Engineers can build motor drives and power supplies that deliver very high levels of efficiency by combining frugal, fast SiC Super Junction Transistors with optimised gate drivers, argues Ranbir Singh from GeneSiC Semiconductor.

Two of GeneSiC’s SiC devices: a 1200 V/ 6A SJT chip and a 10 kV/10 A  SJT chip



Today the silicon IGBT reigns supreme in motor drives and industrial automation systems, where it is used to process voltage and current waveforms and deliver optimum power for DC-DC conversion and AC-DC/DC-AC power conversion. But this device is under increasing threat from transistors built from SiC and GaN.

Both of these alternatives have several similar, attractive characteristics. For example, they have breakdown electric fields that are around an order of magnitude higher than that for silicon – these unlock the door to new device designs, which are much thinner than the incumbent and feature blocking layers with higher doping levels. Other strengths of these wide bandgap transistors, which stem from bandgaps that are around three times that of silicon, include the ability to operate at much higher temperatures and stand up to high-radiation environments.

What’s more, in the case of SiC, the thermal conductivity of this material is much higher than that for silicon, so dissipated heat can be more readily extracted from the device. This, in turn, allows more power to be applied to the device before it exceeds a certain temperature.


GaN versus SiC

Today, GaN power switches continue their relentless progress by increasing their blocking voltage capabilities, but are still limited to  ratings less than 400 V in commercial offerings. The switches offered to date are normally-on (depletion mode) devices that require a negative gate bias to turn them off. Circuit designers find this a significant hindrance, so some switch makers deploy a low voltage silicon MOSFET in a Cascode configuration with the normally-on GaN FET to achieve a normally-off operation. Such a circuit element can be driven using standard silicon MOSFET drivers, but may suffer from parasitic inductances associated with such a connection.

When it comes to SiC, many companies are developing and producing different types of transistors, with their own advantages and disadvantages. At GeneSiC of Dulles, VA, we are pioneering SiC Super Junction Transistors (SJTs), which are gate-oxide free, normally-off, majority carrier devices. They are competing for sales with the likes of power MOSFETs (including planar DMOSFETs and trench-MOSFETs) and (normally-on and normally-off) JFETs.

Vast differences in the intrinsic material properties between SiC and silicon mean that although a particular device stole the show with the incumbent material, it is not necessarily destined to be the outright leader in the SiC arena. Instead, an alternative device may be more promising, because when it is made from SiC, it may exploit the best material properties of SiC, and minimise the use of properties where SiC lags behind silicon.

For example, the silicon BJT (and the IGBT) exhibit minority carriers in the drain region, which are not present in the SiC equivalent. This allows the wide bandgap device to operate at very high frequencies, like a majority carrier device, so it is not plagued by dynamic breakdown issues, such as a poor reverse-bias safe-operating-area (RBSOA). Meanwhile, contemporary SiC MOSFETs have channel mobilities that are just 5-10 percent of that of silicon MOS devices, and the high doping levels found in the drain regions of normally-off SiC JFETs have made it very challenging to manufacture this device with high yields and uniform characteristics. The SJT, in comparison, is hallmarked by: a very high current gain, which can be in excess of 100 and allows low gate currents; and a good RBSOA profile, which is indicative of the robustness of this device.

Figure 1: Several different SiC switch technologies have been developed. In addition to the junction transistor, a form of which is made by GeneSiC, there are planar MOSFETs, trench MOSFETs and JFETs

Driver considerations

These comparisons of device performance have limited worthiness, because any critical assessment of the suitability of the device for industrial deployment must not be restricted to simply its standalone performance, or even the combination of this and its cost. Instead, meaningful judgement of the merit of any class of transistor must include an analysis of how easily it can fit in with existing drive infrastructure.

Dominance of the silicon IGBT in many motor controls and power supplies has led to widespread use of voltage-controlled drivers in these applications. Modern gate drivers generally switch at +15 V levels and feature higher current sourcing/sinking capabilities than their predecessors. Current levels are now several amperes, to accommodate high operating frequencies and large gate capacitances, in both IGBTs and high-current MOSFETs.

