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

2. 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

3. Copyright DR Gawie van der Merwe www.planmypower.co.zaa Inverter background Inverter types –Square wave –Modified sinewave –Pure sinewave General topologies 50Hz Transformer Inverter High frequency BATTERY QS OUTPUT 50HZ TRANSFORMER

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

5. Copyright DR Gawie van der Merwe www.planmypower.co.zaa Inverter Functions Protection –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%

6. Copyright DR Gawie van der Merwe www.planmypower.co.zaa Research Industrial inverters disseminated Various loads analysed Conditions of operation investigated Production complexity and cost investigated

7. Copyright DR Gawie van der Merwe www.planmypower.co.zaa –Disadvantages Heavy Expensive Design Power 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

8. Copyright DR Gawie van der Merwe www.planmypower.co.zaa SINGLE SWITCH FLYBACK CONVERTER High frequency topology Flyback or feed forward as input stage? FEED FORWARD CONVERTER 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.

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

10. Copyright DR Gawie van der Merwe www.planmypower.co.zaa Tests done to compare, frequency harmonic content of varying DC peak voltage. Peak 230 300 250 275 Peak 230V RMS Varying pulse width Output voltage control - fixed or PWM

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

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

13. Copyright DR Gawie van der Merwe www.planmypower.co.zaa –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 Voltage Current Output load protection Voltage In Pulls Current Limitation -100 0 100 200 300 0 1 2 3 4 Current 200 s/div

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

15. Copyright DR Gawie van der Merwe www.planmypower.co.zaa Results Comply to specifications such as overload, overheat, battery low. Weighted efficiency = 90% Pre-lim manufacturing cost = 0.21$/W 050100150200250 0 0.2 0.4 0.6 0.8 1 Input Power (Watt) Efficiency Efficiency with input power Efficiency

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