PROGRAMMABLE CURRENT LIMIT CIRCUIT FOR A PROGRAMMABLE POWER SUPPLY

Abstract

A control circuit of a power supply is provided. The control circuit includes a circuit and a PWM circuit. The circuit generates a limit signal in response to an output voltage of the power supply for limiting a switching current of a transformer of the power supply. The PWM circuit generates a switching signal in response to a feedback signal and the limit signal for switching the transformer and regulating the output voltage of the power supply. A level of the feedback signal is related to a level of the output voltage of the power supply. The output voltage of the power supply is programmable.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a programmable current limit for a power supply, and more particular to a power supply with a programmable output voltage (variable output voltage).

2. Description of the Related Art

Technique of current limits is utilized to meet safety requirements. The detail skill of current limits for a power supply can be found in the prior arts of U.S. Pat. No. 6,611,439 titled “PWM controller for controlling output power limit of a power supply”, U.S. Pat. No. 6,674,656 titled “PWM controller having a saw-limiter for output power limit without sensing input voltage”, U.S. Pat. No. 6,721,192 titled “PWM controller regulating output voltage and output current in primary side”, and U.S. Pat. No. 7,054,170 titled “Power-mode controlled power converter”.

BRIEF SUMMARY OF THE INVENTION

An exemplary embodiment of a control circuit of a power supply is provided. The control circuit comprises a circuit and a PWM circuit. The circuit generates a limit signal in response to an output voltage of the power supply for limiting a switching current of a transformer of the power supply. The PWM circuit generates a switching signal in response to a feedback signal and the limit signal for switching the transformer and regulating the output voltage of the power supply. A level of the feedback signal is related to a level of the output voltage of the power supply. The output voltage of the power supply is programmable.

An exemplary embodiment of a method of generating an output voltage of a power supply is provided. The method comprises the steps of generating a limit signal in response to an output voltage of the power supply for limiting a switching current of a transformer of the power supply; and generating a switching signal in response to a feedback signal and the limit signal for switching the transformer and regulating the output voltage of the power supply. A level of the feedback signal is related to the level of the output voltage of the power supply. The output voltage of the power supply is programmable.

A detailed description is given in the following embodiments with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein:

FIG. 1 shows an exemplary embodiment of a power supply in accordance with the present invention;

FIG. 2 shows an exemplary embodiment of a control circuit of the power supply in FIG. 1;

FIG. 3 shows an exemplary embodiment of a circuit of the control circuit in FIG. 2;

FIG. 4 shows an exemplary embodiment of a PWM circuit of the control circuit in FIG. 2; and

FIG. 5 shows a curve of a current-limit signal versus an output voltage of the power supply in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined

FIG. 1 is an exemplary embodiment of a power supply in accordance with the present invention. A transformer 10 is coupled between an input voltage V_IN of the power supply and a transistor 20. The transistor 20 is coupled to switch the transformer 10 for transforming a power. The power is related to the input voltage V_IN. The transformer 10 includes a primary winding N_P, a second winding Ns and an auxiliary winding N_P. When the transformer 10 is switched, the power is transformed from the primary winding N_P to the second winding Ns, and an output voltage V_O of the power supply is generated. In other words, the secondary winding N_S of the transformer 10 will generate the output voltage V_O through a rectifier 40 and a capacitor 45 when the transformer 10 is switched. A control circuit 100 generates a switching signal S_W coupled to drive the transistor 20 for regulating the output voltage V_O of the power supply. When the transistor 20 is turned on, a switching current of the transformer 10 will generate a switching current signal CS via a resistor 25. The switching current signal CS is coupled to the control circuit 100. Resistors 31 and 32 are coupled to the auxiliary winding N_A of the transformer 10 for detecting a reflected signal V_S. The reflected signal V_S is coupled to the control circuit 100. The reflected signal V_S represents a reflected voltage of the transformer 10. The level of the reflected signal V_S is related to the level of the output voltage V_O during the demagnetizing period of the transformer 10. Therefore, an output-voltage signal E_O (shown in FIG. 2) is produced according to the reflected signal V_S. In other words, the output-voltage signal E_O is correlated to the level of the output voltage V_O of the power supply.

An error amplifier 50 is coupled to receive the output voltage V_O via resistors 51 and 52. The error amplifier 50 has a reference voltage 70 (V_R). The output of the error amplifier 50 generates a feedback signal V_FB coupled to the control circuit 100 through an opto-coupler 80. Therefore, the control circuit 100 will generate the switching signal S_W according to the feedback signal V_FB to regulate the output voltage V_O as shown in Equation (1).

