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A Smart Gate Driver with Active Switching Speed Control for Traction Inverters.

INTRODUCTION

To meet the more stringent fuel efficiency requirement, the automotive industry is going through an evolutionary transformation to make smarter and more efficient electric vehicles (EV) and hybrid electric vehicles (HEV). The traction inverter in the electrified powertrain, as shown in Fig. 1, is one of the components that generate considerable losses. To further increase the vehicle fuel economy, it is desirable to improve the traction inverter efficiency. In a traction inverter, the IGBT and diode generate the majority energy losses in the form of switching and conduction losses. Over these years, the high power semiconductor devices have made significant performance improvements. The gate driver which is used to control the IGBT becomes more crucial if further performance improvement is desired. As shown in Fig. 1, each IGBT has a gate driver circuit to control its turn-on and turn-off. The gate driver circuit can adjust the switching transient, thus impact the switching loss.

This paper focuses on improving the IGBT turn-off switching performance. Fig. 2 shows a generic conventional gate driver circuit schematic. It simply includes a buffer stage, and turn-on and turn-off gate resistors. The gate driver receives a gate input signal from micro-processor and its output is connected to the IGBT gate terminal. The gate input signal is amplified by a buffer stage composed of BJTs or MOSFETs. In Fig. 2, the buffer stage includes a NPN BJT [Q.sub.1] and a PNP BJT [Q.sub.2]. When the gate input signal is high, [Q.sub.1] is turned on. The gate of IGBT Q is connected to gate driver power supply, e.g. 15V, and gate capacitance is charged through turn-on gate resistor [R.sub.on]. When gate input signal is low, the gate capacitance is discharged through turn-off gate resistor [R.sub.off] and [Q.sub.2]. [R.sub.on] and [R.sub.off] determine the IGBT turn-on and turn-off switching speed respectively. [L.sub.s] is the equivalent stray inductance existed in the IGBT high voltage loop. "E" represents the IGBT power emitter terminal and "e" represents the IGBT Kevin emitter terminal.

A typical turn-off switching waveform using this circuit is shown in Fig. 3. The turn-off transient mainly includes four phases. Once the turn-off process begins, gate voltage [V.sub.ge] starts to decrease. Before [V.sub.ge] reaches miller plateau voltage, IGBT collector-emitter voltage [V.sub.ce] and collector current [i.sub.c] haven't changed a lot. This phase is turn-off delay phase. The longer the turn-off delay, the larger dead-time is needed to avoid potential shoot-through problem. Large dead-time could degrade the traction inverter output quality. The second phase is [V.sub.ce] rising period. [V.sub.ce] reaches the dc link voltage at the end of this phase. During the third phase, [i.sub.c] starts to decrease. Because of the stray inductance in the power circuit, di/dt will cause surge voltage across the IGBT. Therefore, [V.sub.ce] continues to increase and exceeds dc link voltage. Due to the [V.sub.ce] surge voltage, it raises the IGBT voltage rating requirement. In the fourth phase, [V.sub.ge] discharges to zero and there is a tail current.

In designing the conventional gate driver for EV/HEV traction inverter, one needs to consider the tradeoff between switching losses and device stresses. For example, the gate resistances are selected at worst case scenario, i.e. maximum load current and highest dc bus voltage, to guarantee the device peak surge voltage always below its rating. This usually leads to large gate resistances. However, the selected large gate resistors unnecessarily slow down the switching speed at low current operation points. Since the traction inverter mostly operates at low current levels in typical driving cycles, the overall vehicle fuel economy is sacrificed to meet the worst case requirement.

The state-of-the-art advanced IGBT gate drivers target to improve the switching performances, and they can be classified as passive, open-loop and closed-loop types [1]. The passive method with added external capacitances or gate voltage shape generator [2] is simple, but the controllability of the switching waveforms is still limited. The open-loop control gate drivers incorporate switchable gate resistors, gate current sources or voltages [3, 4]. For each interval of the switching transient, a specific gate resistor, current source or gate voltage is selected to independently control the waveform during that interval. Due to the dependence of IGBT characteristics on operation conditions, an operating point dependent profile is needed to control the switchable elements so that optimized performance can be achieved at each operation point. Usually, a lot of off-line calibration effort is required to generate the aforementioned control profiles. The closed-loop control concept can adjust the di/dt and dv/dt in real time during the transient by adding extra feedback circuits, control and logic circuits [1, 5, 6, 7], which increase the circuit complexity. Moreover, the added extra circuits could introduce delays, which make it not suitable for very fast switching applications.