One of the downsides of the contemporary SiC MOSFET is that it requires a higher drive voltage than that produced by many, but not all, modern gate drivers: It needs +20 V to achieve a sufficiently low on-resistance. This higher-voltage requirement results from poor transconductance, which can be traced back to the low channel mobilities of SiC. Far lower drive voltages are possible with some classes of SiC transistor that involve a junction-based approach. Junction transistors and normally-off JFETs require just a +4 V drive, but may require non-zero continuous gate currents; while normally-on JFETs may need a negative bias of up to 30 V to turn them off.

This brief overview of voltage requirements for many different classes of SiC transistor appears to imply that all devices require a non-standard gate driver. Given that, it is not surprising that many SiC device manufacturers are actively working on optimum gate drivers for their switch offerings. However, it is possible to use off-the-shelf IGBT drivers with our SiC SJTs because their continuous gate current requirement can be supplied by such ICs. The magnitude of this current can be controlled with a series gate resistance, similar to that used in an IGBT drive, and its addition can also provide the requisite gate-source voltage (3-4 V) for operating this class of junction transistor.

One option for driving all these switches is to combine a commercial gate driver IC with an isolated input signal and a resistor-capacitor output network (see Figure 2). Using this approach, our SJTs can be driven with gate voltages as low as 8–10 V. One additional benefit of using this particular type of SiC switch is that it does not require a negative gate voltage to remain off.
 
Figure 2: The SJT produced by GeneSiC, which can be driven by a gate drive IC, must be capable of supplying a continuous current of 0.5 A to the gate of this transistor. The external parallel gate resistor, RGP, should be adjusted to meet this requirement, while the external parallel capacitor, CGP, can be chosen to ensure an optimum level of dynamic gate current during turn-on and turn-off initial transients. This dynamic current is essential for fast charging of the internal gate-source capacitance. The presence of this paralleled resistor and capacitor on the output of the gate driver can increase the device switching speed, reduce its switching loss and also cut driver losses



Due to the high voltages being switched, the input signal source from potential high drain voltages needs to be protected with an optocoupler or isolator. The isolation rating of this component should greatly exceed the predicted DC voltages in use, particularly with an inductive load present. Choke coils can also be inserted if common-mode noise in the circuit on voltage supplies and gate driver inputs and outputs is too high.

Dynamic considerations

These gate drive considerations only take into account steady state, on-state operation, and it is much more important to consider dynamic losses at high operating frequencies. This is because SiC switches lead to the biggest gains in efficiency over the silicon incumbents when they operate at tens or hundreds of kilohertz. Operating in this regime, the losses associated with the driver and the entire system are dominated by charging and discharging of the gate-source and the Miller capacitances (the capacitances seen looking into the input).

Driver switching losses are directly proportional to the product of the gate-source (CGS) capacitance and the square of the voltage swing. This swing is typically 4-5 V for SJTs and normally-off JFETs, but it can be as high as 20-30 V for MOSFETs and normally-on JFETs – implying that dynamic capacitive losses in the latter devices can be up to 50 times higher. Meanwhile, the device switching loss is governed by the gate-drain (CGD, Miller) capacitance and the square of the device voltage swing, which could be as high as 800 V. Today, the value of CGD can be two-to-three times lower for SJTs and normally-off JFETs, compared with a MOSFET of a similar current rating. So, in summary, SJTs and normally-off JFETs are significantly ahead of their SiC rivals, when it comes to efficient operation in driver circuits.


Figure 4: An industry standard, double-pulse switching test demonstrates GeneSiC’s SJT switching performance using the gate drive circuit detailed in Figure 2. During testing, the SJT is turned on with the application of a gate current IG and the drain current ID is ramped up linearly while flowing through the inductor and SJT in series until ID hits 6 A. At that point the SJT is switched off, and then switched back on after a 2 µs delay to record device turn-on

We have assessed the switching performance of our SJT in the gate drive circuit outlined in Figure 2 using an industry standard, double-pulse switching test. Measurements reveal a current rise time, tr, of only 16 ns, and a fall time, tf, of 26 ns (voltage and current waveforms of the SJT are shown in Figure 3).  Total device switching energy loss is only 97 µJ per cycle, equating to less than 10 W of device switching loss at 100 kHz, while switching 600 V/6 A (3.6 kW).

