Analysis and Design Aspects of a DesaturationDetection Circuit for Low Voltage Power MOSFETs

Bjoern Wittig, Matthias Boettcher, Friedrich W. FuchsChristian-Albrechts-University of Kiel, Germany

Institute of Power Electronics and Electrical DrivesKaiserstr. 2, 24143 Kiel, Germany

Abstract— An analysis of a desaturation detection circuitspecially designed for low voltage power MOSFETs is presentedand design aspects are carried out. The characteristics are a lowdetection threshold hysteresis, a fast shutdown reaction and ajunction temperature compensation. The proposed desaturationdetection circuit is described in detail and experimental resultsare given. The influence of the junction temperature and strayinductances of the package leads on the short circuit detectionthreshold are pointed out.

Keywords— MOSFET, Power semiconductor device, Automo-tive electronics

I. INTRODUCTION

Nowadays there is a trend to more hybrid or electricalvehicles and power MOSFETs with high current ratings play asignificant role in battery fed converters. Typical applicationsare dc/dc converters or dc/ac converters for feeding a threephase ac motor like in cars and lift trucks [1]–[3]. Due tothe high drive power and low voltages of i.e. down to 24V in some applications high currents follow. Thus there isa very high demand for low voltage power MOSFETs witha low drain-source on-state resistance RDS(on) and a lowtemperature dependence on the market.There are many logic integrated circuit (IC) solutions ofdesaturation detection for IGBTs on the market. Due to thelow drain-source on-state resistance RDS(on) of a low voltagepower MOSFET and therefore the very low drain-source on-state voltage drop, most of these ICs cannot be used inlow voltage power MOSFET applications because of the toohigh desaturation detection threshold voltage of the ICs. Forexample the collector-emitter on-state voltage of an IGBT inthe case of a short circuit is about 5 to 10 volts and higher. Incomparison the drain-source on-state voltage of a low voltagepower MOSFET can be just about a few hundreds of millivoltsin case of a short circuit due to the low on-state resistance.There are only a few ICs, which can be used for low voltagepower MOSFET applications, i.e. the IR2127 from Interna-tional Rectifier [4]–[6]. These ICs have the disadvantage ofa very high detection hysteresis due to component variations,which makes a precise adjustment of the current detectionthreshold difficult. Furthermore an additional high current gatedrive is necessary in many applications due to the low peakcurrent ability of the current sensing ICs.For the design of desaturation detection circuits applicationsnotes with design guidelines are available like in [7]. These

design guidelines mainly do not consider the effect of theinduced voltages at the stray inductances of the packageleads. At low voltage and high current applications theseinduced voltages have an relevant influence on the short circuitdetection threshold and should be taken into account forcorrect design calculations.Another undesired effect is the high temperature depen-dence of the power MOSFET drain-source on-state resistanceRDS(on), which increases with higher temperatures. Thereforea desaturation detection circuit with a low detection thresh-old hysteresis, and a junction temperature compensation isrequested.In this paper an analysis of a desaturation detection circuit forlow voltage power MOSFETs with a low detection thresholdhysteresis, a fast shutdown reaction and a junction temperaturecompensation is presented and design aspects are carried out.The paper is structured as follows: In section II a shortexplanation of the theoretical switching behaviour and theeffect of the stray inductances of the commutation path on thecurrent slopes and the influence of the package inductanceson the detection threshold will be given at first. Furthermorean overview about short circuit types in a half bridge configu-ration is given in III. In section IV the proposed desaturationdetection circuit is described in detail. Finally experimentalresults with a low voltage power MOSFET are shown andanalysed in section V. At the end a conclusion is given.

II. SWITCHING BEHAVIOUR OF A LOW VOLTAGE POWERMOSFET IN A HALF BRIDGE CONFIGURATION

In Fig. 1 a typical conventional gate drive circuit with a lowvoltage power MOSFET is illustrated. The theoretical currentand voltage characteristics at turn-on and turn-off of a powerMOSFET are illustrated in Fig. 2 [8]–[10]. The dotted linesshow the theoretical characteristics considering the sum of allstray inductances Lσ in the current path. The widely useddefinitions of the current and voltage rise and fall times andthe turn-on and turn-off delay times for power MOSFETs aredepicted here also.

