Reverse recovery at the system Level:

A SiC MOSFET integrated with a soft-body diode increases a converter circuit’s operating frequency and efficiency while decreasing the number of components.

Figure 1 shows a full bridge topology of a single-phase two-level converter and a pulse pattern that will cause a reverse recovery event. At t₀, all switches start in the off state. S₁ and S₄ are initially turned on during t_1, letting the current pass through the load. During t₂, S₄ returns to the off-state. The current must then change to the freewheeling path, which utilizes the body diode in S₂. This time is known as dead time, and the current will decay due to the path resistance. During the transition period between t₂ and t₃, S₄ turns back on, causing a shoot-through scenario that forces the body diode of S₂ to undergo reverse recovery. After the recovery instant, the parasitic inductance in the current path results in a voltage overshoot to maintain the current in the path.

Reverse recovery and softness factor

A snappy or reverse recovery occurs when a SiC diode transitions from “forward-conduction” to an “off-state.” To simplify the reverse recovery event, Figure 2 shows a diode’s ideal recovery current and voltage waveform (Fig. 2a) and a non-ideal current waveform for a MOSFET (Fig. 2b).

Fig. 2a shows two regions of time based on I_diode. From t₀ to t_{1, the reverse voltage VR (dashed line) application} forces the current to drop at a constant rate, dI/dt. During this period, the rate at which dI/dt changes is determined mainly by the applied V_R, circuit elements such as the complementary device’s external R_G, and parasitic circuit inductance. At the start of t_{1, excess carriers are removed from the drift region, and a depletion region begins to form, which build}s the voltage across the diode. The voltage reaches its target value V_R when I_rrm is met at t_{2, and there is no additional bias from the voltage source VR that increases} the current magnitude further. From t₂ to t_3, the voltage overshoots its target value as the parasitic inductance opposes the decreasing loop current, eventually settling at V_R. The voltage overshoot peak depends on the circuit’s parasitic inductance and rate of change of recovery current dI_r/dt_(max).

Typically, we use two formulas to evaluate the softness factor of a recovery event. Below is S1, a single-parameter ratio:

where t_a = t_2­– t₁ and t_b = t₃ – t₂.

When S₁ = 1, the time it takes for the current to reach Irrm equals the time it takes to return to 0 A or leakage values.

A second method of measuring the softness of a reverse recovery event is defined in the equation below:

Where: dI/dt is the current at the initial zero-crossing of the commuting current, and dI_r/dt_(max) is the max return current during t_b.

When S₂ = 1, the current flow rate into and out of the body diode is equivalent. Most devices never achieve an ideal S₁ and S₂ value. A snappy recovery will occur when S1 and S2 are less than 1, while a value greater than 1 is considered a soft recovery.

Figure 3 shows a half-bridge test circuit used to perform reverse recovery characterization. Like the pulse pattern described in Figure 1, the high-side device will initially switch on and off to allow a controlled amount of current to conduct through the body diode of the low-side MOSFET. The high-side device then turns back on, forcing the freewheeling current to commutate, overshoot, and eventually settle, completing the reverse recovery event. Test boards and other external circuitry should limit the influence on body diode characterization. Do your best to minimize the test board’s stray inductance in accordance with good PCB layout practice and ensure that the external circuitry is not limiting the switching capabilities of the MOSFET. Minimizing the area of the power and gate loops will reduce inductance and achieve greater switching control.

Managing reverse recovery and EMI

Temperature dependence is the major factor for V_DS overshoot and peak I_DS values during the reverse recovery event. Tests performed at high temperatures will provide “worst-case scenario” results. The free-wheeling current through the body diode slowly dissipates over time as heat. This heat causes a temperature change in the junction, decreasing the conductive path’s resistance and thus increasing the initial dI/dt.

