Primary-side controlled flyback power converter

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.

Description

BACKGROUNDING OF THE INVENTION 1. Field of Invention The present invention relates to a switching mode power converter and more particularly relates to a flyback power converter. 2. Description of Related Art A flyback power converter typically includes a PWM controller, a power MOSFET (Metal Oxide Semiconductor Field Effect Transistor), a transformer, and a feedback-control circuit. The purpose of the feedback-control circuit is to sense the output voltage and / or the output current of the secondary-side of the power supply, and to supply a feedback signal to the PWM controller through an isolated device such as an optical-coupler. FIG. 1 shows a traditional flyback power converter. Although this circuit is able to regulate the output voltage and output current, it has several drawbacks. One drawback of this circuit is its large size, due to the need for an optical-coupler and a secondary feedback-control circuit. Another drawback of this flyback power converter is high power co...

AI Technical Summary

Benefits of technology

A principal object of the present invention is to provide a flyback power converter under primary-side PWM control. The flyback power converter according to the present invention supplies a well-regulated constant voltage and constant current output. However, unlike prior-art PWM controllers, it does not require a secondary-side feedback circuit or an optical-coupler. This enables the device count, the size, and the cost of the power supply to be reduced.

Another feature of the flyback power converter according to the present invention is an offset current source. The offset current source is used to compensate for the temperature dependence of the output rectifier. This allows the PWM controller to be supplied with a more accurate feedback signal. The offset current source of the PWM controller pulls an offset current from an input of the double sample amplifier in response to the operating temperature of the flyback power converter. The offset current will produce a voltage drop across a detection resistor to compensate for voltage variations across the output rectifier.

Problems solved by technology

One drawback of this circuit is its large size, due to the need for an optical-coupler and a secondary feedback-control circuit.

Another drawback of this flyback power converter is high power consumption.

The current-sense resistor significantly increases the power consumption of the power converter.

One drawback of this prior-art invention is that the feedback control voltage is sensed from a high voltage source.

This method results in a loss of accuracy, and it increases the cost of the controller.

Another drawback is that the voltage drop of the output rectifier is not compensated for.

The drawback of this prior-art is that the optical-coupler and the secondary feedback circuit are still required for loop control.

One principal drawback of these two prior-arts is inaccurate feedback control.

However, this reflected voltage signal is not directly proportional to the output voltage, because of the spike voltage generated by the leakage inductance of the transformer.

Furthermore, the voltage drop of the output rectifier is not compensated for in the feedback loop.

When load changes occur, this problem will introduce additional distortion into the output voltage.

Another drawback of these two prior-art inventions is high power consumption.

However, the resistor in the filter burns the majority of the reflected power, even if the power consumption required for PWM control is low.

Therefore, the power consumption of the power supply is high.

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