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LCS708HG Arkusz danych(PDF) 11 Page - Power Integrations, Inc. |
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LCS708HG Arkusz danych(HTML) 11 Page - Power Integrations, Inc. |
11 / 26 page Rev. B 062011 11 LCS700-708 www.powerint.com Key Design Details The LLC converter is a variable frequency resonant converter. As input voltage decreases, the frequency must decrease in order to maintain output regulation. To a lesser extent, as load reduces the frequency must increase. When the converter is operating at the series resonant frequency, the frequency changes very little with load. The minimum operating frequency required occurs at brownout (minimum input voltage), at full load. Operating Frequency Selection For lowest cost, and smallest transformer size with the least amount of copper, the recommended nominal operating frequency is ~250 kHz. This allows the use of low-cost ceramic output capacitors in place of electrolytic capacitors, especially at higher output voltages (≥12 V). If the core and bobbin used exhibits too much leakage inductance for 250 kHz, operation at 180 kHz also results in excellent performance. For optimal efficiency at 250 kHz, AWG #44 (0.05 mm) Litz is recommended for the primary, and AWG #42 (0.07 mm) for the secondary winding. Thicker gauge lower cost Litz can be used at the expense of increased copper loss and lower efficiency. Litz gauge (AWG #38 or 0.1 mm) is optimal for very low frequencies (60-70 kHz), requires much larger transformers and greater lengths of Litz wire. For nominal operating frequencies even as low as 130 kHz, the use of PC44 or equivalent core material is recommended for reduced losses. For a given transformer design, shifting the frequency up (by substituting a smaller resonant capacitor), will reduce core loss (due to reduced AC flux density B AC) and increase copper loss. Core loss is a stronger function of flux density than of frequency. The increased frequency increases copper loss due to eddy current losses. Nominal operating frequencies >300 kHz start to lose significant efficiency due to increased eddy current losses in the copper, and due to the fact that a more significant percentage of time is spent on the primary slew time (ZVS transition time) which erodes the percentage of time that power is transferred to the secondary. Resonant Tank and Transformer Design Please refer to the Application Note AN-55 for guidance on using the PIXls HiperLCS spreadsheet which assists in the entire design process. Primary Inductance The optimal powertrain design for the HiperLCS uses a primary inductance that results in minimal loss of ZVS at any steady- state condition. Some loss of ZVS during non-steady-state conditions is acceptable. Reducing primary inductance produces higher magnetizing current which increases the range of ZVS operation, but the increased magnetizing current increases losses and reduces efficiency. The calculation of the primary inductance to be used for a first-pass design is based on device size, rated load, minimum input voltage, and desired operating frequency. It is provided in the PIXls spreadsheet. L PRI is the primary inductance of an integrated transformer (high leakage inductance), or in the case of the use of an external series inductance, the sum of this inductance and the transformer primary inductance. Leakage Inductance The parameter K RATIO is a function of leakage inductance: K L L 1 RATIO RES PRI =- The recommended K RATIO is from 2.5 - 7. This determines the acceptable range of leakage inductance. L RES is the leakage inductance in an integrated transformer; if a separate series inductor is used, it is the sum of this inductance and the leakage inductance of the transformer. A low K RATIO (high leakage inductance) may not be capable of regulation at the minimum input voltage, and may show increased transformer copper losses due to the leakage flux. A high K RATIO (low leakage inductance) will have high peak and RMS currents at low-line, and require a lower primary inductance to achieve ZVS operation over a suitably wide range, which increases the resonant circulating current, reducing efficiency. The core and bobbin designs available to the designer may limit the adjustability of leakage inductance. Fortunately, excellent performance can be achieved over a relatively wide range of leakage inductance values. The K RATIO directly affects the frequency range that the LLC needs to operate in order to maintain regulation over the input voltage range. Increasing K RATIO increases this frequency range, lowering f MIN. A low f MIN is only a potential problem for low frequency designs which typically run at higher nominal B AC. This may allow the core to reach saturation when operating at f MIN. Operating at f MIN occurs when the input voltage is at a minimal (input brown-out). For a design with a separate resonant inductor, running the inductance on the low side of the range (K RATIO = 7), minimizes the size and cost of the inductor. Adjusting Leakage Inductance Sectioned bobbins (separated primary and secondary) are commonly used for LLC converters. Increasing or decreasing both primary and secondary turns (while maintaining turns ratio) will change the leakage inductance proportionally to the square of primary turns. If the leakage inductance is too high, one possible solution is to use a 3-section bobbin, where the secondary is in the middle section, and the primary winding is split into 2 halves connected in series. Lastly, if the leakage inductance is too low an external inductor may be added. |
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