DESIGN SOLUTIONS

POWER

oers protection from accidental electrocution. According to

SELV/FELV regulations, input voltages below 60V are consid-

ered inherently safe to touch. Yet, the need for isolation in this

operating range is still pervasive. In this case, the element to

protect is the power-supply electronic load, typically a very del-

icate and expensive microcontroller that would readily self-de-

struct if accidentally exposed to high voltage.

Isolation also prevents ground loops, which occur when two

or more circuits share a common return path. Ground loops

produce parasitic currents that can disrupt the output voltage

regulation, as well as introduce galvanic corrosion of the con-

ducting traces, a phenomenon that degrades the equipment

reliability. Accordingly, isolated power supplies are routinely

utilized in industrial, consumer, and telecom applications con-

cerned with the protection of sensitive loads and long-term re-

liability of the equipment.

The Digital I/O Module System

A single module can have up to 64 channels and a factory ﬂoor

may utilize several modules. Figure 2 illustrates a generic digi-

tal I/O module system block diagram. A central hub takes the

AC line power and converts it to 24V DC, delivered to the I/O

modules together with the corresponding input (DI) and output

(DO) data. The factory environment is harsh, with electric and

magnetic interference and over-voltages, requiring protection

for sensitive electronics. Each module’s programmable logic

controller (PLC) is powered via an isolated step-down voltage

regulator. At the digital input module (DIM), a rugged, volt-

age-level translator interface powers the sensor, receives its

information, and passes it along to the PLC via a digital isolator

or optocoupler. A similar power, signal, and isolation chain on

the digital output module (DOM) leads to the on-board driver,

interfacing to the external actuator.

Isolated Step-Down Architectures

Flyback and forward-converter topologies are commonly used

in industrial, switch-mode power-supply, isolated, step-down

designs below 40W. The ﬂyback converter utilizes a gapped

transformer to both transfer and store energy, thereby minimiz-

ing the number of output components. However, the high peak

Introduction

Automated factories employ many sensors and I/O modules

to control industrial processes (Figure 1). Designing the power

circuit for ever shrinking modules that operate in harsh indus-

trial environments is challenging, due to the variety of module

applications and the requirements for electrical isolation, small

size, and low-power consumption. For each module, designers

often scramble to ﬁnd the right voltage regulator controller, the

right transformer, inductor, and discrete transistors to ﬁt their

application. In this article, we take a look at a new power meth-

odology that accelerates isolated power supply design, as high-

lighted by six readily available reference designs that support

applications from 2W to 40W.

The Need for Isolation

In general, isolation between a high-voltage input and a

low-voltage output is needed for safety-reasons. In oine ap-

plications where the input voltage is the powerline, isolation

Power Your Isolated System Eortlessly from 2W to 40W

Figure 1. 24V Programmable Logic Controller Input/Output Module

Active-Clamp Forward Converter Architecture

The forward converter (Figure 4) architecture is more complex

and more ecent than the ﬂyback converter, and is preferred

for higher current and higher power.

OUT

N:1

TL431A

OCD

OCT

FORWARD

LOAD

Figure 4. Active-Clamp Forward Block Diagram

In Figure 4, the current is continuously mantained on the sec-

ondary side. During the ‘ON’ time of the transistor T1, the cur-

rent on the secondary side circulates via the transistor T4, and

the inductor L1. During the ‘OFF’ time, the current is held via T3

and L1. Instead of diodes, the use of transistors T3 and T4 on

the secondary side enhances the eciency of this architecture.

An active-clamp reset via T2 and C1 also enhances eciency,

as the energy stored in the primary leakage inductance during

the ‘ON’ time of T1 is subsequently stored in C1 rather than be-

ing dissipated in a passive network. Here again, if external tran-

sistors are used, they need be chosen for low on-resistance and

low switching losses. The transformer should have low leakage

and the inductor should have minimal ohmic losses.

In all cases, the PCB layout should be done with great care to

avoid noise pickup and coupling of the traces, which can result

in parasitic oscillations.

currents inherent in its discontinuous operation relegate its use

to low-power applications. As power increases, the forward

converter becomes preferable, since the inductor following the

transformer provides a smoother secondary side current. Both

architectures are discussed in more detail in the following sec-

tions.

Flyback Converter Architecture

The ﬂyback converter (Figure 3) is a simple, accurate, and

cost-eective isolated architecture. During the ‘ON’ time of the

transistor T1, the voltage across the primary winding is positive

(equal to V

), and the voltage across the secondary winding is

negative. Consequently, the Shottky diode SD prevents the ener-

gy from passing to the output and is stored in the gapped trans-

former. The capacitor C

OUT

assures continuous feed to the output

load. During the ‘OFF’ time of T1, the primary winding inverts its

voltage, allowing the energy to be released to the output, feeding

the load and recharging C

OUT

. In this phase, the primary winding

is reset via the R1/C1/D1 passive network, while an optocoupler

provides the necessary isolated feedback to close the loop with

the primary side with good accuracy (±5%). To minimize power

loss, the system designer should select a Shottky diode with a

very low forward-drop voltage and a low-leakage gapped trans-

former. If the transistor T1 is not integrated, then it needs to be

selected for very low ‘ON’ resistance and low switching losses.

Figure 3. Flyback with Integrated Power Transistor

Figure 2. Digital I/O Module System Block Diagram

CONTROL

OUT

N:1

TL431A

OCD

OCT

FLYBACK IC

LOAD

DIGITAL INPUT MODULE

24V

24V FIELD BUS

PLC

SIGNAL

ISLOLATION

DRIVER

ACTUATOR

AC/DC

DIGITAL OUTPUT MODULE

CONTROL

CENTER

SENSOR

PLC

SIGNAL

ISLOLATION

TRANS-

LATOR

24V

DO 24V FIELD BUS

ISOLATION

ISOLATED

STEP-DOWN

ISOLATED

STEP-DOWN