TW201009532A - Control circuit and power converter using the same and control method - Google Patents

Abstract

A control circuit applicable to a power converter. A current comparison circuit compares via a multi-function terminal a current flowing through a power switch with a reference value when the power switch is turned on. An under voltage detection circuit detects via the multi-function terminal whether an input voltage of the power converter is under a predetermined value.

Description

201009532 IX. Description of the Invention: [Technical Field] The present invention relates to a control circuit, and more particularly to a control circuit having a multi-function terminal. [Prior Art] Fig. 1 shows a conventional flyback converter 10, which is coupled to a primary-side winding Lp of a transformer TX via a filtered and rectified input voltage vin. A power switch 101 is controlled by a pulse width modulation (PWM) control signal Vg'. A sensing resistor RS is used to sense the primary current Ip flowing through the power switch 101, a control circuit 1〇决定Determining the duty cycle of the control signal Vg according to the feedback signal Vcomp and the primary side current Ip corresponding to one of the outputs of the converter 10, thereby stabilizing the output voltage Vo ° The resistors 130 and 140 form a voltage dividing circuit ( Voltage divider), the control circuit 100 detects a sensing voltage Vbno through the voltage dividing circuit, so the sensing voltage Vbno value is proportional to the input voltage Vin, when detecting that the sensing voltage Vbno is lower than a preset value, The control circuit 1 stops the switching of the power switch 1〇1 in order to prevent the converter 10 from operating in an abnormal condition in which the input voltage is too low, thereby causing damage. SUMMARY OF THE INVENTION 201009532 One embodiment of the present invention provides a control circuit suitable for use in a power converter. The control circuit includes a multi-function terminal, a current comparison circuit, and an over-voltage detection circuit. When the power switch of the power converter is turned on, the current comparison circuit compares the current of the power switch through the multi-function terminal and a reference value. When the current reaches the reference value, the current comparison circuit makes the current comparison circuit The power switch is off. When the power switch is turned off, the over-voltage detection circuit detects whether the input voltage of one of the power converters is lower than a preset value through the multi-function terminal, when the input voltage is lower than the preset value, The power switch is fixed in an off state. Another embodiment of the present invention provides a control method, including: (1) providing a multi-function endpoint; (2) when one of the power converters is turned on, the multi-function endpoint is relatively The current of the power switch and a reference value; (3) when the current reaches the reference value, the power switch is turned off; (4) generating a ratio of the sensing voltage to the multi-function terminal Point (5) detecting whether the sensing voltage is lower than a preset value when the power switch is off; and (6) fixing the power switch to a closed state when the input voltage is lower than the preset value . [Embodiment] FIG. 2 is a circuit diagram of a flyback converter 20 according to a preferred embodiment of the present invention. The converter 20 mainly includes a control circuit 200, a power switch 201009532 201, and a voltage dividing circuit 205. The voltage circuit 205 is composed of a resistor R1, a resistor R2 and a sensing resistor Rs. In this embodiment, the resistance of the resistor R1 is much higher than the sum of the resistance of R2 and the resistance of the sense resistor Rs. The control circuit 200, for example, is an integrated circuit including a multi-function terminal CS, a current comparison circuit 210, and an under voltage detection circuit 220. The resistor R1 is disposed between the input voltage Vin of one of the converters 20 and the multi-function terminal CS. The resistor R2 and the capacitor C1 form a low pass filter, which is disposed between the sensing resistor Rs and the multi-function terminal CS for filtering high frequency noise of the sensing voltage Vs. When the power switch 201 is turned on, the primary side current Ip of the converter 20 flows through the power switch 201 and the sensing resistor Rs, so that one of the sensing voltages Vs (~Ip*Rs) on the sensing resistor Rs is proportional to the primary side. Current Ip. When the power switch 201 is turned on, the current comparison circuit 210 compares a sense voltage Vcs with a reference voltage Vcomp, and the sense voltage Vcs is the detected voltage on the multi-function terminal CS, which can be derived according to the following formula (1). Learned. (1) m+R2 Since the resistance value of the resistor R1 is much higher than the sum of the resistance value of R2 and the resistance value of the sensing resistor Rs, the contribution of Vin in the formula (1) can be neglected, and the sensing voltage Vcs is substantially equal to The voltage Vs is sensed. Voltage reference 201009532

Vcomp is a feedback voltage corresponding to one of the converter 2 turns out voltage. When the S sense/voltage Vcs reaches the reference voltage, the current comparison circuit 210 triggers the -SR flip-flop 231, - pwM controls the logic level of the signal % and thus transitions, causing the power switch 2〇1 to end the on state. In other words, when the primary side current Ip reaches or exceeds a reference value, the current comparison circuit 210 turns off the power switch 201. The low voltage detection circuit 220 includes a comparator 222 and a sample and hold circuit. The sample and hold circuit includes a sampling switch 224 and a voltage holding circuit 226. In this embodiment, the sampling switch 224 can be an N channel gold oxide. The semiconductor, voltage hold circuit 226 can be a capacitor. When the power switch 201 is turned off, the over-voltage detecting circuit detects the sensing voltage Vcs on the multi-function terminal CS through the resistor R1, and at this time, the sensing voltage

