MENU

AC/DC converters – empowered by SiC MOSFETs

By Wisse Hettinga


Introduction

AC/DC voltage converters are fundamental components of power electronics, with several applications in the industrial, medical, consumer electronics, railway, e-mobility (charging station) and solar sectors. A high degree of efficiency is extremely important in the power conversion process. The reduction of power losses and heat produced by high-power devices offers relevant benefits, including: economic savings, lower environmental impact and greater ease of meeting the maximum absorption requirements imposed by the most recent international regulations (think, for example, to PC power supplies). In this article we will present an innovative solution, developed by ROHM Semiconductor, which integrates a high voltage SiC MOSFET with an AC/DC QR converter. Silicon carbide is a wide bandgap (WBG) semiconductor capable of overcoming the limitations achieved by silicon in high power applications. The physico-chemical properties of SiC MOSFETs allow operation at higher switching frequencies and voltages than the corresponding silicon-based components, to which are added greater energy efficiency and the ability to withstand high operating temperatures without requiring external heat sinks.

AC/DC solutions for industrial applications

Applications of AC/DC converters in the industrial sector can be grouped in two categories: main power supply and auxiliary power supply. In both cases the input voltage is of the alternating type, with values that vary between about 400VAC and 690VAC; the difference between the two applications is that the main power supply mainly concerns the management of motors and other power loads (servo drives, factory automation and battery management system), while the auxiliary power supply is intended for low power applications which require continuous voltages up to a few tens of volts (embedded control systems, single board computers, sensors, etc.). As shown in Figure 1, the solution proposed by ROHM combines a SiC MOSFET with an AC/DC converter (here in the classic flyback configuration) to supply power to auxiliary systems in the industrial sector.

Infineon SIC MOSFET

 

Figure 1: ROHM’s AC/DC converter with integrated SiC MOSFET

 

Since ROHM released its BM2SCQ12xT-LBZ solution that is shaped in a “Through Hole type device (THD)” to the market in May 2019, many customers globally have shown interest in this product solution. Furthermore, ROHM has released package-upgraded products that is named as BM2SC12xFP2-LBZ in SMD; TO263-7pins. The new package is intended to be applied inside the auxiliary power supply industry. ROHM has early on observed industrial market requirements for the high creepage and clearance distance. Hence, ROHM developed an additional new SMD package with the same technology to address the application, which needs a high pollution degree. ROHM’s BM2SC12xFP2-LBZ series (SMD) has exactly the same specification as the BM2SCQ12xT-LBZ series (THD). They include a 1.7kV SiC MOSFET and allow to obtain unprecedented levels of efficiency, reliability and miniaturization, proposing itself as the ideal solution in industrial applications powered with 400VAC voltage in 3 phases, including street lamps, commercial AC systems, general-purpose AC servos and inverters used in high power equipment.

 

SiC MOSFET

Controller

FB OLP

VCC OVP

BM2SC121FP2-LBZ

 

1700V / 4A 

Rdson 1.2W

 

Quasi Resonant

(Fsw_max 134kHz)

Auto Restart

Latch

BM2SC122FP2-LBZ

Latch

Latch

BM2SC123FP2-LBZ

Auto Restart

Auto Restart

BM2SC124FP2-LBZ

Latch

Auto Restart

Figure 2: The lineup of BM2SC12xFP2-LBZ (SMD Type)

Benefits of integrated SiC MOSFET

The BM2SC12xFP2-LBZ series converters, the first on the market to include a 1700V SiC MOSFET, offer several benefits over traditional solutions made with discrete components. ROHM’s solution combines the power of a SiC MOSFET transistor, capable of withstanding high working voltages and offering a very low RDSON value, with the high efficiency achievable through an almost resonant integrated controller. The main advantages of this innovative component can be summarized as follows:

 

•      Enormous reduction regarding the number of required components: compared to traditional discrete type solutions, ROHM’s integrated solution allows to reduce the many number of required components even by integrating clamping circuit for Gate driver as well as SiC+QR Controller. A single monolithic component also reduces the likelihood of faults occurring in the circuit and, thanks to the use of power transistors based on silicon carbide, it is possible to achieve better thermal management.

•      very suitable for industrial applications with 690VAC input voltage in 3 phases (approx 1000VDC). 1700V SiC MOSFET is enough to cover the sum of high input voltage, reflected voltage and drain spike voltage.  

