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Closed-Loop Gate Drive for High Power IGBTs Lihua Chen and Fang Z. Peng Michigan State University 2120 Engineering Building East Lasing, MI 48824 USA Abstract-To overcome the drawbacks of the conventional gate drive, in this paper a closed-loop gate control method is proposed. In this novel method, the switching speed, specifically di/dt, is measured from the voltage across the parasitic inductance of the IGBT module and a closed-loop control is employed to adaptively adjust the gate drive voltage to control the switching speed according to a preset control reference. As a result, both the voltage overshoot and current overshoot can be effectively controlled. This new gate drive method enables programmable control of switching speed and allows full use of the capability of the power devices. The oscillations during IGBT switching are also reduced and associated EMI problems can be mitigated. The relationships between the controlled voltage overshoot, current overshoot and associated energy losses are derived and can provide guidelines for practical design. Also, the proposed gate drive fully utilizes gate drive signal information of amplitude and duty cycle, and provides a new capability for the system modulator to effectively control both the power electronic circuit’s steady-state and transient behaviors simultaneously. I. INTRODUCTION Based on the IGBT switching transient analysis [1], it was found that there exist two major issues during the switching of IGBTs. 1. during the IGBT turn-on, there occurs a current overshoot caused by the fast snap-off of the freewheeling diode (FWD). 2. during the IGBT turn-off, there occurs a voltage overshoot caused by the fast turn-off of the IGBT and parasitic inductances. Other problems associated with the switching of the IGBTs, like ringing, EMI, etc. are derived from these two major issues. The research on the gate drive circuit for the IGBT started almost as early as the application of this type of semiconductor device [2]. Much work has been done and focused on open- loop gate control to improve the transient performance of the IGBT [3]-[15]. In summary, most solutions were limited to empirical tests and discussion of the results. However, conventional gate drive methods cannot solve these two problems without compromising with lengthened delays, low noise immunity, and high switching energy losses. A closed- loop active gate drive concept to control the switching transient of the IGBT has been introduced to reduce EMI by providing less rapid switching [16]. Also an active voltage ramp control for IGBTs has only been implemented with traditional closed- loop methods for the applications which have IGBTs connected in series [17]-[24]. However, the large power loss associated with the slow voltage rise ramp is usually unacceptable in real applications. To overcome the drawbacks of the conventional gate drive, in this work, a novel closed-loop gate drive method featuring active control of switching speed for high power IGBTs is proposed. To achieve this control, the switching speed, specifically di/dt, is derived from a measurement of the voltage across the parasitic inductance of the IGBT module and a closed-loop control is employed to adaptively adjust the switching speed according to preset references. As a result, both the voltage overshoot and current overshoot can be effectively controlled. In the following discussion, the principle of the proposed gate drive method is explained in detail and experimental results are provided to justify the validity and features of the proposed novel method. The advantages associated with this new method are summarized in the conclusion section. II. PROPOSED CLOSED-LOOP GATE CONTROL To achieve bidirectional current flow, an IGBT module is actually configured with an IGBT and an anti-parallel power diode. The power diode's dynamic behaviors impact significantly the IGBT switching transient. The analysis of the dynamic characteristics of power diodes reveals that the peak reverse recovery current, I rr , is proportional to the rate of change of the reverse-recovery current, di R /dt. And this relationship can be approximately explained in (1) [1]. 6 2.8 10 / BD F R I BV I di dt rr - × (1) where I F is diode forward current, BV BD is diode breakdown voltage. It should be pointed that during the snap-off of the anti-parallel diode, the di R /dt is the same as the rate of change of the opposite IGBT collector current, di C /dt. During the turn-off of the IGBT, the voltage overshoot, Vos, is also proportional to the rate of change of the IGBT collector current and this relationship can be explained in (2). dt di L V C S OS = (2) where L S is the stray inductance of power electronic circuit. Equations (1) and (2) indicate that the control of the di C /dt of the IGBT can directly control the current overshoot during IGBT turn-on, and control the voltage overshoot during IGBT turn-off. The flexible controllability of modern IGBTs enables the control of the di C /dt via gate drive methods [25], and these relationships can be described as follows. 978-1-422-2812-0/09/$25.00 ©2009 IEEE 1331

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Closed-Loop Gate Drive for High Power IGBTs

Lihua Chen and Fang Z. Peng Michigan State University

2120 Engineering Building

East Lasing, MI 48824 USA

Abstract-To overcome the drawbacks of the conventional gate

drive, in this paper a closed-loop gate control method is proposed.

