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PI2061-00-QEIG Datasheet(PDF) 11 Page - Vicor Corporation |
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PI2061-00-QEIG Datasheet(HTML) 11 Page - Vicor Corporation |
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11 / 16 page  Picor Corporation • picorpower.com PI2061 Rev 1.4 Page 11 of 16 N-Channel MOSFET Selection: Several factors affect MOSFET selection including cost and following ratings; on-state resistance (RDS(on)), DC current, short pulse current, avalanche, power dissipation, thermal conductivity, drain-to-source breakdown voltage (BVDSS), gate-to-source voltage (VGS), and gate threshold voltage (VGS (TH)). The first step is to select a suitable MOSFET based on the BVDSS requirement for the application. The BVDSS voltage rating should be higher than the applied Vin voltage plus expected transient voltages. Stray parasitic inductance in the circuit can also contribute to significant transient voltage condition, particularly during MOSFET turn-off after an over current fault has been detected. In a disconnect switch application when the output is shorted, a large current is sourced from the power source through the MOSFET. Depending on the input impedance of the system, the current may get very high before the MOSFET is turned off. Make sure that the MOSFET pulse current capability can withstand the peak current. Also, such high current conditions will store energy even in a small parasitic inductance. The PI2061 has a very fast response time to terminate a fault condition achieving 120ns typical and 200ns maximum. This fast response time will minimize the peak current to keep stored energy and MOSFET avalanche energy very low to avoid damage (electrical stress) to the MOSFET. Peak current during output short is calculated as follows, assuming that the input power source has very low impedance and it is not a limiting factor: Where: : Peak current in the MOSFET right before it is turned off. : Input voltage at MOSFET drain before output short condition occurred. : Over current turn-off time. This will include PI2061 delay and the MOSFET turn off time. : Circuit parasitic inductance The MOSFET avalanche energy during an input short is calculated as follows: Where: : Avalanche energy : MOSFET breakdown voltage MOSFET RDS(on) and maximum steady state power dissipation are closely related. Generally the lower the MOSFET RDS(on), the higher the current capability and the lower the resultant power dissipation for a given current. This leads to reduced thermal management overhead, but will ultimately be higher cost compared to higher RDS(on) parts. It is important to understand the primary design goal objectives for the application in order to effectively trade off the performance of one MOSFET versus another. Power dissipation in load switch circuits is derived from the total drain current and the on-state resistance of the selected MOSFET. MOSFET power dissipation: Where : : MOSFET Drain Current : MOSFET on-state resistance Note: In the calculation use RDS(on) at maximum MOSFET temperature because RDS(on) is temperature dependent. Refer to the normalized RDS(on) curves in the MOSFET manufacturer’s datasheet. Some MOSFET RDS(on) values may increase by 50% at 125°C compared to values at 25°C. The Junction Temperature rise is a function of power dissipation and thermal resistance. Where: : MOSFET Junction-to-Ambient thermal resistance Not Recommended for New Designs |
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