Uninterruptible Power Supply Reference Design--30450c.pdf

PICREF-1

Uninterruptible Power Supply Reference Design

INTRODUCTION

PICREF-1 OVERVIEW

At times, power from a wall socket is neither clean nor

uninterruptible. Many abnormalities such as blackouts,

brownouts, spikes, surges, and noise can occur. Under

the best conditions, power interruptions can be an

inconvenience. At their worst, they can cause loss of

data in computer systems or damage to electronic

equipment.

It is the function of an Uninterruptible Power Supply

(UPS) to act as a buffer and provide clean, reliable

power to vulnerable electronic equipment. The basic

concept of a UPS is to store energy during normal

operation (through battery charging) and release

energy (through DC to AC conversion) during a power

failure.

UPS systems are traditionally designed using analog

components. Today these systems can integrate a

microcontroller with AC sine wave generation, offering

the many beneﬁts listed below.

The Microchip Technology PICREF-1 UPS Reference

Design offers a ready-made uninterruptible power sup-

ply solution with the ﬂexibility of a microcontroller.

The PIC17C43 microcontroller handles all the control

of the UPS system. The PIC17C43 is unique because

it provides a high performance and low cost solution not

found in other microcontrollers.

The PIC17C43 PWM controls an inverter whose out-

put, when ﬁltered, results in a sinusoidal AC output

waveform. Fault signaling can be initiated internal or

external to the PIC17C43 depending on the type of

fault. A fault will disable the entire inverter. The output

voltage and current will be monitored by the PIC17C43

to make adjustments “real-time” to correct for DC offset

and load changes.

The PIC17C43 controls all module synchronization as

well as inverter control and feedback. The PIC17C43

uses zero crossing for synchronization of input voltage/

phase to output voltage/phase. All internal module syn-

chronization is handled by the PIC17C43.

The control algorithms and software are written in C for

maintainability and transportability.

PIC17C43 Microcontroller Beneﬁts

• High Quality Sine Wave - High throughput allows

for high quality output

• Flexibility - core control features and operations

can be changed with software modiﬁcations only

• Transportability of Design

• Variable Loop Response

• Digital Filtering

• Parts and Complexity Reduction

• Peripheral Integration

• Ease of Interfacing

• Testability

• Time to Market

PICREF-1 Key Features

• True UPS Topology

• True Sinusoidal Output

• Point-to-Point Output Correction

• 1400 VA Rating

• 120/240 V Input

Information contained in this publication is intended through suggestion only and may be superseded by updates. No

representation or warranty is given and no liability is assumed by Microchip Technology Inc. with respect to the accu-

racy or use of such information, or infringement of patents arising from such use or otherwise. It is the responsibility

of each user to ensure that each UPS is adequately designed, safe, and compatible with all conditions encountered

during its use. “Typical” parameters can and do vary in different applications. All operating parameters, including

“Typicals”, must be validated for each customer application by the customer's technical experts. Use of Microchip's

products as critical components in life support systems is not authorized except with express written approval by

Microchip. No licenses are conveyed, implicitly or otherwise, under any intellectual property rights.

1997 Microchip Technology Inc.

DS30450C-page 1

PICREF-1

TABLE OF CONTENTS

System Overview .......................................................................................................................................3

Hardware Overview ...................................................................................................................................6

Software Overview .....................................................................................................................................9

Test Results .............................................................................................................................................13

Design Background .................................................................................................................................15

Design Modifications ...............................................................................................................................17

Appendix A: System Specifications .......................................................................................................19

Appendix B: Schematics..........................................................................................................................19

Appendix C: Software Listing..................................................................................................................37

Appendix D: PCB Layout & Fab Drawing ...............................................................................................41

Appendix E: Bill Of Materials (BOM) .......................................................................................................45

Appendix F: UPS Demo Unit....................................................................................................................53

ACKNOWLEDGMENTS

Project Lead Engineer:

