ST AN3215 Application note

AN3215

Application note

Boost for solar applications with 3 kW fixed off time (FOT)

Introduction

The following application note describes the modifications implemented on the STEVALISF001V1 demonstration board, originally designed to work as a PFC rated for a power of 3 kW, to be used as a front-end boost stage for photovoltaic applications.

In recent years the field of solar energy, the production of electric power using solar cells, is requiring power electronic solutions to manage the power delivered by the panels.

The DC voltage provided by the photovoltaic field needs, in many cases, to be boosted before supplying a second electronic power stage needed to convert the DC source into AC voltage, required by domestic appliances or for grid connection.

The power conversion must be done with a solution capable of working at high efficiency in order to avoid energy waste. Each and every watt is important!

The availability of new power devices with lower voltage drop and higher switching capability allows a very efficient solution to be obtained, wasting only a few watts while managing power in the range of thousands of watts.

Figure 1. 3 kW boost power board

August 2010

Doc ID 17446 Rev 1

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www.st.com

Contents	AN3215

Contents

1	FOT boost for solar front-end applications . . . . . . . . . . . . . . . . . . . . .	. 4
2	Fixed off time boost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	5
3	Technical specifications and design rules . . . . . . . . . . . . . . . . . . . . . . .	6
4	Circuital modifications and schematic . . . . . . . . . . . . . . . . . . . . . . . . . .	9
5	Lab test and measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	11
	5.1 Efficiency curves at different input voltages . . . . . . . . . . . . . . . . . . . . . . .	12
6	Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	14

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AN3215	List of figures

List of figures

Figure 1. 3 kW boost power board. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Figure 2. Block diagram of a photovoltaic inverter with power boost front-end . . . . . . . . . . . . . . . . . . 4 Figure 3. Circuit board schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Figure 4. Internal block diagram of L6563 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Figure 5. Output power vs. V control at different input DC voltages . . . . . . . . . . . . . . . . . . . . . . . . . 10 Figure 6. Boost efficiency at 190 Vdc input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Figure 7. Boost efficiency at 250 Vdc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Figure 8. Boost efficiency at 300 Vdc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Figure 9. Boost efficiency at 350 Vdc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

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FOT boost for solar front-end applications	AN3215

1 FOT boost for solar front-end applications

The front-end power boost can be found in most solar inverter solutions.

The functions of this stage are two. The first is to boost the voltage coming from the solar string, in many cases it is mandatory to have a DC bus with a value sufficient to supply a sinusoidal inverter which is able to give 220 Vac at its output, or to be connected on the mains. The second function is to adapt the impedance of the solar string with the impedance of the inverter. This concept can be better understood when remembering that the power delivered by the solar strings is variable according to a wide range of factors, first of all the solar power incidence on the photovoltaic field. For these reasons the front-end power stage connected to the solar strings must give the possibility of modulating the power drained from the string according to a well known algorithm called MPPT (maximum power point tracking). The MPPT calculates run time, the power delivered by the panels under different voltages, and current conditions. The power boost stage is able to modulate the power absorbed from the strings according to the MPPT algorithm. The MPPT algorithm is implemented via firmware in the microcontroller involved in managing the whole solar inverter.

The boost stage receives an analog voltage in the range of 0-2.5 V from the microcontroller, the boost drains power according to this control voltage from the solar string. Figure 2 below shows this concept with a block diagram:

Figure 2. Block diagram of a photovoltaic inverter with power boost front-end

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The boost converter acts as a current generator.

The output voltage of the boost stage is not controlled, so it can be connected in parallel with another boost board in order to increase the power range.

The voltage at the output is regulated by the load (the inverter) that, connected to the mains line, supplies the right power amount in order to maintain the output boost voltage to a fixed value of 400 V.

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AN3215	Fixed off time boost

2 Fixed off time boost

Using the hardware demonstration board, designed for the fixed off time 3 kW PFC ISF001V1 (see AN2951; 3 kW fixed-off-time (FOT) power factor correction), and implementing some simple modifications to the control part, it is possible to realize a boost converter working in the same power range. The idea is to maintain the fixed off time modulation strategy and to modify the control part in order to eliminate the output control voltage loop, maintaining overvoltage protection, and to give the possibility of controlling, through an analog voltage, the power delivered by the boost on the DC bus.

The Toff constant strategy gives a variable frequency control according to the input voltage. In fact, fixing the input voltage, as the output is fixed by the inverter, the duty cycle is dictated by the relationship:

Equation 1

Vout	=		1	where δ is duty cycle
		1−δ
V
in

Variation against the power delivered is really small. Only the peak current on the boost inductor, dictated by the power requested by the inverter, is variable.

As the Toff is fixed at a low input voltage, when the Ton is higher with respect to the continuous mode voltage relation, between input and output, the switching frequency is

reduced. At high input voltage the Ton is reduced to respect the same input/output voltage ratio, so the frequency is increased. This gives a reduction in the working frequency when the switched current is higher, reducing switching losses (lower input voltage at maximum power delivered), and an increased frequency when the current is reduced, reducing voltage ripple on the boost inductor.

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