But less string inverters application environment, the higher the external "Small Cooling Fan" protective performance requirements. Current Outdoor fan protection class generally only reach IP54 / IP55, external fan design resulting in an overall system protection level can not achieve IP65. Meanwhile, in order to avoid direct rain erosion "Small Cooling Fan", designed cooling solution, the ducts become very complex, duct form is limited, once the fan failure, severe decay heat capacity, which will make the inverter output power derating generating capacity decrease, seriously affecting the interests of customers. More critical is because "Small Cooling Fan" perennial exposure to rain and dust, and corrosion serious, life falling sharply, many times as necessary to replace the fan within the life cycle of the inverter, high maintenance costs.