Inverting Energy Concerns Through Efficient Designs

By Martin Schiel, Future Electronics Germany

Solar power promises to address energy shortages without contributing to global warming. Making the most of the sun’s power demands efficient inverter electronics.

READ THIS TO LEARN ABOUT The key engineering challenges facing developers of solar panel systems The different inverter topologies that can be used with solar panel arrays

There is increasing conflict between our future energy demands and the need to limit emissions of greenhouse gases that contribute to global warming. We know we need more power – in Europe alone we will need an extra 250-300GW of capacity by 2020 – but we cannot afford to generate it by burning yet more fossil fuels.

Engineers and scientists believe the solution is to supplement our traditional sources of energy with clean, renewable energy sources such as wind, waves and solar energy. Of these, solar power offers the most promise. Solar panel technology is mature and the politicians are doing their part to encourage its uptake. Germany in particular is encouraging homeowners to install solar panels to supplement the public power supply. Homeowners can even sell any excess electricity generated back to the German domestic energy market (at a government-specified premium rate) by feeding it directly to the local grid.

But there is plenty of work still to be done. The main challenge at present is energy conversion efficiency. Engineers are continually improving solar panels and efficiency figures are now at 20% under laboratory conditions. The overall efficiency figure, however, hinges not only on the efficiency of the solar panels but also on how well the DC output is converted to a practical AC supply by the inverter circuit. The right components and clever design can significantly improve efficiency.

 

 

Converting the Sun’s Power

An inverter’s job is to transform the DC derived from the solar panels into AC, so that it can be consumed by household appliances or fed into the grid.

There are several types of inverter arrangements used for solar panels: central, string, multi-string and AC modules (see Figure 1). In the central inverter arrangement, the solar panels are connected in series to form “strings.” These strings are connected in parallel and fed to a central inverter. The disadvantage with this arrangement is that the large currents flowing in the strings demand a bulky inverter. In addition, each string requires a blocking diode to prevent complete shutdown should the string fail, and the panel must be made up of identical modules.

In the string arrangement, each string is connected to a separate photovoltaic (PV) inverter. The disadvantage is that several inverters are required, pushing up costs. The most common topology today is the multi-string inverter. It uses an arrangement whereby each string is connected to a separate DC-to-DC boost converter in the inverter and tracked separately with Maximum Power Point Trackers (MPPT). This arrangement allows the strings to be made up of different modules and configurations. The DC output of each converter is then fed into one inverter to convert it to an AC voltage, which is then fed into the grid.

Finally, in the AC module arrangement, each solar module is attached to its own small inverter. This solution is both flexible and requires only very cheap inverters. As such, it looks set to become a popular alternative.

 

Choosing a Topology

While there are many inverter topologies, a few simple examples can illustrate the important design elements. The most common topology is an inverter employing a 50Hz transformer. The use of a high frequency transformer reduces its size (and allows the use of smaller inductors), but adds more complexity and cost to the semiconductor side of the inverter.

Transformers are relatively heavy and expensive, and add about 2% to the system’s power losses, but are thought to improve safety. Nonetheless, while most contemporary designs still employ transformers, some newer designs do not. (Transformer-less designs are allowed in Germany and other countries, but not the US due to a different grounding approach.) These types employ a booster to raise the input voltage from the solar panel.

The power side of the inverter circuit has a major impact on the conversion efficiency of the inverter. Modern power devices have been designed specifically to minimize losses and include devices such as Punch-Through (PT) and Non-Punch-Through (NPT) Insulated Gate Bipolar Transistors (IGBT) featuring reduced conduction and switching losses. Trench IGBTs have also come into vogue. These feature an insulated trench in the transistor’s pregion and boast ultralow collector emitter saturation voltage.

While single-phase designs normally use 600V components, higher output 3-phase designs need 1,200V devices. 3-phase designs also boast systematically lower ripple currents through the DC link capacitor, allowing a smaller device to be used. A designer should pay special attention to the breakdown voltage of the switching devices, particularly at low temperatures.

 

 

Minimizing Losses

All inverters employ microcontrollers to supervise modulation and communication, of which modulation control is especially important. This determines how the power devices are driven by a pulse width modulation signal to shape the output current of the inverter according to the mains voltage.

Freescale’s MCF523x microcontroller, for instance, integrates both modulation and communication functions into a single chip. The MCF523x family features an eTPU module that performs the modulation and a host of communication channels are available (including Ethernet, UART, SPI, I²C and CAN). Within the MCF523x family there are numerous derivatives to meet specific system requirements depending on the number of eTPU channels needed and which communication protocols are required. Alternatively, a combination of Freescale’s inexpensive digital signal controller, the 56F801x series (for modulation) and a derivative from the low cost ColdFire MCF5223x family (for communications) could be implemented as a less expensive two-chip solution.

Each of the topologies described above use a different driver circuit. For example, Avago Technologies produces IGBT drivers with internal optical isolation, such as the HCPL-316J. This device features the drive capability of IGBTs with IC up to 150A, 1,200V collector/emitter voltage, optical isolation, fault status feedback, 500ns switching speeds and integrated fail-safe IGBT protection. It is suitable for use with a single microcontroller.

If the inverter topology employs dual controllers, the safety isolation can be implemented using comparatively inexpensive optocouplers at the interface between the two controllers. It is possible to link the input side of the inverter with the power circuits using functional isolation.

Recently, IGBT technology has evolved from PT technology to NPT and then to Trench devices. While NPT IGBTs have low conduction and switching losses, the trench types are better still. In addition, Trench IGBTs have lower V_CE(ON) and E_(OFF) than PT and NPT IGBTs; this reduces losses considerably across a range of switching frequencies from 4kHz to 20kHz (see Figures 2 and 3). Trench devices also deliver higher RMS current per mm² than PT and NPT IGBTs. The combination of reduced losses and higher current density means that Trench IGBTs enhance inverter efficiency and power density.

Trench devices are also easy to design with. No major changes to the PT or NPT IGBT gate drive is necessary because Trench IGBTs have a 20V gate rating, the threshold voltage is similar to that of PT and NPT IGBTs and the devices feature shorter T_D(ON)+T_RISE and T_D(OFF)+T_FALL timings. In addition, no modification to dead time and minimum pulse width settings are needed. Finally, trench IGBTs are immune to high dV/dt induced spurious turn-on.

Some good examples of popular devices used in solar inverters are the Fairchild NPT IGBT FGL40N120AND in a TO264 housing and the 600V Fairchild SuperFet MOSFETs.

 

 

A Bright Market Ahead

Environmental pressures, combined with high fossil fuel costs and government encouragement, are finally moving solar power into the mainstream. This has served to encourage the commercial development of solar energy, which is becoming a lucrative market. For example, worldwide solar power in 2004 equated to about 1GW capacity, by 2020 this is expected to grow 1,000% to 10GW. This equates to an average annual market growth rate of around 30%.

The emphasis now is on improving efficiency. While the efficiency of the panels themselves will gradually improve, considerable gains can be made from good design of the inverters that convert the panel DC into usable AC voltage.

Future Electronics’ applications engineers are able to offer advice on solar inverter design while balancing efficiency against cost, and the company is able to supply a range of controller and power semiconductors from leading semiconductor firms to help the designer optimize their circuits.