Summary

＜Design Example of Isolated Quasi-Resonant Converters Using SiC MOSFET＞

1. Introduction

   Key Points

   ・Design example of a quasi-resonant isolated AC-DC converter

   ・A SiC MOSFET is used as a power switch.

2. Power Supply ICs Used in Design: Optimized for SiC MOSFETs

   Key Points

   ・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.

3. Design Example Circuit

   Key Points

   ・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.

4. Transformer T1 Design – 1

   Key Points

   ・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)”.

5. Transformer T1 Design – 2

   Key Points

   ・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)”

6. Selecting Critical Components: MOSFET Q1

   Key Points

   ・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.

7. Selecting Critical Components: Input Capacitor and Balancing Resistor

   Key Points

   ・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.

8. Selecting Critical Components: Switch Setting Resistors for Overload Protection Points

   Key Points

   ・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.

9. Selecting Critical Components: VCC-Related Components of Power Supply ICs

   Key Points

   ・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.

10. Selecting Critical Components: Components Related to Power Supply IC BO (Brownout) Pins

    Key Points

    ・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.

11. Selecting Critical Components: Components Related to Snubber Circuits

    Key Points

    ・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.

12. Selecting Critical Components: MOSFET Gate Drive Adjustment Circuit

    Key Points

    ・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.

13. Selecting Critical Components: Output Rectifying Diode

    Key Points

    ・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.

14. Selecting Critical Components: Output Capacitors, Output Setting and Control Components

    Key Points

    ・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.

15. Selecting Critical Components: Current Sense Resistors and Components Related to Detection Pins

    Key Points

    ・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.

16. Selecting Critical Components： Components for Dealing with EMI and Output Noise

    Key Points

    ・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.

17. PCB Layout Example

    Key Points

    ・PCB layout rules are essentially the same even when using a SiC MOSFET or in the case of a quasi-resonant converter.

18. Example Circuit and Component List

    Key Points

    ・The example circuit is used to measure and examine efficiency.

    ・The circuit components described are merely one example, and there are other options.

19. Evaluation Results: Efficiency and Switching Waveform

    Key Points

    ・The circuit example shown is used to measure and study efficiency.

    ・Circuit components are examples, and others can be selected.