Figure 1. The MAXREFDES113# reference design block diagram.

Detailed Circuit Description

The MAXREFDES113# reference design contains all the control circuitry and electric components

required for designing a 17V to 36V wide-input, 12V, 20W output isolated flyback converter power

supply. When the MOSFET is turned on, the current flows through the primary winding of the

transformer, which stores the energy. The CS-pin input and the current sense resistor set the peak

current limit and provide overcurrent protection. When the MOSFET is turned off, the energy stored in

the primary side transfers to the secondary side.

The secondary side of the circuit consists of a diode, and a few output capacitors. When the

MOSFET on the primary side is off, the transformer pushes the current to the load at the output and

the remaining energy is stored in the output capacitors. When the MOSFET is on, the diode prevents

the current flow back into the transformer and discharges the output capacitors.

The feedback network is typical of most isolated flyback converters. It is constructed using a TL431

programmable shunt regulator, a 3000V isolation opto-coupler, and other RC components.

Startup Voltage and Input Overvoltage Protection Setting (EN/UVLO, OVI)

The EN/UVLO pin in the MAX17596 serves as an enable/disable input, as well as an accurate

programmable undervoltage lockout (UVLO) pin. The MAX17596 does not begin startup operations

unless the EN/UVLO pin voltage exceeds 1.21V (typ). The MAX17596 turns off if the EN/UVLO pin

voltage falls below 1.15V (typ). A resistor divider from the input DC bus to ground can be used to

divide down and apply a fraction of the input DC voltage to the EN/ UVLO pin. The values of the

resistor divider can be selected so that the EN/UVLO-pin voltage exceeds the 1.21V (typ) turn-on

threshold at the desired input DC bus voltage. The same resistor divider can be modified with an

functionality. When the voltage at the OVI pin exceeds 1.21V (typ), the MAX17596 stops switching.

Switching resumes with soft-start operation, only if the voltage at the OVI pin falls below 1.15V (typ).