LOW SIDE GAN FET DRIVER FOR SPACE APPLICATIONS

This paper reports the results of preliminary single event effects (SEE) testing of the Intersil ISL70040SEH, a single low side driver specifically designed to drive enhancement mode power GaN FETs.


Introduction
Recently, wide band gap semiconductor devices such as GaN and SiC have been gaining interest in power management for space applications.These devices feature higher breakdown voltage, lower RDS(ON), and very low gate charge enabling power management systems to operate at higher switching frequencies while still achieving higher efficiency and a small solution footprint.There is an added benefit, especially from GaN devices, that make these attractive to the space market.Studies have shown that these devices are inherently radiation hard to total ionizing dose.However, GaN power transistors have a limited gate to source voltage range and with that comes the complexities of providing the right grate dive, a low inductive path and the correct timing.In this paper we report results of destructive and non-destructive SEE testing of the ISL70040SEH, a low side GaN FET driver designed for space applications, together with a brief discussion of the part's electrical specifications and how the device solves many of the problems associated with using GaN FETs in power management space applications.

Figure 1. ISL70040SEH block diagram
The ISL70040SEH is a low side driver designed to drive enhancement mode Gallium Nitride (eGaN) FETs in isolated topologies, boost type configurations and synchronous rectification.The ISL70040SEH offers both inverting and non-inverting inputs to satisfy requirements for inverting and non-inverting gate drive in a single device type (refer to Fig. 1).
Inputs of the ISL70040SEH can withstand the input voltages up to 14.7V regardless of the VDD voltage.This allows inputs of the ISL70040SEH to be connected directly to most radiation hardened PWM controllers.The split outputs of the ISL70040SEH offer flexibility to adjust the turn-on and turn-off speed independently by adding additional impedance in either the turn-on path and/or the turn-off path.The nominal operating supply voltage range is 5V to 12V and includes an internal 4.5V series regulator with an under-voltage lockout (UVLO) circuit.When the voltage is below the UVLO threshold, the IN and INB are ignored and an internal pull down device keeps the GaN FET off.The ISL70040SEH operates over the military temperature range from -55°C to +125°C and is offered in a 8 pin hermetically sealed 6mm x 6mm ceramic leadless chip carrier (CLCC) package.The package was designed to reduce parasitic inductance to mitigate the risk of excessive overshoot when driving the gate of the GaN FET.See Fig. 2 and Tab. 1 for pin configuration and pin description.
The ISL70040SEH is targeting a total dose (TID) rating of 100krad(Si) at a high dose rate (50-300rad(Si)/s) , 75krad(Si) at a low dose rate (< 0.01rad(Si)/s) and will be acceptance tested on a wafer by wafer basis.The ISL70040SEH is also SEE rated for both destructive and non-destructive effects to a linear energy transfer (LET) value of 86MeV•cm 2 /mg.Single-event transient (SET) response has emerged as a key issue in power management parts, and this paper will concentrate on ISL70040SEH SEE results.

Performance of the ISL70040SEH
GaN FET devices have special gate drive requirements that must be considered when choosing the correct driver.Typically enhancement mode GaN FETs have a lower rated maximum gate voltage compared to silicon MOSFETs.Therefore, the driver should have a wellregulated gate drive voltage that is high enough to fully turn-on the GaN FET but low enough that it does not damage the gate of the transistor.The driver should also have high source and sink current capability to fully utilize the high switching speed capability of GaN devices.Inversely, high current drive capability also means low pull-up and pull-down resistance output stages.A low resistance turn-off path is essential to mitigate Miller turn-on.Lastly, sometimes the fast turnon capability can lead to loss of efficiency due to overshoots or increase in EMI.In these cases it is required to slow down the turn-on of the GaN device.This is typically this is achieved with a diode and a parallel resistor series with driver output stage.However, this is not recommended due the low threshold of the GaN device and diode forward voltage drop.A driver with a split output stage allows to tune the turn-on and the turn-off speed without under driving the gate.