In this novel method, the switching speed, specifically di/dt, is

measured from the voltage across the parasitic inductance of the

IGBT module and a closed-loop control is employed to adaptively

adjust the gate drive voltage to control the switching speed

according to a preset control reference. As a result, both the

voltage overshoot and current overshoot can be effectively

controlled. This new gate drive method enables programmable

control of switching speed and allows full use of the capability of

the power devices. The oscillations during IGBT switching are

also reduced and associated EMI problems can be mitigated. The

relationships between the controlled voltage overshoot, current

overshoot and associated energy losses are derived and can

provide guidelines for practical design. Also, the proposed gate

drive fully utilizes gate drive signal information of amplitude and

duty cycle, and provides a new capability for the system

modulator to effectively control both the power electronic

circuit’s steady-state and transient behaviors simultaneously.

I. INTRODUCTION

Based on the IGBT switching transient analysis [1], it was

found that there exist two major issues during the switching of

IGBTs. 1. during the IGBT turn-on, there occurs a current

overshoot caused by the fast snap-off of the freewheeling diode

(FWD). 2. during the IGBT turn-off, there occurs a voltage

overshoot caused by the fast turn-off of the IGBT and parasitic

inductances. Other problems associated with the switching of

the IGBTs, like ringing, EMI, etc. are derived from these two

major issues.

The research on the gate drive circuit for the IGBT started

almost as early as the application of this type of semiconductor

device [2]. Much work has been done and focused on open-

loop gate control to improve the transient performance of the

IGBT [3]-[15]. In summary, most solutions were limited to

empirical tests and discussion of the results. However,

conventional gate drive methods cannot solve these two

problems without compromising with lengthened delays, low

noise immunity, and high switching energy losses. A closed-

loop active gate drive concept to control the switching transient

of the IGBT has been introduced to reduce EMI by providing

less rapid switching [16]. Also an active voltage ramp control

for IGBTs has only been implemented with traditional closed-

loop methods for the applications which have IGBTs

connected in series [17]-[24]. However, the large power loss

associated with the slow voltage rise ramp is usually

unacceptable in real applications.

To overcome the drawbacks of the conventional gate drive,

in this work, a novel closed-loop gate drive method featuring

active control of switching speed for high power IGBTs is

proposed. To achieve this control, the switching speed,

specifically di/dt, is derived from a measurement of the voltage

across the parasitic inductance of the IGBT module and a

closed-loop control is employed to adaptively adjust the

switching speed according to preset references. As a result,

both the voltage overshoot and current overshoot can be

effectively controlled. In the following discussion, the

principle of the proposed gate drive method is explained in

detail and experimental results are provided to justify the

validity and features of the proposed novel method. The

advantages associated with this new method are summarized in

the conclusion section.

II. PROPOSED CLOSED-LOOP GATE CONTROL

To achieve bidirectional current flow, an IGBT module is

actually configured with an IGBT and an anti-parallel power

diode. The power diode's dynamic behaviors impact

significantly the IGBT switching transient. The analysis of the

dynamic characteristics of power diodes reveals that the peak

reverse recovery current, Irr, is proportional to the rate of

change of the reverse-recovery current, diR/dt. And this

relationship can be approximately explained in (1) [1].

62.8 10 /BD F RI BV I di dtrr−

≈ × (1)

where IF is diode forward current, BVBD is diode breakdown

voltage. It should be pointed that during the snap-off of the

anti-parallel diode, the diR/dt is the same as the rate of change

of the opposite IGBT collector current, diC/dt.

During the turn-off of the IGBT, the voltage overshoot, Vos,

is also proportional to the rate of change of the IGBT collector

current and this relationship can be explained in (2).

dtdi

LV CSOS = (2)

where LS is the stray inductance of power electronic circuit.

