Control method and circuit with indirect input voltage detection by switching current slope detection

Abstract

The present invention provides a method and a control circuit to detect an input voltage for the control and protections of a power converter. It includes a current sense circuit for generating a current signal in response to a switching current of a transformer. A detection circuit is coupled to sense the current signal for generating a slope signal in response to a slope of the current signal. A protection circuit is further developed to control the switching signal in accordance with the slope signal. The level of the slope signal is corrected correlated to the input voltage of the power converter.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to power converters, and more specifically relates to the control of switching power converters.

2. Description of Related Art

Switching power converters have been widely used to provide regulated voltage and current. A transformer (an inductive device) is used in the power converter for energy store and power transfer. FIG. 1 shows a circuit schematic of a traditional power converter. A controller 15 generates a switching signal S_W at an output terminal OUT to regulate the output of the power converter in response to a feedback signal V_FB. In general, the feedback signal V_FB is obtained at a feedback terminal FB of the controller 15 by detecting the output voltage V_O of the power converter through an optical-coupler or a feedback circuit including an auxiliary winding (Figure not shown).

The switching signal S_W drives a power transistor 12 for switching a transformer 10. The transformer 10 is connected to an input voltage V_IN of the power converter. The energy of the transformer 10 is transferred to the output voltage V_O of the power converter through a rectifier 17 and a capacitor 18. A resistor R_S is connected serially with the power transistor 12 to generate a current signal V_I in response to a switching current I_P of the transformer 10. The current signal V_I is coupled to a current-sense terminal VS of the controller 15 for the control and protections of the power converter. A resistor 19 is further connected from the input voltage V_IN to an input terminal IN of the controller 15 for over-voltage and under-voltage protections, etc.

Furthermore, the over-power protection of power converter requires sensing the input voltage V_IN to control the maximum output power as a constant. The approach was disclosed as “PWM controller for controlling output power limit of a power supply” by Yang et al., U.S. Pat. No. 6,611,439. The drawback of this prior art is the power loss caused by the resistor 19 especially when the input voltage V_IN is high. The object of the present invention is to sense the input voltage V_IN for the control and protections without the need of the resistor 19 for saving power. Moreover, reducing input terminals of the controller 15 is another object of the present invention.

SUMMARY OF THE INVENTION

The present invention provides a method and a control circuit to detect an input voltage for the control and protections of a power converter. It includes a current sense circuit to generate a current signal in response to a switching current of a transformer. The transformer is operated as an inductive device. A detection circuit is coupled to sense the current signal for generating a slope signal in response to a slope of the current signal. When a power transistor of the power converter is turned on, the detection circuit will sample the current signal during a first period to generate a first signal. After that, sampling the current signal during a second period will generate a second signal. The slope of the current signal is determined in accordance with the first signal and the second signal. A protection circuit is further utilized to control the switching signal in accordance with the slope signal. The level of the slope signal is corrected correlated to the input voltage V_IN of the power converter. The slope signal V_SD is increased in response to the increase of the input voltage V_IN.

FIG. 8 shows the schematic circuit diagram of the pulse generator 230. The pulse generator 230 comprises a constant current-source 232, a transistor 231, a capacitor 235 and an NOR gate 236 to produces the pulse signal S_P in response to the falling edge of the oscillation signal IPS. The gate of the transistor 231 is coupled to receive the oscillation signal IPS. The oscillation signal IPS is used to control the transistor 231. The source of the transistor 231 is coupled to the ground. The constant current-source 232 is coupled between the drain of the transistor 231 and the supply voltage V_CC. The capacitor 235 is coupled from the drain of the transistor 231 to the ground. The input terminals of the NOR gate 236 are coupled to the capacitor 235 and the oscillation signal IPS respectively. The pulse signal S_P is generated at the output terminal of the NOR gate 236. The constant current-source 232 is used to charge the capacitor 235 when the transistor 231 is turned off in response to the falling edge of the oscillation signal IPS. The pulse signal S_P is enabled during charging the capacitor 235. The current of the constant current-source 232 and the capacitance of the capacitor 235 determine the pulse width of the pulse signal S_P.