Figure 3: Turn-on (top) and turn-off (bottom) switching waveforms of a 1200 V / 6 A SJT (GA06JT12-247)

Lower losses are possible by turning to a parallel resistor and capacitor on the gate driver IC output, similar to that used in high frequency IGBT drivers. With this change, a dynamic gate current waveform is introduced – due to the presence of a transient gate current peak from the charging of the gate capacitor – and this turns the SJT on and off more quickly (see Figure 5 for an example).

Figure 5: The waveform of the transient gate current, IG, while driving a 1200 V / 6 A SJT. Similar gate current switching transients are observed in MOSFET and IGBTs as well 

It is possible to alter the static and dynamic performance of the SJT – and to ultimately trade-off the switching speed to the device and the driver losses to fit the particular application demands – by adjusting the gate resistor, capacitor, and gate driver output voltage. For a fixed driver output voltage, higher capacitance leads to higher current peaks (see Figure 6(a)) with shorter rise and fall times (see Figure 6 (b)). However, increasing capacitance can have its downsides, such as higher device and driver losses (see Figures 6 (c)). So it is important to hit a sweet spot, where the gate capacitance is low enough to trim device and driver losses but still high enough to obtain desired switching speeds. Care must also be taken to avoid ringing, which may occur in the gate drive output network due to interplay between the gate capacitance and the parasitic inductances in the gate drive circuit. One good remedy is to place a low-inductance resistor in series with the gate capacitance.
 
Figure 6: The external gate capacitance, CGP, impacts: (a) the peak gate current, IG,pk; (b) device turn-on tr and turn-off tf times;(c) and device energy loses

Another variable is the gate driver output voltage, which impacts SJT performance. This voltage must be high enough to bias the SJT gate-source junction on – it has a built-in voltage of about 2.8 V – and supply the steady state gate current that follows the gate current peak. There are no trade-offs associated with increasing the voltage, which leads to a nearly linear decrease in total energy and rise and fall times (see Figure 7).

Figure 7: A higher gate voltage reduces transition times and energy losses

However, using an excessively high gate driver output voltage is not to be recommended, because this can be a large contributor to gate driver loss and steady state driver losses. A superior solution, called a 2-level driver, is provided by Rabkowski et. al. (see further reading). This uses two current levels to drive SJTs – one during the transients, and another during the steady state operation of these devices.

When we drive our 1200 V/6 A SJT with a judicious choice of gate driver output voltage and gate capacitance and resistance values, we obtain very low power losses. Using a duty cycle of 0.7 and a frequency of 500 kHz, the steady state loss for the driver is 3.85 W, while the switching losses for the driver and SJT are 0.54 W and 45.6 W, respectively. These switching losses are frequency dependent, and dominate below 70 kHz (see Figure 8).
 
Figure 8: Gate driver dependent system power loss as a function of frequency for a fixed duty cycle of D = 0.7. A 1-level gate driver is as shown in Figure 2. A 2-level driver circuit uses two gate-driver ICs, supplying different currents during transients and steady-state operation. Note that the SJT conduction loss component is not considered here

The discussion provided here illustrates that replacing silicon IGBTs with SiC switches may not always give engineers of power circuits the ease of use that they may have anticipated, due to the drive requirements of particular classes of this wide bandgap device. However, our measurements show that one type of device, our SJT, can work very well with a conventional gate drive. Its strengths include fast switching speeds and ultra-low losses, and it is not plagued with many of the drawbacks of other SiC transistors and bipolar silicon devices.


In industrial applications, higher efficiencies are being reached through greater deployment of fast, power-semiconductor switches in variable speed drive motors

Further reading



D. Veereddy et. al. Bodo´s Power Systems, pp. 36–38 Oct-2011.
S. Sundaresan et. al. Power Electronics Technology, pp. 21–24 Nov-2011.
“IXD_614 Low-Side Driver Datasheet.” IXYS Inc. http://www.ixysic.com/home/pdfs.nsf/www/IXD_614.pdf/$file/IXD_614.pdf
J. Rabkowski et. al. Power Electronics, IEEE Transactions on 27 2633 (2012)
“GA06JT12-247 Datasheet.” GeneSiC Semiconductor Inc.

http://www.genesicsemi.com /index.php/sic-products/SJT