Equations (1) and (2) indicate that the control of the diC/dt of

the IGBT can directly control the current overshoot during

IGBT turn-on, and control the voltage overshoot during IGBT

turn-off. The flexible controllability of modern IGBTs enables

the control of the diC/dt via gate drive methods [25], and these

relationships can be described as follows.

978-1-422-2812-0/09/$25.00 ©2009 IEEE 1331

During IGBT turn-on the diC/dt is expressed as:

( / 2 )

11

GG T

S

V V I gdi p mc

dt R Cg iesL

gm

− −

=

+

(3)

During IGBT turn-off the diC/dt is expressed as:

( / 2 )

1

T L GG

S

di V I g Vc m

dt R Cg iesL

gm

+ −=

+

(4)

Where VGG is gate drive voltage; IL is load current; gm is IGBT

transconductance; Rg is gate drive resistance; Cies is IGBT

input parasitic capacitance; VT is IGBT threshold voltage, LS1 is

the stray inductance between the IGBT chip emitter and the

Kelvin emitter. Within the two parts in the denominator on the

right side of (3) and (4), although LS1 is usually very small, it is

still comparable to the other component of the denominator,

since the transconductance, gm, has a large value for high

power IGBTs. With the conventional gate drive methods, the

amount of control over the di/dt obtained by changing the gate

resistance, Rg, is very limited because this effect is attenuated

by a large value of gm.

Unlike the conventional methods, in the proposed closed-

loop gate drive, by adaptively varying the gate drive voltage,

VGG, shown in the numerator of (3) and (4), the gate drive can

control the IGBT switching speed, specifically di/dt, and

therefore the current overshoot and voltage overshoot across

the IGBT.

Fig. 1 shows the schematic of the proposed closed-loop gate

drive. For the on-state and off-state control of the IGBT, the

positive inputs of the Op Amps, OP1 and OP2, are zero volts,

and the control input signal is amplified by OP1, OP2 and a

power stage. The IGBT gate voltage is firmly clamped to either

turn-on voltage VCC, in most applications +15V, or turn-off

voltage VEE, usually -8V to -15V. This leads to stable on and

off states for the IGBT.

Fig. 1. Proposed close-loop gate drive circuit.

During the IGBT turn-on and turn-off transients, a

substantial voltage appears across the parasitic inductance, LEe

(shown with a dotted line in Fig. 1), of the main emitter and

Kelvin emitter of the IGBT module. This voltage is linearly

proportional to the rate of change of the IGBT collector current,

diC/dt, specifically expressed as:

dtdi

LV CEeEe = (5)

Also, as shown in (2), this voltage is linearly proportional to

the voltage overshoot. The voltage is measured and adjusted by

Op Amp OP3, and fed to the positive input of OP1. During the

switching transient, the error between the control input voltage

and the measured voltage overshoot is amplified and used to

adaptively adjust the IGBT gate drive voltage, VGG. As

expressed in (3) and (4), the IGBT switching speed can be

actively manipulated, and hence the current overshoot and

voltage overshoot can be controlled by varying the control

input voltage reference, Vref1 and Vref2. Specifically, change

the amplitude of Vref1 to control the turn-on switching speed

and change the amplitude of Vref2 to control the turn-off

switching speed.

To overcome the latency behavior of the IGBT and improve

the stability of the closed-loop control, a phase-lead

compensation is implemented by measuring the derivative of

the collector voltage with the capacitor C1 and resistor R1, and

feeding this to the positive input of OP2. This compensation

should be kept as small as possible to avoid dramatically

slowing down the rate of change of the IGBT collector voltage,

dv/dt, since a slow voltage ramp causes additional energy loss,

which is not desired in real applications. Also, theoretically

there are no relationship between the dv/dt and current

overshoot and voltage overshoot. It was also found that the

gain of OP2 should be kept small to improve noise immunity

and avoid oscillations. In the hardware implementation,

optimized results were achieved when the gain of OP2 was set

to two by the ratio of the resistances of R6 over R7, and the gain

OP1 was set to five by the ratio of the resistances R4 over R5.

The modeling and stability analysis of the proposed closed-

loop gate control will be explained in future works.

Experimental results will be provided next to validate the

feasibility of the proposed closed-loop gate control for IGBTs.