FIG. 9 shows signal-waveforms. The oscillation circuit 100 generates the timing signals S₁ and S₂ in accordance with threshold voltages V₁ and V₂ respectively (shown in the FIG. 5). The first timing signal S₁ includes the first period T₁. The timing signal S₂ has the second period T₂. The detection circuit 210 samples the current signal V_I during the first period T₁ generates the first signal (shown in the FIG. 7). Sampling the current signal V_I during the second period T₂ generates the second signal. The slope signal V_SD is determined in accordance with differential voltage of the first signal and the second signal.

FIG. 10 shows the circuit diagram of the signal generation circuit 250. It includes a V_IN signal generator 300 and a protection signal generator 350. The V_IN signal generator 300 has an operational amplifier 310 coupled to amplify the slope signal V_SD for generating the input-voltage signal V_V. The positive input terminal of the operational amplifier 310 is coupled to receive the slope signal V_SD. A resistor 315 is coupled between the negative input terminal of the operational amplifier 310 and the ground. A resistor 316 is coupled from the negative input terminal of the operational amplifier 310 to the output terminal of the operational amplifier 310. Resistors 315 and 316 determine the gain of the amplification.

The protection signal generator 350 serves as a protection circuit to control the switching signal S_W in response to the input-voltage signal V_V. The protection signal generator 350 includes comparators 320, 325 and operational amplifiers 330, 340 coupled to receive the input-voltage signal V_V. A resistor 335 is coupled between the negative input terminal of the operational amplifier 330 and the input-voltage signal V_V. A resistor 336 is coupled from the negative input terminal of the operational amplifier 330 to the output terminal of the operational amplifier 330. Resistors 335 and 336 determine the gain for operational amplifier 330. A resistor 345 is coupled between the negative input terminal of the operational amplifier 340 and the input-voltage signal V_V. A resistor 346 is coupled from the negative input terminal of the operational amplifier 340 to the output terminal of the operational amplifier 340. Resistors 345 and 346 determine the gain for operational amplifier 340. A reference voltage V_R connects the positive input terminals of the operational amplifiers 330 and 340.

Threshold voltages V_TH and V_TL are coupled to comparators 320 and 325 respectively. The over-voltage threshold V_TH is coupled to the positive input terminal of the over-voltage comparator 320. The negative input terminal of the over-voltage comparator 320 is coupled to receive the input-voltage signal V_V. The over-voltage comparator 320 is used to detect the over-voltage of the input-voltage signal V_V. The over-voltage comparator 320 generates an over-voltage signal when the input-voltage signal V_V is higher than the over-voltage threshold V_TH. The under-voltage threshold V_TL is coupled to the negative input terminal of the under-voltage comparator 325. The positive input terminal of the under-voltage comparator 325 is coupled to receive the input-voltage signal V_V. The under-voltage comparator 325 is used to detect the under-voltage of the input-voltage signal V_V. The under-voltage comparator 325 generates an under-voltage signal when the input-voltage signal V_V is lower than the under-voltage threshold V_TL. Input terminals of an AND gate 360 are connected to the output terminals of the comparators 320 and 325. The output terminal of the AND gate 360 generates the control signal ENB through a delay circuit 370. The delay circuit 370 provides a time delay for the disable of the control signal ENB when the over-voltage or the under-voltage of the input-voltage signal V_V is occurred.

