The MPPT DC/DC power stage performs the function of translating multiples of MPPT voltage of a panel (depending on the number of panels in a string) to a stable voltage level suitable for the inverter or DC/DC stage for battery input. The input voltage is regulated by MPPT which is either performed through software or through external add-ons like a power optimizer. A PV panel acts as a current source where the short circuit current is approximately proportional to the irradiation available. The open circuit voltage across a PV panel is dependent on the ambient temperature conditions. For residential use cases, PV panels usually depict an output MPPT voltage of 33V for a 400W panel and 40V or higher for 500W or 600W rated panels.

Since a string inverter is a cost-sensitive application, a non-isolated boost converter is the preferred topology for conversion of the input string voltage to a stable DC link voltage. Figure 2-2 depicts such an example present in TIDA-010938. This input voltage exists as multiples of 33V or 40V depending on the type and wattage of panels used. The DC link voltage can vary depending on whether it is a single-phase application or a three-phase application. For single-phase, the bus can be rated up to 500-550V and for three-phase usually up to 1200V. A buck or buck-boost stage will be less efficient due to the higher current to be supported with a lower DC link voltage. To increase power level of this stage, multiple strings can be added as independent inputs. Each input can be designed independently and symmetrically. Usually a CLLLC stage or synchronous boost is not employed here since it will not be cost effective.

Figure 2-2 DC/DC MPPT Stage Block Diagram

A boost converter needs one controlled switch (MOSFET, IGBT, etc) in combination with an uncontrolled switch (diode) and an inductor to realize it. This topology has several benefits such as lower number of components, high efficiency, simple implementation, etc. At higher power levels, the diode is replaced by another controlled switch (used as a synchronous switch) to reduce conduction losses. Thus it becomes the synchronous boost converter.

With even higher power levels (as encountered in string inverters), we end up paralleling the power devices so as to reduce conduction losses. Multiple stages of synchronous boost converters are used with phase-interleaved PWMs driving each converter. For ‘n’ number of interleaved stages, the phase difference between the individual PWMs is 360° /n. This significantly reduces ripple currents and helps reduce the overall size.

It is important to keep in mind that the switching transistors need to be appropriately rated depending on the type of application. This translates to 650V drain-source voltage rating for single-phase and up to 1200V rating for three-phase applications (higher drain-source voltage rating required for systems with higher DC bus voltage). Going higher with DC link voltage to beyond 1000V will reduce power losses in system and allow more panels to be added in series. However, the devices need to be chosen with appropriate ratings.