AN3095 datasheet(45/55 Pages) STMICROELECTRONICS

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AN3095

Experimental results

Doc ID 16555 Rev 3

45/55

Experimental results

Control issues have been thoroughly investigated and the possibility of implementing the

algorithm using a 32-bit ARM-based microcontroller from STMicroelectronics is verified. The

dedicated control board, developed for this purpose, is equipped with an STM32F103xx

microcontroller, characterized by a 32-bit CORTEX TM-M3 core with suitable peripherals.

The core, running at 72 MHz, is able to perform up to 90 MIPS. A high performance CPU,

based on Harvard architecture, plus suitable peripherals such as two advanced PWMs, fast

and accurate 12-bit A/D conversions with double S and H circuit and high resolution timers,

allows the implementation of very sophisticated control algorithms.

The control loop has been synchronized with the A-D conversions triggered by the ON

states of the two PWM timers. This brings benefits in terms of accuracy, avoiding the

acquisition of analog quantities (e.g. currents) during commutations of the power devices.

The execution time of the most relevant tasks is reported in Table 7.

The entire control loop is executed in about 30 µs (50 % CPU-load) with a sampling time of

57 µs. Further code may be executed in the remaining 50 %, allowing the implementation of

a HMI (human machine interface) such as LCD driving or a graphical user interface via SPI,

in order to have a complete smart-platform.

For this application, the three main control issues regarding a PV converter, namely, MPPT,

grid synchronization and power management control, have been included within the

firmware. All the PWM signals, necessary for power management, are generated with

proper dead-time, settable with a resolution of 16.6 ns by acting on the firmware developed

for this application. The algorithm may control both the active and reactive power in the DQ

synchronous frame, while the implemented MPPT algorithm is based on the P and O

method and may be optimized with simple modifications to the source code. The inverter

current is transformed, using Park equations, in the two components referred to the rotating

DQ reference frame of the grid voltage. These components, Id and Iq, are proportional to

active and reactive generated power, respectively. The reference current value of q axis, I*q,

is calculated in order to regulate the voltage of the DC bus Vbus. Reactive power is

maintained at zero through I*d, as only the injection of active power into the mains is

allowed, according to international standards. PI outputs are transformed back into AC

quantities, using the inverse park transformation, providing the signals for inverter

modulation. The most critical task of the power management control is the estimation of the

Table 7.

Execution time of the main control functions

Function

Execution time

n.5 A/D acquisitions

1 µs

MPPT

1.5 µs

DQ_PLL+internal PID

10 µs

Direct park transf

5 µs

PI regulator

3 µs each

Reverse park transf

5 µs

Sine modulation

1.5 µs

Total control loop execution time @ 72 MHz

≅ 30 µs

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