Three-level modulation for wide output voltage range isolated dc/dc converters

A power converter having first and second input terminals to receive an input signal from a voltage source to provide an output voltage or an output current to at least one load, comprising:

• (a) a transformer having primary and secondary windings;
• (b) a primary-side circuit, comprising:
• (i) first, second, third, and fourth series-connected switching devices connected across the first and second input terminals, the first and second switching devices being controlled by first and second switch control signals, respectively, and the third and the fourth switching devices being controlled by a third and a fourth complementary switch control signals, respectively;
• and
• (ii) a resonant tank circuit, the resonant circuit being coupled (1) between a first electrical node, being a common electrical node between the first and second switching devices, and a second electrical node, being a common electrical node between the third and fourth switching devices;
• and (2) to the primary winding of the transformer;
• (c) a secondary-side circuit coupled to the secondary winding of the transformer, the secondary-side circuit comprising a filter capacitor providing the output voltage or output current to the load;
• and
• (d) a control circuit that, based on the output voltage or the output current, is configurable to generate the first, second, third, and fourth switch control signals according to a first modulation scheme in which (i) the first and the second switch control signals are substantially complementary with each other and the third and the fourth switch control signals are substantially complementary with each other, (ii) the first, second, third, and fourth switch control signals are periodic with a common switching period, (iii) within the common switching period, (1) each switch control signal has two rising edges and two falling edges;
• and either (2) the first rising edge of the first switch control signal lags the first rising edge of the fourth switch control signal by a first predetermined time, while the second rising edge of the first switch control signal leads the second rising edge for the fourth switch control signal by a second predetermined time, or (3) the first rising edge of the first switch control signal leads the first rising edge of the fourth switch control signal by a first predetermined time, while the second rising edge of the first switch control signal lags the second rising edge for the fourth switch control signal by a second predetermined time, and (iv) the control circuit varies the switching period, the first predetermined time, and the second predetermined time.