Figure 4a shows the temperature dependence of the reverse recovery current. The test parameters include an R_G(ext) = 5 Ω, V_DS = 800 V, and I_D = 40 A. Increasing external gate resistance is recommended to achieve softer recovery characteristics such as reduced Q_rr, I_rrm, and dampened ringing. Improvements in reverse recovery obtained from increasing R_G(ext) are shown in Figure 4b). Higher gate resistance reduces the risk of snappy reverse recovery and can increase switching losses due to increased t_rr if overly dampened. Figure 4b) shows the reverse recovery current plotted versus time for various external R_G values. The reduced ringing effect in the current waveform will reduce unwanted EMI.

Table 1 demonstrates that increasing R_G will decrease dI/dt and Q_rr and dampen the initial oscillatory peak current level. In contrast, increasing R_G also increases t_rr, creating a tradeoff between overshoot and switching times. Always visually inspect the waveform  after measuring it.

Impact of reverse recovery on voltage and energy

You must also consider reverse recovery effects on voltage to ensure a power circuit won’t exceed the device’s safe operating area (SOA). Parasitic inductance in the commutating current path causes an overshoot in the voltage waveform. If ignored, you will violate SOAs and reduce the system efficiency and lifetime of the semiconductor device.

Figure 5a shows the I_SD recovery waveform of the low-side device as a function of time at T = 125°C and V_DS = 800 V. Figure 5b shows the V_DS recovery waveform as a function of time and Figure 5c shows the peak V_DS value as a function of external gate resistance. The devices tested are in a half-bridge configuration with 4 dies in parallel per switch position. As expected, the V_DS peak decreases as R_G(ext) increases. An R_G(ext) >3 Ω is required to remain within the device’s SOA.

Conclusion

The circuits shown help you mitigate overshoot voltage and unwanted EMI during the reverse recovery of a SiC MOSFET body diode. Reverse recovery is an inherent occurrence in MOSFET body diodes, and negative effects are amplified by increased junction temperature. Board or module circuit parasitics create oscillatory voltage spikes that can break device SOA limitations. You should accurately characterize the softness factor of a MOSFET body diode to understand the benefits gained from mitigation techniques fully. Increasing external gate resistance is the most common method for softening recovery characteristics and managing V_DS overshoot.

References

1993. J. B. Mohit Bhatnagar, “Comparison of 6H-SiC, 3C-SiC, and Si for Power Devices,” IEEE Transactions on Electronic Devices, vol. 40, no. 3, pp. 645-655, 1993.Singh R., S. Ryu, J.W. Palmour, A.R. Hefner. J. Lai, “1500 V, 4 Amp 4H-Sic JBS Diodes,” in International Symposium on Power Semiconductor Devices, Toulouse, 2000.
Romero, A., “Capacitance Ratio and Parasitic Turn-on,” Wolfspeed Inc., Durham, 2023.
Yuan, X., S. Walder and N. Oswald, “EMI Generation Characteristics of SiC and Si Diodes: Influence of Reverse-Recovery Characteristics,” IEEE Transactions of Power Electronics, vol. 30, no. 3, pp. 1131-1136, 2015.

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• With the automotive world witnessing a surge in electrification and digitalization, the importance of ESD protection cannot be overstated. The ESD2CANx-Q1 ESD protection diodes by Texas Instruments are AEC-Q101 qualified solutions rated to dissipate contact ESD strikes as specified by the ISO 10605 automotive standard. They feature a 36V working voltage and 2.8pF typical low IO capacitance per channel with a pin-out to suit two automotive controller area network (CAN) bus lines (CANH and CANL). These protection diodes offer long-term protection for in-vehicle networks (IVNs) like CAN-FD, low fault-tolerant CAN, and high-speed CAN systems, as well as in industrial control networks like DeviceNet and CANopen.
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• The AQ1205-01UTG bidirectional discrete TVS diodes by Littelfuse are made using its exclusive silicon avalanche technology. These diodes have versatile applications, such as automotive use, battery protection and management systems, computers and associated peripherals, medical equipment, consumer electronics, and test instrumentation. With the AQ1205-01UTG TVS diode, circuits can safely absorb repetitive ESD strikes of ±30kV (contact and air discharge) as per IEC 61000-4-2 standards without experiencing any performance degradation. The diodes have a reverse leakage current of 20nA (at a voltage of 4.5V) and a maximum capacitance of 9pF. They are also AEC-Q101 qualified and production part approval process (PPAP) capable, ensuring a smooth integration into suitable designs.
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Be sure to see EE World’s APEC 2024 coverage on datacenter power and test equipment.

