1. Field of the Invention
The present invention relates generally to an over-voltage protection circuit, and more specifically to the over-voltage protection circuit used for a switching mode power converter.
2. Description of Related Art
Various power converters have been widely used to provide a regulated voltage and current. For the sake of safety reasons, an over-voltage protection circuit must be provided to protect both the power converter itself and the system it powers. Thus an output voltage is limited in order to avoid the output voltage for the power converter is too high, which will cause an over-high output voltage during the period of feedback open loop or a higher supply voltage.
FIG. 1 illustrates the topology of a conventional power converter. The power converter includes a switching controller U1 coupled to control the power converter. A resistor RST and a capacitor CST provide an initial power source to start up the switching controller U1. In order to regulate the output voltage VO of the power converter, the switching controller U1 generates a switching signal VPWM to a power switch Q1 so as to switch a transformer T1. Wherein, the transformer T1 includes an auxiliary winding NA, a primary winding NP, and a secondary winding NS.
After switching the transformer T1, the auxiliary winding NA generates a reflect voltage VAUX, which provides a further power source to a VDD terminal of the switching controller U1 via a rectifier DA. Since the reflect voltage VAUX is correlated to the output voltage VO, a supply voltage VDD at the VDD terminal is correlated with the output voltage VO too. The pulse width of the switching signal VPWM is regulated in accordance with a feedback signal VFB at a FB terminal of the switching controller U1. An error amplifier 80 is connected to the output of the power converter for receiving the output voltage VO. The feedback signal VFB is coupled to the output of the power converter through the error amplifier 80 and an optical-coupler 90. Thus the output of the power converter is regulated in accordance with the feedback signal VFB. Wherein the supply voltage VDD can be expressed as equation (1):
                    VDD        ≈                              [                                                            TN                  AUX                                                  TN                  S                                            ×                              (                                                      V                    O                                    +                                      V                    F                                                  )                                      ]                    -                      V            D                                              (        1        )            
where VF is the voltage drop of the output rectifier DO; VD is the voltage drop of the rectifier DA; TNAUX is the winding turns of the auxiliary winding NAUX of the transformer T1; and TNS is the winding turns of the secondary winding NS of the transformer T1.