AC coupled bias circuit for power converters
A switching power system for converting an input voltage to an output voltage across a transformer is disclosed. The system includes a control circuit. The control circuit has a control input coupled for receiving the output voltage and an output coupled to a primary winding of the transformer. The system includes a bias circuit for supplying an operating voltage to the control circuit. The bias circuit includes a capacitor. The capacitor has a first terminal coupled to a first terminal of a first secondary winding of the transformer. A first diode is coupled between a second terminal of the first secondary winding and a second terminal of the first capacitor. A second diode is coupled between the second terminal of the capacitor and a bias input of the control circuit. A method of making the same is disclosed.
1. A switching power supply, comprising:
a transformer including a primary winding having a first terminal receiving an input voltage, a first secondary winding providing an output voltage of the switching power supply, and a second secondary winding having a center tap connected to a ground terminal;
a switching transistor having a conduction path coupled to a second terminal of the primary winding;
a pulse width modulated control circuit having a control input coupled for receiving the output voltage of the switching power supply and an output coupled to a control input of the switching transistor; and
a bias circuit having an output for supplying an operating voltage to the pulse width modulated control circuit, the bias circuit consisting of:
(a) a first capacitor having a first terminal coupled to a first terminal of the second secondary winding,
(b) a second capacitor having a first terminal coupled to a second terminal of the second secondary winding, the first and second terminals of the second secondary winding being on opposite ends of the winding with the center tap between the first and second terminals of the second secondary winding,
(c) a first diode having a cathode coupled to a second terminal of the first capacitor at a first node and an anode coupled to a second terminal of the second capacitor at a second node,
(d) a second diode having an anode coupled to the first node and a cathode coupled to the output of the bias circuit,
(e) a third diode having a cathode coupled to the second node and an anode coupled to the ground terminal, and
(f) a third capacitor having a first terminal coupled to the output of the bias circuit and a second terminal coupled to the ground terminal.