150W Inverter - an optimal

1. 150W Inverter - an optimal design in solar home systems. Presenter: Dr Gawie van der Merwe

2. 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

3. QS OUTPUT 50HZ TRANSFORMER 50Hz Transformer Inverter BATTERY High frequency Inverter background • Inverter types • Square wave • Modified sinewave • Pure sinewave • General topologies

4. Inverter Specification 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

5. Protection Over load Short circuit Overheat Reverse polarity Input over voltage Input battery low voltage Inverter Functions • Performance • Inverter must be able to supply load of up to 3 times rated power • PV application, efficiency as high as possible • Weighted efficiency above 80% • No load consumption <100mA (1,2W)

6. Industrial inverters disseminated Research • Various loads analysed • Conditions of operation investigated • Production complexity and cost investigated

7. Power transfer topology 1) 50Hz Transformer Too ‘Heavy’ - for export potential Output voltage variation due to high battery voltage fluctuations Design • Advantages • Simple design • Robust • Disadvantages • Heavy • Expensive 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

8. DC OUTPUT DC OUTPUT BATTERY HF TRAFO BATTERY • Advantages • Advantages INDUCTOR • Short period overload is “unlimited” • 100% of core is used • Physical transformer size is smaller • Simple design • One switch on primary • Primary devices are short circuit proof • Reduction of silicon devices • Disadvantages • Disadvantages • More complex • Additional line inductor is required • Additional component count • 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 High frequency topology • Flyback or feed forward as input stage? SINGLE SWITCH FLYBACK CONVERTER FEED FORWARD CONVERTER Due to overload specification of the Feed Forward converter this was our choice.

9. Other design decisions FUSE DIODE CAP BATTERY • 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 • 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

10. Output voltage control - fixed or PWM Peak 230V RMS Varying pulse width Peak 230 250 275 300 • Tests done to compare, frequency harmonic content of varying DC peak voltage.

11. Decision • 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)

12. Output load protection DCLINK 265 V DC B A SECONDARY H BRIDGE C D E D 100UF CAPACITOR OUTPUT E D E D D E D E 3) CAPACITIVE 2) INDUCTIVE 4) RECTIFIED 1) SHORT OR RESISTIVE AND COMBINED CAPACITIVE TYPICAL LOADS • 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

13. Output load protection In Pulls Current Limitation 4 300 Voltage Current 3 200 Voltage Current 2 100 1 0 0 -100 200 s/div • 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

14. Overload protection • 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

15. Efficiency with input power 1 0.8 Efficiency 0.6 Efficiency 0.4 0.2 0 0 50 100 150 200 250 Input Power (Watt) Results • Comply to specifications such as overload, overheat, battery low. • Weighted efficiency = 90% • Pre-lim manufacturing cost = 0.21$/W

16. 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!