Posted on 18 September 2014

SiC: The Reliability Aspect and Practical Experience (Test)

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By Jochen Hüskens, Product Marketing Manager, ROHM Semiconductor GmbH

Reliability of SiC SBDs

Obviously, breakdown issues in the outer periphery structure of SiCSBD caused by high dV/dt were reported for conventional products but such breakdowns have not been observed in ROHM’s SiC SBDs at dV/dt up to 50 kV/us. In life tests, the SiC SBDs were proofed with regard to the high temperature reverse bias, temperature humidity bias, temperature cycle, pressure cooker, high and low temperature storage. In stress tests, they were examined in terms of resistance to solder heat, solderability, thermal shock, terminal strength (pull as well as bending). Furthermore, Si-FRDs exhibit breakdown due to the very large reverse recovery current induced by high dI/dt. This is extremely unlikely with SiC-SBDs since they have much lower recovery current.

Reliability of SiC MOSFETs

Oxide is used as gate insulating layer. Its reliability directly affects SiC MOSFETs’ reliability.

Development of high-quality oxide has been a challenging problem for the industry. ROHM solved this issue by a combination of appropriate oxide growth process and device structures. As the CCS-TDDB (Constant Current Stress Time Dependent Dielectric Breakdown) data show, its SiC MOSFETs have achieved quality equivalent to that of Si-MOSFETs and IGBTs.

Referring to Figure 1, QBD serves as quality indicator of the gate oxide layer.

ROHM could confirm 1.000 operating ours without any failures

The value of 15 - 20C/cm2 is equivalent to that of Si-MOSFETs. Even with high quality gate insulating layer, there still remains crystal defects that may cause digital failures. ROHM uses unique screening technologies to identify and eliminate defective devices from the production chain. As the results of HTGB (High Temperature Gate Bias) tests conducted at +22V and 150°C, ROHM could confirm 1.000 operating ours without any failures and characteristic fluctuations in 1.000 devices and a lapse of 3.000 hours in 300 devices.

At the current technology level electron traps are formed at the interface between gate insulating layer and SiC body. Electrons can be trapped and in consequence, increase the threshold voltage if a continuous positive gate voltage is applied for an extended period of time.

Shift in threshold voltage is very small, 0.2 – 0.3V, after 1.000 operating hours at 150°C and Vgs = + 22V

However, the shift in threshold voltage is very small, 0.2 – 0.3V, after 1.000 operating hours at 150°C and Vgs = + 22V. This shift is the smallest in the industry. Since most of the traps are all filled in the first several tens of hours, the threshold is fixed and remains stable after that.

The threshold drops due to trapped holes when continuous negative voltage is applied to the gate for an extended period of time. This threshold shift is larger than that caused by positive gate voltage, e.g., the threshold drops by 0.5V or more when Vgs is set to -10V or more. With Rohm’s second-generation MOSFETs (SCT2xxx series and SCH2xxx series), the shift does not exceed 0.3V, provided that the gate is not reverse biased beyond -6V. Negative gate voltage lower than -6V causes a significant drop in the threshold. In normal operation, gate voltage alternates between positive and negative biases and thus repeatedly charges and discharges the traps making unlikely to have significant changes in the threshold.

Reliability of body diodes

Another mechanism that affects SiC MOSFET’s reliability is the degradation caused by its body diode’s conduction. If forward current is continually applied to SiC P-N junction such as body diodes in MOSFETs, a plane defect called stacking fault will be extended due to the hole-electron recombination energy. Such faults block the current pathway, thus increasing on-resistance and Vf of the diode. Increasing the on-resistance by several times disrupts the thermal design. Furthermore stacking faults may degrade the blocking voltage. For this reason, using SiC MOSFETs whose body diodes degrade with conduction in circuit topologies that causes commutation to the body diode, e.g. bridge topologies in inverters, might result in serious problems. This reliability problem only occurs with bipolar devices, not with SiC-SBDs and the first-quadrant operation of SiC-MOSFETs.

ROHM has reduced crystal defects in SiC wafers and epitaxial layers and developed the proprietary process that prevents propagation of stacking faults, ensuring the reliability of body diode conduction. This is confirmed in 8A DC, 1.000-hour conduction tests which show no degradation in all characteristics, including on-resistance and leakage current. This ensures worry-free use of SiC-MOSFETs in circuits that cause commutation in the body diodes. Furthermore, reverse conduction reliability tests with Vgs = 18V and Id = 15 DC (also, 1.000 hours) shows no significant changes in electrical characteristics.

8A DC, 1.000-hour conduction tests which show no degradation

Since SiC-MOSFETs have a smaller chip area and higher current density than Si devices, they tend to have lower short circuit withstand capability (thermal fracture mode) compared to the Si devices. 1,200V SiC-MOSFETs in TO247 package have short circuit withstand time (SCWT) of approximately 8 to 10 us when Vdd is set to 700V and Vgs is set to 18V. SCWT is longer with lower gate voltage, which reduces saturation current and lower power supply voltage, which generate less heat.