PROPOSED GATE DRIVER CIRCUIT

This paper proposed a gate driver circuit that can actively adjust the turn-off switching speed based on different current levels. The circuit is shown as in Fig. 4. To increase the system reliability, the IGBT usually include a current mirror sensing pin which flows a fractional value of the IGBT collector current [i.sub.c]. The signal is used for over-current and/or short circuit current protection [8]. In the proposed circuit, the signal is utilized to actively control the turn-off speed of the IGBT. When the current level is low, fast turn-off is enabled. When the current level is high, fast switching is disabled. To achieve fast turn-off under low current conditions, a fast gate discharge path composed of a MOSFET [Q.sub.3] and a small turn-off resistor [R.sub.off_fast] is added.

The current mirror sensing pin is connected to a sensing resistor [R.sub.s] to convert the current signal to a voltage signal. Because the current mirror signal has spikes during the IGBT switching transient, a RC filter formed by [R.sub.f] and [C.sub.f] is used to filter the noise. The output of the RC filter is compared with a reference voltage [V.sub.ref] which is corresponding to a current level threshold. If the current is lower than the threshold, the comparator outputs low. MOSFET [Q.sub.4] is turned off. [Q.sub.3] gate is pulled up by the +15V power supply through [R.sub.p]. Therefore, the gate of IGBT Q can be discharged through fast discharge path, as shown in Fig. 5(a). When the IGBT current increases to a level that the output of RC filter is higher than [V.sub.ref], the comparator outputs high so that [Q.sub.4] is turned on. Because [Q.sub.4] is on, the gate of [Q.sub.3] is pulled down to ground. Therefore, the fast discharge path is shut down. The IGBT gate can only be discharged through [R.sub.off], as shown in Fig. 5(b). The proposed circuit does not require a high bandwidth comparator. Before the turn-off event, the current flowing through the IGBT current mirror pin charges the [C.sub.f] and the capacitor will hold the voltage till the end of the switching transient.

EXPERIMENTAL RESULTS

The proposed circuit is verified based on the Power Module used in Ford HEV traction inverter in production. The package outline is shown in Fig. 6. All switches in the power module have current sensing and on-die temperature sensing capabilities. The test was done with one of the switches in the power module. In the test, the over-current protection threshold was selected at 520A to accommodate the switch output capability. [V.sub.ref] was selected at 0.3V which corresponded to 400A. 400A was selected because it covered the inverter most operation regions. Moreover, the surge voltage at 400A with fast turn-off switching was the same as the surge voltage at maximum load current (close to over-current level 520A) with slow switching. Therefore, the IGBT voltage stress was not increased with the proposed gate driver circuit.

Fig. 7 provides the turn-off waveforms for the conventional and proposed gate drivers at 65A, 220A and 520A current levels. At 65A, the IGBT current was lower than the 400A threshold, so the IGBT was turned off through the fast discharge path. The waveforms of the two methods are put together. It is obvious that the turn-off delay was significantly reduced and the dv/dt was increased for the proposed method. The di/dt was similar for the two methods. This is because when dv/dt was increased, more current was shifted from the MOS channel to expand the depletion layer. When load current was small, the MOS channel current became small. The gate control over di/dt became very limited. Fig. 8 shows the turn-off waveforms at 220A. Similar to the 65A case, the IGBT was turned off with fast turn-off path. The turn-off delay was reduced, and dv/dt was increased. It is seen that di/dt was increased for the proposed method in this case. Because the di/dt was increased, the surge voltage was higher. Fig. 9 shows the turn-off waveforms at 520A. Because the current was higher than the pre-set threshold 400A, the fast turn-off path was turned off as expected. The [v.sub.ce] and [i.sub.c] waveforms almost overlapped with each other for the proposed and conventional methods.

