Saving household energy use helps protect the environment and has a big impact on electricity bills. The refrigerator is powered 24 hours a day, 7 days a week, so it is important to have an efficient switching power supply. In particular, light load efficiency is a big problem with the switching power supply of the refrigerator, because the refrigerator door is kept closed most of the time. In order to reduce wasted energy, the industry requirement for high-end refrigerators is that the efficiency of switching power supplies should be higher than 90% at 7.7% to 23.2% load. At the same time, when other power loads are used, higher efficiency is required as much as possible. New power semiconductor technologies help increase efficiency. This article describes a comprehensive solution for high-end refrigerator power supplies. By combining state-of-the-art shielded gate Trench Power MOSFET technology with charge-balanced high-voltage MOSFET technology, switching power supplies with over 90% efficiency at light loads have been designed.
Rated power, topology and target equipmentHigh-end refrigerators typically have a power rating of approximately 50W and a maximum power of 65W. Within this power range, the flyback topology is the best choice for both performance and cost. In addition, synchronous rectifiers under heavy load conditions are necessary to achieve high efficiency. For the main power switch, selecting a super-junction MOSFET improves efficiency. Figure 1 shows the power loss of the initial design.
Figure 1 Efficiency curve
In the initial design, a 100V 8.5mOhm shielded gate Trench Power MOSFET was applied to the secondary synchronous rectifier, and a 600V 190mOhm super junction MOSFET was used for the main switch. However, the efficiency at 7.7% load is still below 90%. The power loss in the entire system is only 454mW under this load condition. This means that each small power loss should be reduced to meet the requirements.
Figure 2 Power loss details
Figure 2 shows the power loss details of the initial design. It shows that switching losses dominate at light loads, and conduction losses are almost negligible. Even under full load conditions, switching losses account for a significant portion of total losses. Based on this loss analysis, a 380mOhm MOSFET was evaluated with much smaller parasitic capacitance. The synchronous rectifier MOSFET is also replaced by a 15mOhm MOSFET. Figure 3 shows the efficiency curve after equipment replacement. The efficiency at 7.7% load is 89.93%. It is also possible to use a higher on-resistance device, but it will cause a drop in efficiency over a heavy load range. The 55W load has been reduced by 0.8%. Therefore, a higher on-resistance is not feasible for the main switch. At this point, you can resort to new technology MOSFETs.
Figure 3 efficiency curve
Latest MOSFET technologyOne of the efforts to overcome silicon limitations is to use ultra-junction technology in high voltage power MOSFETs. This technique can significantly reduce on-resistance and parasitic capacitance at the same time, and there are usually trade-offs. Due to the small parasitic capacitance, these super-junction MOSFETs have extremely fast switching characteristics, which can reduce switching losses. In small power ratings such as 50W, the stored electrical energy in the output capacitor is a very important parameter for higher efficiency levels. SuperFET® II technology reduces stored energy in the output capacitor by more than 25% (compared to the previous generation SuperFET® technology, as shown) to 600V.
Figure 4 The stored energy in the output capacitor, the device with a rated impedance of 190mOhm
In the 50W power range, the load current in the MOSFET is extremely small. This extremely low current level results in a long charging time for the output capacitor when turned off . In this case, the switching loss of the MOSFET junction is minimal, and most of the switching losses occur when the hard switch is turned on due to the discharge of the output capacitor. Figure 5 shows an example of shutdown when a small load current is present. Even if the gate voltage (CH1) is already zero, the drain current (CH3) is still flowing. This current is actually charging the output capacitor. Therefore, less stored energy in the output capacitor is a key factor at this point, and SuperFET® II technology should have less switching losses in a given system.
Figure 5 1A shutdown current waveform
90+ efficiencyIn switching power supplies for high-end refrigerators, the main goal is to achieve 90% or higher efficiency at 7.7% to 23.2% load. Traditional ultra-thin technology devices are not easy to achieve this goal. SuperFET® II technology solves this problem. When the FCP380N60 is applied to the main switch, the efficiency at 7.7% load becomes 90.53%, as shown in Figure 3, which is due to less stored energy and excellent switching performance in the output capacitor. Under heavy load conditions, the efficiency difference from the traditional 190mOhm ultrapolar junction MOSFET is much smaller than the traditional 380mOhm ultrapolar junction MOSFET.
in conclusionThe new SuperFET® II technology MOSFETs, shielded gate-channel MOSFETs, and synchronous rectifier controllers include a complete solution for high-efficiency switching power supplies for high-end refrigerators. It achieves over 90% efficiency over most load ranges.
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