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DN05131/D

Oct.. 2019, Rev. 0 www.onsemi.com 1

Design Note – DN05131/D

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

Converter

Description

This design note provides elementary information

about a two-switch forward 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

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 M

driven by the controller in order to

refresh the bootstrap capacitor. The current is sensed

via a 87-mΩ resistance. The switching frequency of

100 kHz and the soft-start duration are set by two

individual components (R

for f

and C

for SS).

The power stage is made of two switching N-channel

transistors M

and M

. These two transistors are

switched in same time and seen the input voltage as

maximum. In the secondary side, three MOSFETs are

self-driven by the transformer but require regulation

transistors Q

and Q

to limit the maximum V

these

elements can accept. The three forward MOSFETs M

and M

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

• Latched OVP/OTP input for improved robustness

• +0.9 A / -1.2 A peak source/sink drive capability

Devices Applications

Input

voltage

Output power

Topology Board Size

NCL30125 Lighting 176-265 V rms 300 W

Two-Switch

Forward

148 x 152 x 53

Output spec. Turn on time Efficiency

Operating

temperature

Cooling

Standby

power

5 V/60 A < 200 ms

above 89 %

@ full load

0 – 40

Open Frame

in Still Air

Does not apply