BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an over-current protection module and related over-current protection method, and more particularly, to an over-current protection module and related over-current protection method capable of compensating a flyback power supply under a current mode control.

2. Description of the Prior Art

A conventional over-current (over-power) protection device for a flyback power supply usually has a fixed current limit. This is illustrated inFIG. 1, which is a schematic diagram of a current value Ipri of a primary side of a transformer of a conventional over-current protection device with different input voltages versus an operating time. A fixed current limit value CL for the over-current protection device may be set for performing an over-current protection mechanism. When the input voltage of the over-current protection device is a low input voltage (e.g., 110V), the over-current protection device is configured to perform the over-current protection mechanism when the current value Ipri reaches the fixed current limit value CL. The current value Ipri is affected by a circuit delay and usually inhibited or cut off when the current value Ipri slightly exceeds the fixed current limit value CL. When the input voltage of the over-current protection device is a high input voltage (e.g., 220V), since an increment speed of the current value Ipri is also high, if the over-current protection device performs the over-current protection mechanism until the current value Ipri reaches the fixed current limit value CL, the current value Ipri will only be inhibited or cut off after the current value Ipri hugely exceeds the fixed current limit value CL, which causes damage to or may even burn the elements. Therefore, the conventional over-current protection device's operation quality is easily affected by different input voltages or loadings.

To solve the above problems, different current limit values with respect to different conditions (e.g., different input voltages) may be set in the conventional over-current protection device, or with respect to different conditions (e.g., different input voltages), different current values Ipri are set corresponding to different initial values to ensure that the current values Ipri of the over-current protection device may be inhibited or cut off at a stable current value no matter whether the input voltage is low or high.FIGS. 2 and 3are schematic diagrams of the current value of the primary side of the transformer of the conventional over-current protection device with different input voltages versus the operating time for compensation. In the example shown inFIG. 2, a compensation current limit CL2for different input voltages is not identical to the fixed current values CL shown inFIG. 1, and is varied with respect to different conditions, to ensure that the current value Ipri will be inhibited or cut off at a stable current value (e.g., the fixed current limit value CL) for the low input voltage or the high input voltage. In the example shown inFIG. 3, different initial values of the current value Ipri with respect to different input voltages are set to ensure that the current value Ipri will be inhibited or cut off at a stable current value (e.g., the fixed current limit value CL) for the low input voltage or the high input voltage.

When the conventional over-current protection device is applied in different systems or environmental conditions, the current limit value or the initial value is required to be reset and circuit elements are required to be replaced, meaning the system cost is high and lacks flexibility. In addition, the circuit for the example shown inFIG. 2is complicated and hard to implement. In the example shown inFIG. 3, a compensation current for the initial values of the current value Ipri is required, which increases jitter within the circuit and generates unnecessary ripples to an output voltage of the flyback power supply, directly affecting the output power quality. Therefore, improvements to the prior art are needed.

SUMMARY OF THE INVENTION

The present invention provides an over-current protection module and related over-current protection method to compensate a flyback power supply under a current mode control to solve the above problems.

In an aspect, the present invention discloses an over-current protection module, for a flyback power supply having a transformer, comprising: a switch control unit, configured to generate a control signal in a first period to control a transistor switch coupled to a primary side of the transformer, wherein the transistor switch is turned on in a first duty cycle of the control signal and the transistor switch is turned off in a second duty cycle of the control signal; a transformation unit, coupled to the switch control unit, configured to generate a compensation current according to the second duty cycle; a timing control unit, coupled to the transformation unit, configured to output the compensation current to an impedance unit to generate an impedance cross voltage in a shut-off period of the transistor switch of the first period; and a current control unit, coupled to the timing control unit and the impedance unit, configured to determine an over-current reference voltage according to the impedance cross voltage for the over-current protection module in a second period following the first period.

In another aspect, the present invention discloses an over-current protection method for controlling a flyback power supply having a transformer, comprising: generating a control signal in a first period to control a transistor switch coupled to a primary side of the transformer, wherein the transistor switch is turned on in a first duty cycle of the control signal and the transistor switch is turned off in a second duty cycle of the control signal; generating a compensation current according to the second duty cycle in the first period; outputting the compensation current to an impedance unit to generate an impedance cross voltage in a shut-off period of the transistor switch of the first period; and determining an over-current reference voltage according to the impedance cross voltage, so as to determine whether a current flowing through the transistor switch is higher than a current limit or not in a second period following the first period according to the over-current reference voltage.

These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.

