5-V/300-A Two-Switch Forward Ac-Dc Converter

2021-07-16
●Description
■This design note provides elementary information about a two-switch fbrward converter built with the NCL30125 operated in current-mode control. This controller offers many features to build an energy-efficient converter with all the needed protections like cycle-by-cycle current limit with a 500-mV sense voltage, over temperature protection with a dedicated NTC pin and brown-out feature. In addition to the low side MSOFET drive, the controller integrates also an high-side section to drive the floating N-channel power MOSFET. Dedicated pins are available to adjust the switching frequency (RT pin - pin 6) or the soft-start duration (SS pin - pin 7). Finally, a high-voltage current source with Dynamic-Self Supply (DSS) is embedded to quickly start the power supply and maintain the V-cc voltage in light load or standby.
■The primary-side section drives a transformer whose primary inductance is 3.8 mH. The energy accumulated in this inductance is sending back to the bulk capacitor thanks to the freewheel components. One of the classical freewheel diode has been replaced by a MOSFET M9 driven by the controller in order to refresh the bootstrap capacitor. The current is sensed via a 87-mQ resistance. The switching frequency of 100 kHz and the soft-start duration are set by two individual components (R-23 for f-sw and C-22 for SS).
■The power stage is made of two switching N-channel transistors Mi and Mg. These two transistors are switched in same time and seen the input voltage as maximum. In the secondary side, three forward MOSFETs are self-driven by the transformer but require regulation transistors Q9 and Q10 to limit the maximum V-GS these elements can accept. The three forward MOSFETs M-2:, M-3 and M-4 are driven by the NCP4306 synchronous rectification controller. The regulation is ensured by a TL431 wired in a type-2 configuration. Loop gain measurements show a 74° phase margin at a 60-A output obtained with a 230-V rms source.
●Key Features
■Integrated High-side driver
■Adjustable switching frequency up to 300 kHz
■Peak current-mode control
■Skip mode to maximize performance in light load conditions
■Internal 600-V start-up source operated in dynamic self-supply during start-up or skip mode.
■Brown-out (BO) detection
■Adjustable soft-start duration
■15-ms timer-based short-circuit protection with auto-recovery or latched operation
■Auto-recovery or Latched OVP on V-cc
■Latched OVP/OTP input for improved robustness
■+0.9 A / -1.2 A peak source/sink drive capability

ON Semiconductor

NCL30125

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Part#

Two-Switch Forward Ac-Dc Converter

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Application note & Design Guide

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Please see the document for details

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English Chinese Chinese and English Japanese

October.2019

Rev. 0

DN05131/D

2 MB

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