Wall Industries SMPS and Microcontrollers 1

Wall Industries SMPS and

Microcontrollers

Introduction

Wall Industries’ commitment to the

latest technologies is evident in its recent

announcement to add microcontrollers to

their already advanced designs. The

purpose of this white paper is to educate

Wall’s customers of the uses and

capabilities on modern microcontrollers

when used in power electronics

products. We will start by describing

what exactly a microcontroller is; what

some of its capabilities are; site some

examples of microcontroller uses in

power electronics; and, finally, the

functions of the microcontroller. Wall

Industries’ TP Wide Input Range

Isolated Converter will be examined as

an example.

Microcontrollers

Microcontrollers are programmable

devices that can perform digital

processing and replace analog circuitry.

Unlike PC microprocessors, micro-

controllers are mostly self-contained in

that they require no external parts such

as memory or oscillators. Their

relatively low-cost and numerous

internal peripherals make them ideal to

replace analog circuitry in power

supplies. Furthermore, once the

microcontroller is embedded in the

design, new capabilities and flexibilities

can be realized. Also, due to continuous

performance improvements, some

microcontrollers now can be used in

applications that once required high-end

digital signal processors (DSP’s).

Practical Uses in Power

Supplies

There are many practical uses of

microcontrollers in power supplies. The

uses range from mimicking existing

analog circuitry to adding never -before-

available features to the power supply.

The simplest use is in replacing

supervisory control circuits. For

example, the micro can control the

behavior of the power supply during

start-up - including when to begin

synchronous rectification to prevent

back-bias glitches. The micro can

monitor the enable pin and react

appropriately particularly when the pin

is used after fault conditions or during

start-up. Handling these corner

conditions can add costs to a power

supply without a microcontroller. With

respect to fault conditions,

microcontrollers are particularly agile at

handling the various protection

requirements of a modern power

supplies (e.g. resettable latch, limited

hiccup, combination faults).

Microcontrollers can also get involved in

the feedback control mechanisms of the

power supply. While it would require a

very fast microcontroller for a typical

converter to have complete digital

control, there are many hybrid options

that make much more sense. For

example, the analog PWM can be

digitally controlled inside the

microcontroller. This would allow the

PWM to be easily shut down when

required, its frequency be digitally

Wall Industries SMPS and Microcontrollers 2

controlled (even modulated for noise

reduction pusposes), or give fault

information directly to the controller

firmware. Other alternates to complete

digital control include control element

tuning and hybrid control where the

slower outer voltage loop is digital but

the fast inner current loop is still analog.

While microcontrollers can replace

existing analog circuitry, they can also

add some functions to a power supply

that have normally been reserved for

higher end systems due to cost

considerations. Now with powerful in-

expensive microcontrollers available,

even the simplest converters can get

advanced digital capabilities like

communication and logging. These

improvements are allowing computer

systems to communicate with its power

systems and gather information and

control behavior. It is even possible now

to log into a power supply to check its

statistics.

Indirect Advantages

The advantage of adding microcon-

trollers to our products goes beyond the

new capabilities that microcontrollers

will now facilitate. While new features

are important, solving issues associated

with traditional design methods are

critical. Adding microcontrollers can

have several indirect advantages

including cost, reliability, and flexibility.

Direct costs can be reduced because

many analog components such as

window detectors, comparators and

latches can be replaced with a single

microcontroller. Integrating many com-

ponents into one increases reliability, but

also saves board space. This extra board

space can be used to save costs by

producing a smaller product. It can also

be used to improve the performance of

the power supply by increasing current

carrying copper or the size of the

magnetics.

Since the software is flexible, a single

design can accommodate several part

types saving money. For example, one

hardware design can handle positive or

negative enable logic, latching or hiccup

behavior. Custom behavior can be added

with no added manufacturing costs.

Having a programmable micro also

allows us to quickly meet changing

customer requirements. Before using

microcontrollers, a small change in a

timing specification or recovery

behavior would require a change in the

bill of materials, PCB change or even a

new design. Now many of the changes

can be handled with only a firmware

update. These changes are quicker,

cheaper and can be performed on

existing hardware either in the factory or

sometimes in the customer’s box.

Types and C apabilities

While there are many semiconductor

manufacturers of microcontrollers (e.g.

Atmel, Cypress Microsystems, Freescale

Semiconductors, Texas Instruments), in

the interest of brevity, we will focus now

only on Microchip’s PIC micro-

controller line

(www.microchip.com). The

extensive PIC line microcontrollers can

be roughly divided into three groups:

basic, advanced and high-end. As the

microcontroller capabilities increase and

cost decreases each year, what functions

are included in each of these categories

changes. The following is a current

snapshot.