System Board 5858
MAXREFDES115#: ISOLATED 24V TO 5V 20W POWER
SUPPLY
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.
MAXREFDES115# is an efficient active clamp topology design with 24V input, and a 5V at
4A (20W) of power. The design features the MAX17599, an active clamp, current-mode
PWM controller optimized for industrial supplies; this entire circuit fits on a 20mm x 55mm
board. As with all Maxim reference designs, the BOM, schematics, layout files, Gerber files
are all available. In addition, all boards feature through-hole pins for immediate board
placement and accelerated prototyping, and are available for purchase.
Features
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Functional Insulation
Compact and Flexible
Low Power Dissipation
Minimal External Components
Robust Operation in Adverse Industrial Environments
5V at 4A Output with 20% Over-Range Current
Applications
• PLCs
• Industrial Process Control and Sensors
• Telecom and Datacom Power Supplies
MAXREFDES115# System Board
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MAXREFDES115# Reference Design Block Diagram
System Board 5858
MAXREFDES115#: ISOLATED 24V TO 5V 20W POWER
SUPPLY
Introduction
Though 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 MAXREFDES115# is a compact, isolated 24V input flyback converter module that
features 5V at 4A (20W) output. Multiple transformers have been qualified for this design,
simplifying the process of transformer selection. In addition, through-hole pins on the
bottom side of the module allow quick and easy integration of this power converter into any
system. Pins are 15.24mm from power to ground, and 63.5mm from the input side to the
output side.
MAXREFDES115# System Board
Enlarge+
The MAXREFDES115# reference design (Figure
1)
demonstrates the application of the
MAX17599 low IQ, wide-input range, active-clamp current-mode PWM controller. The
MAX17599 contains all the control circuitry required for the design of wide-input isolated
forward-converter industrial power supplies. The reference design features the active-
clamp transformer reset topology for forward converters. This reset topology has several
advantages including reduced voltage stress on the switches, transformer size reduction
due to larger allowable flux swing, and improved efficiency due to elimination of dissipative
snubber circuitry. These features result in a compact and cost-effective isolated power
supply. The design is set to switch at 250kHz. For EMI-sensitive applications, the user can
program the 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 16.5V 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 the maximum allowed value (set to 37.0V in the design).
To control inrush current, the device incorporates a soft-start (SS) pin to set the soft-start
time for the regulator. Power dissipation under fault conditions is minimized by hiccup
overcurrent protection (hiccup mode). The SS feature provides safe discharging of the
clamp capacitor when the device is turned off, and allows the controller to restart in a well-
controlled manner.
Additionally, the negative current limit is provided in the current-sense circuitry, helping
limit clamp switch current under dynamic operating conditions. An overtemperature fault
triggers thermal shutdown for reliable protection of the device. The reference design
delivers a peak efficiency of 91.5% 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/datacom power supplies, isolated battery chargers, servers,
and embedded computing.
System Diagram
Figure 1. MAXREFDES115# reference design block diagram.
Detailed Circuit Description
The MAX17599 low IQ active-clamp current-mode PWM controller contains all the control
circuitry required for designing wide-input isolated forward-converter industrial power
supplies.
The device includes an AUX driver that drives an auxiliary MOSFET (clamp switch) that
helps implement the active-clamp transformer reset topology for forward converters. This
reset topology has several advantages, including reduced voltage stress on the switches,
transformer size reduction due to larger allowable flux swing, and improved efficiency due
to elimination of dissipative snubber circuitry. Programmable dead time between the AUX
and main driver allows for zero voltage switching.
Primary Power Stage
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