•      an increase in efficiency of 5% and a reduction in power losses of 28%. By virtue of these characteristics, the converter allows to obtain a significant reduction in size, with substantial improvements in terms of reliability and power absorption – essential requirements in industrial applications.

•      the high voltage that the SiC MOSFET can withstand and its high noise immunity allow to reduce the size of the components used for noise suppression.

•      availability of multiple integrated protection functions that further increase reliability, including overload protection (FB OLP), overvoltage protection on the power supply pins (VCC OVP) and high precision thermal shutdown (TSD). These functions are particularly relevant for industrial power supplies that require continuous operation, contributing to a significant improvement in system reliability.

•      the control circuit, implemented through a quasi-resonant controller (QR), allows a more efficient operation compared to traditional circuits based on the PWM technique, minimizing the effects produced by noise in industrial equipment.

•      devices are available in the THD package for powers up to 100W (over 36w the application of a heat sink is required) and in the SMD TO-263-7L package for powers up to 48W without requiring any heat sink. This last package, particularly compact, has been especially designed to meet the high creepage and clearance requirements.

 

The use of a Through Hole Device (THD) type is very limited to particular applications, such as industral applilcations, due to creepage and clearance package issues. To avoid high voltage electrical damage on the package the International Stand IEC60950 defines minimum creepage distance with three categories; Pollution degree(PD), Material Group(Index) and Input Voltage (RMS). ROHM’s new SMD package is assumed to be used in pollution degree PD=2, follows material group 2 and Input Voltage 950Vrms (Vin 900V + VOR 100V + Vspike 400V = 1400V). Therefore, a creepage distance between 5.6mm ~ 7.1mm is necessary to meet the requirements. The creepage of BM2SC12xFP2-LBZ is measured 6.8mm(typ), in order to meet market requirements. Besides, the addional cost of a heatsink is not an issue for SMD. Radiation Noise because of drain pin in THD package doesn’t occur.

In comparison to through hole (THD) package that has heat sink design flexibility, TO263 package shall be mounted directly onto PCB, and insertion of heat sink to dissipate its heat is therefore not possible. As a result, the output power the IC can provide is strictly related on how the PCB layout has been designed. Regarding this relevant aspect, ROHM has prepared dedicated boards for thermal evaluation. The thermal behaviour of three boards have been measured by different PCB patterns size for IC, 78mm2, 300mm2 and 600mm2 as shown in Figure 3. Their system specification are: Vin=300-900VDC, Vout=24V, Iout=2A, Po=48W, Ta=25°C.

Apart from the fact that increasing Vin contribute IC’s surface temperature to rise, it is clear  that the wider PCB patterns, the lower IC’s surface temperature. For instance, a temperature of 84.2°C@Vin=900V with a PCB pattern of 78 mm2 decreases to 70.7°C on a PCB pattern of 600mm2. System specifications are referred to a room temperature condition of 25°C. Since ambient temperature greatly affects the IC’s surface temperature, designers should pay full attention on handling IC and designing PCB with SMD type without heat sink, reserving enough PCB space to dissipate heat for system safety and output performance. Figure 3 shows a thermal scan analysis with the three boards operating with different input voltages.

Figure 3: thermal scan analysis on the three boards

 

Applications

Unlike traditional silicon-based devices (such as MOSFETs and IGBTs) which limit the energy savings achievable in auxiliary power systems, SiC MOSFETs offer a number of significant advantages such as: lower switching and conduction losses, better power management and possibility to operate at higher temperatures. By reducing the number of parts required, designers have more PCB surface available to implement new and innovative functionalities. ROHM’s converters, optimized for 400 VAC secondary power supply circuits in industrial environments (see Figure 4, where the converter is located in the primary side of the circuit), are particularly suitable for applications such as: general-purpose inverters, AC servos, PLCs (Programmable Logic Controllers), manufacturing equipment, robots, industrial lighting (street lamps), auxiliary power supplies, charging station, solar application and UPS.

Figure 4 Typical application circuit of the AC/DC converter with integrated SiC MOSFET

 

 

Figure 5 shows the simplified circuit diagram relating to the component. As can be seen, thanks to the presence of the integrated SiC MOSFET (connected between the drain and source terminals), the circuit is extremely simple and compact, requiring only a small number of external passive components. The feedback signal, necessary for a correct regulation of the output voltage when the load conditions are variable, is connected to the input pin FB (feedback) of the converter.