Robert Schreiber, Microchip Technology

Reference Design Documentation:

Beth McLoughlin, Microchip Technology

System and Code Development:

Airborne Power (Consultants)

Guy Gazia (guyg@airbornepower.com),

David Karipides (davek@airbornepower.com),

Terry Allinder

DS30450C-page 2

1997 Microchip Technology Inc.

PICREF-1

SYSTEM OVERVIEW

The OR’d DC bus voltage is fed into the free running

chopper which both isolates the DC bus from the

H-Bridge inverter and doubles the DC voltage for the

inverter to operate at 120 or 240 volts. The output of the

chopper is ﬁltered to remove switching noise and then

fed into the H-Bridge.

The PIC17C43 microcontroller controls the inverter by

driving the H-bridge through Hardware Protection cir-

cuitry and Insulated Gated Bipolar Transistor (IGBT)

drivers. The output of the H-bridge is ﬁltered and drives

the load with an AC sine wave that is synchronized to

the input AC voltage.

An A/D converter provides feedback to the PIC17C43

for output monitoring.

All module synchronization, control, and fault detection

are handled through the PIC17C43.

The power ﬂow for the PICREF-1 system is shown in

Figure 1 . The Uninterruptible Power Supply (UPS) is

either supplying power based on the input power, if the

unit is plugged in, or based on the batteries.

Power Flow

When available, the input power is ﬁltered for common

mode noise and is protected from surges/spikes by

input power protection circuitry. The power then goes

into the power factor correction (PFC) module which

forces the input current to be sinusoidal so that power

utilization is more efﬁcient. The PFC module also recti-

ﬁes the input AC power to produce voltage-regulated

DC power which is used by the rest of the functional

modules.

This rectiﬁed AC power is OR’d through diodes with the

DC voltage generated from the battery boost circuit.

The battery boost voltage is set slightly lower than the

rectiﬁed AC input voltage so that, under normal condi-

tions, the rectiﬁed AC input power provides power to

the load. Once the voltage from the rectiﬁed AC input

source drops below the battery boost DC voltage, the

power is drawn from the battery boost module. In this

mode the battery charger is turned off so as not to

cause an additional load on the battery (i.e., so the bat-

tery is not charging itself).

FIGURE 1:

PICREF-1 UNINTERRUPTIBLE POWER SUPPLY (UPS) BLOCK DIAGRAM

Power Flow

Current Sense

Charge Control

A/D

Voltage Sense

BBC

Sync

Battery &

Battery

Charger

Battery

Boost

Circuit

Enable

PWM

Hardware

Protection

PIC17C43

Pos_Neg

Fault

Zero

Crossing

Inverter

Fault

Inverter

Control

PFC

Sync

IGBT

Drivers

Input Power

120/240 V

50/60 Hz

Output Power

120/240 V

50/60 Hz

Chopper

Sync

Power

Factor

Correction &

Rectiﬁcation

Input

Power

Protection

Free

Running

Chopper

Output

Filtering

H-Bridge

(Inverter)

1997 Microchip Technology Inc.

DS30450C-page 3

PICREF-1

Inverter Operation

As long as none of the out-of-saturation signals

(Q9/Q10/Q11/Q12 OC Alarm) are LOW, the power

stage can be enabled. When the PIC17C43 is ﬁrst pow-

ered up, the ENABLE line (PORTC, bit0) will be in a

high impedance state. A pull-down resistor keeps the

ENABLE line held LOW so that any spurious signals

which may be generated while the system is initializing

will not drive the H-Bridge. If any of the out-of-satura-

tion signals go LOW, the FAULT signal goes HIGH,

reporting to the PIC17C43 that an external fault

occurred. This will disable the H-Bridge.

The inverter may be re-enabled by cycling the ENABLE

line LOW and then HIGH to reset the ﬂip-ﬂop and allow

the PIC17C43 to drive the H-Bridge again.

