IEEE Transactions on Nuclear Science, Vol. NS-28, No. 6, December 1981

DOSE RATE TOLERANT HEXFET POWER SUPPLY

W. E. Abare and W. K. MartindaleHarris Corporation

Government Information Systems DivisionMelbourne, Florida 32901

Abstract

A summary of flash x-ray testing of power HEXFETtransistors (IRF 351's) and a power supply employingHEXFETs is presented. HEXFETs are shown to exhibit adose rate damage sensitivity which is triggered by lessthan one hundreth of the normal single pulsed energyrating. The power supply testing demonstrates thatHEXFETs can be used in high power applications in ahigh dose rate environment if a current limiting con-figuration is employed.

Introduction

Airborne power supplies have always required min-imum volume and weight. A large portion of the volumeand weight has been contributed by the magnetics. Con-ventional linear dissipative supplies using line fre-quency magnetics are large and inefficient thus themain drive has been toward high frequency, high effi-ciency switching power supplies.

The maximum operating frequency of most switchingsupplies has been limited to less than 50 KHz by theswitching speeds of bipolar transistors. The advent ofthe power MOS field effect transistor technology hasallowed increased switching speeds with simple and lowcost gate drive circuits. Operation at frequencies of100 KHz and higher have become feasible.

Inherent to the HEXFET structure are bipolar NPNtransistors formed by the N substrate (collector), Pwells (base) and N sources (emitter). During normaloperation the NPN transistors are biased off since thesource metalization makes contact with the base andemitter forming a low resistance base emitter shunt.

Test Set-Up

Figure 2 is a schematic of the HEXFET test con-figuration. The fixture houses two separate HEXFETswhich can be biased under various drain and gate vol-tages. The drain photocurrent is monitored using pas-sive current probes. The source capacitance (Cl) is alow ESR film capacitor (TRW Type 35).

RIVDs *- I -c. I'104 ct

VGSI - ___ 0

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EXFET TEST FD(TUEFigure 2

This paper describes the application and flashx-ray testing of the International Rectifier powerFET's (HEXFETs) in a conventional switching powersupply. Dose rate test results indicate a destructivemechanism exists when the devices are used in low impe-dance, high surge current circuit topologies. Thissupply utilizes a switching regulator operating at225 KHz providing power to a DC/DC converter operatingat half this frequency.

HEXFET Structure

Figure 1 illustrates the structure of the HEXFET.Normal current flow is vertical through the body of thedevice then horizontally through the channel region andvertically out the source. A voltage applied betweenthe gate and source terminals control the device'scurrent flow. Another power MOSFET feature is thedevice's reverse body-drain diode.

Test Data

Figure 3 is a plot of HEXFET drain current versustransient dose for several HEXFET test samples. Thedata points are keyed to indicate approximate drainphotocurrent pulse widths and device failures.

W0.

0

aC

DOSE RADS CS1)100 &ADS 1000 RADS

1r6R/S

-100 WAVEFORM B /D

-60

o . . 1 ..

1O0R/S

LEGEND

PULSE WIDTH *- O.2p0- 1 Ps®. IOIs

DEVICE DAMAGE X

HEXFET STRUCTUREFigure 1

HEXFET DRAIN CURRENT VERSUS TRANSIENT DOSE

Figure 3

Two observations concerning device failure can bemade from Figure 3. First, device damage thresholdsare widely dose rate distributed. Secondly, devicefailure is always coincident with a several microsecondextended photocurrent waveform.

0018-9499/81/1200-4380$00.75(31981 IEEE

_ . . . . ..... . . . . ... .. . ..... . ...

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Figure 4, Waveform A & B, are typical of HEXFETdrain current waveforms. The two HEXFETs are both ex-posed with 50 volts drain to source and the gategrounded. The radiation dose for both devices is ap-proximately 300 rads delivered in 50 ns. Negative gatevoltages had no effect on the observed drain currents.The source capacitance (Cl) is 10 uf. HEXFET S/N 1 wasundamaged following this pulse, however, S/N 4 wasseverely damaged (drain to source shorted, gate tosource low resistance).

