Published March 16, 2015

1200V IGBTs “M series” in Trench Gate Field Stop for Rugged, Efficient and Reliable Industrial Power Drives

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ABSTRACT:
The third generation of IGBT trench-gate field-stop technology is at the heart of the M series 1200V IGBTs product family from STMicroelectronics. It has been specifically designed to minimize the environmental impacts of pollution saving more energy and increase reliability in applications such as industrial motor drives – representing the largest energy demanding – as well as solar inverters, uninterruptible power supplies, welding equipment, and more generally, in any hard-switching circuits operating at up to 20 kHz.

The series’ current rating ranges in massive production shows 15A, 25 and 40A in standard TO-247 and TO-247 long-lead packages. Several are the size of IGBT chips in M series available for die form.

INTRODUCTION:
With the ever-increasing demand of energy, most governments are now trying to minimize the impact on environmental pollution by introducing regulations that require an increase of efficiency in applications such as industrial pumps, fans, and various motor drives.

In industrial motor drive systems in general, IGBTs are commonly used to drive the motor in both discrete and module options.

In solar inverters, welding equipment, uninterruptible power supplies, and more generally, in any hard-switching circuits operating up to 20 kHz, IGBTs in M series represent the switch option silicon based to yield the best system efficiency.

This article presents the 1200V IGBT M series in third generation trench-gate field-stop technology as an excellent power device solution for the above listed applications.

Starting from a technology description of how these specific advantages have been obtained, both static and dynamic behaviors will be analyzed with a benchmarking of the trade-off performance the best competition offers for the same market applications.

TECHNOLOGY FEATURES:
1200V IGBT in M series shows an optimized trade off V_CE(sat) versus E_off including the short circuit capability. ST tailors the properties of this new series in its trench-based field stop IGBTs using the following:

• Float zone (FZ) silicon substrates and well controlled thin wafer technologies;
• Fine tuning backside processing dose, annealing temperatures and other processing parameters;
• Optimized horizontal layout with introduction of dummy cell concept for a limited saturation current to ensure exceptional device ruggedness during unexpected events.

As a further result of these improvements, the maximum operating junction temperature guaranteed by this new technology has been raised to 175°C.

Fig 1 – descriptive section of M series IGBTs

A new generation of diodes, optimized for a high level of softness maintaining a fast recovery, has been introduced in the duo pack option of the new 1200V M family. This provides excellent performance in EMI behavior while minimizing the contribution to losses by the diode.

Furthermore, the manufacturing process for the Trench gate field stop technology ensures tight control over parameters such as the IGBT conduction loss (V_CE(sat)) and its threshold voltage (V_GE(th)). Combined with a positive temperature coefficient of saturation voltage, this feature makes parallel connections of many devices safe when building high power applications.

ELECTRICAL POSITIONING
The new 1200V IGBTs in M series shows a large improvement in trade off performance compared to the latest competitor IGBT devices available on the market and tailored for the same target application. Figure 2 shows comparative trade off measurements of the normalized turn off (E_off) versus conduction (V_CE(sat)) losses as result of same size devices characterization carried out at 150°C in order to take the temperature performance de-rating into account. As is shown, the new 1200V IGBT in M series, against a comparable V_CE(sat) values to the best competitor (competitor 1), shows an important reduction in turn off loss of approximately -10% leading to best in class devices. Milestones are the improvement in performance against the other best competitors in the market (2, 3).

Fig 2 – trade off diagram E_off versus V_CE(sat)

STATIC AND DYNAMIC PERFORMANCE
In order to analyze how the improvements in technology translate into electrical performance of the M series IGBTs, the 40A size at 100°C (STGWA40M120DF3) results have been considered as the family champion; other sized devices belonging to the same series have the same behavior at their own nominal current.

Static (V_CE(sat), I_CES): The output characteristics at 25°C and 175°C are shown in Figure 3.

Fig 3 – Output characteristic of STGWA40M120DF3

The 1200V M series device shows a V_CE(sat) of 1.85V at nominal collector current 25°C and 2.3V at the same current rating and 175°C. The positive temperature coefficient shown results in a negative feedback during functionality, so making the devices in M series suitable for an easier paralleling.