The Vcs value is 'the ratio is proportional to the input voltage vin, so it can be used to detect whether the input voltage Vin is lower than a preset value. The sampling switch 224 is controlled by the PWM control signal Vg' as well as the power switch 201. When the power switch 201 is turned off, the sampling switch 224 is turned on, so that the input voltage of one of the comparators 222 can detect the sensing voltage Vcs. If the sense voltage Vcs is higher than a reference voltage Vth1, the comparator 222 determines that the current input voltage Vin is normal, so a logic high level signal is output, and the PWM control signal Vg will still vary with the output of the SR flip-flop. If the sense voltage vcs is lower than a reference voltage Vth1, the comparator 222 determines that the current input voltage vin 201009532 is too low, so a logic low level signal is output, so that the PWM control signal Vg outputted by a logic gate 235 is fixed to a logic low. At the level, the power switch 201 thus stops switching and is fixed in the off state. When the power switch 201 is turned on, because of the isolation of the sampling switch 224, the voltage holding circuit 226 stores the sensing voltage Vcs at the end of the last power-off state of the power switch 201, so that the output signal of the comparator 222 can be maintained at one. At the logic high level, the power switch 201 is normally switched. Since the over-voltage detecting circuit 220 only operates when the power switch 201 is turned off, the operation of the current comparing circuit 210 when the power switch 201 is turned on is not affected. If the control circuit 200 of the embodiment is integrated on a chip, the over-voltage detection circuit and the current comparison circuit 210 can detect the sensing voltage Vcs through the same multi-function terminal CS, as compared with FIG. By controlling the circuit 100, a wafer pin can be omitted. In addition, according to the above embodiments, a corresponding PWM control method can be obtained, and the steps thereof can be briefly described as follows: (1) providing a multi-function terminal; (2) when one of the power converters is turned on, Comparing the current through the power switch with a reference value through the multi-function terminal; (3) turning off the power switch when the current reaches the reference value; (4) generating a sense of the input voltage Measuring voltage at the multi-function terminal; (5) detecting, when the power switch is off, detecting whether the sensing voltage is lower than a pre-201009532 setting value through the multi-function terminal; and (6) when the input voltage is lower than When the preset value is set, the power switch is fixed in an off state. The above is only the preferred embodiment of the present invention, and the scope of the present invention is not limited thereto. It is to be understood that the scope of the present invention is not limited by the spirit and scope of the present invention, and should be considered as a further embodiment of the present invention. ® [Simple description of the diagram] Figure 1 is a conventional flyback converter. Figure 2 is a flyback converter in accordance with a preferred embodiment of the present invention. [Main component symbol description] Converter 10 Transformer TX Primary winding Lp _ Power switch 101 Control circuit 100 Pulse width modulation control signal Vg Resistance 130 Resistance 140 Converter 20 Control circuit 200 11 201009532

Power switch 201 voltage divider circuit 205 resistor R1 resistor R2 sense resistor Rs multi-function terminal CS current comparison circuit 210 low voltage detection circuit 220 flip-flop 231 comparator 222 sampling switch 224 voltage hold circuit 226 logic gate 235 capacitor C1 12

Claims (1)

201009532 X. Patent application scope: 1. A control circuit for a power converter, the control circuit comprising: a multi-function terminal, a current comparison circuit, wherein when one of the power converters is turned on, The current comparison circuit compares the current through the power switch with a reference value through the multi-function terminal, and when the current reaches the reference value, the current comparison circuit turns off the power switch; and an over-voltage detection circuit When the power switch is turned off, the over-voltage detection circuit detects whether the input voltage of one of the power converters is lower than a preset value through the multi-function terminal. 2. The control circuit of claim 1, wherein the over-voltage detection circuit includes a comparator for detecting whether the input voltage is lower than the preset value. 3. The control circuit of claim 2, wherein the over-voltage detection circuit further comprises a sample-and-hold circuit, the sample-and-hold circuit comprising: a voltage hold circuit for storing a voltage value; and a sampling switch Connected between the voltage holding circuit and the multi-function terminal, wherein when the power switch is turned off, the sampling switch is turned on, so that the comparator detects the input voltage. 4. The control circuit of claim 3, wherein the voltage holding circuit comprises 13 201009532 having a capacitor. 5. The control circuit of claim 3, wherein the sampling open relationship is controlled by a pulse width modulation control signal. 6. A power converter comprising: a control circuit as claimed in claim 1; and a voltage dividing circuit 'generating a sensing voltage proportional to the input voltage at the multi-function terminal, wherein the The low voltage detecting circuit determines whether the input voltage is lower than the preset value according to the sensing voltage. 7. The power converter of claim 6, wherein the voltage dividing circuit comprises: a current sensing resistor coupled to the power switch; and a first resistor coupled to the input voltage at one end The other end is coupled to the multi-function endpoint. 8. The power converter of claim 7, wherein the voltage dividing circuit further comprises a -second resistor' having a terminal handle connected to the current sensing resistor, and the other end coupled to the multifunction End point. 9. A control method, suitable for the power converter, the control method comprises: providing a multi-function terminal; when the power conversion H - power _ conduction, through the multi-function terminal comparison - through the The current of the power switch and the reference value; when the electric muscle reaches the reference value of the sea, the power switch is turned off; 201009532 When the power switch is turned off, the hard-hitting gold is crying and the end of the power is too much Detecting whether the wheeling voltage of the electric/original conversion thief is lower than a preset value; and when the wheeling voltage is lower than the preset value, fixing the power switch to a closed state. 10. The control method further includes: generating a sensing voltage proportional to the input voltage to the multi-function terminal; and determining, according to the sensing voltage, whether the input voltage is lower than the preset value.