# Analyzing Bi-Directional NBMs Using the Whiteboard

November 29, 2016
By

The ability of our Non-Isolated Bus Converter Modules (NBMs) to operate in either direction makes designing many power systems simpler, and they can also provide significant cost, size and efficiency benefits. If this capability is to be deployed effectively, however, the power system design tools need to support this innovative capability.

Fortunately, the PowerBench online tools enable the analysis of NBMs in reverse operation, making it very straightforward to analyze the performance of these power components in either mode using the powerful Whiteboard.

Analyzing forward operation simply requires the placement of the component, a power source and load on the whiteboard. In this example, we’re using an NBM with an input range of 36 V to 46 V and a K factor of 1/3. We’ve placed the NBM in forward operation, taking a 42 V input and driving a 100 A load.

Figure 1 -NBM in Forward Operation (click to enlarge)

By clicking analyze, we can see that in this configuration the NBM will deliver 100 A at 13.86 V, with an efficiency of 98.6%.

Figure 2 – Analyzing Performance of NBM in Forward Operation (click to enlarge)

But what if we want to use the NBM in reverse operation, stepping up the voltage using a K factor of 3/1?

In the diagram below we’ve placed another NBM, and clicked on it to configure the operation. In the box that pops up, there is a simple radio button to select forward or reverse operation. So we just need to select reverse operation.

Figure 3 – Setting NBM for Reverse Operation (click to enlarge)

There is also an option to flip the symbol for the NBM horizontally, allowing the power to flow from left to right while the NBM operates in reverse. The image below shows the flipped component, with the high voltage side now on the right, rather than the left.

Figure 4 – NBM in Reverse Operation, Flipped to Allow Power to Flow from Left to Right (click to enlarge)

Now we have configured the NBM, it’s a simple job to add a power source and load. Clicking Analyze again will let you look at the performance of the component in both directions of operation.

Figure 5 – Analyzing Performance of NBM in Both Directions of Operation (click to enlarge)

The results above show the NBM operating in reverse, producing 41.63 V at 30 A from a 14 V power source.

One common application for bi-directional components is stepping up voltages for transmission along a long cable. For example, tethered drones use a long cable that must be as light as possible. It’s easy to simulate this application using the whiteboard.

Figure 6 – Analyzing Performance of Back-to-Back NBMs (click to enlarge)

In this example we’re delivering 100 A at 14 V by stepping up the voltage to 42 V and then stepping it back down to 14 V using a pair of NBMs. This reduces the current to be carried by a factor of three, allowing the cabling to the drone to be smaller and lighter.

This approach does have a cost: the NBMs dissipate just under 40 W. However, even if the resistance of the cabling is fairly low, stepping up the voltage can increase system efficiency, as power loss in the cable is proportional to I2R.

## Example – Analyzing Back-to-Back NBMs for a UAV

Let’s look at a realistic example: a tethered drone (or UAV). In our example, the drone needs a 14 V supply at 10A, and the resistance of the cabling is a relatively conservative 0.5 Ohms.

Figure 7 shows the performance of two different approaches to this problem: one using the NBMs to step up the voltage, and the other using the cable to directly transmit the 14 V supply.

Figure 7 – Improving Efficiency by Using Back-to-Back NBMs (click to enlarge)

In the analysis above, it’s obvious that the back-to-back NBMs is a much better solution. Not only is the total power loss much lower (18.16 W compared with 50 W), but the voltage drop in the cable is also reduced considerably. In fact, the 5 V drop seen without the NBMs will almost certainly mean the voltage of the power source will need to be increased, further increasing cost and power loss, while the solution using NBMs exhibits a drop of only a little over 0.5 V.

This example shows that, with just a few clicks, the PowerBench Whiteboard allows power developers to analyze fairly complex power scenarios and quickly identify the best solution for their application.

Tags: , , ,