III. EXPERIMENTAL RESULTS AND DISCUSSION

A. Experiment Setup and Preliminary Test Results

To validate the proposed closed-loop gate control for IGBTs,

a gate drive board has been built and tested on a chopper

circuit with a laminated bus-bar structure. Two single IGBT

modules, CM1000HA-24H from Powerex, rated at 1 kA, 1200

V, were chosen to build this circuit. Fig. 2 shows the test

circuit configuration and signal monitoring.

A double-pulse method has been used for the test of the

closed-loop gate drive, e.g., 1 kA. The first long pulse is used

to raise the load current to a desired value. The second short

pulse, 10µs, is used for characterizing the IGBT switching

behaviors. The switching transients are captured by the

oscilloscope with a small time scale and high resolution for the

evaluation of the proposed closed-loop gate control.

978-1-422-2812-0/09/$25.00 ©2009 IEEE 1332

Fig. 2. Test circuit configuration and signal monitoring.

The preliminary testing was first conducted with a

conventional gate control (CGD) by simply disconnecting the

feed back inputs to the proposed gate drive board. To avoid

damaging the IGBTs, an applied dc bus voltage, Vdc, of 300 V

was used and the load inductor current was initially charged to

1 kA Fig. 3 shows the turn-off waveforms of the gate-emitter

voltage, Vge, collector current Ic, and collector-emitter voltage

Vce. Using the math function of the oscilloscope, the turn-off

energy loss was measured by integrating the product of the Vce

and Ic. A voltage spike as high as 780 V was detected during

the testing. The magnitude of the rate of change of the IGBT

collector current is as high as 3700 A/µs. Fig. 3 also shows that

the energy loss for each turn-off transient is about 0.17 J.

Fig. 3. IGBT turn-off waveforms with CGD (Vdc=300V).

Fig. 4 shows the turn-on transient waveforms. A current

spike as high as 160 A was detected during the testing. The

rate of change of the IGBT collector current, di/dt, is 1600

A/µs, and the measured energy loss, Eon, for this turn-on

transient is about 29 mJ. There are also oscillations which

appear on the Vce waveform before the IGBT turn-on which

are a result of the turn-off of the IGBT. Because there is only

10 µs between the end of the first pulse and the start of the

second pulse, and the CGD turn-off control is employed in the

preliminary test, the oscillation caused by the IGBT turn-off

transient has not yet been fully damped.

Fig. 4. IGBT turn-on waveforms with CGD (Ic=1 kA).

B. Test Results of Closed–Loop Turn-off Control

For comparison, the closed-loop turn-off control tests were

performed, at first with a dc bus voltage of 300 V, which is the

same as that used for the preliminary test. The load inductor

current was also initially charged to 1 kA. Fig. 5 through 7

show the turn-off waveforms using different control voltage

references, Vref2, of -10V, -5V, and -2.5 V, respectively. As

shown in the figures, the voltage overshoot has been

effectively controlled to 310 V, 180V, and 100V; while the rate

of change of the IGBT collector current, di/dt, are 2150 A/µs,

1020 A/µs, and 580 A/µs, respectively. However, Fig. 5

through 7 also show that the initial rate of change of the

collector voltage, dv/dt, is nearly the same in all three tests, and

is comparable to that of CGD result shown in Fig. 3. Therefore,

the energy loss associated with the collector voltage rise ramp

is minimized in the proposed gate drive.

From the test waveforms, the parasitic inductance of the test

circuit can be estimated at about 150 nH. Compared to the

CGD Vge waveform shown in Fig. 3, the gate terminal

voltages shown in Fig. 5 through 7 have been adaptively pulled

up during the turn-off transient while the IGBT collector

current is decreasing. As a result, the gate discharge current

was decreased and the di/dt is limited; and the voltage

overshoot was controlled according to the reference Vref2. It

should be pointed out here that the measured Vge waveform is

the voltage across the IGBT external gate-Kelvin emitter

terminals, and that this is slightly different from the gate

voltage of the internal IGBT chip because of additional voltage

drops on the parasitic components within the gate drive path.