Another important feature of the new 1200V IGBT in trench gate field-stop technology is the low current leakage (I_CES) during off-state resulting less than 100μA at 1200V up to 175°C.

Turn-off: The switching off waveforms of the new 1200V M series 40A in third technology generation is showed in figure 4. As illustrated, devices in the M series show a smooth switching behavior for the current waveform resulting in a small over voltage transients. This one, together with the linear voltage rise, minimizes the EMI.

Fig 4 – turn off of STGWA40M120DF3 at T_J = 175°C

A low gate charge (Qg = 125nC @ V_CC = 960V, I_C = 40A, V_GE = 15V) is also shown by the device STGWA40M120DF3; it results in a lower gate drive power when compared to competitive IGBTs.

Turn-on: A new generation of diodes with a high level of softness has been introduced for the optimal match of the new 1200V IGBT family in M in the duo pack option to enhance turn-on efficiency for the above listed applications.

The results of this coupling are excellent performance in turn-on also at very high di/dt thanks to the softness showing limited switching-on loss (turn-off of the diode) and very good EMI behavior. A turn-on characteristic of the 40A size at a temperature of 175°C is shown in figure 5.

Fig 5 – turn on of STGWA40M120DF3 at T_J = 175°C

Together with a very good waveform in both softness and EMI, a very low turn-on loss – higher than 35% against the best competitor – results from devices belonging to M series.

Robustness: The major benefit in robustness of the new M series family of 1200 V IGBTs is the guaranteed minimum short circuit withstand time of 10 µs at a starting junction temperature of 150 degrees Celsius. The waveform in figure 6 shows the ability of the STGWA40M120DF3 to sustain and switch-off the short circuit event without any impairment of the IGBT characteristics.

Fig 6 – short circuit behavior of STGWA40M120DF3 at T_Jstart = 150°C

The 1200V device in the M series also shows a wide square Reverse Bias Safe Operating Area at T = 175°C: 1200V and twice the nominal collector current.

Reliability: To increase the maximum operating junction temperature (Tj max) to 175°C, the following quality tests have been performed at high temperature: High Temperature Reverse Bias (HTRB), High Temperature Gate Bias (HTGB), and Humidity. 1200V IGBTs device in M series demonstrate outstanding stability and ruggedness with regards to elevated junction temperature and tough environmental conditions.

COMPARISON ANALISYS ON TYPICAL APPLICATION
The design example in this paper using STGWA40M120DF3, 1200V/40A in TO-247 package in a sixpack topology, shows that the M series IGBT silicon technology offers the lowest overall losses for motor drive.

In industrial motor drive systems, IGBTs – in both discrete and modules option – are commonly used to drive the motor.

These must be able to deliver excellent performance with the lowest power losses. Typically, the motor will switch in the range of 8 – 12 kHz. Therefore, the IGBT silicon must be optimized for this condition.

The comparison is based on measurements taken on the STGWA40M120DF3 and the three main competitors of the same size currently available in the market. It is assumed that these devices can operate on a system with nominal output current of 20 Arms, V_OUT=600V, f_sw=10kHz, PF=0.8, T=100°C. Figure 7 shows the split of power loss due to their conduction for both IGBT and co-packed diode, and the total switching losses (on and off) at 30A that represent approximately the peak of current each phase of the motor.

In particular, figure 7 shows the quality of the STGWA40M120DF3 in switching versus the other devices in the market today, keeping almost the same loss during conduction. The M series IGBT shows 8-10% lower power dissipation compared to the closest one, competitor 1, and 15-20% lower versus competitor 2. This results in a lower case temperature increasing the FIT for a more robust design.

Fig 7 – Power loss comparison in a motor control application at current level of 30A, V_BUS=600V, f_sw=10kHz, PF=0.8, T=100°C.

PRODUCT LINE
As summarized in the below table, a wide offering in terms of current capability in M series can fit any power requirement packaged in TO-247 and TO-247 Long Lead including freewheeling diode package option onboard and in die form. For the die form, the packing format suitable is D7 (tested 100%, cut, on ring and stick foil) and D8 (tested 100%, cut and packed in Tape & Reel):