The turn-off loss and surge voltage data at different current points are provided in Fig. 10 and Fig. 11 respectively. When the current was lower than 400A, the proposed method reduced the turn-off loss by 10-20% compared with the conventional method. After 400A, the turn-off loss was kept the same between the two methods. When the load current was smaller than 100A, the surge voltage is similar between the proposed method and conventional method. The reason is explained above. Most load current is shifted to expand the depletion layer. The MOS channel current is small, so the gate has limited control over di/dt. When the current was between 100A and 400A, the surge voltage was higher for the proposed method due to faster turn-off transient. When the current is above 400A, the surge voltage was the same as the conventional method because the fast turn-off path was shut down. The surge voltage at 400A was designed the same as the surge voltage at 520A. Therefore, the proposed method did not impose higher voltage stress across the IGBT.

CONCLUSIONS

The proposed gate driver circuit can reduce the traction inverter loss by reducing the turn-off switching losses at most frequent operation regions. By utilizing the IGBT current sensing signal, it enables fast switching at low current levels and slow switching at high current levels. The circuit is very simple and reliable, and the added cost is very low. The experimental verification was done based on power module used in Ford HEV traction inverter in production. It showed 10%-20% reduction in turn-off losses in most frequent operation current regions. The highest surge voltage was kept the same as in the conventional method. Therefore, increase of the IGBT device voltage rating was not needed.

REFERENCES

(1.) Lobsiger Y. and Kolar J. W., "Closed-Loop di/ dt and dv/dt IGBT Gate Driver," IEEE Transactions on Power Electronics, vol. 30, no. 6, pp. 3402-3417, Jun. 2015.

(2.) Grbovic, P. J., "An IGBT Gate Driver for Feed-Forward Control of Turn-on Losses and Reverse Recovery Current," IEEE Transactions on Power Electronics, vol. 23, no. 2, pp. 643-652, Mar. 2008.

(3.) Volke A. and Hornkamp M., IGBT Modules--Technologies, Driver and Application, 1st ed. Munich, Germany: Infineon Technologies AG, 2011.

(4.) John V., Suh Bum-Seok and Lipo T. A., "High-performance active gate drive for high-power IGBT's," IEEE Transactions on Industry Applications, vol. 35, no. 5, pp. 1108-1117, Sep/Oct 1999.

(5.) Chen L., "Intelligent gate drive for high power MOSFETs and IGBTs," Ph.D. dissertation, Dept. Electr. Comput. Eng.,Michigan State Univ., East Lansing, MI, USA 2008.

(6.) Chen L., and Peng F. Z., "Closed-loop gate drive for high power IGBTs," in Proc. 24th IEEE Appl. Power Electron. Conf. Expo.,Washington, DC, USA, Feb. 2009, pp. 1331-1337.

(7.) Lim, T. C. Williams, B. W. Finney, and S. J. Palmer, P. R., "Series-connected IGBTs using active voltage control technique," IEEE Transactions on Power Electronics, vol. 28, no. 8, pp. 4083-4103, Aug. 2013.

(8.) Infineon, "1EDI2001AS Datasheet," (Jul. 2015). [Online]. Available: http://www.infineon.com/dgdl/Infineon-1EDI2001AS-DS-v03_01-EN.pdf?fileId=5546d462503812bb01505fa4c8d6059c

CONTACT INFORMATION

Yan Zhou

Ford Engineering Laboratory Bldg, Ford Motor Company

Dearborn, MI, 48124

yzhou77@ford.com

Yan Zhou, Lihua Chen, Shuitao Yang, Fan Xu, and Mohammed Khorshed Alam

Ford Motor Company

doi:10.4271/2017-01-1243
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Article Details
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Author:Zhou, Yan; Chen, Lihua; Yang, Shuitao; Xu, Fan; Alam, Mohammed Khorshed
Publication:SAE International Journal of Alternative Powertrains
Article Type:Report
Date:Jul 1, 2017
Words:2126
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