System Board 6261
MAXREFDES112#: ISOLATED 24V TO 12V 10W
FLYBACK POWER SUPPLY
Overview
Maxim's power supply experts have designed and built a series of isolated, industrial
power-supply reference designs. Each of these power supplies efficiently converts 24V
into useful voltage rails at a variety of power levels. Every power rail is isolated with a
readily available transformer from multiple, global vendors, providing for quick, convenient
transformer selection. Each design has been tested for load and line regulation, as well as
efficiency and transient performance. As with all Maxim reference designs, the BOM,
schematics, layout files, and Gerber files are all available. In addition, boards are available
for purchase; most boards feature through-hole pins for immediate board placement and
accelerated prototyping.
MAXREFDES112# is an efficient flyback topology with 24V input, and a 12V output at
10W of power (0.8A). The design features the MAX17596, a peak-current-mode converter
with flexible switching frequency. This entire circuit fits on a 20.3mm (0.8in) x 59.7mm
(2.35in) board.
Features
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Functional insulation
Compact and flexible
Minimal external components
Robust operation in adverse industrial environments
12V 800mA with 20% over range current
±5% output accuracy
Applications
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Industrial control and automation
Process control
PLC
Telecom and Datacom power supplies
MAXREFDES112# System Board
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MAXREFDES112# Reference Design Block Diagram
System Board 6261
MAXREFDES112#: ISOLATED 24V TO 12V 10W
FLYBACK POWER SUPPLY
Details
Introduction
Isolated power supplies provide reliable power regulation for a variety of applications.
Transformer selection is often the most difficult step in isolated power design. The
MAXREFDES112# is a compact 24V input flyback converter module that features 12V at
800mA output. Multiple transformers (Halo Electronics TGSP-P145EP10LF, Wurth
Electronics 750315882, HanRun HR051075, and Sumida 12387T070) have been qualified
for this design, simplifying the process of transformer selection. In addition, through-hole
pins on the bottom side of the module allows quick and easy integration of this power
converter into the main system. Pins are 0.6in from power to ground, and 2in from the
input side to the output side.
MAXREFDES112# System Board
The MAX17596 is a peak-current-mode controller for design of wide input-voltage flyback
and boost regulators. The reference design operates over a 17V to 36V input voltage
range, and provides up to 800mA (plus 20% overhead) at 12V output. The MAX17596 has
an internal amplifier with 1% accurate reference, eliminating the need for an external
reference. The switching frequency is programmable from 100kHz to 1MHz with an
accuracy of 8%, allowing optimization of magnetic and filter components, resulting in
compact and cost-effective power conversion. This design is set to switch at 500kHz. For
EMI-sensitive applications, the MAX17596 incorporates a programmable frequency
dithering scheme, enabling low-EMI spread-spectrum operation.
The input undervoltage lockout (EN/UVLO) is provided for programming the input-supply
start voltage (set to 17V in the design), and to ensure proper operation during brownout
conditions. The EN/UVLO input is also used to turn on/off the IC. The overvoltage input
(OVI) protection scheme is provided to make sure that the controller shuts down when the
input supply exceeds its maximum allowed value (set to 36V in the design).
A capacitor is connected to the SS pin to program the soft-start period hence reduces the
input inrush current during startup. Hiccup-mode overcurrent protection and thermal
shutdown are provided to minimize dissipation in overcurrent and overtemperature fault
conditions.
The reference design delivers a peak efficiency of 88% with the supplied components
when the input is 24V. This general-purpose power solution can be used in many different
types of power applications, such as programmable logic controllers (PLC), industrial
process control, industrial sensors, telecom power supplies, isolated battery chargers,
servers, and embedded computing.
System Diagram
Figure 1. MAXREFDES112# block diagram.
Detailed Circuit Description
The MAXREFDES112# reference design contains all the control circuitry and electric
components required for designing a 17V to 36V wide-input, 12V 10W output isolated
flyback converter power supply.
When the MOSFET is turned on, the current flows through the primary winding of the
transformer and 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 tuned off, the
energy stored in the primary side is transferred 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
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