$\begin{array}{cc}{V}_O=\frac{R_{51}+R_{52}}{R_{52}}\times V_R& \left(1\right)\end{array}$

The reference voltage 70 (V_R) is programmable for determining the output voltage V_O of the power supply. Furthermore, the resistor 52 is adjustable for programming the output voltage V_O. A resistor 56 and a capacitor 57 are coupled to the error amplifier 50 for feedback loop compensation. The capacitance of the capacitor 57 will determine the bandwidth of the feedback loop for the regulation of the output voltage V_O.

FIG. 2 is an exemplary embodiment of the control circuit 100 in accordance with the present invention. In the control circuit 100, a level-shift circuit includes a transistor 120 and resistors 110, 125, and 126. The level-shift circuit is coupled to receive the feedback signal V_FB for generating a signal V_B. In other words, the level of the signal V_B is related to the level of the feedback signal V_FB. The signal V_B, the switching current signal CS, and a limit signal I_X are coupled to a pulse width modulation circuit (PWM) 300 for generating the switching signal S_W. A circuit 200 generates the limit signal I_X in accordance with the output-voltage signal E_O, an input-voltage signal E_IN, and an initial-timing signal S_T.

A sample-hold circuit (S/H) 150 receives the reflected signal V_S for generating the output-voltage signal E_O and the input-voltage signal E_IN. The output-voltage signal E_O is correlated to the output voltage V_O. The input-voltage signal E_IN represents the input voltage V_IN. The detail skill of sampling the reflected voltage V_S of the transformer 10 and generating the output-voltage signal E_O can be found in the prior arts of U.S. Pat. No. 7,349,229 titled “Causal sampling circuit for measuring reflected voltage and demagnetizing time of transformer”, and U.S. Pat. No. 7,486,528 titled “Linear-predict sampling for measuring demagnetized voltage of transformer”. The detail approach for generating the input-voltage signal E_IN through the detection of the transformer's voltage can be found in the prior art of U.S. Pat. No. 7,671,578 titled “Detection circuit for sensing the input voltage of transformer”.

FIG. 3 is an exemplary embodiment of the circuit 200 in accordance with the present invention. The output-voltage signal E_O generates a signal V_G through a voltage divider. The voltage divider is formed by resistors 251, 252 and 253. The enabling/disabling of the resistor 253 is controlled by a transistor 265. A comparator 260 will turn on the transistor 265 once the input-voltage signal E_IN is higher than a threshold V_T1. In other words, when the input-voltage signal E_IN is higher than the threshold V_T1, the resistor 253 is enabled; and once the input-voltage signal E_IN is lower than the threshold V_T1, the resistor 253 is disabled. Therefore, when the input-voltage signal E_IN is lower than the threshold V_T1, the signal V_G is shown as,

$\begin{array}{cc}{V}_G=\frac{R_{252}}{R_{251}+R_{252}}\times E_O& \left(2\right)\end{array}$

When the input-voltage signal E_IN is higher than the threshold V_T1, the signal V_G can be expressed as,

$\begin{array}{cc}{V}_G=\frac{R_P}{R_{251}+R_P}\times E_O& \left(3\right)\\ R_P=\frac{R_{252}\times R_{253}}{R_{252}+R_{253}}& \left(4\right)\end{array}$

An amplifier 210, a resistor 215, and a transistor 220 will generate a current I_AG according to the signal V_G.

$\begin{array}{cc}{I}_{\mathrm{AG}}=\frac{V_G}{R_{215}}& \left(5\right)\\ I_X=\left(K\times I_{\mathrm{AG}}\right)+\left(K_0\times I_{240}\right)& \left(6\right)\end{array}$

As shown in Equation (6), the limit signal I_X is generated according to the current I_AG, wherein K and K₀ are constant and related to a ratio of current mirrors formed by transistors 231, 232, 234, 235, 238, and 239; I₂₄₀ is a current of a current source 240. The maximum value of the limit signal I_X is clamped by a current source I_MAX. The current source 240 is enabled by a switch 245. The switch 245 is controlled by the initial-timing signal S_T. That is to say the initial-timing signal S_T will enable the current source 240 when the power supply is turned on and during the output voltage V_O is programmed to a higher value. Therefore, the value of the limit signal I_X is determined by the output-voltage signal E_O, the input-voltage signal E_IN, and the initial-timing signal S_T. In detailed, When the switch 245 is turned off by the initial-timing signal S_T, the mirror composed of the transistors 238 and 239 mirrors only the current I₂₃₅ flowing through the transistor 235 to generate the limit signal I_X. When the output voltage rises (that is during the power-on period of the power supply), the switch 245 is turned on by the enabled initial-timing signal S_T. At this time, the mirror composed of the transistors 238 and 239 mirrors the currents. The currents sum of the currents I₂₃₅ and I₂₄₀ to generate the limit signal I_X. Thus, according to the above description, the level of the limit signal I_X is increased during the power-on period of the power supply.

