error amplifier 170 generates the feedback signal V_F in accordance with the output of the power converter and the reference signal V_REF for generating the feedback signal V_FB (as shown in FIG. 2). Accordingly, the feedback signal V_F is related to the output voltage V_O. The feedback signal V_F is modulated in response to the change of the compensation signal I_COMP because the compensation signal I_COMP is used to compensate the reference signal V_REF. In other words, the compensation signal I_COMP is coupled to modulate the feedback signal V_F for compensating the voltage drop of the output cable 70 (as shown in FIG. 2) of the power converter. A positive input terminal and a negative input terminal of the error amplifier 170 receive the reference signal V_REF and the voltage V_A, respectively. An output terminal of the error amplifier 170 generates the feedback signal V_F. The capacitor 175 is coupled between the negative input terminal of the error amplifier 170 and the output terminal of the error amplifier 170.

FIG. 4 is a circuit diagram of an embodiment of the compensation circuit 200 according to the present invention. The sense signal V_S is coupled to a voltage-to-current converter (V/I) 280 to generate a charging current I_IN in accordance with the level (amplitude) of the sense signal V_S. The charging current I_IN is correlated to the level of the input voltage V_IN of the transformer 10 (as shown in FIG. 2). The sense signal V_S is further coupled to a comparator 210 to generate an on-time signal S_ON when the level of the sense signal V_S is higher than a threshold V_R2. The on-time signal S_ON is related to the on time T_ON of the switching signal S_PWM (as shown in FIG. 2). A positive input terminal and a negative input terminal of the comparator 210 receive the sense signal V_S and the threshold V_R2, respectively. An output terminal of the comparator 210 outputs the on-time signal S_ON.

Through a switch 231, the charging current I_IN is coupled to charge a capacitor 250 for generating a signal V_T1. The signal V_T1 is a voltage and utilized to generate the compensation signal I_COMP. The on/off of the switch 231 is controlled by the on-time signal S_ON. Therefore, the charging current I_IN charges the capacitor 250 in response to the sense signal V_S for generating the signal V_T1. The on-time signal S_ON is further coupled to a pulse generator 215 for generating a sample signal S₁ and a clear signal S₂. The waveforms of the on-time signal S_ON, the sample signal S₁ and the clear signal S₂ are shown in FIG. 6. The sample signal S₁ is enabled when the on-time signal S_ON is disabled. Once the sample signal S₁ is disabled, the clear signal S₂ is enabled after a delay time. The clear signal S₂ is applied to enable a switch 233 coupled between the capacitor 250 and the ground for discharging the capacitor 250.

A switch 232 is coupled between the capacitor 250 and a capacitor 270. The sample signal S₁ is utilized to enable the switch 232 for sampling the signal V_T1 to the capacitor 270. Thus, the capacitor 270 produces a signal V_T2. Because the signal V_T1 is generated in accordance with the charging current I_IN and the period of on-time signal S_ON, the level of the signals V_T1 and V_T2 are correlated to “the level of the input voltage V_IN of the transformer 10” and “the period of the on time T_ON of the switching signal S_PWM”.

The signal V_T2 is coupled to an operational amplifier 300. The operational amplifier 300, the resistor 115, transistors 310, 311, and 312 develop a voltage-to-current converter for converting the signal V₇₂ to the compensation signal I_COMP. The capacitor 270 is coupled to a positive input terminal of the operational amplifier 300. A negative input terminal of the operational amplifier 300 is coupled to a source of the transistor 310 and the resistor 115 through the terminal R_P. The source of the transistor 310 is coupled to the resistor 115 through the terminal R_P. A gate of the transistor 310 is controlled by an output terminal of the operational amplifier 300. A current I₃₁₀ is generated at a drain of the transistor 310 in response to the signal V_T2. The current I₃₁₀ is further coupled to a current mirror formed by the transistors 311 and 312. The current mirror generates the compensation signal I_COMP. Sources of the transistors 311 and 312 are coupled to a supply voltage V_CC. Gates of the transistors 311 and 312 and drains of the transistors 310 and 311 are coupled together. A drain of the transistor 312 generates the compensation signal I_COMP.

The resistor 115 is used for programming the level of the compensation signal I_COMP. A current source 315 is connected between the supply voltage V_CC and the drain of the transistor 310 for providing an offset current I₃₁₅. Therefore, the signal V_T2 must be higher than a specific value for generating the compensation signal I_COMP. This specific value is related to the offset current I₃₁₅.

The compensation signal I_COMP is generated in accordance with the demagnetizing time (T-discharge) of the transformer 10 (as shown in FIG. 2), which is shown in equations (1) and (2). Therefore, the compensation signal I_COMP is increased in response to the increase of the output current I_O of the power converter. The compensation signal I_COMP is coupled to compensate the reference signal V_REF for modulating the feedback signals V_F and V_FB (as shown in FIG. 2 and FIG. 3). The feedback signal V_FB is thus increased in response to the increase of the compensation signal I_COMP. Because the output voltage V_O is increased in response to the increase of the feedback signal V_FB, the output voltage V_O can be increased in response to the increase of the output current I_O for the output cable compensation.