Examining Fig. 5 through 7, it can be shown that when the

switching speed and voltage overshoot are controlled to be

lower, the Vce oscillation amplitude after the first voltage

overshoot hump becomes smaller. Actually there are almost no

oscillations appearing in Fig. 7, because the energy stored in

the parasitic inductances and capacitances has been effectively

discharged during the switching transient (an equivalent circuit

model to study this oscillation phenomenon has been

developed, but omitted here due to page limitation). As a result,

the EMI problems associated with fast switching and

oscillations can also be mitigated when the switching speed is

controlled.

978-1-422-2812-0/09/$25.00 ©2009 IEEE 1333

Also, the IGBT tailing current becomes much smaller in Fig.

7 and this is attributed to the slow switching speed transient,

during which more carriers stored in the IGBT drift region

have been recombined. Thereby the additional energy loss

caused by the IGBT tail current is minimized.

Fig. 5. IGBT turn-off waveforms with proposed gate drive

method (Vref2=-10V, Vdc=300V).

Fig. 6. IGBT turn-off waveforms with proposed gate drive

method (Vref2=-5V, Vdc=300V).

Fig. 7. IGBT turn-off waveforms with proposed gate drive

method (Vref2 =-2.5V, Vdc=300V).

After confirming that the proposed closed-loop gate drive can

effectively control the voltage overshoot, the dc bus voltage

was increased to 600 V, and similar tests were conducted. Fig.

8 through 10 show the turn-off waveforms for control voltage

references of -10V, -5V, and -2.5V, respectively.

Fig. 8. IGBT turn-off waveforms with proposed gate drive

method (Vref2 = -10V, Vdc=600V)

Fig. 9. IGBT turn-off waveforms with proposed gate drive

method (Vref2 = -5V, Vdc=600V).

Fig. 10. IGBT turn-off waveforms with proposed gate drive

method (Vref2= -2.5V, Vdc=600V).

978-1-422-2812-0/09/$25.00 ©2009 IEEE 1334

These results shown in Fig. 8 through 10 demonstrated that

the Vos and di/dt was also effectively controlled according to

the reference Vref2, and the numerical values of Vos and di/dt

are same as those shown in Fig 5 through 7, respectively. It can

conclude that the proposed closed-loop gate drive can

effectively control the voltage overshoot and di/dt according to

a control reference independent of dc voltages. Also the Vce

oscillation mitigation phenomena are examined in the test

results shown in Fig 8 through 10.

C. Test Results of Closed-Loop Turn-on Control

During the closed-loop turn-on control tests, the load

inductor current was also initially charged by the first long

pulse to 1 kA, which is the same as that used for the

preliminary test with CGD control. The turn-on transient of the

second short pulse is captured for performance evaluation. Fig.

11 through 13 show the turn-on waveforms using different

control voltage references, Vref1, of 10V, 5V, and 2.5 V,

respectively. As shown in the figures, the current overshoot has

been effectively controlled to 115 A, 60 A and 20 A; while the,

di/dt, are 1333 A/µs, 1140 A/µs, 730 A/µs and 340 A/µs,

respectively.

Fig. 11. IGBT turn-on waveforms with proposed gate drive

method (Vref1 =10V).

Fig. 12. IGBT turn-on waveforms with proposed gate drive

method (Vref1 =5V).

Fig. 13. IGBT turn-on waveforms with proposed gate drive

method (Vref1 =3V).

Compared to the CGD Vge waveform shown in Fig. 4, the

gate terminal voltages shown in Fig. 11 through 13 have been

reduced during the turn-on transient while the IGBT collector

current is increasing. As a result, the gate charge current was

limited and the di/dt or turn-on speed is controlled. Also,

before the IGBT turn-on, in the Vce waveforms shown in Fig.

11 through 13, the oscillations are much smaller compared to

that shown in Fig. 4. This is attributed to the closed-loop gate

control that has been used to turn off the IGBT at the end of

first long pulse and the oscillation has been reduced.