FIG. 4 is an exemplary embodiment of the pulse width modulation circuit (PWM) circuit 300 according to the present invention. An oscillator (OSC) 400 will generate a clock signal CLK and a ramp signal RMP. The clock signal CLK is coupled to clock a flip-flop 420 via an inverter 410 for generating the switching signal S_W through an AND gate 425. An adder 411 will generate a signal S_P in response to the ramp signal RMP and the switching current signal CS. The signal S_P is coupled to a comparator 415. The comparator 415 will reset the flip-flop 420 and turn off the switching signal S_W via an AND gate 419 when the feedback signal V_B is lower than the signal S_P. The limit signal I_X and a current source I_MIN are coupled to a resistor 35 to develop the current-limit signal V_LMT.

According to the operation of the limit signal I_X, the current source I_MIN, and the resistor 35 in FIG. 4, the current-limit signal V_LMT represents the limit signal I_X. In other words, the current-limit signal V_LMT is in the direct proportion to the limit signal I_X. So that, the current-limit signal V_LMT is coupled to limit the switching current of the transformer 10. A comparator 416 is coupled to reset the flip-flop 420 and turn off the switching signal S_W via the AND gate 419 when the switching current signal CS is higher than the current-limit signal V_LMT.

FIG. 5 shows a curve of the current-limit signal V_LMT versus the output voltage V_O of the power supply. When the level of the output voltage V_O is programmed to a higher value (e.g. 20V), the current-limit signal V_LMT will be set to a higher value (e.g. 1V). If the level of the output voltage V_O is programmed to be a lower voltage (e.g. 5V), then the current-limit signal V_LMT will be automatically set as be lower level (e.g. 0.5V) to meet the safety requirement. As described in above, the current-limit signal V_LMT represents the limit signal I_X. In other words, the level of the limit signal I_X is decreased in response to the decrease of the level of the output voltage V_O.

Besides, when the level of the input voltage V_IN is higher, the current-limit signal V_LMT will be automatically adjusted to a lower value. In other words, the level of the limit signal I_X is decreased in response to the increase of the level of the input voltage V_IN.

While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.

Claims

1. A control circuit of a power supply comprising: a circuit generating a limit signal in response to an output voltage of the power supply for limiting a switching current of a transformer of the power supply; anda PWM circuit generating a switching signal in response to a feedback signal and the limit signal for switching the transformer and regulating the output voltage of the power supply,wherein a level of the feedback signal is related to a level of the output voltage of the power supply, and the output voltage of the power supply is programmable.

2. The control circuit as claimed in claim 1, wherein the limit signal is generated in response to an input voltage of the power supply.

3. The control circuit as claimed in claim 1 further comprising: a sample-and-hold circuit to sample a reflected voltage of the transformer for generating an output-voltage signal,wherein the output-voltage signal represents the output voltage of the power supply.

4. The control circuit as claimed in claim 1 further comprising: a sample-and-hold circuit coupled to the transformer for generating an input-voltage signal,wherein the input-voltage signal represents an input voltage of the power supply.

5. The control circuit as claimed in claim 1, wherein a level of the limit signal is decreased in response to decrease a level of the output voltage of the power supply.

6. The control circuit as claimed in claim 1, wherein the control circuit decreases a level of the limit signal when an input voltage is increased.

7. The control circuit as claimed in claim 1, wherein a level of the limit signal is increased during a power-on period of the power supply.

8. The control circuit as claimed in claim 1, wherein a level of the limit signal is increased during a period when the output voltage is programmed to a higher value.

9. A method of generating an output voltage of a power supply, comprising: generating a limit signal in response to an output voltage of the power supply for limiting a switching current of a transformer of the power supply; andgenerating a switching signal in response to a feedback signal and the limit signal for switching the transformer and regulating the output voltage of the power supply,wherein a level of the feedback signal is related to the level of the output voltage of the power supply, and the output voltage of the power supply is programmable.

10. The method as claimed in claim 9, wherein the limit signal is generated in response to an input voltage of the power supply.

11. The method as claimed in claim 9 further comprising: generating an output-voltage signal by sampling a reflected voltage of the transformer, wherein the output-voltage signal represents the output voltage of the power supply; andgenerating an input-voltage signal by sampling the reflected voltage of the transformer, wherein the input-voltage signal represents an input voltage of the power supply.

12. The method as claimed in claim 9, wherein a level of the limit signal is decreased in response to decrease a level of the output voltage of the power supply.

13. The method as claimed in claim 9, wherein a level of the limit signal is decreased when an input voltage is increased.

14. The method as claimed in claim 9, wherein a level of the limit signal is increased during a power-on period of the power supply.

15. The method as claimed in claim 9, wherein a level of the limit signal is increased during a period when the output voltage is programmed to a higher value.