AN2039| Application Note
Maxim/Dallas > App Notes > POWER-SUPPLY CIRCUITS Keywords: off line, foward converter, isolated supply, voltage mode, pwm controller, forward converter, offline, power supplies
May 16, 2003
APPLICATION NOTE 2039
50W Voltage-Mode Forward Converter Design with the MAX8541
This application note details the design of a 50-watt, isolated forward converter using the MAX8541 synchronizable, high-frequency, voltage-mode PWM controller. Design procedures for both the power stage and controller are presented, along with actual performance measurements. The converter delivers 20 amperes of load current at an output voltage of 2.5 volts, and employs synchronous rectifiers for secondary rectification. The input voltage range for the converter is 36-75VDC. This design is available as an evaluation board. The evaluation board demonstrates how easy it is to implement the features network and telecom applications require. The design methods can easily be adapted to the design of high performance, full featured off-line power supplies. Key features of the application are:
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300kHz switching frequency Programmable input UV/OV Protection Programmable hiccup mode and latch type current limit protection Programmable maximum duty cycle clamp with feedforward Adjustable ramp magnitude of MAX8541 voltage mode controller Synchronization to external clock Adjustable current limit threshold Active low-enable feature for easy turn on and turn off Internal leading edge blanking on the current sense pin Output overvoltage protection Space saving 16-pin QSOP
Description of Application Circuit Operation
Figure 1 shows the circuit diagram of a 2.5V, 20A, isolated forward converter that uses the MAX8541 voltagemode controller (U1). At start-up, the total capacitance at the Vcc pin is charged through MOSFET Q7 and the parallel combination of resistors R30 and R22 from the DC input voltage VIN. When Vcc exceeds the undervoltage lockout threshold of the MAX8541, it goes through the soft-start mode and pulses of gradually increasing duty cycle are applied to the gate drive IC, U8. The MOSFET Q1 starts to switch the input dc voltage across the power transformer T1, used to provide isolation and to step the input dc voltage down to the required level. (Selection of the power transformer turns ratio is dealt with in the following section.)
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