ADC-DC converter that can operate from a high input voltage is needed for future high-power space applications. However, the selection of space-qualified, high-voltage transistors and filter capacitors for such a converter are very limited. The available high-voltage components have lower performance than lower-voltage components. One possible solution to this problem is connecting in series the inputs of multiple converters to lower the input voltage at the individual converter inputs. However, because of component tolerances, performance degradation, and transient events, this can result in an unbalanced voltage distribution throughout the various inputs. Excessive voltage on any of the stacked converters can damage components and cause a catastrophic failure. A circuit that could inherently balance the voltage between the inputs of multiple low-voltage DCDC converters would have better performance and reliability.
One circuit that meets these requirements is known as the Auto-Balancing Series-Stacked (ABSS) converter. Any number of inverters can be stacked in series on the input to distribute the input voltage. Diagonally opposed transistors in each inverter are switched simultaneously and alternately. This generates magnetic flux in the transformer core that couples equally to multiple identical primary windings wound on the same core, forcing the voltage to be balanced between the primary windings and thus on the individual inputs to the converters. Any transient operation or component degradation in the transformer or transistor will impact all converters equally and maintain a balanced voltage distribution.
In one implementation of the ABSS converter, four full-bridge inverters were stacked in series to a common transformer, allowing operation at a maximum input of 600 V. Each individual inverter operates at 150 V using high-performance transistors rated for 250 V. The power transformer has four identical primary windings for each stacked inverter and two identical secondary windings connected in series that can generate up to 400 V.
A pulse width modulation controller operating at a frequency of 30 kHz uses peak-current control, an inner current loop measuring output filter inductor current, and an outer voltage control loop to provide tight output regulation and over-current protection. The implementation employs primary current transformers in each leg of each full-bridge inverter to ensure that a fault in one bridge will be properly sensed and depends on identical operation of the two bridges.
Tests demonstrated balanced and symmetrical operation of all inverters under a variety of steady-state, input, and transient load conditions. Preliminary performance measurements demonstrated efficiencies in the order of 95% at a maximum output power of 4 kW. An analysis concluded that the ABSS converter has minimum reliability impact when compared to a single full-bridge converter.
This work was done by Luis Pinero and Robert Scheidegger of Glenn Research Center and Arthur Birchenough of Vantage Partners, LLC. NASA is seeking partners to further develop this technology through joint cooperative research and development. For more information about this technology and to explore opportunities, please contact http://technology.grc.nasa.gov.LEW-19172-1A .