AN4070
Application note
250 W grid connected microinverter
By Rosario Attanasio
Introduction
This application note describes the implementation of a 250 W grid connected DC-AC
system suitable for operation with standard photovoltaic (PV) modules. The design is
associated to the STEVAL-ISV003V1 demonstration board which demonstrates the
possibility of implementing a full microinverter solution (MIC) using STMicroelectronics
products.
In fact, both the components used to implement the power, control and communication
section belong to the product portfolio offered by STMicroelectronics.
The design is based on two power stages, namely, an interleaved isolated boost DC-DC
converter and a mixed frequency DC-AC converter. The control section is based on an
STM32F103xx microcontroller which ensures proper maximum power point tracking
(MPPT) on the input side of the system and decoupled control of the active and reactive
power on the output. The control algorithm has been developed to allow system operation
both with 230 V AC, 50 Hz grids and with 240 V AC, 60 Hz without any hardware
modifications. The connection to a 120 V AC, 50/60 Hz grid requires few hardware
modifications to ensure the best system performance. An image of the STEVAL-ISV003V1
demonstration board is shown in
Figure 1.
Figure 1.
Image of the 250 W MIC
December 2012
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Contents
AN4070
Contents
1
2
3
4
5
System description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
DC-DC converter design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Schematic description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
STM32F103xx based current control for inverter grid connection . . . 29
Experimental test results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
Appendix A Magnetic components datasheets . . . . . . . . . . . . . . . . . . . . . . . . . . 42
Appendix B Alternative DC-DC converter magnetic components with low
profile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
6
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
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List of tables
List of tables
Table 1.
Table 2.
Table 3.
Table 4.
Table 5.
Table 6.
Table 7.
Table 8.
Summary of main electrical specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
DC-DC converter main specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
DC-DC converter MOSFET main characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
DC-DC converter rectifier diodes main characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
HF transformer specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
RM14 core with N87 Ferrite main characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
STB11N65M5 MOSFET main electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
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List of figures
AN4070
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Figure 13.
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Figure 16.
Figure 17.
Figure 18.
Figure 19.
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Figure 21.
Figure 22.
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Figure 24.
Figure 25.
Figure 26.
Figure 27.
Figure 28.
Figure 29.
Figure 30.
Figure 31.
Figure 32.
Figure 33.
Figure 34.
Figure 35.
Figure 36.
Figure 37.
Figure 38.
Figure 39.
Figure 40.
Figure 41.
Image of the 250 W MIC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Block scheme of the 250 W grid connected system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
DC-DC converter topology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Gate driving signals of the DC-DC converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Currents through the input inductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
MOSFET MOS1 and MOS2 currents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Rectifier diode currents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
HF transformer current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Basic scheme of the DC-AC converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Schematic of the two main power stages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Schematic of the filtering and relay circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
DC-DC converter driver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
DC-AC converter driver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Auxiliary power supply of the demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
AC line current sensing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
AC line voltage sensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
PV current sensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
PV voltage sensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Block diagram of the implemented control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Implemented PLL structure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Block diagram of the control algorithm implemented on the 250 W MIC STM32F103xx . . 32
DC-DC converter MOSFET current (green) and drain to source voltage (purple) @250 W35
DC-DC converter input current ripple cancellation and inductor currents. . . . . . . . . . . . . . 36
DC-DC converter HF transformer waveforms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
DC-DC converter rectifier diode waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
Inverter modulating signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
DC-AC converter output voltage and current waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . 38
DC-DC converter efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
Microinverter efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
MIC output current and voltage waveforms during grid connection . . . . . . . . . . . . . . . . . . 40
Output current THD% . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
Power factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
DC-DC boost converter inductors (part1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
DC-DC boost converter inductors (part2) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
AC voltage TV. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
DC-DC converter HF transformer (part 1). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
DC-DC converter HF transformer (part 2). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
Inverter filter inductor (part 1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
Inverter filter inductor (part 2) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
DC/DC inductor for 250 W microinverter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
DC-DC transformer for 250 W microinverter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
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System description
1
System description
The block diagram reported in
Figure 2
shows the main concepts behind the proposed
microinverter solution.
Figure 2.
Block scheme of the 250 W grid connected system
AM12044v1
Although the characteristics of an MIC may change according to the modules’ electrical
specifications, its structure can be composed by up to three stages to perform the MPPT
function and deliver power to the grid. The very first MICs used three stages to perform such
a conversion but, nowadays, the trend is to reduce the stages to two or even one. The
reduction of the number of stages dramatically increases efficiency, up to 96%. On the other
hand, the single-stage conversion scheme is not capable of controlling reactive power,
performing power factor correction according to specific needs that may be dictated by the
utility. These considerations, together with the possibility of reducing the size of electrolytic
capacitors, thanks to the use of a high voltage DC link, have mainly driven the design of the
250 W MIC, which then belongs to the two-stage category. The DC-DC stage is used to
boost the output voltage of the PV module up to about 400 V DC and is also responsible for
implementing the maximum power point tracking (MPPT). High efficiency and high input to
output voltage step ratio are the most important requirements for this stage. High voltage
gain can be obtained through capacitor multiplier systems or high-frequency transformers
but, since galvanic isolation is required for MIC applications, the HF transformer is always
necessary. The transformer turns ratio depends on the input and output voltage, but also on
the topology choice which can be either a voltage source one or a current source one.
However, special care must be taken when designing and manufacturing the HF transformer
to keep the leakage flux as low as possible. The leakage flux mainly depends on the winding
construction which, in turn, is affected by the transformer turns ratio. The lower the turns
ratio is, the simpler it is to realize a transformer with low leakage flux. This consideration has
led to the use of a capacitor voltage doubler on the output stage of the DC-DC converter.
The DC-AC converter is a full bridge characterized by a high frequency leg switching with
sinusoidal PWM and low frequency leg switching at grid frequency. The adoption of this
modulating strategy allows optimization of the efficiency of the MIC in the low load part of
the operating profile since a sensible reduction in switching losses is achieved. The
selection of the power devices is also crucial for the correct operation of the topology in
terms of efficiency.
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