As described in [11], [12] the current rise and fall times arestrongly influenced by the stray inductances in the commuta-tion path of the current. Taking the transconductance gfs, theinput capacitance of the power MOSFET Ciss and the gatethreshold voltage VGS,th into account the relationship of the

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Fig. 1. Typical conventional gate drive circuit of a power MOSFET

Fig. 2. Theoretical current and voltage characteristic at turn-on and turn-off of a power MOSFET, without (continuous line) and with (dotted line)consideration of the commutation path stray inductance Lσ

current slope during turn-on and the stray inductance LSσ1

can be approximated as follows [10]–[12]:

dIDdt

≈Vgg − VGS,th − ID

2gfs

(RGCiss 1gfs

) + Lsσ1

(1)

At turn-on during the current rise time the induced positivevoltages at the stray inductances in the commutation path leadto a lower voltage stress of the power MOSFET M2 [13]. Inthis phase the voltage decrease is:

Vind = Lσ ·diDdt

(2)

During the turn-off process and the current fall time theinduced voltages at the stray inductances are negative. TheConsideration of the turn-on overvoltage VFRM of the inversebody-diode of M1 leads to the following overvoltage peak atturn-off [8]:

Vpk = Lσ ·diDdt

+ VFRM (3)

During short circuit all stray inductances and load induc-tances of the whole current path are the main current slopedetermining parameters. But especially LDσ1 and LSσ1 havean essential effect on the reaction and performance of thedesaturation detection circuit, which will be shown in theexperimental measurements in this paper. For example thestray inductances LDσ1 and LSσ1, whose sum is about 6to 10 nH in a typical D2PAK package, have an influenceon the short circuit detection threshold ID,th. During shortcircuit the sum of the induced voltage drops at LDσ1 andLSσ1 can be much higher than the reference voltage of thedesaturation detection circuit. This can lead to a lower shortcircuit detection threshold compared to the theoretical valuewithout considering the stray inductances. Furthermore at ahalf bridge shoot trough an immediate fault condition of thedesaturation detection circuit can occur at the turn-on moment.In this case the response time and therefore the turn-off currentmagnitude are just determined by the propagation delay timesand filter time constants of the desaturation detection circuit.

III. SHORT CIRCUIT TYPES IN A HALF BRIDGECONFIGURATION

In Fig. 3 the two types of a possible short circuit ina half bridge configuration are shown [13]–[15]. The firsttype is a short circuit at the load, i.e. a short circuit of thewindings of an induction motor. In this case the current slopeis determined by the inductance of the load wire, motor cableor the rest inductance of the motor windings. Therefore thereis a lower current slope in the transistors in comparison tothe following described short circuit type 2. For experimentalverification two different cases of the short circuit type 1 willbe analysed in this paper. First a short circuit near the loadwill be investigated. Additionally a short circuit type 1 nearthe converter, which leads to a higher current rise due to thelower inductance, is analysed.

Fig. 3. Short circuit types in a half bridge configuration

In case of a failure in the controller or signal transmittersfor the switch-on and switch-off signals, i.e. optocouplers, anoverlapping conduction time of both switches, the high-sideand the low-side switch of the half bridge, and a resultingshoot trough may occur. As mentioned before in this case theshort circuit type 2 current slope is much higher compared tothe short circuit type 1 due to the very low stray inductancein the current path. For this reason the short circuit type 2

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Fig. 4. Proposed desaturation detection circuit for low voltage power MOSFETs

is more difficult to handle because of the need of a very fastreaction of the desaturation detection circuit.

IV. DESATURATION CONTROL CIRCUIT FOR LOWVOLTAGE POWER MOSFETS

In Fig. 4 the principle structure of the proposed desaturationdetection circuit is illustrated. During the on-state of the powerMOSFET a small additional current is injected through R1 andthe diode D1 into the power stage. This way the actual drain-source voltage of the low voltage power MOSFET VDS andthe forward voltage drop VF of the Diode D1 is measuredand divided by a voltage divider. Then this voltage over R3 iscompared with a reference voltage Vref by a fast comparator.If the measured voltage VDS+VF is higher than the referencevoltage, which means that a failure has occurred or theMOSFET is blocking, the output voltage V1 of the comparatoris set to the high level of 5 V. To decrease the temperaturedependence of the desaturation detection circuit the referencevoltage Vref depends on the measured case temperature TC ofthe power device, which is nearly the junction temperature TJin steady-state, by a thermistor. With increasing temperaturethe resistance of the thermistor decreases and leads to a higherreference voltage Vref .After a detection of a too high drain-source voltage, which isalso present during the blocking time of the power MOSFET,the fault signal of the comparator is filtered with a timeconstant of a few picoseconds. The following RS flip-flophas the function to hold the failure signal and to disable thedetection circuit during the blocking time by means of IN=0.The flip-flop is being reset by the next high level of the logicinput signal IN. Using a RS flip-flop as an integrated circuitinstead of a discrete solution leads to a lower circuit volume,a low propagation delay and low costs.The input signal IN is just applied to the driving stage, when

there is no detected failure, which is done by an AND-gatter.Otherwise the input signal of the driving stage is hold downat the low level. Finally the control signal is applied to theMOSFET driver.For a satisfying operation the delay-times of all used com-ponents, especially the comparator, should be as small aspossible. Therefore a comparator LMV7219M5 from NationalSemiconductors with a low propagation delay time of typical4.5 ns is used [16]. Furthermore the time constant of the usedfilter for the measurement of VDS + VF should be just in therange of a few hundred nanoseconds or less due to the fastcurrent rise during the short circuit type two.

