Switched-mode power converters are usually less efficient as their switching speed rises. This is important to keep in mind because the transient power dissipation that occurs in switched-mode supplies as voltages and currents swing between high and low levels can peak in the kilowatts. These transient losses are directly proportional to switching frequency, so faster switching increases them. In modern converters switching at 100kHz or more, transient dissipation still needs to be actively managed and minimized.
The latest SiC switches are fast – up to ten times faster than silicon IGBTs for parts with similar ratings. Their speed stems, in part, from the much smaller die sizes with lower device capacitances, enabled by the ability to have very low on-resistance per unit area.
The fact that SiC can sustain electric fields before breakdown that are ten times greater than for silicon allows the design of on-resistances ideally 100X lower than unipolar Si devices. Device architecture is another contributor to superior performance. UnitedSiC cascodes use SiC JFETs, which are half the size of any available SiC MOSFETs.
The theoretical advantages of high switching speeds are always tempered by the physical realities of implementation. For example, the edge rates for wide band-gap (WBG) devices such as SiC FETs can commonly be more than 100kV/µs and 3000A/µs. Just measuring these signal edges, with rise and fall times measured in nanoseconds, demands very high bandwidth oscilloscopes. Once you have been able to characterize such waveforms, the next question is, can you put them to practical use?
Challenges with Inductance
Take a half-bridge rectifier in a TO-247 package. It’s likely to present a series inductance of up to 50nH. Since V = –Ldi/dt, we can work out that a 50nH inductance will drop 150V when hit with a 3000A/µs edge-rate waveform, and that the voltage will appear as a drain voltage overshoot. Similarly, a stray drain capacitance of just 10pF will cause current pulses of 1A from a waveform with a 100kV/µs edge rate, which can lead to ohmic losses in heat sinks.
FETs have their problems, too. Their source inductance can cause a transient voltage that opposes the gate-drive signal, with a consequent risk of spurious turn-on. Fast transitions can also cause oscillations and chaotic behavior within FETs. For these reasons, internal gate resistances are often added to SiC FETs to slow edge rates. It is customary to use external resistances to slow the edge rate of on- and off-drive voltages.
Many of these issues can be overcome by using SiC cascodes (Figure 1) with snubbers, which provide a fast, normally-On device with effectively zero gate-drain capacitance. External gate resistor RGEXT used in conjunction with device RC snubbers can provide the needed overshoot and dV/dt control with minimal loss impact. This solution can be used to upgrade the efficiency of systems built with standard Si MOSFETs or IGBTs, using the same gate drive circuitry.
Figure 1. SiC JFET cascode UF3C half-bridge with snubbers.
Relying only on external gate resistors does have issues, though – the resistors effectively introduce turn-off delays, limiting the circuit’s minimum on-time and hence its control range and operating frequency. This matters for new designs that need to switch at high frequency to make the most of WBG device characteristics.
Utilizing JEFTs with RC Snubbers
Recent research at UnitedSiC has shown that ‘taking the brakes off’ a SiC JFET cascode by using faster JFETs, low values for the external gate resistors, and simple RC snubbers, increases switching speed and power conversion efficiency while limiting voltage overshoots. You might think that this approach just transfers the power dissipation issues from the FET to the snubber, but our tests show that the snubbers can be quite small to achieve the voltage-limiting effect. Improvements to the JFET have also halved its lower reverse recovery charge, Qrr, which leads to lower turn-on losses relative to our UJ3C general-purpose devices.
Devices from our UF3C series can be used with snubber resistor values of five or ten ohms and capacitors down to 47pF. The actual values vary with the device type and application, with hard-switched active rectifiers, totem-pole power factor correction and similar circuits seeing the most benefit. These characteristics mean that the devices can easily be used to upgrade existing designs because they’ll usually already have positions for snubbers.
Measuring Switching Losses for TO-347 Packaged Devices
Figure 2 shows some comparative total switching losses for various TO-247 packaged devices in the 1200V/35mOhm class. The UF3C120040K3S (PDF) device with a 33ohm gate resistor and a snubber of 330pF and 5ohm shows excellent results across the entire load range.
Figure 2. The comparative values of total switching loss (EON+EOFF) including snubber loss.
Figure 3 shows the measured loss in the snubber resistor for the UF3C120040K3S. The losses are a small fraction of the total switching loss, since the capacitances are small.
Figure 3. Snubber resistor loss as a fraction of EON+EOFF switching loss
These results show that it is possible to have the efficiency benefits of high-speed switching without the risk of voltage stress from overshoots with small snubbers using the UF3C series SiC cascodes. The fact that the devices are compatible with a wide range of Si and SiC gate-drive voltages and also have guaranteed avalanche ratings is a bonus.
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