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MNPC or NPC – best topology for a multi-level inverter?

Mixed-voltage Neutral Point Clamped (MNPC) and Neutral Point Clamped (NPC) modules are power module topologies used in high power applications such as three-phase solar/photovoltaic (PV) inverters and uninterruptible power supplies (UPS). The choice between the two topologies is mainly a question of switching frequency. NPC delivers at higher switching frequencies, while MNPC delivers at lower switching frequencies. However, the threshold frequency between the two is always getting lower and there are other factors to consider…

Advantages of MNPC and NPC power semiconductor modules

MNPC and NPC semiconductor modules have significant advantages when used as as multilevel inverters:

  • Low EMI caused by dv/dt issues
  • High efficiency
  • Reduced stress caused by multiple voltage levels on the DC bus.
  • Low-disturbance input current
  • Lower common state voltage

How MNPC and NPC modules work

When MNPC and NPC modules are used as inverters, the DC voltage can be converted into a variable alternating voltage and variable frequency. Unlike a half-bridge or sixpack topology, these topologies offer an additional voltage level at the output. The potential can also have a status of 0, as well as DC+ and DC-.
At real or active power, these are switched at just 50Hz; therefore, they correspond to the positive or negative sinusoidal half-wave. Usually operated at 8kHz for MNPC and 16kHZ for NPC, the outer switches generate the sine wave so they require only half the blocking voltage capability required for conventional topologies. This is significant because semiconductors with a high blocking voltage capability are slower at switching. MNPC and NPC modules with 600V or 650V components can be operated at higher switching frequencies than, for example, 1200V half-bridges.

MNPC topology

The classic MNPC stage comprises four IGBTs and four diodes. The topology is also known as T-type, or NPC2. The blocking voltage is 600V or 650V for the horizontal (neutral point) switches and 1200V for the outer switches. Some modules come with 1200V and 1700V components. Modules are typically equipped with an NTC or PTC alongside the semiconductors.

NPC topology

Historically called a three-level module (though, confusingly, the MNPC topology also has three levels), the classic NPC stage uses four IGBTs and six diodes. The blocking voltage is 600V, 650V or 1200V.

 

classic MNPC topology (left) and classic NPC topology (right)

So, which is better – MNPC or NPC?

The choice between the two is largely based on the switching frequency of the application, though there are other factors (see the list below). Each topology has better loss characteristics at different frequencies.

NPC allows higher switching frequencies. NPC enables faster switching than MNPC. So above a certain frequency, it makes sense to choose NPC. This depends on the IGBTs used, but as a rule of thumb it was traditionally 16kHz. However, this is getting ever lower and now it is more like 10kHz. Manufacturers have considered discontinuing MNPC but some designers prefer to stick with this topology because they are familiar with it.

NPC allows a little more power. A higher current range due to smaller switches means an NPC can have a higher nominal current rating inside the module. Consequently, NPC allows a little more power.

MNPC makes emergency switch-off easier. Emergency switch-off is easier with MNPC because the switching order is not important. In the case of NPC, the IGBTs must be switched in a particular order – typically the outer IGBTs followed by the inner ones to avoid too much voltage across one of the 600V/650V-rated IGBTs.

It depends (indirectly) on the application power rating. Manufacturers typically offer both types for each power rating. However, higher power applications tend to operate at lower switching frequencies, and vice versa. Therefore, while there is a power rating correlation this is more directly related to switching frequency. For high power ratings such as 500kW or above (e.g. solar inverters), MNPC tends to be the better choice because the switching frequencies are usually quite low (e.g. 4kHz or 8kHz). For lower power applications, NPC makes more sense because the switching frequency is likely to be higher. Higher switching frequencies mean smaller passive components, which save cost, weight and size.

It depends on the input voltage. Even at high power ratings the choice still depends on input voltage. A solar inverter might typically have an input of 1000V, so MNPC is an option. But many designers want up to 1500V, which requires 1700V-rated chips. Since the performance of MNPC semiconductor dies are not as good as NPC dies at 1700V, the NPC topology is a better choice.

Summary of advantages – MNPC v NPC

MNPC

  • Easier emergency switch-off
  • Familiarity (more traditional topology)
  • Best for switching at <10kHz

NPC

  • Allows a little more power (higher current range)
  • Best for switching at >10kHz

 

 

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