FEATURE POWER ELECTRONICS EVALUATING THE OPTIONS
Walter Balzarotti, field application engineer and Vladimir Scarpa, applications engineer of power, both at Rohm Semiconductor explore how the latest 1700V SiC MOSFET and Quasi-resonant controller brings compactness and lower system cost to auxiliary power supplies for advanced application requirements
P
ower converters used in industrial systems, such as photovoltaic (PV)
inverters, uninterruptable power supplies (UPS) and industrial motor drives, need an auxiliary supply unit (AUX). These units provide the required power for system peripherals – i.e. Microprocessor, LCD display, sensors, fans - as well as for gate drivers inside the main power circuit.
The output power of such AUX is the range of some tens of Watts with low DC voltage. Standard topology is Flyback and input voltage levels in 3- phase systems can go up to 480Vac or 900Vdc. Considering the reflected voltage in the primary side, which is added during blocking state, a power switch with breakdown voltage above 1500V is normally required. By using standard silicon devices, the possible implementations become either too complex, i.e. using the series of lower voltage devices, or very inefficient, i.e. using 1500V rated silicon MOSFETs that show high losses and require bulky and expensive heat-sinks. In order to overcome these issues, a 1700V rated SiC MOSFET is used as a single, high performance device for AUX applications. Rohm offers a family of these devices, as shown in Fig. 1. This includes parts with on-resistances of 0,75Ω and 1,15Ω, available in two different packages, namely TO-268-2L and TO-3PFM. The former is in a SMD package, while the latter is in a full moulded, isolated package. Both have extended creepage, respectively 5mm and 5.45mm. In order to overcome these issues, a 1700V rated SiC MOSFET is used as a single, high performance device for AUX applications. This article presents an evaluation board developed by Rohm Semiconductor, focused on AUX applications. It is based on Flyback topology, and contains the device SCT2H12NZ as the main switch, in combination with the BD7682FJ-LB, a quasi-resonant Flyback controller. This controller further reduces the dynamic losses on the SiC MOSFET and so lowers its operating temperature. The board is able to operate with both AC and DC input voltages. It is
18 JULY/AUGUST 2017 | ELECTRONICS Figure 1:
1700V SiC MOSFET family from Rohm semiconductor
be useful for solar PV inverters, enabling the power to be extracted directly from the PV panels or after the step-up stage. Electrical parameters of the AUX evaluation board can be found in the table of Figure 2. In order to bring the MOSFET
Figure 2:
Top and bottom views of the AUX evaluation board, and its main electrical parameters
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switching losses even lower, the quasi- resonant ACDC controller BD7682FJ-LB is used. This family of devices is a compact and effective solution especially designed to fulfill the requests of a SiC MOSFET. It operates with variable frequency, according to the load conditions. This can be seen in Figure 4, which shows measured drain- to source voltage VDS at different output power values. The turn-on time is dynamically modified in order to have the switch turn-on in an oscillation valley. This minimises the dynamic losses of the SiC MOSFET, reducing its temperature and increasing system efficiency. For no load condition, the controller goes in burst mode to avoid unnecessary energy losses. A datasheet of the BD768xFJ-LB, is
available:.
http://rohmfs.rohm.com/en/p roducts/databook/datasheet/ic/power/is olated_converter/bd7682fj-lb-e.pdf
therefore possible to derive the power directly from the grid or from the system DC link, e.g. after the PFC stage. In case of AC input, the accepted input voltage range goes from 210VAC to 690VAC. This option can be interesting for applications like UPS and industrial drives, that derives the power for the AUX from the AC grid. In case of DC input, the input range goes from 300VDC to 900VDC. This can
Figure 3:
Schematic view of the AUX evaluation board, including main functions of BD768F controller
Figure 4:
VDS measurements (200V/div) at different load conditions
EXPERIMENTAL RESULTS The AUX evaluation board has been tested at three different DC input voltage conditions, namely 300V, 600V and 900V. For each case, the output power range was from zero to nominal power 40W. In case of low input voltage 300V, the over load protection was activated above 30W. For VIN
=300V, maximum achieved
efficiency is 87%. Higher the input voltage, higher the dissipation on the resistor dividers used in the circuit. Therefore the efficiency decreases slightly with the increasing of the input voltage. Nonetheless, it is above 80% of efficiency in most of the load conditions. As the maximum case temperature of the SiC MOSFET was 80°C, no dedicated heat-sink was necessary during the complete measurement set.
Rohm Semiconductor
www.rohm.com/eu T: 01908272400
/ ELECTRONICS
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