Design Example of Isolated Quasi-Resonant Converters Using SiC MOSFET
Power Supply ICs Used in Design: Optimized for SiC MOSFETs
- AC/DC converter design
- Gate driving
- Isolated AC/DC converter design
- Quasi-resonant converter design
- SiC on-resistance
- Silicon Carbide
In considering design, we begin by discussing power supply ICs used in design. As mentioned in the "Introduction", this chapter addresses the two new issues of the design of quasi-resonant converters and the use of SiC MOSFETs as power transistors. Hence power supply ICs used in the design will be quasi-resonant converter ICs that can use SIC MOSFETs as switches.
In Designs Using Power Supply ICs, a Specialized Power Supply IC Must be Used with SiC MOSFETs
The power supply IC to be used in the design is ROHM's BD7682FJ-LB IC. The BD7682FJ-LB is a quasi-resonant controller for AC/DC converters, and is the world's first* IC optimized for driving SiC MOSFETs. (*3/25/2015)
The reader will have noticed that, in order to use a SiC MOSFET as a switch, a power supply IC designed for this purpose is needed. This means that gate driving of a SiC MOSFET is different from that for a Si MOSFET. One may immediately wonder just what is different; but before discussing the power supply IC, we must first explain the differences in gate driving of SiC MOSFETs and Si MOSFETs.
The main difference is that the VGS to drive SiC MOSFETs is somewhat higher, and the internal gate resistance is higher, so that an external gate resistor Rg with a smaller value is used. Rg is an external resistor, and so properly belongs in a discussion of circuit design. However, the gate driving voltage nearly always depends on the IC specifications, and so although it may not always be strictly necessary, the best option is to select a power supply IC that has been optimized for use with a SiC MOSFET.
Specifically, the driving voltage of general IGBTs and Si MOSFETs is VGS = 10 to 15 V, and the gate driving voltage (OUT pin high voltage) of a power supply IC, such as for example the BM1P061FJ PWM controller IC for AC/DC converters used in " Design Method of PWM AC/DC Flyback Converters ", is 10.5 V (min) to 14.5 V (max), with a typical value of 12.5 V.
In contrast, because a SiC MOSFET gradually saturates at VGS voltages of 20 V and above, it is recommended that driving be at VGS=18 V. The gate driving voltage (OUT pin clamp voltage) of the BD7682FJ-LB used here is 16.0 V (min) to 20.0 V (max), and typically 18.0 V.
Below are shown the on-resistance vs. VGS characteristics of the R8005ANX, an N channel, 800 V, 5 A Si MOSFET used in the design of the BM1P061F (left), and the SCT2H12NZ, the N channel, 1700 V, 3.7 A SiC MOSFET used in this section (right). We can see that the each IC gate driving voltage is a VGS value that is somewhat lower than the level at which the respective MOSFETs reach saturation.
This comparison is not accomplished in equivalent specifications and conditions, and so should be regarded as merely illustrating the above-described differences in VGS.
Power Supply IC Used in Design: The BD7682FJ-LB AC/DC Converter Controller IC for SiC MOSFET Driving
The above explanation should have clarified the most important aspects of the BD7682FJ-LB for use with SiC MOSFETs. Below we summarize the features of the IC.
- Small 8-pin SOP-J8 package
- Quasi-resonant method (low EMI)
- Frequency reduction mode
- Low current consumption during standby: 19uA
- Low current consumption at no load (burst operation at light load)
- Maximum frequency : 120kHz
- CS pin Leading-edge blanking
- VCC under-voltage lockout (UVLO) and VCC over-voltage protection (OVP)
- Cycle-by-cycle over-current protection circuit
- ZT trigger mask function and ZT over-voltage protection (OVP)
- Input under-voltage protection (brownout)
- Gate clamp circuit for SiC-MOSFETs
- Operating power supply voltage range (VCC) : 15.0V～27.5V
- Normal operating current : 0.80mA (typ)
- Burst operating current : 0.50mA（typ）
- Maximum oscillation frequency : 120kHz（typ）
- Operating temperature range: -40℃～105℃
The principal features are the design for use with SiC MOSFETs, and the quasi-resonant method. Quasi-resonant method results in lower noise compared with PWM method due to soft switching operation, and can reduce EMI while achieving high efficiency.
Moreover, numerous protection functions are incorporated to enable high-voltage (up to 690 VAC) operation, accommodating a broad range of industrial equipment applications. The IC is provided with overvoltage protection for power supply pin, brown-in/out protection for input voltage pin (function to prohibit operation at low input voltages), overcurrent protection, secondary-side overvoltage protection, and the like.
In high-voltage applications, SiC MOSFETs have advantages over Si MOSFETs, such as lower switching and conduction losses and smaller temperature-induced characteristic fluctuations. These advantages are useful when pursuing power conservation and miniaturization, which in recent years have been important goals, for example through higher-efficiency power conversion, smaller heat sinks, and miniaturization of transformers and capacitors by raising operation frequencies.
The graph on the right compares the efficiency of a SiC MOSFET and a Si MOSFET in an AC/DC converter. As the graph indicates, an improvement in efficiency of up to 6% can be expected.
In addition to the BD7682FJ-LB used this time, there are three other variations, differentiated by their FB pin overload protection and VCC pin overvoltage protection.
|FB pin OLP||VCC pin OVP|
Finally, although not directly related to characteristics or performance, these are products with long-term supply guarantees specifically for the industrial equipment market. They are power supply ICs that offer excellent performance and the protection functions necessary for industrial equipment, among other features, and so this commitment to long-term supply should also be an important plus.
Next time, we will explain the quasi-resonant method.
・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.