The operational amplifier 330 serves as a current-limit adjustment circuit for adjusting a current limit of the transformer 30 (shown in the FIG. 2) in response to the input-voltage signal V_V. It is also adjusting the current limit of the power transistor 20 (shown in the FIG. 2). The operational amplifier 330 produces the current-limit signal V_M to disable the switching signal S_W for limiting the switching current I_P (shown in the FIG. 2) The operational amplifier 340 serves as a signal adjustment circuit and generates the blanking adjustment signal V_B for adjusting the blanking time of the switching signal S_W in response to the input-voltage signal V_V. The current-limit signal V_M is decreased in response to the increase of the input-voltage signal V_V. The blanking adjustment signal V_B is increased in response to the decrease of the input-voltage signal V_V.

FIG. 11 shows a circuit schematic of the blanking circuit 80. The blanking circuit 80 comprises a constant current-source 85, a transistor 82, a capacitor 83, a comparator 87, an inverter 81 and an NAND gate 89 to produce the blanking signal S_K in response to the rising edge of the switching signal S_W. The constant current-source 85 is coupled from the supply voltage V_CC to the drain of the transistor 82. The gate and the source of the transistor 82 are coupled to the output terminal of the inverter 81 and the ground respectively. The capacitor 83 is coupled between the drain of the transistor 82 and the ground. The switching signal S_W is coupled to the input terminal of the inverter 81 to control the transistor 82 through the inverter 81. Therefore the constant current source 85 will start to charge the capacitor 83 once the switching signal S_W is turned on.

The capacitor 83 is connected to the negative input terminal of the comparator 87 to compare with the blanking adjustment signal V_B coupled to the positive input terminal of the comparator 87. The output terminal of the comparator 87 is connected to the input terminal of the NAND gate 89. Another input terminal of the NAND gate 89 is connected to the switching signal S_W. The blanking signal S_K is thus generated at the output terminal of the NAND gate 89. The current of the constant current-source 85, the capacitance of the capacitor 83 and level of the blanking adjustment signal V_B determine the blanking time of the blanking signal S_K. The blanking time of the blanking signal S_K is therefore increased in response to the decrease of the input voltage V_IN.

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 control circuit of a power converter including an input voltage detection, comprising:

a power transistor coupled to a transformer for switching the transformer;

a current sense circuit generating a current signal in response to a switching current of the transformer;

a switching circuit coupled to receive the current signal and a feedback signal to generate a switching signal for controlling the power transistor and regulating the output of the power converter;

a detection circuit coupled to sense the current signal for generating a slope signal in accordance with a slope of the current signal;

a signal generator generating an input-voltage signal in accordance with the slope signal; and

a protection circuit coupled to control the switching signal in response to the input-voltage signal;

wherein the input-voltage signal is corrected correlated to the input voltage of the power converter.

2. the control circuit as claimed in claim 1, wherein the slope of the current signal is detected when the power transistor is turned on.

3. The control circuit as claimed in claim 1, wherein a first signal is generated by the detection circuit sampling the current signal during a first period when the power transistor is turned on, a second signal is generated by the detection circuit sampling the current signal during a second period when the power transistor is turned on, the slope of the current signal is determined in accordance with the first signal and the second signal.

4. The control circuit as claimed in claim 1, wherein the detection circuit comprises:

a first capacitor coupled to sample-and-hold the current signal through a first switch during a first period after the power transistor is turned on;

a second capacitor coupled to sample-and-hold the current signal through a second switch during a second period after the power transistor is turned on; and

a third capacitor coupled to sample-and-hold the differential voltage of the first capacitor and the second capacitor for generating the slope signal.

5. The control circuit as claimed in claim 4, wherein the first switch is controlled by a first sample signal, the second switch is controlled by a second sample signal, the first sample signal and the second sample signal are generated by an oscillation circuit of the power converter.

6. The control circuit as claimed in claim 5, wherein the oscillation circuit further generates an oscillation signal coupled to enable the power transistor, the first sample signal and the second sample signal are synchronized with the oscillation signal.

7. The control circuit as claimed in claim 1, wherein the signal generator comprises:

an operational amplifier coupled to amplify the slope signal for generating the input-voltage signal.

