Switched mode power supplies can be used to create a direct current (DC) voltage from an alternating current (AC) voltage by switching current through an energy storage element such as a transformer. The duty cycle of the switching is controlled to regulate the output voltage to a desired level. Switched mode power supplies are generally efficient at heavier loads but less efficient at lighter loads. Two popular types of isolated switched mode power supplies are forward mode and flyback mode converters.
Flyback converters are common in AC voltage to DC voltage applications. A flyback converter is based on a flyback transformer that alternately builds up flux in the magnetic core and transfers energy to the output. When current is switched through the primary winding, the primary current in the transformer increases, storing energy within the transformer. When the switch is opened, the primary current in the transformer drops, inducing a voltage on the secondary winding. The secondary winding supplies current into the load. A controller varies the on- and off-times of a primary switch in series with the primary winding to regulate the output voltage to a desired level.
Flyback converters can be configured to switch additional reactive elements in parallel to the primary winding using a topology known as active clamp flyback (ACF). ACF converters can reduce electric stress on components and improve efficiency by achieving close to zero volt switching (ZVS) of the primary switch and to produce clean drain waveforms without any ringing. They also allow soft increase in secondary current. However while ACF converters have high efficiency at medium and heavy loads, their efficiency decreases at lighter loads due to continuous conduction losses from magnetizing current that continuously circulates on the primary side of the transformer due to the additional reactive elements. Moreover, ACF converters are not suitable for other techniques that improve efficiency at light loads such as cycle skipping and frequency foldback.
In flyback converters, it may be advantageous in certain applications to increase the frequency of operation to 1 megahertz (MHz) and beyond. When the frequency of operation is increased, the primary magnetizing inductance and the output capacitance may be reduced. However there is a tradeoff between size reduction and increased frequency on the one hand and low standby power on the other. Converters consume less standby power if switching losses are reduced when operating at light loads or no load. Recently, certain regulatory agencies have set strict standards for efficiency and have encouraged the development of power supply controllers that are capable of both high frequency and low frequency operation to operate at low frequencies at light loads and no load. Thus the frequency change from no load to full load may vary widely based on the chosen frequency of operation. In addition, users may want to start or stop the frequency foldback under various load conditions in different applications. Known converters, however, are dedicated to certain operating conditions and are unable to satisfy these requirements at the same time.
The use of the same reference symbols in different drawings indicates similar or identical items. Unless otherwise noted, the word “coupled” and its associated verb forms include both direct connection and indirect electrical connection by means known in the art, and unless otherwise noted any description of direct connection implies alternate embodiments using suitable forms of indirect electrical connection as well.