Many gate driver ICs incorporate functions that simplify detection and management of short circuit condition. For example, Rohm’s BM6103FV-C can shutdown the switch in approximately 2 us once over current is detected. It has soft turn-off capability to gradually reduce the gate voltage during turnoff to prevent high surge voltage, which is induced by high dI/dt across the drain and source inductance. It is advised to pay careful attention not to apply over voltage by using such a soft turn on function or other preventative measures.

Si-MOSFETs involve a breakdown mode in which high dV/dt causes transient current to pass through the capacitance Cds and turn on the parasitic bipolar transistor, leading to device breakdown. This is less likely an issue with SiC-MOSFETs since the current gain of their parasitic bipolar transistors are low. So far such breakdown mode has never been observed with ROHM’s SiC-MOSFETs operating with dV/dt at up to 50 kV/us. Since SiC-MOSFETs generate exceptionally low recovery current, reverse recovery current also will not cause high dV/dt. Consequently, SiC-MOSFETs are considered unlikely to cause this breakdown mode.

How to use SiC Power modules and their reliability

Since SiC modules support high switching speed and handles high currents, surge voltage (V= L x DI/Dt) is generated due to wire inductance L in the module or at its periphery and may exceed the rated voltage. Below is a list of recommendations to prevent or mitigate this problem. However, these measures may have an impact on the switching performance:

  • Reduce wire inductance by using thick and short wirings in both main and snubber circuits.
  • Place capacitors close to MOSFETs to reduce wire inductance.
  • Add snubber circuit
  • Increase gate resistance to reduce dI/dt

Referring to Figure 4, when the MOSFET M1 of the upper arm of a half bridge turns on, reverse recovery current flows through the freewheeling diode (external SiC-SBD or body diode) of the MOSFET M2 of the lower arm and raises the drain-source voltage of M2.

When the upper MOSFET M1 turns on, reverse recovery current fl ows through the freewheeling diode of the lower MOSFET M2 and raises the drain-source voltage of M2

Due to this dV/dt, transient gate current (I = Crss x dV/dt) through the reverse transfer capacitance Crss of M2 flows into the gate resistance, thus resulting in a rise in the gate voltage of M2. If this voltage rise exceeds the gate threshold voltage of M2, short-circuit current flows through both the upper and the lower arms.

While the threshold voltage of SiC-MOSFETs defined at several mill-amperes is as low as around 3V the gate voltage required to conduct high current is 8V or higher. As a result, withstand capability of bridge arm short circuit is not significantly different from that of IGBTs. However, to prevent this unexpected short circuit, it is recommended to take measures listed below which are also valid for Si power modules. The following measures may influence the switching performance, so adjustment of the circuit with monitoring waveforms to prevent self turn-off is advised:

  • Increase negative gate bias voltage to turn OFF the MOSFET.
  • Add a capacitor between the gate and the source.
  • Add a transistor between the gate and the source that clamps Vgs to ground when the switch is off
  • Increase the gate resistance to reduce the switching rate.

Like IGBT modules, the RBSOA (Reverse Bias Safe Operating Area) of SiC power modules covers the entire range of twice the rated current.

Summary - examples of applications and benefits of using SiC

Due to the above described characteristics, the deployment of SiC can be beneficial in many ways and in a broad number of applications: The usage in power factor correction (PFC) circuits (CCM – continuous conduction mode) leads to improvement of the conversion efficiency and noise reduction due to elimination of reverse recovery current and the downsizing of passive filter components under high frequency operation achieved by low Err. No significant improvement is expected for critical conduction mode PFCs as reverse recovery current from the diode does not influence the total conversion loss. When used in solar inverters , reduction in Eoff, Err and conduction loss at low load condition can be achieved as well as the downsizing of the cooling system for power devices. In DC/DC converters it can lead to the reduction in Eoff, Err and downsizing of a cooling system for power devices and the downsizing of transformers under high frequency operations.

DC/DC converters

Using SIC in DC/DC converters leads to the reduction in Eoff, Err and downsizing of a cooling system for power devices and the downsizing of transformers under high frequency operations

In bi-directional converters the downsizing of passive filter components in high frequency operations as well as the reduction in Eoff, Err and size reduction of cooling systems for power devices are advantageous.

Bi-directional converters

SIC in bi-directional converters allow downsizing of passive fi lter components in high frequency operations as well as the reduction in Eoff, Err and size reduction of cooling systems

Usage in inverters for induction heating equipment benefit from the enlargement of operable conditions by increased frequency, reduced Eoff, Err and downsizing of the cooling system for the power devices. When deployed in motor drive inverters reduction in Eoff, Err and equally, downsizing of the cooling system for power devices can be observed. Buck converters operating in DCM (discontinuous conduction mode) and BCM (boundary conduction mode; also called critical conduction mode) do not benefit from SiC SBD’s recovery performance but generally, buck converters avail of the reduction in Eoff and downsizing of a cooling system for power devices and the downsizing of passive filter components.

 

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