Figure 3. ISL70040SEH typical rising waveform
On the ISL70004SEH the gate drive voltage on the VDRV pin is regulated by the internal linear regulator to 4.5V.This tightly regulated gate voltage is high enough to fully turn-on enhancement mode GaN FETs but low enough to allow some overshoot, alleviating some of the complications of PCB design.The ISL70004SEH also has active damping output stages which changes the output impedance of the pull-up and pull-down paths to dampen overshoot and prevent the gate voltage from exceeding the maximum gate-source rating of enhancement mode GaN FETs.The device under test (DUT) was mounted in the beam line and irradiated with heavy ions of the appropriate species.The parts were assembled in 8 lead ceramic leadless packages with the metal lid removed for beam exposure.The beam was directed onto the exposed die and the beam flux, beam fluence, and select voltages and current in the device pins were measured.
The tests were controlled remotely from the control room.All input power was supplied from portable power supplies connected via cable to the DUT.The supply currents were monitored along with the device outputs.All currents were measured with digital ammeters, while all the output waveforms were monitored on a digital oscilloscope for ease of identifying the different types of SEE displayed by the part.Events were captured by triggering on changes in the output.Tab. 2 summarizes the test conditions during heavy ion radiation for the destructive test.The schematic of the test board is given in Fig. 5.  Destructive testing results indicated no failures of the ISL70040SEH for a supply voltage of 14.7V at an LET of 86MeV•cm 2 /mg.The final DUT was retested with a VDD supply of 15.1V.That DUT exhibited an increase in operating supply current larger than 5% and was considered a failure.Tab. 4 summarizes the results of destructive SEE testing.

SET testing of the ISL70040SEH
Single event transient tested utilized the same schematic as shown in Fig. 5.Both static and dynamic tests were conducted to characterize the SET behavior of the ISL70040SEH.
During the static test the driver was supply voltage was 4.5V and 13.2V, the min and max recommended operating range.The outputs were set in a high or low condition and a digital oscilloscope connected to the output was used to trigger on deviations greater than ±500mV.There was no capacitive load connected to output.
For dynamic testing the supply voltage was 4.5V and 13.2V.The input was driven with 0-5V square wave at 500kHz.A digital oscilloscope was connected to the output and use to trigger on a SET event that led to change of pulse width greater than ±50ns.There was no capacitive load connected to output.
Tab. 5 summarizes the test conditions for the static test in both the low and high states.Four devices were subjected to the test and no triggers were captured with oscilloscope.The ISL70040SEH is SET immune in the static condition.This is a very positive results as in a typical applications the driver will not unintentionally turn-on or turn-off the device and the output state solely relies on the input condition.13.2V 0-5V @ 500kHz 0V PW ± 50ns

Summary
The ISL70040SEH is radiation hardened low side GaN FET driver which contains all the features necessary to drive enhancement mode GaN FETs in space applications.The single event effect testing demonstrating no SEB or SEL with a supply voltage of upto 14.7V at an LET of 86MeV•cm 2 /mg.
Single event characterization demonstrated that in a static condition the output does not change states with regards to the input condition.This means the device is SET immune and will not inadvertently turn-on or turn-off the GaN power transistor due to a heavy ion strike.Dynamic characterization results demonstrate that the output pulse width will not change more than 50ns.For low switching frequency applications 500kHz an below that is less than 5% change of a 50% duty cycle pulse width.As the switching frequency in increased the percentage is much larger and may cause and issue.Follow up SET characterization is planned for next year and the pulse width trigger may be reduced to 20ns or lower detect the minimum change in pulse width due to an ion strike.

Fig. 3
Fig.3shows a typical rising waveform of the ISL70040SEH with a load capacitance of 1000pF and the VDD supply voltage at 12V.The input signal is on channel 1 (C1) and the driver output is on channel 2 (C2).Some of the key performance highlights are:1.Gate drive voltage steadies to 4.5V with a peak of approximately 4.6V.2. The rise time for 10% to 90% is 4.3ns.3.The turn-on delay is 30ns.Some of the delay is due to the internal SET mitigation circuitry.4. The dampen response of the output due to increased impedance of the output stage.

Figure 4 . 4 .
Figure 4. ISL70040SEH typical falling waveform Fig.4shows a typical rising waveform of the ISL70040SEH with a load capacitance of 1000pF and the VDD supply voltage at 12V.The input signal is on channel 1 (C1) and the driver output is on channel 2 (C2).Some of the key performance highlights are:1.The fall time for 90% to 10% is 3ns.2.The turn-off delay is 25ns.Some of the delay is due to the internal SET mitigation circuitry.3. The dampen response of the output due to increased impedance of the output stage.4.Under shoot peaks at -3V, due to the strong pulldown capability.To minimize undershoot one may add an external resistor on the OUTL pin.The added impedance will also slow down the fall time.

Figure 5 .
Figure 5. ISL70040SEH SEE test schematic The supply current, input pin (IN) current and gate output voltage were monitored pre and post heavy ion irradiation.If any of those monitored specifications changed more than the specified criteria the device was considered a failed unit.Tab. 3 summarizes the specifications, conditions and the failure criteria used during destructive SEE testing.

Table 3 .
Destructive SEE testing failure criteria

Table 4 .
ISL700040SEH destructive SEE test summary

Table 5 .
Static SET test conditions