2019.07.10
This summary concludes our discussion, spanning 19 articles, of “Design Example of Isolated Quasi-Resonant Converters Using SiC MOSFET“.
This design example involves two major points. One is the fact that SiC MOSFETs are used as power switches. Compared with Si MOSFETs, SiC MOSFETs offer lower losses and superior operation characteristics at elevated temperatures. The other point is that a quasi-resonant design was selected for the switching topology. Features of quasi-resonant designs include low noise and high efficiency. By combining these features, an AC-DC converter with high efficiency and low noise even when handling high voltages can be designed. This design example should provide confirmation of the contributions of SiC power devices to energy efficiency, which has become a major issue in recent years.
The power supply IC used is a quasi-resonant controller that employs an external SiC MOSFET. However, ROHM is currently developing a converter IC with an internal SiC MOSFET. While there are numerous converter ICs with internal Si MOSFETs, this will be the world’s first device with an internal SiC MOSFET.
Below are summarized all issues addressed and key points.
・Design example of a quasi-resonant isolated AC-DC converter
・A SiC MOSFET is used as a power switch.
・In order to use SiC MOSFETs in designs employing power supply ICs, a power supply IC specifically designed for the purpose is necessary.
・The gate driving voltages VGS of SiC MOSFETs and Si MOSFETs are different.
・In this design, the BD7682FJ-LB, an AC-DC converter controller IC used to drive a SiC MOSFET, is employed.
・A self-excitation type flyback converter using a quasi-resonant design utilizes the voltage resonance between the transformer primary winding inductor and a resonance capacitor.
・The quasi-resonant design makes possible reduced switching loss and noise levels.
・The core size, primary inductance and number of turns for a transformer T1 are calculated according to the procedure described.
・Calculations can be performed according to more or less the same approach as in “Designing Isolated Flyback Converter Circuits: Transformer Design (Calculating Numerical Values)”.
・The core size, primary inductance and number of turns for a transformer T1 are calculated according to the procedure described.
・Calculations can be performed according to more or less the same approach as in “Designing Isolated Flyback Converter Circuits: Transformer Design (Calculating Numerical Values)”
・As the MOSFET Q1, a SiC MOSFET, which is a recurring theme of these design examples, is used.
・When selecting the MOSFET, the maximum Vds, peak current, loss due to ON-resistance, maximum allowable power dissipation for the package, and other factors are considered.
・The I_{D} rating is selected to be roughly Ippk×2.
・Vds is calculated according to an equation.
・Series-connected input capacitors are used to obtain the required rated voltage.
・Balancing resistors are inserted in order to obtain uniform voltages when capacitors are series-connected.
・Balancing resistors result in simple IR losses, so care must be taken in choosing resistance values.
・An overload protection correction function of this IC reduces power losses by lowering the current limit level when the input voltage rises above a setting value, to provide more reliable protection during overloading.
・The switching voltage is set through the resistance value of the resistor R20, calculated according to the equation provided.
・The IC power supply VCC is generated by the VCC windings using the secondary-side output.
・Upon startup, a secondary-side output is not generated, and so a circuit to supply a voltage for startup is provided separately.
・In order to avoid erroneous VCC OCP activation, a resistor to limit the VCC windings surge voltage is necessary.
・A brownout function is a protective function that stops switching operation when the VIN falls below the voltage necessary for normal operation.
・A voltage obtained by resistive division of the VIN is applied to the BO pin, to set voltages for starting and stopping operation.
・Component values are calculated according to given formulae.
・A snubber circuit is incorporated in order to suppress surges arising from the leakage inductance of a transformer at the input.
・A basic snubber circuit has an RCD configuration, but for more complete protection, TVS diodes can be added.
・The gate driving signal is adjusted to optimize the loss and noise of the switching transistor.
・If the switching rise and fall times are made faster, losses are reduced, but the switching noise increases.
・As output rectifying diodes, fast recovery diodes or Schottky barrier diodes are used.
・Diodes should be selected such that the device is used within 70% of the rated voltage and within 50% of the rated current.
・An output capacitor is selected using the peak-to-peak ripple voltage (ΔVpp) that can be tolerated by the output load and the ripple current.
・The output voltage setting resistor can be calculated from an equation given by the data sheet.
・Feedback signal adjustment components should conform to values given on the data sheet.
・Components necessary for the different detection pins should be set according to the data sheet and the design manual.
・If noise enters the detection pins, erroneous operation and other problems may result. The addition of a capacitor or RC filter should be considered.
・EMI countermeasures include addition of an input filter, Y capacitors, and snubbers for output rectifying diodes.
・To deal with output noise, an LC filter can be added to the output.
・In all cases, component values must be adjusted while checking the influence of noise.
・PCB layout rules are essentially the same even when using a SiC MOSFET or in the case of a quasi-resonant converter.
・The example circuit is used to measure and examine efficiency.
・The circuit components described are merely one example, and there are other options.
・The circuit example shown is used to measure and study efficiency.
・Circuit components are examples, and others can be selected.
Basic studies to understand AC-DC converters and to go designing.
Basic studies to understand AC-DC converters and to go designing.