Grid connected inverter

AN3152

8/17

Doc ID 17056 Rev 1

continuous current mode, so the power MOSFET is turned on during the recovery time of

the diode. The BOOST is driven from a microcontroller in order to implement the MPPT.

Some inverter modules adopt a push-pull topology instead of the boost topology, to elevate

the panel solar voltage and achieve galvanic insulation. In this case, more expensive

devices sized for two times the input voltage must be used. If the BUS voltage is 500 V, the

devices must be able to manage at least 1000 V, so 1200 V devices are needed. IGBTs are

commonly used for this voltage range, due to acceptable conduction loss levels.

Unfortunately, IGBTs are not fast devices and they must be used at relatively low

frequencies in order to minimize switching losses. The boost topology permits the use of

more efficient 600-650 V power MOSFET devices, and pushes up the switching frequency.

A higher frequency means smaller and cheaper inductor and bulk capacitors.

The second stage is a “power transformer” stage, as shown in

Figure 3. This is used to

provide galvanic insulation between the panels and the grid. This block is not mandatory in

some countries but is generally used for safety reasons. Its frequency of operation is about

100 kHz in order to minimize transformer size. The power MOSFETs are connected in a

typical bridge configuration, and the devices work on a modified zero-voltage switching

(ZVS) modulation to minimize the switching losses. The bus voltage is 450 V (typ).

The “high performance bridge rectifier” stage is used to rectify the alternating voltage

coming from the “power transformer.” It is composed of four SiC diodes connected in a

Graetz bridge, as shown on the right side of

Figure 3. These diodes are able to work at very

high di/dt, typically greater than 1000 A/µs. The operating frequency is the same as that of

the “power transformer” stage, about 100 kHz.

The “current generator” stage is used to provide and regulate the power generated from

solar panels onto the grid. It consists of a typical buck topology composed of an MDmesh™

V power MOSFET and a SiC diode, as shown in

Figure 4. The operating frequency is about

100 kHz, and the drive signal is modulated in order to follow the 100 Hz of the grid.

The last stage is the “sync bridge rectifier.” This is a simple Graetz bridge used to rectify the

grid and allow the “current generator” to introduce power to the grid during the full sinusoid.

It is composed of four power MOSFETs specially driven and used as synchronous rectifiers

(see

Figure 5). Please note that this stage can be realized with simple diodes, but for

efficiency issues we use a MOSFET as a synchronous rectifier, grid-commutated at twice

the grid frequency. Grid-commutated operation is possible because the input current to the

stage is modulated to the rectified sinusoidal by the “current generator” stage.

Thanks to this solution, a single switch current generator can be adopted using the

STW42N65M5 or STW77N65M5 650 V power MOSFETs. Currently, most solutions adopt a

half-bridge current generator topology instead of a single switch. These solutions are able to

follow the grid voltage on a full sinusoid, but 600-650 V power MOSFETs cannot be used

due to their low breakdown voltage. In fact, in this case the power MOSFETs must be able to

manage at least 750 V on the bus voltage (double the maximum peak-to-peak grid voltage ±

373 V). If a full-bridge inverter topology is used to design a current generator (to unfold the

power to grid, synchronized to the sine wave), PWM modulation must be used. Both fast

switches and diodes must be used in order to increase efficiency. IGBT devices with a

paralleled discrete fast diode are generally used, due to better performance with respect to

the power MOSFET solution. IGBTs are preferred based on the higher performance of the

discrete fast diodes with respect to a power MOSFET internal body diode. Due to the circuit

solutions adopted on this proposed new inverter design, 650 V MDmesh™ V MOSFETs can

be safely used (STW42N65M5 or STW77N65M5). Using MDmesh™ combined with 600 V

SiC diodes (STPSC1006) allows us to increase the frequency of operation and work at high

dI/dt, from 800 A/µs to 2 kA/µs, and more. The capability to operate at a high frequency

translates into being able to use the smallest (and magnetic) transformer, and even smaller

capacitors.