1. Field of the Invention
The present invention relates generally to power supplies and, more specifically, the present invention relates to a switched mode power supply controller.
2. Background Information
Electronic devices use power to operate. Switched mode power supplies are commonly used due to their high efficiency and good output regulation to power many of today's electronic devices. In a known switched mode power supply, a low frequency (e.g. 50 Hz or 60 Hz mains frequency), high voltage alternating current (AC) is converted to high voltage direct current (DC) with a diode rectifier and capacitor. The high voltage DC is then converted to high frequency (e.g. 30 to 300 kHz) AC, using a switched mode power supply control circuit. This high frequency, high voltage AC is applied to a transformer to transform the voltage, usually to a lower voltage, and to provide safety isolation. The output of the transformer is rectified to provide a regulated DC output, which may be used to power an electronic device. The switched mode power supply control circuit provides usually output regulation by sensing the output controlling it in a closed loop.
A switched mode power supply may include an integrated circuit power supply controller coupled in series with a primary winding of the transformer. Energy is transferred to a secondary winding from the primary winding in a manner controlled by the power supply controller to provide the clean and steady source of power at the DC output. The transformer of a switched mode power supply may also include another winding called a bias or feedback winding. The bias winding provides the operating power for the power supply controller and in some cases it also provides a feedback or control signal to the power supply controller. In some switched mode power supplies, the feedback or control signal can come through an opto-coupler from a sense circuit coupled to the DC output. The feedback or control signal may be used to modulate a duty cycle of a switching waveform generated by the power supply controller or may be used to disable some of the cycles of the switching waveform generated by the power supply controller to control the DC output voltage.
In order to compensate for process variations, analog integrated circuits such as power supply controllers are commonly trimmed for critical parameters during wafer sort, using trim pads on the wafer before being assembled in plastic packages. Trimming is done at the wafer level because the trim pads are not usually accessible after assembly (e.g. after encapsulation in plastic). Some of the tests are also only done at the wafer level because they require access to internal circuitry through test pads, which are accessible (through probes) only at wafer sort.
The disadvantage of trimming at wafer sort is that the trimmed parameters are subject to shifts due to physical stresses that the die is subjected to after assembly by the encapsulation material such as plastic. This limits the accuracy to which they can be guaranteed independent of how accurately it is trimmed at wafer sort. Consequently, the assembled parts are then tested again at final test to eliminate those parts that have shifted too much or were damaged during assembly.