ST AN3152 APPLICATION NOTE

AN3152

Application note

The right technology for solar converters

Introduction

Following a short overview of types of solar power systems and converters, this application
note introduces a fully working, grid-connected solar inverter prototype suitable for rooftop
applications. This solar inverter has been equipped with STMicroelectronics’ MDmesh™
and silicon carbide (SiC) devices to allow evaluation of these products in “green”
technologies.

Thanks to ST’s newest technologies (MDmesh™ V and SiC) and a new converter
philosophy, a 650 V fast power MOSFET can be used to boost performance and reduce the
size of the converter. A stage-by-stage explanation is presented here, as well as the
topologies adopted, and a detailed analysis of the front-end stage of the inverter is
discussed. The BOOST stage is used as a case study to validate the performance of the
new power MOSFETs and diodes.

Everyone is thinking “green’ these days. Adding to the new green conscience are the
increasing costs of fossil and nuclear fuels, prompting the market to turn to creative
technologies that use renewable resources to produce energy. Energy produced from
renewable resources are becoming an important contribution to the world’s total energy
demand and will increase in the next decades. Solar photovoltaic technology represents one
of the most promising energy resources, due to its low environmental impact and high
reliability. Every photovoltaic (PV) system consists of a module array and an inverter. The
inverter module is mandatory on all grid-connected applications, in order to amplify the low
DC voltage generated by the module to the higher AC level required by the grid. If several
modules are connected in series it might not be necessary to include amplification, but in
any case a maximum power point tracking (MPPT) function is required.

This prototype has a wide operating input voltage range (from 200 V to 400 V), and 3 kW of
maximum power output, since it is intended for rooftop applications. Total efficiency must be
at least 97%, and an MPPT function is implemented in order to enable panels to work at
their highest efficiency. In fact, the PV module (or modules, if connected in series) changes
its maximum power point continually during normal operation due to the variances in solar
radiation caused by shade or weather.

May 2010 Doc ID 17056 Rev 1 1/17

www.st.com

Contents AN3152

Contents

1 Solar power systems and inverters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2 Grid connected inverter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3 Measurements on the BOOST stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

4 SiC technology vs. silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

6 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2/17 Doc ID 17056 Rev 1

AN3152 List of figures

List of figures

Figure 1. Block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Figure 2. BOOST stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Figure 3. Power transformer and rectification stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Figure 4. BUCK current generator stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Figure 5. 100 Hz bridge sync rectifier stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Figure 6. Efficiency vs. power output on BOOST stage @ R
Figure 7. Typical waveform during normal operation on BOOST stage @ P
Figure 8. Turn-on waveform on BOOST stage @ P

OUT

Figure 9. di/dt vs. gate resistance value @ total parasitic inductance of 110 nH on the power loop 12
Figure 10. I
Figure 11. Silicon vs. SiC diode comparison @ I

vs. di/dt @ P

peak

= 1900 W . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

OUT

= 5 A, di/dt = 840 A/µs. . . . . . . . . . . . . . . . . . . . . . 14

F

=8.2 Ω and di/dt = 1400 W . . . . . . . . . 9

g

= 1300 W . . . . . . . 10

OUT

= 1700 W, VIN = 200 V . . . . . . . . . . . . . . . . 11

Doc ID 17056 Rev 1 3/17

Solar power systems and inverters AN3152

1 Solar power systems and inverters

There are a wide variety of topologies employed in the design of converters for solar power
systems, but they can be separated into two main classifications:

● Grid connected:

– These are usually isolated residential PV panels or collections of panels, called

“farms”, connected to a central power facility maintained by a private or public
company. Therefore, the energy produced from these photovoltaic modules during
the day can be used immediately on the main grid. These systems do not require
storage batteries or any backup system to store the energy, because the main grid
continues to supply energy at night or in the absence of sunlight.

● Standalone:

– These are used on systems typically far from a municipal or public power grid.

These systems use a battery, or bank of batteries, to store the energy produced
during the day by the panels, in order to provide a supply of energy at night or
when solar irradiation is low. Grid connected inverters are generally more complex
than the standalone because they must synchronize with the grid sinusoid and
supply the current within the same phase. However, the first stages of design are
quite the same on both types of inverters. Traditionally, solar systems used to
implement a single, shared inverter for all panels. This is called a “centralized”
system. The trend today is toward a “string inverter” system, in which each panel
is equipped with its own low power and high performance inverter. Each of the
inverters mentioned previously can be designed following two different
approaches with respect to frequency operation. If a low frequency approach is
adopted (typically 100 Hz), very slow devices can be used. However, in this case
very heavy transformers and bulk capacitors are necessary. Generally, newer
designs adopt high frequency operation in order to minimize inverter dimensions
and weight and to maximize performance. It is worth mentioning that in our
example we have performed the tests on a 3 kW grid-connected inverter, but the
same benefits can be obtained in each of the types of inverters previously
mentioned.

4/17 Doc ID 17056 Rev 1

AN3152 Grid connected inverter

2 Grid connected inverter

A PV inverter must perform three main functions in order to feed energy from a PV array into
the utility grid:

1. Shape the current into a sinusoidal waveform

2. Invert the current into an AC current

3. Boost the PV array voltage if it is lower than the grid voltage

The way these functions are sequenced within an inverter design determines the choice of
semiconductor and passive components, and consequently their losses, sizes and prices. In
order to validate new technologies, a solar inverter prototype was developed and equipped
with ST MDmesh™ and SiC devices. The block diagram is shown in Figure 1, and block-by­block details are shown in Figures 2, 3, 4 and 5.

Note: The new approach and topologies of each stage allow the use of a 650 V power MDmesh™.

Figure 1. Block diagram

Doc ID 17056 Rev 1 5/17

Grid connected inverter AN3152

Figure 2. BOOST stage

Figure 3. Power transformer and rectification stage

6/17 Doc ID 17056 Rev 1