Bench Talk

Good Things Come in Small Packages: SiC FETs in OBC Chargers

(Source: Qorvo)

Silicon carbide (SiC) MOSFETs are firmly establishing themselves as a contender for the semiconductor switches in all stages of electric vehicle (EV) on-board chargers at power levels of 22kW and higher. The UnitedSiC (now Qorvo) SiC FETs, with their unique cascode construction of a Si MOSFET and SiC JFET, outperform IGBTs in efficiency, and are more attractive than superjunction MOSFETs. It’s not just about overall converter system losses, though. Cost, size, and weight are also important factors that matter to an EV owner.

Designers have a choice of different package styles for semiconductor power switches in EV on-board chargers, including surface mount variants that can be viable up to tens of kW when SiC FETs are used. In this blog, we will look at some SiC FET performance figures.

At the typical power levels seen in EVs, even with 98%+ efficiency, on-board chargers still need to dissipate hundreds of watts from a small housing in a hot environment. As a result, heatsinking is needed, often with liquid cooling. A major design consideration is how the switches connect to this heatsinking arrangement for optimal thermal transfer, yield reliability, and low assembly cost. It is common to find SiC FETs in a TO-247-4L package that delivers excellent thermal performance, around 1.0°C/W from the junction to the cooling fluid, using UnitedSiC (now Qorvo)'s wafer-thinning technology with a silver-sinter die and a ceramic isolator pad. However, a downside of the TO-247-4L package is that it needs mechanical fixing and through-hole soldering. It also has significant package inductance and limited creepage and clearance between its pins. In addition, the package has less distance between PCB pads unless the leads are ‘joggled’ in a complicated and expensive way.

A surface-mount alternative would seem attractive, but at the 22kW level? Actually, yes, it can be viable with UnitedSiC (now Qorvo) D2PAK-7L devices with little or no effect on performance, depending on the power conversion stage considered. Looking at the headline differences between the package styles in Table 1 below, the D2PAK-7L wins except for die pad size, which results in an overall junction to cooling fluid thermal resistance of around 1.3°C/W for an 18-milliohm device bonded to an insulated metal substrate, about 30 percent more than the TO-247-4L package.

Table 1: Comparison between D2PAK-7L and TO-247-4L (Source: Qorvo)

The practical effect of a higher thermal resistance is a higher junction temperature for given power dissipated, all things being equal, but because of the substantial assembly savings with an SMT device, perhaps lower resistance parts can be used, reducing temperature. However, if using only one SMT device reaches thermal limits—Tj becomes too high—paralleling SMT devices is a viable solution. If using two SMT devices in parallel to replace one SMT device, each of the two paralleling SMT devices should have twice the on-resistance compared to using only one SMT device. In this case, the current in each is halved, but on-resistance is doubled for each, so dissipation is half of a single part. The total dissipation of two paralleled SMT devices would be slightly lower compared to using just one SMT device with half the on-resistance. Thermally, each device would be much cooler because for the same thermal management (thermal resistance from junction to ambient or coolant), each paralleling device only dissipates half the loss of a single SMT device. So theoretically, each paralleling SMT device’s temperature rise from ambient- or coolant-to-junction should also be half of a single SMT device. Apart from this, the lower package inductance of the D2PAK-7L might allow faster switching edge-rates for even lower dynamic losses.

It's useful to look at some examples of the package performance comparisons in the different stages of a typical on-board charger using the UnitedSiC online FET-Jet Calculator^™. A ‘Totem Pole PFC’ stage is common, and an example rated at 6.6kW with 400V output, 75kHz, continuous conduction mode (CCM) was evaluated with a range of TO-247-4L and D2PAK-7L SiC FETs for the ‘fast switching’ leg, for a heatsink/fluid temperature of 80°C. The junction temperature differences between the two packages ranged from 3°C to 8°C depending on the class of on-resistance.

At higher power and with a three-phase AC supply, a ‘Vienna rectifier’ might be used with an 800V DC-link at, say, 40kHz (Figure 1). 750V SiC FETs can be used, and if 18-milliohm TO-247-4L and D2PAK-7L parts are compared again, the difference in junction temperature is just 3°C with 0.1% difference in ‘semiconductor’ efficiency. Higher on-resistance parts in this application inevitably show a bigger difference with an unworkable temperature rise for single devices, but at 22kW in a high-value product, the cost of the lower resistance parts is not a large overhead for the benefits gained.

Figure 1: The image illustrates the Vienna rectifier front end. (Source: Qorvo)

D2PAK-7Ls Can Replace TO-247-4Ls in the DC/DC Stage Effectively

The Totem-Pole PFC and Vienna rectifier stages just considered are ‘hard’ switching, and frequency is kept relatively low to minimize dynamic losses. The DC/DC stage in an OBC can be a resonant or ‘soft’ switched converter, such as the CLLC topology with a much higher frequency for small magnetics and low loss, typically 300kHz. For example, at 6.6kW with a 400V DC-link and using 18-milliohm SiC FETS, losses according to FET-Jet Calculator™ per device are 4.1W and 4.2W for TO-247-4L and D2PAK-7L respectively, and the lower inductance of the SMT package makes it a natural choice for the higher frequency used.

Moving to SMT D2PAK-7L packages is a natural progression from TO-247-4L types when total system cost is considered with minimal or no difference in temperature rise or system efficiency, especially if the electrical and mechanical ease of paralleling is factored in. As SMT devices, along with their class-leading Figures of Merit (FoM) and easy gate drive, SiC FETs are inching closer to the ideal switch choice for EV on-board charger applications.

With standard 1700V ratings and better efficiency than IGBTs, SiC FETs are becoming more attractive than super-junction MOSFETs, firmly establishing themselves as contenders in all stages of EV on-board charging. While SiC FETs in a TO-247-4L package provide excellent thermal performance, a downside is that they require mechanical fixing and through-hole soldering. So, migrating to an SMT device like the UnitedSiC D2PAK-7L package is a natural evolution when total system cost is considered with minimal or no impact on temperature rise or efficiency. These SMT SiC FETs not only offer designers significant savings on circuit assembly but also class-leading FoM and an easy gate drive solution, making them the ideal switch choice for on-board chargers for EVs.

Mike Zhu is an application engineer at UnitedSiC Inc (now Qorvo). He received his BS in Electrical Engineering from Chongqing University in 2013, and MS in Electrical & Computer Engineering from The Ohio State University in 2015 and joined UnitedSiC since then. He has 9 year’s research experience in SiC and GaN device evaluation, design of high frequency, high-efficiency and high-power density power electronics as well as EMI solutions for WBG devices.