MNPC and NPC type semiconductor modules deliver 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.
- Can draw low disturbance input current
- Lower common state voltage (so what?)
How MNPC and NPC modules work
When MNPC and NPC modules are used as an inverter, 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 and therefore 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 and therefore requires 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 600 or 650V components can be operated at higher switching frequencies than, for example, 1200V half-bridges.
The classic MNPC stage comprises of four IGBTs and four diodes. The topology is also known as T-type of 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 of PTC alongside the semiconductors.
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 600, 650 or 1200V.
classic MNPC topology (left) and classic NPC topology (right)
So, which is better – MNPC or NPC?
Choosing between the two types is largely based on the switching frequency of the application, though there are other factors. (see below list). Each topology provides better loss characteristics at different frequencies.
NPC allows higher switching frequencies – NPC enables faster switching than MNPC. Above a certain frequency it makes sense to choose NPC rather than MNPC. This does depend on the IGBTs used but as a rule of thumb this was traditionally 16kHz. However, it gets ever lower and now it is more like 10kHz. Manufacturers have considered discontinuing MNPC but some designers prefer to stick with MNPC type because of their familiarity with the topology.
NPC allows a little bit 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 bit more power.
MNPC is easier for emergency switch off – 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 an excessive voltage across one of the 600/650V 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, whilst 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 saving cost, weight and size.
It depends on input voltage – Even at high power ratings it still depends on input voltage. A solar inverter might typically have an input of 1000V and MNPC comes in to play again. But many customers these days want up to 1500V and then you have to use 1700V rated chips and the performance of these 1700V MNPC chips is not so goof do it makes sense to switch to an NPC topology.
Summary of advantages – MNPC v NPC
- Easier emergency switch off
- Familiarity (more traditional topology)
- Best for switching >10kHz
- Allows a little more power (higher current range)
- Best for switching >10kHz