Typical application circuits require low voltage bias source to operate. But sometimes only the utility AC is available. So there is a need to generate this low voltage bias source directly off the AC line.
FIG. 1 schematically shows a prior art linear power supply 10 which provides a low voltage bias source from the offline AC. The linear power supply 10 is simple but lossy as a rectifier resistor-zener diode circuit.
FIG. 2 schematically shows a prior art switch mode power supply 20 which provides a low voltage bias source from the offline AC. However, PWM control may be an overkill approach for just a simple bias circuit.
FIG. 3 schematically shows a prior art step-down transformer power supply 30 which also provides a low voltage bias source from the offline AC. The step-down transformer power supply 30 is large in volume and high in cost, which is not acceptable for most applications.
So there is a need for a simple, efficient and cost effective low voltage dc source converted directly off the AC line.