D. Summary of Closed-Loop Control Test Results

To fully examine the proposed closed-loop gate control, a

series of tests has been conducted to find out the relationship

between the voltage overshoot, current overshoot, and the

IGBT switching speeds by varying the control reference. Table

1 summarizes the test results of turn-off gate control, and

similarly, table 2 summarizes the test results for turn-off gate

control. The test results listed in table 1 and 2 demonstrate that,

with the proposed closed-loop gate drive, the IGBT speed,

voltage overshoot, current overshot can be effectively

controlled according to the preset references. Also, by using

the test results of the conventional gate drive as base values,

the relationships between the voltage overshoot, current

overshoot and IGBT switching speed can be plotted as Fig. 14

and 15 for turn-on and turn-off closed-loop gate control,

respectively. Examining Fig. 14 and 15, approximately linear

relationships between the voltage overshoot, current overshoot

as functions of controlled switching speeds are detected during

the tests, and this confirms the theoretical analysis which has

been discussed in the section 2.

TABLE 1

SUMMARY OF TUNR-OFF CONTROL

Vref2

(V)

Vos

(V)

Vos/

Vos(CGD)

di/dt

(A/µs)

[di/dt]/

[di/dt(CGD)]

-2.5 100 0.208 580 0.157

-3 120 0.25 667 0.18

-4 150 0.313 833 0.225

978-1-422-2812-0/09/$25.00 ©2009 IEEE 1335

-5 180 0.375 1020 0.276

-6 200 0.417 1140 0.308

-7 220 0.458 1290 0.349

-8 240 0.5 1480 0.4

-9 280 0.583 1800 0.486

-10 310 0.658 2150 0.581

CGD 480 1 3700 1

TABLE 2

SUMMARY OF TURN-ON CONTROL

Vref1

(V)

Irr

(A)

Irr/

Irr(CGD)

di/dt

(A/µs)

[di/dt]/

[di/dt(CGD)]

3 20 0.125 340 0.213

4 45 0.281 590 0.369

5 60 0.375 730 0.456

6 70 0.438 940 0.588

7 95 0.594 1140 0.713

8 110 0.688 1230 0.769

10 115 0.719 1333 0.833

CGD 160 1 1600 1

Fig. 14. Voltage overshoot vs. turn-off switching speed.

Fig. 15. Current overshoot vs. turn-on switching speed.

IV. CONCLUSIONS

A closed-loop gate drive method was proposed to actively

control the switching speed according to a preset control

reference. Compared to the conventional gate drive solutions,

the proposed method has the following advantages:

• The switching speed can be adaptively manipulated

according to preset control references. As a result, the

current overshoot and voltage overshoot can be effectively

controlled and the full capability of the power device can

be used with the proposed gate drive;

• This method enables programmable switching speed

control by dynamically changing the PWM signal

amplitudes. This feature can be illustrated in Fig. 16 and

17. The PWM gate signal duty ratio is used to control the

device’s on-state and off-state time and the PWM gate

signal amplitudes can be used to control the device’s turn-

on and turn-off speeds;

Fig. 16. Control both the duty cycle and switching speed by a

gate PWM signal.

Fig. 17. Conventional (top) and proposed (bottom) PWM gate

control signals.

• The proposed gate drive fully utilizes gate drive signal

information of amplitude and duty cycle (two control

freedoms). As a result, both the power electronic circuit’s

steady-state and transient behaviors can be effectively

978-1-422-2812-0/09/$25.00 ©2009 IEEE 1336

controlled by the system modulator. This new concept has

not been seen from previous works and literatures;

• The proposed gate control enables power device

snubberless operation without additional clamping circuits;

therefore, the power conversion systems design can be

simplified;

• Oscillations are reduced during IGBT switching and the

associated EMI problems can be mitigated;

• Simple circuit and low cost allow easy embedding into the

gate drive circuit.

The investigation also find that, compared with the CGD

methods, the proposed gate control method can actively control

the IGBT turn-off speed under over-current conditions caused

by overload operation or fault short circuit, and limit the

voltage overshoot independent of dc bus voltage and fault

current levels. In addition, the relationships between the

voltage overshoot, current overshoot and associated energy

losses as functions of controlled switching speed has been

derived and can provide guidelines for practical design. The

modeling of the IGBT and a mathematical analysis of the

proposed gate control has been also conducted to confirm the

proposed new method. All these results will be presented in

future works.

REFERENCES

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and Applications,” 3rd

edition, Upper Saddle River, NJ,

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