Soft-start apparatus for power supplies

[0001] 1. Field of the Invention

[0002] The present invention relates generally to a soft-start apparatus for a power supply and, more particularly to a soft-start apparatus featuring an increasing pulse width of a pulse width modulation (PWM) signal as the power supply starts up.

[0003] 2. Description of Related Art

[0004] Various power supplies have been widely used to provide a regulated output voltage. FIG. 1 illustrates a block diagram of a traditional power supply comprising a switching circuit 30, a switching controller 10, an output inductor L, an output capacitor C₁, a voltage divider formed by resistors R₁ and R₂, a compensation network formed by a resistor R₃ and a capacitor C₂. An output terminal OUT of the switching controller 10 generates a switching signal V_PWM to control a power switch of the switching circuit 30. Therefore, an output voltage V_O of the power supply can be regulated.

[0005] The switching controller 10 includes a comparator 162, an oscillator 168, a D-type flip-flop 166 and an error amplifier 160. The oscillator 168 generates a saw signal V_SAW, which determines a switching frequency of the switching signal V_PWM. A positive input of the error amplifier 160 is supplied with a reference voltage V_REF. A negative input of the error amplifier 160 is connected to a feedback terminal FB of the switching controller 10 for receiving a feedback voltage V_FB. The error amplifier 160 generates a compensation signal V_COM in response to the reference voltage V_REF and the feedback voltage V_FB. The comparator 162 turns off the switching signal V_PWM via the D-type flip-flop 166 whenever the saw signal V_SAW exceeds the compensation signal V_COM. A pulse width of the switching signal V_PWM is then determined by the operation of the comparator 162.

[0006] Referring to FIG. 1, various waveforms at a start-up transient of the traditional power supply is illustrated in FIG. 2. Since the feedback voltage V_FB is extremely low at the start-up transient of the traditional power supply, the error amplifier 160 outputs a high compensation voltage V_COM. This causes a wide pulse width of the switching signal V_PWM. That is to say, the switching signal V_PWM retains a maximum pulse width when the output voltage V_O is insufficient at the start-up transient of the power supply. When the time to build a normal output voltage extends, voltage and current stress of the power switch of the switching circuit 30 will inevitably increases.

[0007] The present invention proposes a soft-start apparatus for a power supply. The soft-start apparatus provides a compensation signal to a PWM controller at the start-up transient of the power supply. A pulse width of a switching signal of the PWM controller gradually increases from an initial value to a determined value. After the power supply starts up, the pulse width of the switching signal is timely modulated in response to the load conditions. This can effectively avoid over-voltage and over-current stress of a power switch at the start-up transient of the power supply.

[0008] According to the present invention, a first power source starts to charge up a start-up capacitor and to generate a reference voltage across the start-up capacitor at the start-up transient of the power supply. A voltage-regulating unit has a first input supplied with an internal reference voltage and a second input connected to the start-up capacitor. The voltage-regulating unit further has an output with open-drain or open-collector structure. Therefore, the reference voltage can be regulated within the internal reference voltage. An error amplifier has a first input connected to the start-up capacitor and the output of the voltage-regulating unit for receiving the reference voltage. The error amplifier further has a second input connected to a feedback terminal of the power supply for receiving a feedback voltage. An output of the error amplifier generates a compensation signal. A clamping diode is connected between the second input and the output of the error amplifier for clamping the compensation signal at the start-up transient of the power supply.

[0009] At the start-up transient of the power supply, the first power source starts to charge up the start-up capacitor. The reference voltage is therefore generated across the start-up capacitor. This makes the compensation signal increase in response to the increment of the reference voltage. In such a manner, the pulse width of the switching signal gradually increases from an initial value to a determined value. The soft-start function is therefore achieved by the increasing compensation signal.

[0010] 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.

[0017] FIG. 3 shows a power supply having a soft-start apparatus 40 according to an embodiment of the present invention. The soft-start apparatus 40 connects to a PWM controller 50 and a feedback terminal FB of the power supply. A feedback voltage V_FB at the feedback terminal FB is obtained from an output voltage V_O of the power supply via a voltage divider, which is formed by resistors R₁ and R₂.

[0018] The soft-start apparatus 40 further comprises a first power source 407, a start-up capacitor C_SS, a voltage-regulating unit 406, an error amplifier 402, and a clamping diode 404. The first power source 407 is used to charge up the start-up capacitor C_SS and to generate a reference voltage V_REF across the start-up capacitor C_SS. The voltage-regulating unit 406 has a first input supplied with an internal reference voltage V_R. The voltage-regulating unit 406 further has a second input connected to its output. Such a connection forms a unit-gain buffer. The output of the voltage-regulating unit 406 further connects to the start-up capacitor C_SS. The voltage-regulating unit 406 has an output with open-drain or open-collector structure. Therefore, the reference voltage across the start-up capacitor C_SS is regulated within the internal reference voltage V_R.

[0019] A first input of the error amplifier 402 connects to the start-up capacitor C_SS and the output of the voltage-regulating unit 406. A second input of the error amplifier 402 connects to the feedback terminal FB of the power supply. The error amplifier 402 amplifies a difference between the reference voltage V_REF and the feedback voltage V_FB to generate a compensation signal V_COM. The compensation signal V_COM is supplied to the PWM controller 50 via the output of the voltage-regulating unit 406. The clamping diode 404 is connected between the second input and the output of the error amplifier 402 for clamping the compensation signal V_COM at a forward voltage drop V_F of the clamping diode 404 as soon as the power supply starts up.

