3-level NPC topology design challenges and techniques

Recently, the three level Neutral-Point-Clamped topology (NPC) used in high power applications is increasingly being applied in low- and medium-power applications to exploit specific advantages in system level design. Applications requiring filters, like UPS systems or PV inverters, benefit from the improved spectral performance and lower specific switching loss of lower voltage class devices in this topology. Up to now, however, setting up a three level phase leg has only been possible by applying discrete devices or combining at least three modules. By integrating a three level phase leg into a single module, adapting chip technology for slightly higher breakdown voltage, and providing a simple solution for driving this topology are becoming more appealing for new projects.

Operation principles of 3-level NPC topology
The three level phase leg in an NPC topology consists of four IGBTs with its associated anti-parallel diodes, all arranged in series, and two additional diodes DH and DL connecting intermediate nodes to the neutral point of the DC-link. All power semiconductors used exhibit the same blocking voltage. Depending on the sign of the output voltage and current, four different commutation loops are in operation during one period of the output base frequency. With voltage and current in positive direction, T1 and DH operate like a buck chopper, whereas T2 just conducts the output current without switching as shown in Figure 1(a). With the voltage and current being both negative, T4 and DB operate like a boost chopper, with T3 just conducting the current. For these conditions only two devices are within the commutation loop, and this will be referred to as short commutation. But with the output current being negative combined with positive voltage, current flowing through T3 and DB has to commutate to D2 and D1 as shown in Figure 1(b). This commutation involves four devices and will be designated as long commutation. For the remaining case another path of long commutation exists. Managing stray inductances and over voltages for the long commutation is one of the demanding tasks when designing three-level converters.


Figure 1

IGBT modules dedicated for 3-level NPC topology
While integrating in four IGBTs and six diodes is not an option for high power applications, this is feasible in the low power and medium power range as far as number of available power and control pins does permit the use of a standard package.

For the low power range, the EasyPACK 2B package as shown in Figure 2 offers sufficient DBC area to integrate a complete 150A three level phase leg. Due to the facts that pins can be placed freely within the given grid and the pins can be assigned to provide either a power or a control function, suitable interconnection means are provided. There are auxiliary emitter terminals available to enable fast switching. For power terminals up to eight pins are used in parallel to achieve the required current rating as well as to minimize stray inductance and PCB heating.


Figure 2

For the medium power range, the newly introduced EconoPACK 4 package (Figure 3) is an optimal choice for integrating all the power devices. The three terminals are used to enable a low inductance connection to a split DC-link as it is needed for three level converters, whereas the two terminals on the opposing side are used in parallel as phase output terminals. A driver PCB can be connected directly to the control terminals visible at the edge of the module frame. This package is intended to be used for three level phase legs with chip currents up to 300A.


Figure 3

Integrating all devices of a three level phase leg into one module is very promising in regard to minimizing stray inductance, but with only 600V of blocking voltage it is still very difficult to meet typical application requirements due to the non-perfect balance of DC-link voltages and faster switching of 600V devices.

To ease the design and give the customer a larger margin, these modules are equipped with enhanced IGBT and diode chips that can block 650V. These new chips have the exact same conduction and switching characteristic as the well known 600V IGBT3 devices. Also the softness and robustness of both devices (SOA, RBSOA, SCSOA) remains the same. This is enabled by the development of new termination structures for the IGBT and diode. Therefore the VCEsat of the 650V IGBT stays at its excellent value of 1.45V (1.70V) at 25°C (150°C) [1] with low switching losses that contribute only 1/3 of the total inverter losses for switching frequencies of 16kHz. Also the IGBT still has its smooth current tail that even at critical conditions shows no snap-off [2]. The diode also stays at the optimized VF-Qrr trade-off at 1.55V (1.45V) at 25°C (150°C) [1] and keeps its soft switching behavior.

Design challenge
The application of three-level NPC topology in low- and medium-power applications creates some specific driver requirements that have to be considered for optimum system performance.

Arising from high switching frequencies
Due to switching frequencies covering a range form 16kHz to 30kHz, the driver has to provide small and consistent propagation delay so that the deadtime can be minimized. Considering the fast switching times of 650V devices, the main contribution to deadtime requirement arises from variations in driver propagation delay [3]. If deadtime is too large compared to the period of the switching frequency, this will lead to nonlinear behavior of the inverter stage – creating new challenges in control algorithms [4], [5].