VERTICAL 20 A/DIV. HORIZONTAL 1 ps/DIV.WAVEFORM A (S/N 4)

critical temperature for second breakdown, at whichpoint Cl provides additional energy to catastrophi-cally damage the device.

HEXFET Design Advantages

The device used as the power switching element ina Power Supply to a large degree defines the comp-lexity and operating frequency of the supply. The in-creased switching speed of the HEXFET allows operationat switching rates beyond 100 KHz with simple gatedrive circuits. Use of HEXFET's also provides addi-tional benefits. One advantage is that once the gatedrive circuitry has been designed for a fixed primepower voltage and switching speed, the currentsswitched are independent from the gate drive. Thus,any range of load current can be switched from themicroampere level to the maximum drain current of thetransistor. This permits a basic power supply designto be flexible and will accommodate a large range ofoutput powers.

HORIZONTAL 1 ps1DIV.WAVEFORM B (S/N 1)

HEXFET DRAIN CURRENT WAVEFORMFigure 4

HEXFET S/N 1 (Waveform B) shows a short durationdrain current pulse of approximately 35 amperes.HEXFET S/N 4 (Waveform A) starts out with the samedrain current waveform then breaks into a mode wherethe current goes up to 100 amperes and stays there forseveral microseconds. S/N 1 was subsequently exposedto transient dose levels greater than 1400 rads withno device damage. S/N 1 and S/N 4 were electricallyequivalent prior to testing.

HEXFET Damage Discussion

A possible explanation of the dose rate induceddamage observed in some of the HEXFET test samples ispresented below. This explanation followed test datareview, chip visual inspection and discussions withInternational Rectifier. Further flash x-ray testingshould be performed to isolate the exact failure mech-anism.

As stated earlier the observed HEXFET drain photo-currents were independent of gate source bias. A por-tion of the observed drain current is primary photo-current in the large substrate diode. The currentwaveform of S/N 1 (Figure 3) represents less than 600microjoules of energy deposited in the HEXFET from thedose rate exposure. However, S/N 4 exposed to the samedose rate level starts out with a similar waveform thenbreaks into a mode in which it consumes all the energyin capacitor Cl.

We conclude that the drain current of S/N 4 issubstrate diode primary photocurrent and secondaryphotocurrent in a small fraction of the parasitic NPNtransistors. This current localization raises thetemperature of the conducting NPN transistors to the

The HEXFET is essentially free from the usualsecond breakdown effects in normal, non-nuclear envi-ronments since current crowding increases the resis-tance of that region and diverts the current for im-proved and more uniform current distribution. Thusthe devices can handle large peak powers which arenormally encountered in a switching power supply.

Application in a switching power supply involvesthe generation and processing of square waves withvery short rise and fall times which requires widebandwidth devices. The HEXFET is a very wide band-width device and shows no time delay or pulse stretch-ing due to storage or delay times as is produced byconventional bipolar transistors. Apparent storageand delay times as is evidenced by an overdriven HEXFETis simply the time required for the gate drive voltagewaveform to reach the threshold of the HEXFET and canbe made negligible with simple gate drive circuitry.

HEXFET gate drive power is far less than theequivalent bipolar base drive. Switching 10 amperesrequires a base power of 1.2 volts at 1 ampere or 1.2watts for a typical bipolar device. A power HEXFET ata switching rate of 100 KHz and 10 amperes of draincurrent requires driving about 5 nanofarads with a vol-tage change of 12 volts. Assuming a resistive gatedrive circuit, the power is CV2 x f or 72 milliwatts.

These advantages make the HEXFET transistor nearideal for use in switching power supply and were thereasons this device was selected for use in this ap-lication.

Power Supply Description

Figure 5 is a simplified block diagram of thepower supply. Prime power (MIL-STD-704) is rectifiedto produce 150 Vdc. This voltage varies directly withline variations and is applied to the input of aswitching pre-regulator operating at 225 KHz. The out-put of the regulator provides power to the DC/DC con-verter operating at 112.5 KHz. The converter and pre-regulator are synchronized through a common clock asshown. A sense winding on the converter provides afeedback signal to the pre-regulator which is comparedto a reference voltage and controls the on time of thepre-regulator switch and completes the voltage regula-tion loop.