The PIC17C43 microcontroller and hardware protec-

tion circuits are found on the PICREF-1 Inverter Control

Card. IGBT driver circuits are found on the Inverter

Drive Card. Schematics for these cards can be found in

Appendix B .

The H-bridge circuit works by generating the separate

positive and negative cycles needed for sine wave gen-

eration. The PIC17C43 controls all signals to the hard-

ware protection circuitry and IGBT drivers and thus

controls the generation of the sine wave ( Figure 2 ).

Software Fault / No Enable

Driving the FAULT HIGH will disable the inverter’s

power stage.

Hardware Fault

The hardware protection logic automatically disables

the inverter’s power stage in the event any of the IGBT’s

have gone out of saturation, i.e., an external short was

placed on the H-Bridge which was so severe that an

appreciable voltage was developed across one of the

switches that was on. This feature prevents a short

from immediately destroying the switching devices.

FIGURE 2:

INVERTER OPERATION

Q9 Drive

Q9_G

Q10 Drive

Q10_G

ENABLE

Q11 Drive

Q11_G

PWM

Q12 Drive

PIC17C43

Hardware

Protection

IGBT

Drivers

Q12_G

POS_NEG

Q9 OC Alarm

Q9_C

FAULT

Q10 OC Alarm

Q10_C

Q11 OC Alarm

Q11_C

Q12 OC Alarm

Q12_C

DCBus

Simpliﬁed

H-Bridge

(Inverter)

Q10_C

Q9_C

Q10_G

Q9_G

Q10

AC Output

AC HI

Output

Filtering

AC LO

Q12_C

Q11_C

Q11

Q12_G

Q12

Q11_G

DS30450C-page 4

1997 Microchip Technology Inc.

PICREF-1

Normal Operation

The transistor drive signals are fed into the IGBT Drive

circuits (Inverter Drive Card) to determine the state (ON

or OFF) of each transistor. A drive signal of ‘

In normal operation (FAULT is LOW and ENABLE is

HIGH), the states of Q9, Q10, Q11 and Q12 are deter-

mined by POS_NEG and PWM signals. These signals

pass through steering logic which produce transistor

(QX) drive signals (see Inverter Control Card schemat-

ics in Appendix B ).

The steering logic causes the IGBT pairs Q9,Q11 and

Q10,Q12 to be held in the OFF state for one microsec-

ond before allowing them to switch ON. This prevents

shoot-through from occurring during changes of states

from the PWM. This is necessary because of the rela-

tively slow turn off times for the IGBTs. This prevents

complementary pairs from being ON at the same time.

Table 1 describes the drive values for different input val-

ues of POS_NEG and PWM.

’ corre-

sponds to an IGBT being OFF, and a drive signal of ‘

’

corresponds to an IGBT being ON.

From Table 1 , when the POS_NEG signal is

, Q10 is

held ON, Q12 is held OFF, and Q9,Q11 are modulated

in a complementary fashion with the PWM signal. Sim-

ilarly, when POS_NEG is

, Q9 is held ON, Q11 is held

OFF, and Q10,Q12 are modulated in a complementary

fashion with the PWM signal. Therefore, the differential

output signal from the H-Bridge has an average value

proportional to the duty cycle of the PWM signal and a

polarity set by the POS_NEG signal. The output ﬁlter

smooths this pulse train and all that remains is the aver-

age value of the PWM signal ( Figure 3 ).

To understand how the PIC17C43 determines the mod-

ulation for the H-Bridge transistors, please see the sec-

tion

TABLE 1

INVERTER CONTROL SIGNALS

Software Overview

Drive

Q10

Drive

Q11

Drive

Q12

Drive

POS_NEG

PWM

QX Drive =

-> transistor QX is OFF

QX Drive =

-> transistor QX is ON

FIGURE 3:

INVERTER WAVEFORMS

AC Output

AC HI/LO

POS_NEG

PWM

1997 Microchip Technology Inc.

DS30450C-page 5

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