Primary-side controlled flyback power converter
First Claim
1. A flyback power converter comprising:
- a transformer having a secondary winding, a first primary winding, and a second primary winding, wherein said first primary winding has a first terminal and a second terminal, said second primary winding has a first terminal and a second terminal, wherein said second terminal of said first primary winding is connected to said first terminal of said second primary winding, and said second terminal of said second primary winding is connected to a ground reference;
an output rectifier connected from said secondary winding to an output of the power converter;
a switching transistor for controlling the voltage across the primary windings of the transformer, wherein a drain of said switching transistor is connected to an input of the power converter;
a current-sense resistor for generating a current-sense voltage, wherein said current-sense resistor is connected between a source of said switching transistor and said first terminal of said first primary winding;
a PWM controller for regulating a switching frequency and for providing a PWM signal to drive a gate of said switching transistor, wherein said PWM controller has a supply-voltage input for receiving power, a detection input for receiving a flyback voltage, a current-sense input for current detection, a compensation input for frequency compensation, and a ground-reference input connected to said first terminal of said first primary winding, and wherein said current-sense input is connected to said source of said switching transistor;
a power-pin capacitor for starting up said PWM controller, wherein said power-pin capacitor is connected between said supply-voltage input of said PWM controller and said ground-reference input of said PWM controller;
a start-up resistor for charging said power-pin capacitor, wherein said start-up resistor is connected from said input of the power converter to said supply-voltage input of said PWM controller;
a power-pin diode connected from said second terminal of said first primary winding to said supply-voltage input of said PWM controller;
a detection resistor for receiving an offset current to compensate for the voltage drop across said output rectifier, wherein said detection resistor is connected from said second terminal of said first primary winding to said detection input of said PWM controller;
a compensation capacitor connected from said compensation input of said PWM controller to said ground-reference input of said PWM controller;
a transient voltage suppressor connected in parallel with said second primary winding; and
a snubber diode connected in series with said transient voltage suppressor.
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Accused Products
Abstract
The present invention provides a primary-side flyback power converter that supplies a constant voltage output and a constant current output. To generate a well-regulated output voltage under varying load conditions, a PWM controller is included in the power converter in order to generate a PWM signal controlling a switching transistor in response to a flyback voltage sampled from a first primary winding of the power supply transformer. Several improvements are included in this present invention to overcome the disadvantages of prior-art flyback power converters. Firstly, the flyback energy of the first primary winding is used as a DC power source for the PWM controller in order to reduce power consumption. A double sample amplifier samples the flyback voltage just before the transformer current drops to zero. Moreover, an offset current is pulled from a detection input of the double sample amplifier in order to generate a more accurate DC output voltage. The offset current is generated in response to the temperature in order to compensate for temperature-induced voltage fluctuations across the output rectifier. Ultimately, in order to maintain a constant output current, the PWM controller modulates the switching frequency in response to the output voltage.
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Citations
16 Claims
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1. A flyback power converter comprising:
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a transformer having a secondary winding, a first primary winding, and a second primary winding, wherein said first primary winding has a first terminal and a second terminal, said second primary winding has a first terminal and a second terminal, wherein said second terminal of said first primary winding is connected to said first terminal of said second primary winding, and said second terminal of said second primary winding is connected to a ground reference;
an output rectifier connected from said secondary winding to an output of the power converter;
a switching transistor for controlling the voltage across the primary windings of the transformer, wherein a drain of said switching transistor is connected to an input of the power converter;
a current-sense resistor for generating a current-sense voltage, wherein said current-sense resistor is connected between a source of said switching transistor and said first terminal of said first primary winding;
a PWM controller for regulating a switching frequency and for providing a PWM signal to drive a gate of said switching transistor, wherein said PWM controller has a supply-voltage input for receiving power, a detection input for receiving a flyback voltage, a current-sense input for current detection, a compensation input for frequency compensation, and a ground-reference input connected to said first terminal of said first primary winding, and wherein said current-sense input is connected to said source of said switching transistor;
a power-pin capacitor for starting up said PWM controller, wherein said power-pin capacitor is connected between said supply-voltage input of said PWM controller and said ground-reference input of said PWM controller;
a start-up resistor for charging said power-pin capacitor, wherein said start-up resistor is connected from said input of the power converter to said supply-voltage input of said PWM controller;
a power-pin diode connected from said second terminal of said first primary winding to said supply-voltage input of said PWM controller;
a detection resistor for receiving an offset current to compensate for the voltage drop across said output rectifier, wherein said detection resistor is connected from said second terminal of said first primary winding to said detection input of said PWM controller;
a compensation capacitor connected from said compensation input of said PWM controller to said ground-reference input of said PWM controller;
a transient voltage suppressor connected in parallel with said second primary winding; and
a snubber diode connected in series with said transient voltage suppressor. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16)
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Specification