This document describes how the NCP 1216A controller
can be used to design a DC/DC single?ended forward
converter suitable for telecommunication applications. The
requirements for the converter are as follows:
? Input voltage range from 36 V to 72 VDC
? Continuous output power greater than 30 W for a 12 V
output voltage
? Small PCB dimensions
? Efficiency greater then 85%
? Input to output isolation voltage of 1500 V
The NCP1216A controller is an attractive solution for this
application, due to the following features:
? 50% Maximum Duty Cycle Operation
Forward converters usually limit the maximum duty
cycle to 50%. Since the voltage reset is constrained to
be equal to the input voltage (1:1 reset ratio), it is not
desirable to exceed 50% DC to avoid saturating the
transformer core.
? No Auxiliary Winding Operation
The DSS (Dynamic Self?Supply) function allows
the NCP1216A derive power directly from the HV
line without having to supply VCC either from the
secondary output inductance (creepage distance and
isolation issues) or via an auxiliary winding delivering
a variable voltage of N x Vin.
? 500 mA Peak Current Capability
The NCP1216A can drive a MOSFET directly without
any additional driver stage. If the selected MOSFET
gate charge would overload the DSS capability, then an
auxiliary winding could be used solely to supply the
driver pulses.