MAXREFDES113 Datasheet, PDF (3/7 Pages) Maxim Integrated Products – Functional Insulation

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
additional resistor, ROVI, to implement overvoltage input protection in addition to the EN/UVLO
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).
For the expected values of the startup DC input voltage (VSTART) and overvoltage-input protection
voltage (VOVI), the resistor values for the divider can be calculated as follows: