Snubber capacitor generating an auxillary power supply voltage
An integrated circuit (IC) forming a pulse-width modulator controls the switching operation of an output stage of a switching power supply. A snubber capacitor that is coupled to a primary winding of a transformer of the output stage is used for producing a capacitive coupled charging current. The capacitive coupled charging current is coupled to a filter or charge storage second capacitor for producing in the second capacitor a first portion of a second power supply voltage. During a portion of a switching cycle of the output stage, the snubber capacitor is coupled to an inductor to form a resonant circuit. The resonant circuit produces in the second capacitor a second portion of the second power supply voltage for energizing the IC. The second power supply voltage is used for energizing the IC.
1. A power supply, comprising:
a source of an input supply voltage coupled to a first winding of a transformer:
an output switching transistor responsive to a first switching control signal and coupled to said first winding for producing a current in said first winding that is transformer-coupled to a second winding of said transformer to produce a first output supply voltage that energizes a first load circuit, said output switching transistor interrupting said first winding current, during a first portion of a cycle of said first switching control signal, and producing a flyback voltage in said first winding;
a first capacitor coupled to said first winding and to a second capacitor to form with respect to said flyback voltage a capacitive voltage divider of a snubber capacitance network that reduces a rate of change of said flyback voltage and produces a capacitive current in each of said first and second capacitors, said capacitive current storing a corresponding charge in each of said first and second capacitors that develops in said second capacitor a first portion of a second supply voltage, said second supply voltage being coupled to a second load circuit; and
an inductor coupled to said first capacitor during a second portion of said cycle, when said output switching transistor is conductive to form a resonant circuit of said inductor with said first capacitor for generating from the charge stored in said first capacitor a resonant current that is coupled to said second capacitor, said resonant current storing a charge in said second capacitor for developing a second portion of said second supply voltage.
2. A power supply according to claim 1 , further comprising a controllable second switching transistor for selectively interrupting at least one of the currents that store the corresponding charges in said second capacitor in a manner to regulate said second supply voltage.
3. A power supply according to claim 2 , further comprising a comparator responsive to said second supply voltage and coupled to a control terminal of said controllable second switching transistor for changing a conduction state of said controllable second switching transistor when said second supply voltage is outside a range of values.
4. A power supply according to claim 2 , wherein said second switching transistor is coupled in parallel with said inductor.
5. A power supply according to claim 2 , wherein said second switching transistor is coupled to a junction terminal between each of said inductor and said first capacitor.
6. A power supply according to claim 1 , further comprising a rectifier coupled between said first and second capacitors for enabling the corresponding current that charges said second capacitor to flow in a first direction and for preventing a flow of a current in said second capacitor in an opposite direction.
7. A power supply according to claim 1 , wherein said first output voltage is produced in a second winding of said transformer that isolates said first output supply voltage from said second output supply voltage.
8. A power supply according to claim 1 , wherein said first and second capacitors are coupled in series.
9. A power supply according to claim 1 , wherein said second capacitor is larger than said first capacitor.
10. A power supply according to claim 1 , wherein said output switching transistor couples said inductor in parallel with said second capacitor to form said resonant circuit.
11. A power supply according to claim 1 , further comprising a duty cycle modulator for generating said first switching control signal wherein said second supply voltage is coupled to said duty cycle modulator for energizing said duty cycle modulator.