A power converter used in an industrial system such as a photovoltaic (PV) inverter, Uninterruptible Power Supply (UPS) or industrial motor drive will normally include an auxiliary supply unit which supplies power to system peripherals such as a microprocessor, LCD display, sensors and fans, as well as to the gate drivers inside the main power circuit.
Auxiliary supply units commonly support loads of up to 40W, and provide an output at a low DC voltage. The standard topology used in these circuits is flyback, and in three-phase systems they will typically operate from an input voltage of up to 480V AC or 900V DC.
Such a system places considerable demands on the auxiliary supply unit’s power switch. Taking into account the reflected voltage in the primary side, which is added during the blocking state, a power switch with a breakdown voltage of more than 1,500V is normally required. The use of standard silicon MOSFETs for this application entails considerable problems. The designer is faced with one of two undesirable options:
• either to connect several lower-voltage devices in series, which increases the system’s complexity, size and component count
• or to use a 1,500V silicon MOSFET, a device type which suffers from high power losses and which require a bulky and expensive heat-sink
The adoption of a high-voltage Silicon Carbide (SiC) MOSFET in place of a single silicon MOSFET eliminates these drawbacks. SiC MOSFETs with a 1,700V rating are available now from ROHM Semiconductor in surface-mount (TO-268-2L) and fully moulded, isolated (TO-3PFM) packages, as shown in Figure 1. The devices feature an extended creepage distance of 5mm and 5.45mm respectively.
To show the superior characteristics of an auxiliary supply unit based on a SiC MOSFET, ROHM has developed an evaluation board which may
be used to supply peripheral devices in a power converter, and which supports loads up to 40W at an output voltage of 12V. Based on the flyback topology, this AUX Evaluation Board includes the SCT2H12NZ, a 1,700 SiC MOSFET, as the main switch, and the BD7682FJ-LB, a quasi-resonant flyback controller. This controller’s operation helps to keep the dynamic losses in the SiC MOSFET to a minimum, thus reducing its operating temperature.
The board operates from both AC and DC inputs, drawing power either directly from the mains or from the DC portion of the power supply, such as after the PFC stage. For mains-supplied designs, the input-voltage range is wide: from 210V to 690V AC. This is useful in UPS and industrial drives, which commonly supply the auxiliary power unit from the mains.
For DC-fed systems, the input range is 300V-900V DC. In PV inverters, this enables the auxiliary supply unit to draw power directly from the solar panels, or after the boost-conversion stage.
The BD7682FJ-LB quasi-resonant controller used in the AUX Evaluation Board is a compact and effective solution intended to meet the requirements of a SiC MOSFET. It operates at a variable frequency of up to 120kHz; the frequency is adjusted in response to the load conditions. The benefit of this can be seen in Figure 4, which shows the drain-source voltage at various output-power levels.
The turn-on time is dynamically modified to ensure that the switch turns on in an oscillation valley. This minimises the switching losses in the SiC MOSFET, reducing its operating temperature and increasing system efficiency. When there is no load, the controller goes into burst mode to keep energy losses low.
Simple to use and housed in a compact SOP8-J8 package, the BD768xFJ-LB offers many functions and protection features:
• Current sense, implemented via a shunt resistor in series with the SiC MOSFET
• Overload protection configured by the current-sense resistor
• Mask function: avoids erroneous sensing of the ￼￼￼voltage over the auxiliary winding
• Quasi-resonant control for low dynamic losses and EMI
• Frequency-reduction mode to increase efficiency at light loads
• Burst operation in no-load conditions for low-current stand-by operation
• Output over-voltage protection
• Soft start
• Input brown-out protection
• Integrated SiC MOSFET driver
Measured system performance
Figure 5 shows the performance of the AUX Evaluation Board when operating at three DC input voltages: 300V, 600V and 900V. In each case, efficiency was measured across an output-power range from zero to the system’s maximum rated power of 40W. At the lowest 300V input voltage, overload protection was activated above 30W.
At an input voltage of 300V, efficiency peaks at 87%. At higher input voltages, dissipation across the resistor dividers used in the circuit increases. This means that efficiency declines slightly as the input voltage increases. Nonetheless, efficiency remains above 80% across nearly all load conditions.
Over the entire load range at all three input-voltage values, the highest temperature measured at the MOSFET’s case was 80°C. The SCT2H12NZ’s maximum junction temperature is 175°C, so no heat-sink was required for the operation of the test circuit.