Power converter control to avoid imbalance and transformer saturation

#A switched-mode power converter can be implemented using a full-bridge power converter configuration, such as for an application where an alternating current (AC) input or a direct current (DC) input is provided, and a DC output is to be generated (or vice versa, such as for bi-directional conversion). For example, for single-phase AC input, four switches (e.g., transistors) can be used to direct a current through a primary winding of an isolation transformer. During a first switching phase, the switches can be controlled such that current having a first polarity is conducted through the primary winding. During a second switching phase, the switches can be controlled such that the primary winding conducts a current having a polarity opposite the first polarity.

#A secondary side of the isolation transformer can include one or more windings electrically coupled to a rectification circuit, the output of which can be used to provide power to other circuitry. The rectification circuit can be coupled to an inductor. The during switching phases where power is transferred from the primary winding to the second winding, the current in the inductor can be ramped up, and during other portions of the switching cycle, the current in the inductor can ramp down, thereby generating an output voltage.

#As mentioned above, a switched-mode power converter can include a transformer-coupled configuration, such as including an isolation transformer. A bridge network can be used to controllably couple an input voltage (e.g., an alternating-current (AC) source or a direct current (DC) source) to a primary winding of the isolation transformer, such as using a first current polarity during a first switching phase, and an opposite second polarity during a second switching phase. During a first switching phase, for example, a magnetic flux is established in the transformer. Such flux can be concentrated particularly in a highly magnetically-permeable core material of the transformer if the transformer includes such a core material (e.g., steel, iron, ferrite, or one or more other materials). During a second switching phase, an opposing magnetic flux is established. Generally, the time-averaged flux in the transformer should be about zero, to avoid saturation of the core material of the transformer. Such saturation can lead to poor efficiency, such as high losses or heat generation, or may even damage one or more of the switches in the bridge network or the transformer. In general, balancing of the opposing magnetic fluxes induced during first and second switching phases can be referred to as maintaining a “volt-second” balance, because such a flux can be proportional to an integral of a terminal voltage of at a winding of the transformer with respect to time over a duration of a switching cycle, such as can be expressed in units of “volt-seconds.”

#FIG. 1 illustrates generally at least a portion of a switched-mode power converter circuit, such as can include an isolation transformer comprising a primary side coupled to an input voltage via a switching network, and an output secondary side coupled to one or more of a rectifier network or a filter network.

#FIGS. 2A through 2C illustrate generally illustrative examples of an input voltage signal that can include a rectified representation of a time-varying current through a primary side of an isolation transformer of a switched-mode power converter, such as in a balanced operating state as in FIG. 2A, an unbalanced stated as in FIG. 2B, and in a state where the isolation transformer is saturating in a first phase in FIG. 2C.

#FIG. 3 illustrates generally a portion of a control circuit for a switched-mode power converter circuit, such as can include a current-to-voltage converter, a rectifier, one or more analog-to-digital converters, one or more decimators, and a difference determination circuit.

#In the drawings, which are not necessarily drawn to scale, like numerals may describe similar components in different views. Like numerals having different letter suffixes may represent different instances of similar components. The drawings illustrate generally, by way of example, but not by way of limitation, various embodiments discussed in the present document.

#FIG. 1 illustrates generally at least a portion 150 of a switched-mode power converter circuit, such as can include an isolation transformer 100 comprising a primary side (e.g., a primary winding) coupled to an input voltage, “+V,” via a switching network, and an output secondary side (e.g., including one or more windings) coupled to one or more of a rectifier circuit 104 (e.g., a rectifier network) or a filter circuit (e.g., including one or more of an inductor L or a capacitor C). Generally, switched-mode power converter can use the transformer 150 and switching circuitry (e.g., switches A through D) to convert an input signal (such as a direct current (DC) source) provided across an input node +V and a reference node, to a desired output, VO. Such an input can be balanced or single-ended in polarity. For example, an alternating current input can be stepped down and rectified to provide an intermediate-valued DC supply, +V, for input to the circuit shown in FIG. 1.

#The secondary side of the transformer 100 can be coupled to the output VO, such as through a full-wave rectifier circuit 104 or other rectification scheme (as in examples where a DC output voltage is desired at VO). The filter network can include one or more energy storage elements, such as the series inductor L, which can store energy during one or more “charging” phases wherein the current through the inductor L is ramped up, and then such an inductor L can discharge at least a portion of such stored energy into a load connected to the output VO, such as during a “discharge” portion of a switching cycle. A duty cycle of such charging phases can be varied, such as to provide a regulated output VO.