Figure 6 shows the pre-regulator switch schematic.From this schematic it can be seen that short durationsurge currents are controlled by inductor Li. Duringthe flash x-ray pulse, the photocurrent in the HEXFET

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is limited by the magnitude of the input voltage andthe inductance of Li.

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115 VAC _ GTELT (UP TO 10

POWR YJPPLY BLOCK DIAGRAM

Figure 5

+150'

CR1 TO DC/DCCONVERT

fo. 225 KHz_.-Imanyo -RETURN FOR 100V

LI01

RETURN o YZ~FOR 150V

SWITCHING PRE-REGULATORFigure 6

Figure 7 shows the initial power supply topologyused in the first flash x-ray testing of the supply.Q2 and Q3 drive the primary of the converter transfor-mer in a voltgae feed topology. The term "Voltage Fed"describes the impedance the DC/DC converter sees as itsprime power source. In the configuration shown, theconverter is provided power by the LC filter (Li, Cl)of the pre-regulator. At high frequencies, the outputimpedance of this filter is low and is primarily theimpedance of Cl. This capacitor can provide very highpeak currents for a short time (<10 psec) into the con-verter transistors Q2 and Q3. This topology is wellknown and commonly used. Principle disadvantage iscontrol and analysis of the potential core saturationof the converter transformer.

PRE-REGULATOR+150

DC/DCCONVERTER

This topology failed catastrophically during theflash x-ray testing. Visual chip inspection showed Q2and Q3 has been severely damaged. Apparently the de-vices were supplied with enough energy from Cl to causethe physical damage. In several of the tests Qi, thepre-regulator transistor, was operational but showedexcessive drain current in the off state. The deviceapparently was stressed by the failure of the convertertransistors Q2 and Q3. Physical damage of this device,Qi, was less than the converter transistors.

A circuit topology which controlled and limitedsurge currents was then investigated. The topology se-lected is shown in Figure 8 and had little impact onthe existing hardware. The advantages of this topologyare listed below.

a. Short duration surge currents are limited byLi.

b. Since converter core saturation is evidencedby rapidly rising excitation current, coresaturation problems are minimized.

c. Currents on the primary now have a controlledrate of rise of current dependent on the in-ductance of Li and the input voltage.

DC/DCCONVERTER

PRE-REGULATORVDC

iCR1 CR2

PRIME POWER IRF

j3~~51q

01 ~~03i gCR4 CR3

PRIMEPOWMER CNVERTERPOWER ~~~GATE DRIVERETURN 112.5 kHz

FINAL TOPOLOGYCURRENT FED DC/DC CONVERTER

(High Impedance Source for the DC/DC Converter)

Figure 8

Power Supply Testing

The modified power supply was exposed to differentlevels of transient radiation for two DC input voltagesand under half and full load conditions. The tablebelow summarizes the various test levels and test con-ditions. The flash x-ray pulse width was 50 ns.

INPUTLEVEL VOLTAGE LOAD CONFIGURATION COMMENTS

240 rads 158 Vdc Half Current Fed Shut Down OKAYRestarted OKAY

790 rads 158 Vdc Half Current Fed Shut Down OKAYRestarted OKAY

720 rads 137 Vdc Full Current Fed Shut Down OKAYRestarted OKAY

INITIAL TOPOLOGYVOLTAGE FED DC/DC CONVERTER

(Low Impedance Source for the DC/DC Converter)

Fi gure 7

Concl us ion

The high frequency capabilities of HEXFETs permitsswitching mode power supply designs with operating fre-quencies above 100 KHz. The increase in frequencygreatly reduces the magnetics weight and volume. HEX-FETs exhibit a failure mode in a high dose rate environ-ment which is induced by less than one hundreth of thenormal single pulsed energy rating. Device application

4382

VDC , + 100 VDC

in a high dose rate environment is achievable throughthe use of HEXFET current limiting circuit topologieswhich limit photocurrent surges to a safe level.

Acknowledgements

We wish to acknowledge helpful conversations andtest samples suppled by Alex Lidow and Brian R. Pellyof International Rectifier.

We also wish to acknowledge George Moffett ofAerojet (AESC) for his cooperation and assistanceduring flash x-ray testing of the HEXFETs and PowerSupply.

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