[0020] As described above, the reference voltage V_REF across the start-up capacitor C_SS starts to increase when the power supply starts up. A maximum value of the reference voltage V_REF is equal to the internal reference voltage V_R. At the start-up transient of the power supply, the feedback voltage V_FB and the reference voltage V_REF are both zero. Since being clamped by the clamping diode 404, the error amplifier 402 outputs the compensation signal V_COM at the start-up transient of the power supply with a voltage level equal to the forward voltage drop V_F of the clamping diode 404.

[0021] Once the clamping diode 404 is removed from the second input and the output of the error amplifier 402, a soft-start function of the power supply will not take effect anymore. That is, at the start-up transient of the power supply, the feedback voltage V_FB supplied to the second input of the error amplifier 402 will be soon amplified. The error amplifier 402 will output the compensation voltage V_COM to the PWM controller 50 with a saturated level, such as a supply voltage of the error amplifier 402.

[0022] Referring to FIG. 3, the start-up capacitor C_SS is charged up by the first power source 407. A start-up period of the power supply can be determined by a capacitance of the start-up capacitor C_SS. As a higher capacitance of the start-up capacitor C_SS is selected, the start-up period of the power supply extends and a power switch having lower normal voltage and current can be utilized. Therefore, by properly selecting the capacitance of the start-up capacitor C_SS, a delay of the start-up time can become acceptable.

[0023] The PWM controller 50 comprises an oscillator 504, a comparator 502, and a D-type flip-flop 506. The oscillator 504 generates a saw signal V_SAW having an upper threshold voltage V_TH2 and a lower threshold voltage V_TH1. A positive input of the comparator 502 is connected to the output of the error amplifier 402 for receiving the compensation signal V_COM. A negative input of the comparator 502 is supplied with the saw signal V_SAW. The comparator 502 resets the D-type flip-flop 506 as the saw signal V_SAW exceeds the compensation signal V_COM. This further determines the pulse width of the switching signal V_PWM.

[0024] Referring to FIG. 3, various waveforms at the start-up transient of the power supply of the present invention is illustrated in FIG. 4. The reference voltage V_REF across the start-up capacitor C_SS starts to increase as the power supply starts up. Since the output of the voltage-regulating unit 406 is an open-drain or open-collector structure, the maximum value of the reference voltage V_REF will be equal to the internal reference voltage V_R. As the voltage at the first input of the error amplifier 402 is zero, the output of the error amplifier 402 is clamped at the forward voltage drop V_F of the clamping diode 404, such as 0.7 volts. However, the forward voltage drop V_F of the clamping diode 404 is still lower than the lower threshold voltage V_TH1. Meanwhile, the compensation signal V_COM increases in response to an increment of the reference voltage V_REF. The pulse width of the switching signal V_PWM starts to extend from an initial value during the start-up capacitor C_SS is charged up. As the reference voltage V_REF reaches the internal reference voltage V_R, the pulse width of the switching signal V_PWM reaches to a determined value and is then timely modulated in response to the load conditions. This achieves soft-start operation for the power supply.

[0025] FIG. 5 shows the voltage-regulating unit 406 according to an embodiment of the present invention. The voltage-regulating unit 406 comprises the first input, the second input, and the output for outward connection. The voltage-regulating unit 406 further comprises a differential-pair input amplifier formed by transistors 4061, 4062, 4063 and 4064, a current source 4060, a resistor 4065, a capacitor 4060, and an output transistor 4067. The differential-pair input amplifier is connected to the current source 4060. In response to a differential-pair input signal from the first and second inputs of the differential-pair input amplifier, the differential-pair input amplifier generates an output signal V_OUT, which is supplied to a gate of the output transistor 4067. A drain of the output transistor 4067 is the output of the voltage-regulating unit 406, which generates the reference voltage V_REF. The source of the output transistor 4067 is connected to a ground reference. The resistor 4065 and the capacitor 4066 are connected in series between the drain and the gate of the output transistor 4067 for frequency compensation. The drain of the output transistor 4067 is connected to the output of the voltage-regulating unit 406, which is an open-drain or open-collector output structure.

[0026] Further referring to FIG. 3, the soft-start apparatus 40 further comprises a switch 405 connected with the start-up capacitor C_SS in parallel. As the power supply is unable to provide a normal output, a reset signal RST will be generated in response to the feedback voltage V_FB. The switch 405 is turned on by the reset signal RST to discharge the start-up capacitor C_SS for resetting the power supply. The start-up apparatus 40 further comprises a discharge circuit formed by connecting a second power source 408 and a second switch 403 in series. The discharge circuit is connected in parallel with the start-up capacitor C_SS. As abnormal condition, such as over-power, short circuit, and over-voltage, occurs, a protection signal PTN will be generated in response to the feedback voltage V_FB to turn on the switch 403. The protection signal PTN turns on the switch 403 to discharge the start-up capacitor C_SS. Therefore, the power supply is reset, which achieves the protection for the power supply.

[0027] As describe above, the present invention proposes a soft-start apparatus for a power supply. The soft-start apparatus generates a compensation signal to a PWM controller at the start-up transient of the power supply. A pulse width of a switching signal of the PWM controller gradually increases from an initial value to a determined value. After the power supply starts up, the pulse width of the switching signal is then timely modulated in response to the load conditions. This can effectively avoid over-voltage and over-current stress of a power switch at the start-up transient of the power supply.

[0028] 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.