ST AN4070 Installation and operating instructions

December 2012 Doc ID 022934 Rev 1 1/53
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
www.st.com

Contents AN4070
2/53 Doc ID 022934 Rev 1
Contents
1 System description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2 DC-DC converter design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3 Schematic description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
4 STM32F103xx based current control for inverter grid connection . . . 29
5 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

AN4070 List of tables
Doc ID 022934 Rev 1 3/53
List of tables
Table 1. Summary of main electrical specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Table 2. DC-DC converter main specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Table 3. DC-DC converter MOSFET main characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Table 4. DC-DC converter rectifier diodes main characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Table 5. HF transformer specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Table 6. RM14 core with N87 Ferrite main characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Table 7. STB11N65M5 MOSFET main electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Table 8. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

List of figures AN4070
4/53 Doc ID 022934 Rev 1
List of figures
Figure 1. Image of the 250 W MIC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Figure 2. Block scheme of the 250 W grid connected system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Figure 3. DC-DC converter topology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Figure 4. Gate driving signals of the DC-DC converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Figure 5. Currents through the input inductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Figure 6. MOSFET MOS1 and MOS2 currents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Figure 7. Rectifier diode currents. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Figure 8. HF transformer current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Figure 9. Basic scheme of the DC-AC converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Figure 10. Schematic of the two main power stages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Figure 11. Schematic of the filtering and relay circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Figure 12. DC-DC converter driver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Figure 13. DC-AC converter driver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Figure 14. Auxiliary power supply of the demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Figure 15. AC line current sensing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Figure 16. AC line voltage sensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Figure 17. PV current sensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Figure 18. PV voltage sensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Figure 19. Block diagram of the implemented control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Figure 20. Implemented PLL structure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Figure 21. Block diagram of the control algorithm implemented on the 250 W MIC STM32F103xx . . 32
Figure 22. DC-DC converter MOSFET current (green) and drain to source voltage (purple) @250 W35
Figure 23. DC-DC converter input current ripple cancellation and inductor currents. . . . . . . . . . . . . . 36
Figure 24. DC-DC converter HF transformer waveforms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
Figure 25. DC-DC converter rectifier diode waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
Figure 26. Inverter modulating signals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
Figure 27. DC-AC converter output voltage and current waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . 38
Figure 28. DC-DC converter efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
Figure 29. Microinverter efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
Figure 30. MIC output current and voltage waveforms during grid connection . . . . . . . . . . . . . . . . . . 40
Figure 31. Output current THD% . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
Figure 32. Power factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
Figure 33. DC-DC boost converter inductors (part1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
Figure 34. DC-DC boost converter inductors (part2) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
Figure 35. AC voltage TV. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
Figure 36. DC-DC converter HF transformer (part 1). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
Figure 37. DC-DC converter HF transformer (part 2). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
Figure 38. Inverter filter inductor (part 1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
Figure 39. Inverter filter inductor (part 2) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
Figure 40. DC/DC inductor for 250 W microinverter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
Figure 41. DC-DC transformer for 250 W microinverter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

AN4070 System description
Doc ID 022934 Rev 1 5/53
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
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.
AM12044v1

System description AN4070
6/53 Doc ID 022934 Rev 1
The system is powered up by an auxiliary power supply which generates a +15 V and a +5
V bus from the input voltage using an L4951 integrated circuit and few external components.
The startup voltage of the auxiliary power supply is 18 V while the maximum voltage is 55 V.
The STM32F103xx supply voltage is scaled down from the +5 V bus using a low drop linear
regulator. Five feedback signals are used by the microcontroller in order to fully control the
system. In particular, a voltage transformer and a Hall effect sensor are used to sense grid
voltage and current, respectively. Two resistive voltage dividers are used to sense the DC-
DC converter input and output voltage. Since the microcontroller is supplied from the input
side of the system, the high voltage bus feedback signal is opto-isolated to ensure galvanic
isolation between the input and the output of the system. Finally, the input current generated
by the PV module is sensed using a simple current shunt resistor. These five feedback
signals are sampled at 17.4 kHz using the STM32F103xx 12-bit A-D converters.
The control strategy is based on a standard perturb and observe (P&O) MPPT algorithm to
adapt the input system impedance to the PV module electrical characteristics, while it uses
DQ axes control to ensure that MIC generated current and voltage are able to cope with grid
characteristics. The main system specifications are summarized in Ta b l e 1 .
Table 1. Summary of main electrical specifications
Parameter Value
Vin (nominal input voltage) 35.8 V
Vin_max (maximum input voltage) 55 V
Vin_min (minimum input voltage) 18 V
MPPT range 20 V to 40 V
Iin (nominal input current) 7.6 A
Imax (max. input current) 11 A
DC-DC output voltage nominal 380 V
DC-DC max. output voltage 450 V
Vout AC 230 V, 50 Hz - 240 V, 60 Hz
Iout (nominal) 1.1 A (230 V AC) / 1.06A (240 V AC)
Power factor 0.98 @ full load
Nominal output power 250 W
DC-DC switching frequency 35 kHz
DC-AC switching frequency 17.4 kHz

