Reducing Costs & Increasing Efficiency in Power Converter Design

Optimizing Power Converter Design through Simulation 

Optimal power converter design requires a fine balance between design efficiency and physical testing costs. Finding the right efficiency point can be expensive for companies — simply jumping from 98% to 99% efficiency could double the converter cost. In this blog, we’re going to explore the technical approaches power converter design engineers may take to optimize their product designs as well as highlight simulation solutions such as GT-PowerForge.

Reducing Switching Losses 

In power electronic conversion, losses are created by power semiconductor devices in conduction and switching. Conduction losses can be reduced by using a larger semiconductor section, but costs will increase as a bigger die or module needs to be used. Moreover, this solution negatively impacts the switching losses as the parasitic elements are increased.  

When switching losses are reduced, here’s how this impacts costs: 

• Lowering the switching frequency: will increase the filtering needs as it needs more inductance and capacitance to keep the same ripple level. In this case, the inductors and capacitors are going to be more expensive.
• Using wide bandgap (WBG) semiconductor devices: increasing switching speed becomes more expensive (up to 5 to 10 times the cost of silicon devices).
• Using multi-level topologies: the voltage switched by the semiconductor is lower with a lower voltage rating. From a cost standpoint, more semiconductor devices can be used but with a lower voltage rating. The device cost is non-linear to the voltage rating, therefore the semiconductor cost will depend on the applications. The filtering multilevel needs are usually more important in terms of input filters (Flying Capacitance, NPC, T-type topologies) but with smaller output filters. The complexity cost is also to be considered. 
• Reconsider cooling device size: by proactively analyzing and optimizing the power converters to reduce the losses, the cooling demands are also reduced resulting in a lower cost device. 

Electric Vehicle Power Converter Simulation Case Study
To illustrate these previous assertions, let’s explore these solutions on an electric vehicle’s (EV) 3-phase inverter with a sine filter using power converter design software such as GT-PowerForge. (NOTE: for this example, the inverter nominal operating point will be at 200kW with a DC bus of 800V, 400V between phases, a frequency of 50Hz, cos(φ)=0.8 and Space vector modulation)

How using simulation can find the best trade-off between COST and EFFICIENCY  
Si IGBT vs SiC MOSFET  

The figure of merit between Si IGBT and SiC MOSFET in the efficiency vs cost plan allows us to separate the plan in two: 

• The solutions with efficiency below 98.3% equipped with Si IGBT device have the lowest cost 

• The solutions above this efficiency equipped with SiC MOSFET have the lowest cost. To explain this result, while SiC MOSFET are indeed more expensive, increasing the switching frequency has a small impact on the loss, while reducing greatly the sine filter cost. At high switching frequency however, the gain on filter will be compensated by an additional heatsink price and additional losses. 

Si IGBT vs SiC MOSFET

The figure of merit between Si IGBT and SiC MOSFET in the efficiency vs cost plan allows to separate the plan in two :

• The solutions with efficiency below 98.3% equipped with Si IGBT device have the lowest cost
• The solutions above this efficiency equipped with SiC MOSFET have the lowest cost. To explain this result, SiC MOSFET is more expensive but increasing the switching frequency has a small impact on the loss while reducing greatly the sine filter cost. At high switching frequency, the gain on filter will be reduced by an additional heatsink price and additional losses.

2 Level topologies vs Multilevel topologies

The result from this comparison are not intuitive at a first glance. Below 98.7% efficiency, 4L Flying capacitor with Si IGBT are less expensive. Indeed, while being more expensive in terms of semiconductor devices and capacitor, the AC filter is greatly reduced by the apparent output switching frequency. At low frequency, the Si IGBT is more performant in terms of efficiency and cost. In this application T-Type and NPC are not competitive because they need a large amount of capacitor in the DC bus to keep the imposed 1% DC bus voltage ripple between the middle point and the bus voltage, which greatly impacts the cost.

2 Level topologies vs Multilevel topologies vs Si IGBT vs SiC MOSFET

The combination of the topologies and the semiconductor device technology has been tested as shown in Fig. 3. In term of efficiency vs cost, Flying Capacitor with Si and 2 Levels with SiC are forming best solutions.

The solutions presented do not exclude the existence of a better solution. In particular, loosen the ripple constraints on NPC and T-Type middle point, evaluate the modulation strategy influence or other semiconductor device as well as refine the optimal switching frequencies for this converter. Finally, the best presented conclusion only have a meaning for this specific converter specifications and cannot be generalized.

Figure 3: (eff vs cost) 2L vs ML and Si vs SiC

Now that all dimensions have been explored and pareto fronts identified, it is easier to identify a preferred solution for our converter efficiency vs. cost trade-off. But while efficiency and cost are key, we also need to dive deeper into estimating the concrete impacts of this solution into our vehicle system. 

Power Converter Design and Vehicle Systems Simulation  

GT-PowerForge offers the ability to export a loss map destined to be integrated into GT-SUITE’s GT-ISE model. This loss map will evaluate the power converter when its operating condition varies, effectively allowing a simulation in GT-ISE to take more dimensions into account, such as input power or temperature fluctuations. This therefore allows for a more precise and detailed evaluation of the vehicle capabilities such as: the estimation of range capabilities, focus on battery sizing and powertrain optimization. 

Interested in power converter design software? 

If you find this piece insightful, see how GT-PowerForge‘s simulation capabilities can assist with your power converter design needs.