8. The control circuit as claimed in claim 1, wherein the protection circuit comprises:

an over-voltage comparator coupled to receive the input-voltage signal and an over-voltage threshold to generate an over-voltage signal when the input-voltage signal is higher than the over-voltage threshold;

wherein the over-voltage signal is coupled to disable the switching signal.

9. The control circuit as claimed in claim 1, wherein the protection circuit comprises:

an under-voltage comparator coupled to receive the input-voltage signal and an under-voltage threshold to generate an under-voltage signal when the input-voltage signal is lower than the under-voltage threshold;

wherein the under-voltage signal is coupled to disable the switching signal.

10. The control circuit as claimed in claim 1, wherein the protection circuit comprises:

a current-limit adjustment circuit coupled to receive the input-voltage signal to generate a current limit signal to disable the switching signal for limiting the switching current of the power converter;

wherein the current limit signal is decreased in response to the increase of the input voltage of the power converter.

11. The control circuit as claimed in claim 1, wherein the protection circuit comprises:

a signal adjustment circuit coupled to receive the input-voltage signal to generate a blanking adjustment signal for adjusting a blanking time of the switching signal;

wherein the blanking time is increased in response to the decrease of the input voltage of the power converter.

12. A control circuit of a power converter including an input voltage detection, comprising:

a power transistor coupled to a transformer for switching the transformer;

a current sense circuit generating a current signal in response to a switching current of the transformer;

a switching circuit coupled to receive the current signal and a feedback signal to generate a switching signal for controlling the power transistor and regulating the output of the power converter;

a detection circuit coupled to sense the current signal for generating a slope signal; and

a protection circuit coupled to control the switching signal in response to the slope signal;

wherein the level of the slope signal is corrected correlated to the input voltage of the power converter.

13. The control circuit as claimed in claim 12, wherein the slope signal is correlated to a slope of the switching current.

14. The control circuit as claimed in claim 12, wherein a first signal is generated by the detection circuit sampling the current signal during a first period when the power transistor is turned on, a second signal is generated by the detection circuit sampling the current signal during a second period when the power transistor is turned on, the slope signal is generated in accordance with the first signal and the second signal.

15. The control circuit as claimed in claim 12, wherein the detection circuit comprises:

a first capacitor coupled to sample-and-hold the current signal through a first switch during a first period after the power transistor is turned on;

a second capacitor coupled to sample-and-hold the current signal through a second switch during a second period after the power transistor is turned on; and

a third capacitor coupled to sample-and-hold the differential voltage of the first capacitor and the second capacitor for generating the slope signal;

wherein the slope signal is correlated to a slope of the current signal.

16. The control circuit as claimed in claim 12, wherein the protection circuit comprises:

an over-voltage comparator generating an over-voltage signal in response to the slope signal and an over-voltage threshold when the slope signal is higher than the over-voltage threshold;

wherein the over-voltage signal is coupled to disable the switching signal.

17. The control circuit as claimed in claim 12, wherein the protection circuit comprises:

an under-voltage comparator generating an under-voltage signal in response to the slope signal and an under-voltage threshold when the slope signal is lower than the under-voltage threshold;

wherein the under-voltage signal is coupled to disable the switching signal.

18. The control circuit as claimed in claim 12, wherein the protection circuit comprises:

a current-limit adjustment circuit generating a current limit signal to disable the switching signal for limiting the switching current of the power converter in response to the slope signal;

wherein the current limit signal is decreased in response to the increase of the input voltage of the power converter.

19. The control circuit as claimed in claim 12, wherein the protection circuit comprises:

a signal adjustment circuit generating a blanking adjustment signal for adjusting a blanking time of the switching signal in response to the slope signal;

wherein the blanking time is increased in response to the decrease of the input voltage of the power converter.

20. A control circuit of a power converter, comprising:

a switching circuit generating a switching signal for controlling a power transistor and regulating the output of the power converter in response to a switching current and a feedback signal;

a detection circuit coupled to sense the switching current for generating a slope signal; and

a protection circuit coupled to control the switching signal in response to the slope signal;

wherein the slope signal is corrected correlated to an input voltage of the power converter.

