Phase Shift DC-DC Converter with Integrated Magnetics
Overview
This demonstration shows a phase shift DC-DC converter with a magnetically integrated current doubler. The combined transformer and output inductors are implemented in the PLECS Magnetic Domain.
Model
Fig. 1 Phase shift converter
Note
Model
This model shows a full bridge phase shift converter with a current doubler at the output. A stiff \(400\,\mathrm{VDC}\) voltage source at the input represents the rectified utility. A transformer provides galvanic isolation, while the transformer windings ratio is chosen to provide \(48\) to \(50\,\mathrm{VDC}\) at the output. The current doubler rectifier is implemented by providing the output current two paths across separate branches containing an output inductor and diode. To reduce the overall size of the magnetic components, the transformer and filter inductors may share the same core. Due to the phase shift modulation, zero-voltage switching (ZVS) can be achieved, which reduces the turn-on losses of the MOSFETs to zero.
Control
Each switch in the full bridge is on for \(50\,\%\) of its respective switching period. The switch pairs in the two legs all have the same switching period but are operated such that between each switch a phase shift is introduced that varies a small amount over time based on a dynamically calculated modulation index using feedback measurements. An output voltage error signal is generated based on a setpoint value and this is fed through a PI regulator to generate an output current setpoint. The difference between output current setpoint and the measured value is fed into the current controller, which has a simple proportional gain, and generates the phase shift ratio for the PWM modulator.
Magnetic Circuit
The motivation to integrate the output inductors into the transformer core is to increase the power density of the converter. Modeling such a magnetic geometry using an electrical equivalent circuit with coupled inductances can be cumbersome. Using the PLECS Magnetic Domain, however, modeling such a structure becomes intuitive and simple. The parameters of the magnetic model are directly related to the geometry and material characteristics of the core, which in most cases can be obtained from datasheets.
In this model two E 70/33/32 cores from TDK using N87 material are stacked.
Thermal Model
A thermal description is assigned to all four of the MOSFET switches in the full bridge as well as the output diodes. These descriptions can be viewed and edited by double-clicking on the component and selecting Edit… from the drop-down menu of the Thermal description parameter. The thermal parameters were obtained from datasheets provided by Infineon, specifically for the IPW60R280P6 CoolMOS superjunction MOSFET and IDW75E60 Si power diode. For the two different device types, the thermal impedance chain representing the thermal transitions from the junction to the case are entered directly in the thermal descriptions.
Each switch (Q1-Q4) in the H-bridge converter is realized using three MOSFETs connected in parallel. The number of parallel devices is configured in the parameter window. An equal distribution of current is assumed across all parallel devices, and the thermal parameters are automatically scaled to reflect the device count and configuration.
Since the three heat sink components are connected together, all six devices dissipate heat into the same heat sink. A thermal resistance connects the heat sink with the temperature of the ambient air. The thermal descriptions for the MOSFETs and diodes are stored in a private thermal library in the directory /phase_shift_dcdc_converter_with_integrated_magnetics_plecs.
The average switching, conduction and total losses of the semiconductor devices can be calculated easily using the “Switch Loss Calculator” component. Select the switch components of interest in the schematic editor and drag them onto the probed components list of the “Switch Loss Calculator” block. For more details, browse the Help section of this block.
The “Efficiency Calculator” subsystem calculates the converter efficiency using the source power and total switch losses.
For more information on thermal modeling and the calculation of device losses and efficiency, see the demo model Buck Converter with Thermal Model.
Simulation
Run the simulation with the model as provided to view the output signals, the losses and junction temperature for Q1, and the BH curve of the magnetic core.
Steady-State Operation
The final temperature of the junction for Q1 can be calculated with the Steady-State Analysis that is pre-configured in the model. For PLECS Standalone or Blockset (in Simulink) execute the following steps:
Standalone: From the Simulation menu choose Analysis tools…. This opens a dialog, in which the analysis has already been pre-configured. To start the analysis, click on the Start analysis button. You can display the progress of the analysis by clicking on the Show log button.
Blockset: Double-click on the Steady-State Analysis block to open the dialog and click on the Start analysis button. The progress of the analysis is displayed in the MATLAB Command Window.
When the analysis has finished a simulation of five steady-state cycles is displayed for all waveforms preconfigured in the Scopes.
Conclusion
This model discusses the operation of a phase shift DC-DC converter, with both thermal and magnetic circuits included. In addition to a startup transient simulation, a Steady-State Analysis is also configured to quickly determine the final operating temperatures of the devices.
Bibliography
[1]
U. Badstuebner, J. Biela, B. Faessler, D. Hoesli and J.W. Kolar, “An Optimized 5 kW, \(147\,W/in^3\) Telecom Phase-Shift DC-DC Converter with Magnetically Integrated Current Doubler,” Applied Power Electronics Conference and Exposition (APEC), 2009, Twenty-Seventh Annual IEEE, pp. 21-27, 15-19 Feb. 2009.