 

Figure 5: simplified circuit diagram

 

Block diagram

The detailed block diagram of the converter is shown in Figure 6. The devices belonging to the BM2SC12xFP2-LBZ series use a control technique based on PFM (Pulse Frequency Modulation). In particular, the status of the FB, ZT, and SOURCE pins is continuously monitored in order to determine the ON and OFF periods of the SiC MOSFET, thus optimizing efficiency and reducing noise. The integrated circuit also offers an important soft start functionality. Normally, a high current is flowing in the AC/DC converter at the time of switching on. To properly handle this aspect, the BM2SC12xFP2-LBZ series devices include a soft start feature to avoid inrush current during startup phase. This function is activated when the voltage on the VCC pin drops below approximately 14.0V (typical). The soft start feature performs the following actions immediately after power on:

·         from the turn-on instant to less than 1 ms: set the SOURCE limiter value to 25% of the normal value

·         from 1 ms to less than 4 ms: sets the SOURCE limiter value to 50% of the normal value

·         from 4 ms on: normal operation is activated

 

Figure 6: block diagram of the BM2SC12xFP2-LBZ converter

The integrated thermal shutdown functionality serves to prevent heat damage to the integrated circuit. Normal circuit operation should always occur within the maximum IC junction temperature rating. However, if the rating is exceeded for a continued period, the junction temperature (Tj) will rise, activating the TSD circuit which will turn off the power output pins. At this point the IC should be turned off and on again to resume normal operation, as the TSD circuit keeps the outputs OFF even when Tj falls below the TSD threshold.

Quasi resonant feature


The quasi-resonant switching technique, compared to the traditional continuous and discontinuous operating modes used in flyback converters, allows to drastically reduce switching losses, improving efficiency and thermal management. Furthermore, the electromagnetic interference generated with the quasi resonant technique is extremely low, especially at the (relatively low) voltages and currents of the applications in which these converters are used. Electromagnetic interference is very low thanks to the frequency jitter produced by this circuit, allowing the noise to be evenly distributed, simplifying the design and improving the effectiveness of EMI filters. In the classic flyback converter operating in the discontinuous conduction mode (PWM controlled), the MOSFET transistor switches to the ON state at a fixed frequency, without worrying about the effects produced by the frequency jitter. Subsequently, the MOSFET is commanded to the OFF state when the set current level is reached. After this process, the switching cycle is repeated indefinitely. The problem is that the instants of time in which the MOSFET is turned on are not synchronized with the resonance occurring on the drain terminal. It follows that, at times, the MOSFET can switch to the ON state when the voltage on the drain is very high, causing a deterioration in efficiency. Compared to the PWM technique, the quasi resonant switching circuit does not operate with a fixed switching frequency, but switches at the bottom of the resonant operation (see Figure 7). In the case of high input voltages, the quasi resonant technique proves to be superior to the PWM one, allowing to improve efficiency and reduce noise.

 

Figure 7: efficiency comparison between PWM and QR techniques

 

In the case of low power loads, the time in the ON state is reduced, resulting into an increased switching frequency and, potentially, in switching losses. However, this drawback is solved through the frequency reduction functionality, integrated in ROHM’s BM2SC12xFP2-LBZ series converters. This function, programmable through a minimum and a maximum voltage threshold, allows to limit the maximum switching frequency. This achieves the dual benefit of high efficiency and low EMI. The device also integrates protection circuits against overload (FB OLP), overvoltage (VCC OVP) of the supply voltage pin and a very precise thermal shutdown (TSD) function (obtained thanks to the integrated SiC MOSFET), together with both overcurrent and secondary side overvoltage protections. The integration of multiple protection circuits in industrial power supplies that require continuous operation contributes to significantly improved reliability and an extended duration of the system.

 

Conclusion

ROHM’s new BM2SC12xFP2-LBZ solution is very beneficial with regards to auxilary power supply inside the industrial market. The integrated 1700V SiC MOSFET in the IC is enough to substitute the need of Super Junction MOSFET or two series high voltage MOSFET for industrial applications with higher input voltages of up to 900VDC. As the two in one package provides simple design merits, it is also possible to accelerate the power supply design phase. Today’s upgraded SMD package from THD package is compliant with the international standard in terms of creepage and clearance criteria. As a result,  there is no technical barrier to be applicable in the market anymore.

 


Share:

eeNews Europe
10s