Power supply with open-loop protection
A power supply with an open-loop protection according to the present invention comprises a transformer, a switch, a signal generation circuit, a feedback detection circuit, a brown-out detection circuit, and a delay circuit. The transformer receives an input voltage. The switch is coupled to the transformer for switching the transformer. The signal generation circuit generates a switching signal to control the switch. The feedback detection circuit generates a pull-high signal in response to a feedback signal of the power supply. The brown-out detection circuit generates a delay signal in response to the pull-high signal and the input voltage. The delay circuit counts a delay time in response to the delay signal for generating a disabling signal coupled to the signal generation circuit to latch the switching signal. The brown-out detection circuit is utilized to detect whether the input voltage is in the brown-out condition for determining whether the open-loop protection is executed.
1. A power supply with open-loop protection, comprising:
a transformer, receiving an input voltage;
a switch, coupled to the transformer for switching the transformer;
a signal generation circuit, generating a switching signal to control the switch for switching;
a feedback detection circuit, generating a pull-high signal in response to a feedback signal of the power supply;
a brown-out detection circuit, generating a delay signal in response to the pull-high signal and the input voltage; and
a delay circuit, coupled to count a delay time in response to the delay signal for generating a disabling signal, the disabling signal coupled to the signal generation circuit to latch the switching signal.
2. The power supply as claimed in claim 1 , wherein the delay circuit counts the delay time in response to the delay signal for generating the disabling signal to latch the switching signal when the pull-high signal is in a high level and the input voltage being higher than a second signal is detected by the brown-out detection circuit.
3. The power supply as claimed in claim 1 , wherein the brown-out detection circuit controls the delay circuit disabling to count the delay time and the switching signal is not latched by the disabling signal when the input voltage being lower than a second signal is detected by the brown-out detection circuit.
4. The power supply as claimed in claim 1 , further comprising:
a oscillator, generating a pulse signal, the signal generation circuit generating the switching signal in response to the pulse signal.
5. The power supply as claimed in claim 1 , further comprising:
a driving circuit, generating a reset signal to disable the switching signal in response to a current signal of the power supply, a power limit signal and the feedback signal.
6. The power supply as claimed in claim 1 , wherein the feedback detection circuit generates the pull-high signal in response to the feedback signal and a threshold signal.
7. The power supply as claimed in claim 1 , wherein the brown-out detection circuit generates a brown-out signal in response to a saw signal, a current signal and the switching signal, and generates the delay signal in response to the brown-out signal and the pull-high signal, in which the current signal is correlated to the input voltage.
8. The power supply as claimed in claim 7 , wherein the brown-out detection circuit disables the brown-out signal for controlling the delay circuit to stop counting the delay time and the switching signal is not latched when the saw signal is higher than a first signal, the current signal is lower than a second signal and the switching signal is in a high level.
9. The power supply as claimed in claim 7 , wherein the brown-out detection circuit enables the delay signal for controlling the delay circuit to count the delay time for generating the disabling signal to latch the switching signal when the pull-high signal is in a high level as well as the saw signal is lower than a first signal, the current signal is higher than a second signal or the switching signal is in a low level.
10. The power supply as claimed in claim 7 , wherein the brown-out detection circuit further comprises:
a duty comparator, comparing the saw signal with a first signal for generating a duty signal;
a input comparator, comparing the current signal and a second signal for generating an input signal; and
a first logic circuit, generating the brown-out signal in response to the duty signal, the input signal and the switching signal.
11. The power supply as claimed in claim 10 , further comprising:
a second logic circuit, generating the disabling signal in response to the pull-high signal and the brown-out signal, the delay circuit counting the delay time in response to the delay signal for generating the disabling signal.