21. The control circuit as claimed in claim 20, wherein the slope signal is correlated to a slope of the switching current.

22. The control circuit as claimed in claim 20, wherein a first signal is generated by the detection circuit sampling the switching current during a first period when the power transistor is turned on, a second signal is generated by the detection circuit sampling the switching current during a second period when the power transistor is turned on, the slope signal is generated in accordance with the first signal and the second signal.

23. The control circuit as claimed in claim 20, wherein the detection circuit comprises:

a first capacitor coupled to sample-and-hold the switching current through a first switch during a first period after the power transistor is turned on;

a second capacitor coupled to sample-and-hold the switching current through a second switch during a second period after the power transistor is turned on; and

a third capacitor coupled to sample-and-hold the differential voltage of the first capacitor and the second capacitor for generating the slope signal;

wherein the slope signal is correlated to a slope of the switching current.

24. The control circuit as claimed in claim 20, wherein the protection circuit comprises:

a current-limit adjustment circuit adjusting a current limit of the power transistor in response to the slope signal.

25. The control circuit as claimed in claim 24, wherein the current-limit adjustment circuit generates a current limit signal to disable the switching signal for the current limit in response to the slope signal, the current limit signal is decreased in response to the increase of the input voltage of the power converter.

26. The control circuit as claimed in claim 20, wherein the protection circuit comprises:

an over-voltage comparator generating an over-voltage signal in response to the slope signal and an over-voltage threshold when the slope signal is higher than the over-voltage threshold;

wherein the over-voltage signal is coupled to disable the switching signal.

27. The control circuit as claimed in claim 20, wherein the protection circuit comprises:

an under-voltage comparator generating an under-voltage signal in response to the slope signal and an under-voltage threshold when the slope signal is lower than the under-voltage threshold;

wherein the under-voltage signal is coupled to disable the switching signal.

28. The control circuit as claimed in claim 20, wherein the protection circuit comprises:

a signal adjustment circuit generating a blanking adjustment signal for adjusting a blanking time of the switching signal in response to the slope signal;

wherein the blanking time is increased in response to the decrease of the input voltage of the power converter.

29. A control method of a power converter, comprising:

generating a switching signal for controlling a power transistor in response to a switching current;

sensing a slope of the switching current for generating a slope signal corrected correlated to an input voltage of the power converter; and

controlling the switching signal in response to the slope signal.

30. The control method as claimed in claim 29, wherein generating a switching signal further comprises for regulating the output of the power converter in response to a feedback signal.

31. The control method as claimed in claim 29, wherein the step of sensing a slope of the switching current for generating a slope signal comprises:

generating a first signal in response to the switching current during a first period when the power transistor is turned on;

generating a second signal in response to the switching current during a second period when the power transistor is turned on; and

generating the slope signal correlated to the slope of the switching current in accordance with the first signal and the second signal.

32. The control method as claimed in claim 29, wherein the step of controlling the switching signal comprises:

generating an over-voltage signal to disable the switching signal in response to the slope signal and an over-voltage threshold when the slope signal is higher than the over-voltage threshold.

33. The control method as claimed in claim 29, wherein the step of controlling the switching signal comprises:

generating an under-voltage signal to disable the switching signal in response to the slope signal and an under-voltage threshold when the slope signal is lower than the under-voltage threshold.

34. The control method as claimed in claim 29, wherein the step of controlling the switching signal comprises:

generating a blanking adjustment signal for adjusting a blanking time of the switching signal in response to the slope signal.

35. The control method as claimed in claim 34, wherein the blanking time is increased in response to the decrease of the input voltage of the power converter.

36. The control method as claimed in claim 29, wherein the step of controlling the switching signal comprises:

adjusting a current limit of the power transistor in response to the slope signal.