AN4070 datasheet(5/53 Pages) STMICROELECTRONICS

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AN4070

System description

Doc ID 022934 Rev 1

5/53

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

AN4070 Datasheet(PDF) 5 Page - STMicroelectronics