FIG. 5 shows a reference circuit of the voltage-to-current converter 280 according to the present invention. The voltage-to-current converter 280 comprises an operational amplifier 281, a transistor 282, a resistor 283 and a current mirror developed by transistors 286 and 287. A positive input terminal of the operational amplifier 281 receives the sense signal V_S. A negative input terminal of the operational amplifier 281 is coupled to a source of the transistor 282. An output terminal of the operational amplifier 281 is coupled to a gate of the transistor 282. The resistor 283 is coupled between the negative input terminal of the operational amplifier 281 and the ground. A current I₂₈₂ is generated at a drain of the transistor 282. A drain of the transistor 286 is coupled to receive the current I₂₈₂. Gates of the transistors 286 and 287 are coupled each other and they all are coupled to the drains of the transistors 286 and 282. Sources of the transistors 286 and 287 are coupled to the supply voltage V_CC. The charging current I_IN is generated at a drain of the transistor 287 in response to the current I₂₈₂.

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.

Claims

1. A regulation circuit of a power converter, comprising: an error amplifier generating a feedback signal in accordance with an output of the power converter; anda compensation circuit coupled to a transformer of the power converter for generating a compensation signal in response to a transformer signal generated by the transformer;wherein the feedback signal is applied to generate a switching signal for switching the transformer and regulating the output of the power converter; the compensation signal is coupled to modulate the feedback signal for compensating a voltage drop of an output cable of the power converter.

2. The regulation circuit as claimed in claim 1, wherein the error amplifier comprises a reference signal for generating the feedback signal; the compensation signal is coupled to compensate the reference signal for modulating the feedback signal.

3. The regulation circuit as claimed in claim 1, further comprising: a resistor coupled to the compensation circuit for programming the level of the compensation signal.

4. The regulation circuit as claimed in claim 1, wherein the transformer signal is related to an on time of the switching signal and the level of an input voltage of the transformer.

5. The regulation circuit as claimed in claim 1, wherein the compensation signal is generated in accordance with a demagnetizing time of the transformer.

6. The regulation circuit as claimed in claim 1, wherein the compensation signal is increased in response to the increase of an output current of the power converter; the feedback signal is increased in response to the increase of the compensation signal; an output voltage is increased in response to the increase of the feedback signal.

7. The regulation circuit as claimed in claim 1, wherein the regulation circuit is located in a secondary side of the transformer.

8. The regulation circuit as claimed in claim 1, wherein the compensation circuit comprises: a capacitor providing a voltage for generating the compensation signal;a charging current charging the capacitor in response to the transformer signal for providing the voltage, the charging current is correlated to the transformer signal; anda voltage to current converter converting the voltage to the compensation signal;wherein the transformer signal is related to an on time of the switching signal and the level of an input voltage of the transformer.

9. A method for compensating an output cable of a power converter, comprising: generating a feedback signal in accordance with an output of the power converter;generating a compensation signal in response to a transformer signal generated by a transformer of the power converter; andmodulating the feedback signal by using the compensation signal;wherein the feedback signal is utilized to generate a switching signal for switching the transformer of the power converter and regulating the output of the power converter; the compensation signal is coupled to compensate a voltage drop of the output cable of the power converter.

10. The method as claimed in claim 9, wherein the feedback signal is generated by an error amplifier; the error amplifier further comprises a reference signal for generating the feedback signal; the compensation signal is coupled to compensate the reference signal for modulating the feedback signal.

11. The method as claimed in claim 9, further comprising: programming the level of the compensation signal by a resistor.

12. The method as claimed in claim 9, wherein the transformer signal is related to an on time of the switching signal and the level of an input voltage of the transformer.

13. The method as claimed in claim 9, wherein the compensation signal is generated in accordance with a demagnetizing time of the transformer.

14. The method as claimed in claim 9, wherein the compensation signal is generated in a secondary side of the transformer.

15. A regulation circuit of a power converter, comprising: a compensation circuit coupled to modulate a feedback signal in response to a transformer signal generated by a transformer of the power converter;wherein the feedback signal is applied to generate a switching signal for switching the transformer and regulating an output of the power converter; the transformer signal is related to an on time of the switching signal and the level of an input voltage of the transformer; the feedback signal is related to an output voltage of the power converter; the output voltage is increased in response to the increase of an output current of the power converter for compensating a voltage drop of an output cable of the power converter.

16. The regulation circuit as claimed in claim 15, wherein the regulation circuit is located in a secondary side of the transformer.