V. EXPERIMENTAL RESULTS

The proposed desaturation detection circuit has been testedin a half bridge configuration as illustrated in Fig. 3. A photoof the driving stage with the desaturation detection circuit isshown in Fig. 5.In this laboratory setup the power supply of the driving stageis galvanic separated by a dc/dc converter but can also realizedby a bootstrap power supply.

The 60 V power MOSFETs IRFS3206PbF fromInternational Rectifier is used here in this laboratorysetup [17]. A load inductance of 57 µH and a DC linkvoltage VDC of 24 V are chosen. A theoretical short circuitdetection threshold ID,th of 175 A is adjusted. For theanalysis of the short circuit detection thresholds at differentjunction temperatures the power stage is mounted on aregulated heating plate. The drain current is measured with aCWT 15R Rogowski current probe from PEM UK Ltd. witha peak current ability of 3.0 kA and a bandwidth of 17 MHz.In Fig. 6 the turn-on and turn-off process of the low-sidepower MOSFET is shown in case of no short circuit. Atturn-off the output signal V1 of the comparator is set to 5 V

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Fig. 5. Photo of the driving stage with the desaturation detection circuit

Fig. 6. Turn-on and turn-off process of the power MOSFET in case of noshort circuit: Ch. 1: V1 (5 V/DIV), Ch. 3: VGS (10 V/DIV), Ch. 4: VDS (20V/DIV), Ch. A: ID (50 A/DIV), timebase t: 200 ns/DIV @ VDC = 24V,TC = 25 ◦C

during the current fall time. This is caused by the inducedvoltages at the stray inductances in the commutation path. Toavoid a failure command to the driving stage, the flip-flopdisables the fault-signal during the blocking time and turn-offprocess of the power MOSFET.

A turn-off process in case of a too high load currentwith a low positive current slope is shown in Fig. 7. In thiscase the drain current detection threshold is 172.8 A whichis nearly the theoretical value of 175 A and hence can beadjusted precisely by the proposed desaturation detectioncircuit.

A. Short circuit type 1

In Fig. 8 a short circuit type 1 at the load is illustrated.Because of the relative high remaining stray inductance ofabout 1.3 µH due to the load wires, the short circuit currentslope is relative low. At a drain-current of 144.7 A thedesaturation detection circuit detects a failure and the powerMOSFET is turned-off after a shutdown propagation delaytspd of about 43 ns. The lower practical short circuit detectionthreshold compared to the theoretical adjusted value of 175A is caused by the induced voltages, which are in the rangeof 50 - 150 mV in this setup and for this short circuit type.The thermistor compensates the temperature depending on-state drain-source voltage of the power MOSFET due to thetemperature-sensitive on-state drain-source resistance RDS,onat higher junction respectively case temperatures.

Fig. 7. Turn-off process of the power MOSFET in case of a too high loadcurrent: Ch. 1: V1 (5 V/DIV), Ch. 3: VGS (10 V/DIV), Ch. 4: VDS (20V/DIV), Ch. A: ID (50 A/DIV), timebase t: 1 µs/DIV @ VDC = 24V,TC = 25 ◦C

Fig. 8. Desaturation detection of a short circuit type 1 with a short circuit atthe load side: Ch. 1: V1 (5 V/DIV), Ch. 3: VGS (10 V/DIV), Ch. 4: VDS (20V/DIV), Ch. A: ID (50 A/DIV), timebase t: 500 ns/DIV @ VDC = 24V,TC = 25 ◦C

TABLE IMEASURED SHORT CIRCUIT DETECTION THRESHOLDS ID,th,meas. OF

THE DESATURATION DETECTION CIRCUIT FOR DIFFERENT CASE

TEMPERATURES TC WITH AND WITHOUT TEMPERATURE COMPENSATION

AT SHORT CIRCUIT TYPE 1 AT THE LOAD SIDE

ID,th,meas. ID,th,meas.TC with temperature without temperature

compensation compensation25 ◦C 144.7 A 140.9 A50 ◦C 164.2 A 125.3 A75 ◦C 156.7 A 112.8 A

