How to 10X Inverter Hardware

This article is intended to help share industry knowledge on the various ways AC Voltage is made from DC Sources and some of the trade-offs and advantages of various architectures. To keep things simple we will focus on the most common AC voltage of 120Vrms at 60Hz which is standard in North America.

Starting with a DC source such as a battery, commonly available consumer grade inverters are between the 12Vdc to 60Vdc range. The simplest way to convert from direct current (DC) to alternating current (AC) is to quite literally alternate the polarity of the battery between positive and negative. In practice this is accomplished with a simple H-Bridge circuit to automate the flipping and regulate the frequency that it is flipping. To achieve 60Hz it would need to flip the polarity 60 times in 1 second. If we assume a 12V dc input, then the output will be +/-12Vac. To amplify to the desired +/-120Vac we can use a step-up transformer with a 10:1 ratio to boost the output voltage to the desired voltage level. This method is the most crude and typically only used for explaining the operating concept. Total harmonic distortion (THD) or the difference between the inverter output and a pure sine wave is very high on this design, typically around 60% because its producing a square wave. High THD can damage AC devices, cause buzzing, and may even require 20% more power for a given load to operate.

We can improve the THD of this design by introducing another concept called pulse width modulation (PWM). By breaking up the parts of the square wave into smaller segments at a specific pulse duration and then filtering the pulses to smooth the output we can reduce THD to levels of around 20%. This is called a modified sine wave inverter and is a common low cost consumer device but its recommended to use with caution.

Pure sine wave inverters are typically classified as having a THD < 3% and is recommended as a suitable source for almost all devices. This is a similar THD level to what is provided by the utility grid operators. Pure sine wave inverters have an internal oscillator to control the output waveform shape, typically switch at a higher frequency to smooth out the ripple in the wave form, and instead of flipping the battery polarity use a different method where they regulate the output voltage relative the input voltage this creating a synthetic AC Wave. This method reduces large surges in flipping the battery polarity.

When we look at switching technologies for power electronics, the main 3 devices are metal oxide semiconductor field effect transistors (MOSFETs), Insulated gate bipolar transistor (IGBTs), and Silicon Carbide (SiC). Each device has its own pros and cons and it really depends on the overall system objectives. Cost, conductivity (efficiency), and frequency are some of the main factors to assess. Higher frequency is generally better, up to a point, because it allows for smaller supporting inductors and capacitors. The parts can be smaller because each pulse pulse has less energy so there is less energy to store.

MOSFETs are good for switching at high frequencies between 10kHz to 1MHz, have low resistance, but are not able to support higher voltages. It’s a good solution for switching at lower voltages such as 48V systems and lower.

IGBTs can operate at higher voltages, but have higher resistance than MOSFETs and but operate in a lower 1 to 20kHz range.

SiC can operate at higher voltages, has low resistance, and can operate at higher switching frequencies of 10 to 100kHz. The downside is that the designs are typically more challenging to execute.

Design considerations for mobile applications.

Mobile applications are subject to extreme shock and vibration as well as size and weight restrictions. Transformers are large, heavy, fragile and expensive. So the ideal design would be to remove the transformer typically used in boosting the inverter output up to the desired levels. This is what’s called a transformerless inverter architecture.

To create a transformerless inverter, the inverter DC input needs to be high enough to support direct DC to AC conversion. Unlike the square wave example where the peak voltage was also the rms voltage we now have a pure sine wave and need to calculate the minimum input voltage required. This can calculated by taking our desired output voltage, say 120Vac_rms, converting it to Vac_peak by multiplying by square root of 2 or 1.414. Then this needs to be doubled to account for the positive and negative portion of the wave.

DC_in = 120Vac_rms sqrt(2) x 2 = 340Vdc

For best practices, we add at least 10% to allow for voltage trim and transient loading, so it would be recommended to target 375Vdc or above from your DC Source.

Transformerless Inverters and High Voltage Architectures

Utilizing SiC devices in a transformerless architecture, we can simply convert directly from DC to AC at these voltage levels. This removes significant cost and size from the design as well as improves efficiency by elimination of conversion steps. SiC inverters from companies such as ADVANTICS push 98%+ DC/AC conversion efficiencies over a broad load factor range. Additionally, due to the low conduction losses and reduced filter sizes these can be extraordinarily compact. For comparison purposes, an industry leading 48Vdc inverter that creates 120Vac is 10x heavier.

There is a secondary advantage of working at higher voltages as well. Power is a function of voltage multiplied by your amperage.

Power = Voltage x Amps

So, if we want 100kW of power and only have a 48Vdc battery is would require 2083A of continuous current. While there are many sizes for cables there are practical limits for continuous power, for example a non-insulated high voltage transmission line is designed to carry up to 1000A. Per UL standards, insulated 4/0 cable is limited to 230A continuous for power transmission or just 11.5kW per phase. Conversely, if you up the DC voltage to 480Vdc it only needs 208A of current to fully support the system.

Heat generation is predominantly driven by the current. It can be calculated by multiplying the current squared by the resistance. In practical terms, every time the current doubles the heat generation is 4 times higher. In this theoretical example we can estimate the order of magnitude difference in heat generation between a 48Vdc and 480Vdc system at the same power levels. We will use the same resistance assumption of 1 for this dimensional analysis.

P_Loss_48Vdc = (2083^2)*1 = 4,333,889

P_Loss_480Vdc = (208.3^2)*1 = 43,388.89

Order of magnitude 4,333,889/43,388.89 = 100

The difference in heat generation is 100x higher with the 48V system. While this example is exaggerated the underlying physics are still there and points to why we don’t see many higher power 48V inverters.

When we now assess efficiency, its clear to see how given the same power levels a low voltage transformer based inverter system typically achieves only 92% efficiency while a transformerless system can achieve 98% efficiency. While the overall difference may see small, another way to look at it is the transformerless high voltage inverter generates 4 times less heat (8% vs 2% Loss). Thats 4 times less heat to transfer out of your system. Additionally, SiC inverters such as those from ADVANTICS are built with high temperature components. Heat transfer is a function of the temperature delta so having a high source temperature enables easier heat transfer out of the system. Many systems can even be air cooled given this temperature delta.

Lastly, there are compounding system level knock on effects to efficiency. That 6% gap adds up in a multiple places. In order to deliver the same amount of output energy when charging from a source like a diesel generator, a system would need 6% more battery storage in the bill of materials cost to cover inverter losses. If we are analyze a system requiring 100kWh of delivered energy output, it would take 118kWh of energy input for charge and then discharge to achieve that with a 48V system. Conversely, a 480V system would only require 104kWh on energy input for the same 100kWh output. If your customer is paying $0.45/kWh for diesel in a generator this adds an additional $0.06/kWh (13%) tax to the baseline energy cost for the life of the asset. Long term this eats into the return on investment (ROI) for your product.

In Conclusion

The inverter industry is continuing to evolve to meet the application needs that customers are demanding. Off-grid inverters are no longer limited to small consumer device power levels. Transformerless high voltage inverters can offer have a clear path forward and fundamental advantage over older system architectures that cant be beat. A simple comparison between a leading 48V off-grid pure sine wave inverter supplier at the 16kW level shows that the hardware weighs 256 pounds, a 16kW single phase 120Vac inverter from ADVANTICS and the active neutral weighs less than 20 pounds. So as you can see, this is how you 10X a Inverter.

Find out more at www.advantics.fr or send us an email at [email protected]

ADM-PC-BP25 series inverters: https://advantics.fr/products/bp25/