150W Inverter - an optimal Presenter: Dr Gawie van der Merwe design in solar home systems

150W Inverter - an optimal

Presenter: Dr Gawie van der Merwe

design in solar home

systems.

Copyright DR Gawie van der Merwe www.planmypower.co.zaa

Background

Complete “Solar Home” kits are being presented as an option to power remote areas

Kit typically consists of– 1 or 2 solar panels– Small PV regulator– Suited DC or AC Lights– Small 50Hz inverter– Battery and suitable enclosure

Cost and reliability of some of above components are problems

Investigation was done on the optimal design of an inverter to comply with solar home requirements


Inverter backgroundInverter types

– Square wave– Modified sinewave– Pure sinewave

General topologies

SAWTOOTH

HF TRAFO

INDUCTOR

BATTERY

ERROR AMP

FEEDBACKDRIVE CIRCUITAND CONTROLLATCH

PRIMARY SIDE DEVICESCONTROLLED TO OFFERA FIXED DC LINK VOLTAGE

MODIFIED SINEWAVE OUTPUT

REF

50Hz Transformer

Inverter

High frequency

BATTERY

QS OUTPUT

50HZ TRANSFORMER


Input : 12V DC

Output : 230V AC

50Hz

Efficiency: >90%+

Rating : 150W

Waveform: Ideal for the

typical

load such as

TV, VCR

& 11W

Fluorescent

light

Cost : As low as

possible

MTBF : >10 year

Inverter Specification


Inverter FunctionsProtection

– Over load– Short circuit– Overheat– Reverse polarity– Input over voltage– Input battery low voltage

– PV application, efficiency as high as possible

No load consumption <100mA (1,2W)

Performance– Inverter must be able to supply

load of up to 3 times rated power

Weighted efficiency above 80%


Research

Industrial inverters

disseminated

Various loads analysed

Conditions of operation

investigated

Production complexity and

cost investigated


– Disadvantages• Heavy

• Expensive

DesignPower transfer topology

1) 50Hz Transformer• Too ‘Heavy’ - for export potential

• Output voltage variation due to high battery voltage fluctuations

– Advantages• Simple design

• Robust

2) High frequency topology- Advantages

• Lightweight, export• Small• Good design can be cost competitive• Manufactured in Eastern world

- Disadvantages• Design more complex• EMI Interference higher


SINGLE SWITCH FLYBACK CONVERTER

High frequency topology Flyback or feed forward as input stage?

FEED FORWARD CONVERTER

HF TRAFO

INDUCTOR

BATTERY

DC OUTPUT

DC OUTPUT

BATTERY

- Simple design- One switch on primary- Primary devices are short circuit proof- Reduction of silicon devices

Advantages

Disadvantages- Linear overload not possible. Power transfer is limited

to transformer ratio- 50% of ferrite is used- Power is transferred from primary at max 50% duty cycle

Advantages- Short period overload is “unlimited”- 100% of core is used- Physical transformer size is smaller

Disadvantages- More complex- Additional line inductor is required- Additional component count

Due to overload specification of the Feed

Forward converter this was our choice.


Input over voltage protection– All components to input stage is rated 35V– Over voltage shutdown 16V

Reverse polarity– Options:

• Mechanical relay• Series diode - or contactor• Fuse with diode - chosen

– Advantages• Fuse is required in any way, no consumption during working assist with

efficiency

– Disadvantages• Fuse serviceable item

Frequency stability– Crystal controlled for lifetime accuracy

FUSE

DIODE

BATTERY

CAP

Battery low cutout– Required for battery protection

– Must allow short period voltage dip

– Allow battery voltage drop down to 8.8V for up to 10 seconds

– An “additional” supply must be created to allow for above

Other design decisions


Tests done to compare, frequency harmonic content of varying DC peak voltage.

Peak 230 300250 275

Peak

230V RMS

Varying pulse width

Output voltage control - fixed or PWM


Fixed duty cycle

Fixed DC link voltage

V DC = 265V - 60-70% duty cycle found to be best under general conditions.

(refer to other load tests)

Decision


SECONDARYH BRIDGE

OUTPUT

A B

C D

D E

TYPICAL LOADS

2) INDUCTIVEOR RESISTIVE1) SHORT

3) CAPACITIVE4) RECTIFIEDAND COMBINEDCAPACITIVE

D D DE E E

D E

DCLINK 265 V DC

100UF CAPACITOR

Typical load– Consist of input rectifier capacitor and the load– Examples

• Computer• VCR, TV (up to a point)• Fluorescent lights (Electronic)• Stepdown transform for radio’s etc.

– Problems• As a general rule the input capacitor, is rated

for a current ripple, caused by sinewave supply

• General - lifetime reduction occurs when used with QS applications

Output load protection


– Examples - 11W fluorescent lights - 8000 hours expected lifetime reduced to = 3000 hours - etc.

Suggestion - Reduce current peak during, high dV/dt transients

Do “in pulls” current fold back

VoltageCurrent

Output load protectionV

olt

age

In Pulls Current Limitation

-100

0

100

200

300

0

1

2

3

4

Cu

rren

t

200 s/div


Rating 150W

– Non linear loads or loads with a low Power Factor not very “measurable”

– 3 methods of overload protection

• AC output current measure

• AC current peak measure

• Primary side DC current measurement - Input power

Overload protection


Results

Comply to specifications such as overload, overheat, battery low.

Weighted efficiency = 90%

Pre-lim manufacturing cost = 0.21$/W

0 50 100 150 200 2500

0.2

0.4

0.6

0.8

1

Input Power (Watt)

Eff

icie

ncy

Efficiency with input power

Efficiency


Conclusion & further work

A well designed inverter, with

all requirement

First production stage

undergone

Load life time test still being

done with items such as small

fluorescent light.

Inverter lifetime test still to be

done!

Documents

150W Inverter - an optimal Presenter: Dr Gawie van der Merwe design in solar home systems