Protection apparatus and method for an isolated type power supply
An isolated type power supply includes a transformer, and a power switch and a current sense resistor serially connected to a primary coil of the transformer, and a protection apparatus and method monitor a control signal which is used to switch the power switch in normal operation, and trigger a signal to stop the power from switching when the control signal becomes greater than a threshold. This protection apparatus and method can more quickly detect short circuit of the current sense resistor, and prevent the isolated type power supply from overcurrent impact.
1. A protection apparatus for an isolated type power supply including a transformer whose primary coil is serially connected with a power switch to switch a current of the primary coil by a control signal and a current sense resistor to detect the current of the primary coil, the protection apparatus comprising:
a detection circuit connected to the power switch, operative to monitor the control signal to trigger a protection signal when the control signal becomes greater than a threshold; and
a suspension circuit connected to the detection circuit, operative to stop the power switch from switching responsive to the protection signal.
2. The protection apparatus of claim 1 , wherein the detection circuit is enabled by an enable signal when the power switch is turned on for a period and a voltage of the current sense resistor is lower than a second threshold.
3. The protection apparatus of claim 2 , wherein the detection circuit comprises:
a comparator connected to the power switch, for comparing the control signal with the first threshold to determine a comparison signal; and
a logic circuit connected to the comparator, operative to generate the protection signal according to the comparison signal and the enable signal.
4. The protection apparatus of claim 2 , further comprising a comparator connected to the current sense resistor, for comparing the voltage of the current sense resistor with the second threshold to trigger the enable signal when the power switch is turned on for a period and the voltage of the current sense resistor is lower than the second threshold.
5. The protection apparatus of claim 2 , further comprising a counter operative to trigger the enable signal when the power switch is turned on for a period and the voltage of the current sense resistor is lower than the second threshold.
6. The protection apparatus of claim 2 , further comprising a counter operative to trigger the enable signal when it is detected in a plurality of successive cycles where the power switch is turned on for a period and the voltage of the current sense resistor is lower than the second threshold.
7. The protection apparatus of claim 1 , wherein the suspension circuit comprises a flip-flop operative to provide an output signal responsive to the protection signal to stop the power switch from switching.
8. A protection method for an isolated type power supply including a transformer whose primary coil is serially connected with a power switch to switch a current of the primary coil by a control signal and a current sense resistor to detect the current of the primary coil, the protection method comprising the steps of:
(A) monitoring the control signal; and
(B) triggering a signal to stop the power switch from switching when the control signal is greater than a threshold.
9. The protection method of claim 8 , wherein the step (B) comprises the step of enabling the signal to stop the power switch from switching to be triggered only when the power switch is turned on for a period and a voltage of the current sense resistor is lower than a second threshold.