Arising from topology
- Although the devices used only have a blocking voltage of 600V or 650V, the isolation requirements for the driver are similar to a 1.2kV application
- Since the number of driver circuits doubles, it is mandatory to use a design for the driver and its power supply with low part count and low board space requirement.
- Protection features such as short circuit detection and under voltage lockout have to match with the three-level NPC topology. Turning off an inner IGBT first (T2, T3 in Fig 1) would expose this device to the full DC-link voltage and lead to immediate device failure due to SCSOA or RBSOA violation.

With the new integrated IGBT drivers of the EiceDRIVER family, these requirements can be met effortlessly [6], [7]:
- The integrated microtransformer provides basic isolation up to a repetitive isolation voltage of 1.42kVpeak.
- With the integrated Active Miller Clamp feature, this driver can be used with a single supply at high switching speed without the risk of parasitic turn-on [8].
- Compared to typical optocoupler-based drivers, tolerances and variation of propagation delay are significantly reduced by the microtransformer technology.
- The integrated VCEsat-protection may be used for the outer switches, but has to be disabled for the inner IGBTs.

Laboratory test and results
In the following section, switching waveforms of an EasyPACK 2B 3-level module will be shown. The tests have been done using 1ED020I12-F IGBT gate driver for IGBTs. The current has been measured with current transducer either at DC+ or DC-.

Short commutation
Figure 4 shows the switching waveforms of a short commutation at nominal current, a DC voltage of 400V and 25°C junction temperature. With a peak value of 550 V the voltage stays well within limits.


Figure 4

Long Commutation
Figure 5 shows the switching waveforms of a long commutation at the same conditions.
With a voltage peak of 580V, this voltage is only about 30V higher than for the short commutation and still fairly below the 650V breakdown voltage.


Figure 5

First measurement results show that due to integration of a complete three-level phase leg into a single module, switching behavior nearly similar to the short commutation can be achieved for the long commutation. However, to achieve enough headroom to switch at higher currents, a further reduction of circuit stray inductance would be necessary. This can be achieved easily by using several capacitors in parallel and using a multilayer board reducing the spacing between the coplanar power layers connecting module and capacitors. Furthermore it has to be considered that a real application circuit would not contain current transformers within the DC-link connections. The current transformers used here contribute to stray inductance with 15nH, increasing the overvoltage by 45V.


Conclusion
Integrating a complete phase leg into one single module, increasing blocking voltage from 600V to 650V, and providing a highly integrated driver solution, the three-level inverter proves to be an attractive candidate for low- and medium-power low voltage applications requiring high switching frequency, filtering and high efficiency, such as double conversion UPS and PV inverters.

References
[1] Datasheet of FS6R06VE3_B2, available at www.infineon.com
[2] Kanschat, P.; Rüthing, H.; Umbach, F.; Hille F.: 600 V IGBT³: A detalied analysis of outstanding static and dynamic properties, Proceedings of ISPSD
[3] Infineon Technologies AG: AN 2007-04, How to calculate and minimize dead time requirement for IGBTs properly, May 2007
[4] Holmes G.; Lipo, T.: Pulse width modulation for power converters, IEEE Press, Piscataway, 2003
[5] Kalker, T.; Ackva A.; Jansen, U.: Novel digital controller for induction machines considering the inverter swicthing times and a fluctuating DC-link voltage, EPE 1991, Vol. 2, p. 58-62
[6] Strzalkowski, B; Jansen, U.; Schwarzer, U: High performance IGBT-driver in microtransformer technology providing outstanding insulation capability, PCIM 2007
[7] Infineon Technologies AG: Datasheet 1ED020I12-F, Oktober 2008, available at www.infineon.com
[8] Infineon Technologies AG: AN 2006-01, Driving IGBTs with unipolar gate voltage, Dec. 2005, available at www.infineon.com


Captions
Fig. 1: Commutation loops in a three level phase leg: (a) short commutation; (b) long commutation.
Fig. 2: EasyPACK 2B package.
Fig. 3: EconoPACK 4 package.
Fig. 4: Switching waveforms of a short commutation.
Fig. 5: Switching waveforms of a long commutation.