Alpha & Omega Semiconductor

Electronics generate heat. It’s what they do. Alpha & Omega Semiconductor has developed a package that it calls MEGA IPM7 for its intelligent power modules. The SMD package contains an embedded IGBT for motor drives in appliances such as refrigerators, washing machines, and air conditioners. What makes the package different is its surface contains metal, which helps to pull heat away from the embedded device.

Analog Devices

Analog Devices introduced the LT8418, a 100 V half-bridge GaN gate driver. Designed to minimize EMI issues, the LT8418’s split gate drivers let you adjust a GaN FET’s turn-on and turn-off slew rates. The device can source up to 4 A peak and sink up to 8 A.

Bourns

The manufacturer of power magnetics, circuit-protection devices, sensors, switches, and other devices focused on transformers chokes, gate drivers, and toroids.

Brightworks

Many of the power semiconductors seen at APEC have been designed into phone and laptop chargers from well-known manufacturers such as Anker, Belkin, and others, some of which will remain nameless. Brightworks manufactures chargers and power supplies that other companies private brand, using some of those power semiconductors. The photo shows a 20 W USB-C charger that the company let me take home. I’m tempted to crack open the case to see what’s inside.

Empower Semiconductor

The manufacturer of voltage regulators, silicon capacitors, and interposers exhibited an evaluation board for its EP7123 dual-output integrated voltage regulator. Each output supplies up to 6 A with a 3.3 V input. Other parts in the series include one, three, and four outputs.

Infineon

At APEC 2024, Infineon introduced the TDM22544D and TDM22545D high-density power modules designed for data center processors. The modules package a MOSFET with an inductor that’s surrounded by heat-dissipating metal.

Operating from the Infineon booth, AWL Electricity showed a rather unusual demonstration of wireless power transfer. In this video, Cédric Hamel-Bruneau uses a modified lamp with an LED bulb at one end and no power cord on the other. Instead, the base holds a receiver that receives power from a transmitter. The concept uses electric-fields to transfer power through capacitive coupling. That’s different than, say, a wireless charger that uses magnetic fields and inductive coupling.

Menlo Micro

You might think of Menlo Micro’s switching products for communications, 5G, and test & measurement but the company also makes power switches. The video shows a demonstration of the MM9200 power switch. Here, Menlo Micro shows a set of these switches running in a serial-parallel combination that the company says runs cooler than an identical configuration using SiC.

Microchip

Microchip exhibited the MAICMMC40X120 Aviation Power Core module that integrates an SiC MOSFET with drivers and a microcontroller. It can produce AC power with variable frequencies to drive motors for aerospace and defense applications.

Qorvo

The company introduced a series of SiC 1200 V half-bridge and full bridge power modules. The four modules in the UHBxxx12E1BC3N series have drain currents of 17 A, 25 A, 50 A, and 100 A with R_DS(ON) at 70 mΩ, 35 mΩ, 19 mΩ, and 9.4 mΩ, respectively.

Power Integrations

In the video, Andrew Smith explains how the company achieves multiple DC rails with a single chip. The InnoMux 2-EP is a zero-voltage switching (ZVS) flyback switcher that selects which of its loads needs energy.

TDK

TDK’s µPOL (point-of-load) DC-DC converters provide telemetry for parameters such as voltage, current, and temperature. The demonstration in the video shows a 12 A regulator sending telemetry to a computer. The converter’s output range is 0.6 V to 1.8 V.

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