100 ◦C 147.8 A 107.7 A125 ◦C 141.6 A 100.3 A

In Table I the measured short circuit detection thresholdsID,th,meas for different case temperatures TC of the powerMOSFET with and without temperature compensation arelisted. It can be seen, that the variation of the short circuitdetection threshold is very low for different case temperaturesTC in case of the temperature compensation. In contrast tothat the variation without temperature compensation is veryhigh.Compared to a short circuit at the load side, a short circuit

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Fig. 9. Desaturation detection of a short circuit type 1 with a short circuitdirect at the converter: Ch. 1: V1 (5 V/DIV), Ch. 3: VGS (10 V/DIV), Ch.4: VDS (20 V/DIV), Ch. A: ID (50 A/DIV), timebase t: 500 ns/DIV @VDC = 24V, TC = 25 ◦C

directly at the converter leads to a much faster current rise dueto the lower remaining stray inductance of the shorter copperwire. For this case a short circuit current path inductance ofabout 300 nH was used in this laboratory setup. In Fig. 9 thiscase of short circuit type 1 is illustrated. It is clearly to berecognized, that the load inductance leads to a steeper currentslope. At turn-on the output voltage V1 of the comparator isset to 0 V again. After a short time the desaturation detectioncircuit detects a too high drain-source voltage. Because ofto the induced voltage drops at the stray inductances of theMOSFET housing, the short circuit detection thresholds arelower at a short circuit at the converter side than in the caseof a short circuit at the load side. The shutdown propagationdelay tspd is the same as before.

B. Short circuit type 2

The short circuit type 2 leads to a much faster current risecompared to the short circuit type 1. In Fig. 10 this kind ofshort circuit is shown. During the current rise the sum of theinduced voltage drops at the package stray inductances LDσ1

and LSσ1 is about 3 V due to the low ratio between the pack-age inductance (LDσ1 +LSσ1) and the stray inductance of thewhole commutation path. Therefore the measured drain-sourcevoltage VDS,meas at the input of the comparator remains abovethe reference voltage Vref for the whole turn-on period. Afterthe delay time at turn-on caused by the output filter of thecomparator the failure signal V1 is applied to the RS flip-floprespectively the AND-gatter right before the driving stage andthe power MOSFET is turned-off. In this laboratory setup theshort circuit detection threshold is 258 A and the short circuitshutdown current is 300 A at TC = 25 ◦C.

In Table II the measured short circuit detection thresholdsand short circuit turn-off currents at different case temperaturesare listed. It can be seen, that there is no essential temperatureinfluence on the short circuit detection threshold. As men-tioned before in section II the short circuit detection thresholdis only influenced by the filter time constant at the input ofthe comparator.

Fig. 10. Desaturation detection of a short circuit type 2: Ch. 1: V1 (5 V/DIV),Ch. 3: VGS (10 V/DIV), Ch. 4: VDS (20 V/DIV), Ch. A: ID (100 A/DIV),timebase t: 200 ns/DIV @ VDC = 24V, TC = 25 ◦C

TABLE IIMEASURED SHORT CIRCUIT DETECTION THRESHOLDS ID,th,meas. AND

SHORT CIRCUIT TURN-OFF CURRENT ID,sc(off) OF THE DESATURATION

DETECTION CIRCUIT FOR DIFFERENT CASE TEMPERATURES TC WITHOUT

TEMPERATURE COMPENSATION AT A SHORT CIRCUIT TYPE 2

TC ID,th,meas. ID,sc(off)25 ◦C 258 A 302 A50 ◦C 258 A 302 A75 ◦C 256 A 299 A

100 ◦C 254 A 296 A125 ◦C 250 A 292 A

VI. CONCLUSION

A desaturation detection circuit for low voltage powerMOSFETs with junction temperature respectively case tem-perature compensation is presented and analysed. First theswitching behaviour of a power MOSFET and the influenceof the stray inductances in the current commutation path onthe switching process are described. Additionally the influenceof the stray inductances of the MOSFET package on theshort circuit detection threshold is depicted. The differentkinds of a possible short circuit in a half bridge configurationare explained. Furthermore the desaturation circuit speciallydesigned for low voltage power MOSFETs is explained indetail and recommendations of the parameters are given. Theproposed circuit is tested and analysed in the laboratoryfor different types of a short circuit and for different caserespectively junction temperatures. A very fast reaction ofthe desaturation detection circuit and a very low variation ofthe short circuit detection threshold depending on the casetemperature in case of a short circuit type 1 at the load areachieved. In case of a short circuit type 2 the short circuitdetection threshold is mainly determined by the filter timeconstants.

ACKNOWLEDGMENT

The authors would like to thank the Fraunhofer-Gesellschaftand the state of Schleswig-Holstein, which partly founded thisproject. The work was carried out in a combined project of

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the ’Centre of Competence for Power Electronics Schleswig-Holstein’.

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