AN4070 DC-DC converter design
Doc ID 022934 Rev 1 7/53
2 DC-DC converter design
The topology chosen for the DC-DC section of the MIC is an isolated, interleaved boost
converter. The basic electrical scheme of such a converter is shown in Figure 3.
Figure 3. DC-DC converter topology
It consists of an input capacitor C1, two inductors, L1 and L2, two ground referenced
MOSFETs, MOS1 and MOS2, a high frequency transformer, T1, two rectifier diodes, D1 and
D2, and three output capacitors, C2, C3 and C4. The two rectifier diodes, together with C2
and C3, form a voltage doubler circuit which is used to further boost the HF transformer
output voltage. The advantage of such a solution, compared to a standard one using four
rectifier diodes, is to halve the transformer turns ratio, with consequent realization of a
shorter secondary winding which, in turn, means better primary to secondary flux coupling
and lower secondary winding conduction losses. The two driving signals used to command
the two MOSFET gates are 180 degree phase shifted with respect to each other and always
have a duty cycle >0.5.
This introduces the problem of controlling the DC-DC operation at very low output power.
Two solutions can be used to overcome this problem. With the first solution the DC-DC
converter is controlled by keeping the duty cycle at the minimum allowable value and the
switching frequency is used as a control variable. The second solution uses a constant
switching frequency and enables burst mode operation. This second case is the preferred
method of control at low power. The implementation of burst mode is based on the definition
of a suitable hysteresis bandwidth for the output voltage which is used to turn on and off the
modulation of the DC-DC converter.
Figure 3 shows the theoretical gate driving signals used to control MOS1 and MOS2. The
180 degree phase shift produces two phase shifted inductor currents (Figure 4) which in
turn produce the beneficial effect of input and output current ripple cancellation. This allows
the use of small capacitor values to filter the high frequency content of input and output
currents.

DC-DC converter design AN4070
8/53 Doc ID 022934 Rev 1
Figure 4. Gate driving signals of the DC-DC converter
Figure 5. Currents through the input inductors
As the control strategy requires the two gate drive signals to be overlapped during part of
the switching period, there is an operating interval during which both the MOSFET devices
are ON and therefore both the input inductor chokes, L1 and L2, store energy. During this
interval the input current is equally divided in the two MOSFETs, as shown in Figure 6,and
no current flows across the secondary winding and the diodes. The load is supplied by the
output capacitors.
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AN4070 DC-DC converter design
Doc ID 022934 Rev 1 9/53
Figure 6. MOSFET MOS1 and MOS2 currents
During the second switching interval MOS1 is still on while MOS2 is turned off. The energy,
previously stored in L2 is now transferred to the secondary while inductor L1 is charged by
the input voltage and its current continues to build up. The primary current is delivered to the
transformer secondary winding and output capacitor C2 is charged since diode D2 is in
conduction (Figure 7). During the third time interval both MOSFETs conduct again.
Therefore, energy is stored both in L1 and L2 while the load is supplied by the output
capacitors. During the fourth time interval MOS1 is off and MOS2 is kept on, which means
that the energy previously stored in L1 is discharged on C3 with the transformer secondary
current flowing through D1.
In reality, some additional operating intervals must be taken into account. This is mainly due
to the presence of system parasitic elements such as transformer leakage inductance and
semiconductor output capacitances.
Figure 7. Rectifier diode currents
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DC-DC converter design AN4070
10/53 Doc ID 022934 Rev 1
Figure 8. HF transformer current
The main specifications used for the design of the DC-DC stage are reported in Ta bl e 2 .
The design equations used to carry out the implementation of the DC-DC converter are
described and commented on in the following part of this document.
1. Calculation of the switching period:
Equation 1
Table 2. DC-DC converter main specifications
Specification Value
Nominal input voltage 35.8 V
Maximum input voltage 40 V
Minimum input voltage 20 V
Nominal output power 250 W
Nominal output voltage 380 V
Maximum output voltage 430 V
Minimum output voltage 370 V
Target efficiency >97%
Switching frequency 35 kHz
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