Solar Micro Inverter is able to help the solar photovoltaic PV system to achieve per-panel level Maximum Power Point Tracking (MPPT) to improve power yield performance even in unideal conditions such as cloud or tree shades or bird drops and dust on the PV panels. Micro Inverter are once costly specialty products, but they are becoming more and more affordable thanks to technology advancement in recent years, and they are one of the fastest growing market segments in the solar industry. Usually installed under the PV panel, micro inverter is required to have high power conversion efficiency, good thermal performance, small size and long lifetime.
The conventional auxiliary power supply is usually a Flyback, either secondary side regulated (SSR) or primary side regulated (PSR). SSR design needs extra TL431+optocoupler that means extra costs. The optocoupler also introduces some reliability issue due to light attenuation when aging. The PSR design circuit is simpler and cost less but the conventional PSR flyback's output performance is relatively poorer. In addition, conventional PSR Flyback also requires an extra auxiliary winding which usually increases the transformer size and can be another issue for micro inverter application requiring small size.
This article presents a new auxiliary power supply design for micro inverter based on LMR38020 Fly-Buck™, with advantages of ease of design, low counts of components in BOM, low cost, small transformer size and well performance on efficiency, thermal and regulation.
Fly-Buck™ is a trademark of Texas Instruments.
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The output characteristics of solar Photovoltaics (PV) cells reveals that the energy harvest can vary greatly depending on the light radiation intensity and ambient temperature. One basic requirement of PV system is always making each PV panel in the system to output the maximum power available. Hence, Maximum Power Point Tracking (MPPT) techniques are widely used to enhance the energy harvest to the maximum, hence getting the best return of the investment.
The conventional MPPT operation, which are widely used nowadays, is carried out at the PV string level, as shown in Figure 1-1. Since each PV string usually consists of multiple PV panels and each PV module consist of multiple PV cells, which are usually connected in series, it only achieves global string MPPT but not module level MPPT. Therefore, each PV module may not work at its own maximum power point, preventing the ultimate maximum energy harvest. To solve this issue, micro inverters are deployed beneath each PV panel, as shown in Figure 1-2, to achieve per-panel level MPPT thus improving the overall power yield performance. In such a configuration, the micro inverter converts each PV panel’s DC power output to grid ac power rails.
Figure 1-3 shows a typical block diagram of micro inverter. There are two power stages in the micro inverter. The first power stage is DC/DC converter that converts the variable PV panels output voltage to a regulated high-voltage DC link suitable for the DC/AC inverter stage, and the first power stage also achieves the MPPT function. Commonly a micro inverter has 1, 2, or 4 DC/DC blocks allowing connected to 1, 2, or 4 PV panels, accordingly. Examples in Figure 1-2 show micro inverter with DC/DC blocks. The second power stage is the inverter that converts the high-voltage DC link to the grid ac voltage. It can be single phase or three phases inverter based on the targeted system configuration.
Besides the two power stages, there are other function blocks, such as DSP control, gate driver, PV voltage and current sensing, inverter or grid voltage and current sensing, PV current OCP, grid voltage zero-crossing and Insulation detection, wire/wireless communication and temperature sensing, and importantly the bias supply which is critical for the entire system to function.
Generally, the DSP control system and related signal acquisition circuits operates in low voltage referenced to the DC side ground, which is also the PV panel return node. So, no isolation is required for these circuit. However, circuits in the DC/AC side, such as the inverter gate driver, need isolation. This requires the auxiliary bias supply, which takes power from the PV panel, to be able to produce both the non-isolated low voltage bias voltages for the DSP and signal acquisition circuit, and the isolate bias voltages for the inverter gate drivers' use.
Figure 1-4 shows a typical power tree of micro inverter.
Outputs of all PV panels are connected together through the Oring diodes as the input source of the auxiliary power supply. Based on the voltage characteristics of the PV panel, the input voltage range is normally 16 to 60 volts.
Because the auxiliary power supply needs to provide multiple isolated voltages for the inverter gate drivers, the Flyback topology is commonly used, not only because it can easily realize multiple outputs, but also its counts of external components that can achieve competitive bill-of-materials (BOM) cost. For the single-phase inverter gate driver, the conventional flyback converter normally has one primary winding and three secondary windings, of which two windings for the upper and lower gate drivers, and one for the primary circuit use that includes MPPT dc-dc stage's non-isolated driver, relay and other system circuits.
Due to Flyback converter’s cross regulation issue, other system circuits are not powered by the Flyback converter. Additional dc/dc converter or LDOs either on the primary side or secondary side are used to provide the required tightly regulated bias supply voltages to those circuits. Usually, a primary buck is used to produce 5 V for the buffer, amps, Comps and hall circuits etc. A secondary buck converter is used to produce 3.3 V and 1.2 V for the DSP. An LDO is used as well to provide 3.3 V output without switching noise for the wireless communication module (such as Sub-1G).
Table 1-1 lists a 7-watts design requirements example of the auxiliary power supply.
Specifications | Requirements |
---|---|
Input Voltage Range | 16 V approximately 60 V |
Output 1 | 12 V/5 W (non-isolated) |
Output 2 | 12 V/1 W (isolated) |
Output 3 | 12 V/1 W (isolated) |
Thermal | Allows max 30°C up above Ta (Ta max = 80°C). |
Transformer Core | Prefer EP10 or smaller, accepts max EP13. |
Frequency | Target around 300 KHz (250 approximately 400KHz). |
Efficiency | > 85% |
Cross Regulation | < 10% |
Reliability | Solar products need high reliability and long-life time |
The following section includes the conventional Flyback design based on the above design requirements.