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AN-1001 Datasheet(PDF) 2 Page - International Rectifier |
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AN-1001 Datasheet(HTML) 2 Page - International Rectifier |
2 / 4 page A More Realistic Characterization Of Power MOSFET Output Capacitance Coss Introduction: The Power MOSFET has gained popularity and become the dominant switching device in power electronics since 1975. Its fast switching speed has extended power conversion switching frequencies from the 20kHz range of bipolar transistor to beyond 100kHz. in hard switching. With soft switching techniques such as zero voltage switching (ZVS) and zero current switching (ZCS), the switching frequency can exceed Mega Hertz. As switching frequency moves upward, power MOSFET parasitic parameters such as inductance and capacitance should be well defined and understood in order to optimize the particular power conversion design. This design tip focuses on explaining the power MOSFET output capacitance Coss and how it actually affects the power conversion circuit. Other capacitance’s such as input capacitance Ciss, and reverse transfer capacitance Crss, and the related gate charges have been well explained in previous International Rectifier publications. In hard switching circuits, Coss is used to calculate the additional power dissipation of power MOSFET due to discharging this output capacitor every switching cycle. In soft switching circuits, Coss may be used to calculate the resonant frequency or transition time, which is critical in establishing ZVS and / or ZCS conditions. Unfortunately the value of Coss varies non-linearly as a function of drain to source voltage Vds. The value of Coss specified in most manufacturer’s data sheets are at 25V Vds which is not really useful in actual circuit application. In an effort to better aid circuit designers, International Rectifier has taken the extra steps to specify the effective Coss (Coss at Vds of 1V and 80% Vdss) and also extend the capacitance curves to 80% of Vdss , instead of 50V, for IRs new range of high voltage HEXFETs. An example of these curves for a new 600V HEXFET is shown in Fig. 1 below : Fig. 1:. Typical Capacitance Vs. Drain to Source Voltage Listed below are the definitions of Coss effective and Coss @ 80% of Vdss, how they are measured and how to apply them in circuit design calculations. Coss effective: Coss effective is defined as a fixed capacitance that would give the same 1 10 100 1000 10000 100000 1 10 100 1000 DS V , Drain-to-Source Voltage (V) A V = 0V, f = 1MHz C = C + C , C SHORTED C = C C = C + C GS iss gs gd ds rss gd oss ds gd Ciss Coss Crss |
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