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
Table 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