ELEC5206 Chap.7 Buck-Boost and Flyback PV Interfaces
Buck-Boost and Flyback PV Interfaces
When the MPP voltage range and the battery or bus range overlap, neither a buck nor a boost can regulate in all conditions. The third PV side converter lecture introduces the buck-boost, which steps both ways at the cost of inverted output polarity and chopped currents at both ports, and ranks it as the last option.
The fourth lecture derives the flyback from the buck-boost by replacing the inductor with a coupled transformer of turns ratio one to n. The flyback suits a large voltage ratio, galvanic isolation and power below about 500 W, which is why it appears in microinverters, often interleaved to raise capacity. Both use the same inductor and capacitor equations; the flyback duty ratio gains a turns-ratio term.
Watch for the case-study slides, which print boost formulas beside correct buck-boost numbers. The chapter works a buck-boost charger across an overlapping range and a flyback front end for a module-level inverter, and compares the ideal gain curves of all four topologies against duty ratio so the choice of topology becomes a question of gain as well as direction.
What this chapter covers
- 01
Overlapping voltage windows in PV-battery systems
- 02
Buck-boost polarity and the plus or minus 20% battery swing
- 03
Buck-boost duty ratio, inductance and input capacitance
- 04
Why both converter ports need heavy filtering
- 05
From buck-boost to flyback: turns ratio and magnetising inductance
- 06
The flyback duty ratio with n
- 07
Isolation, the low-power limit and interleaving
- 08
Ranking the four PV side converter topologies
Worked example · free
Buck-boost for matched PV and battery voltages
- 1Duty ratio: D = 24/(24 + 24) = 0.5.
- 1Inductor: L = 24 × 0.5/(1 × 50,000) = 240 µH.
- 1Input capacitor: Cin = 6 × 0.5/(0.2 × 50,000) = 300 µF.
Key terms
- Buck-boost converter
- A converter that can step voltage up or down, with output polarity inverted relative to the input.
- Flyback converter
- An isolated converter derived from the buck-boost in which a coupled transformer stores and transfers energy.
- Turns ratio
- The ratio n of secondary to primary turns in a flyback transformer, which multiplies the voltage gain.
- Magnetising inductance
- The inductance embedded in a flyback transformer that plays the role of the buck-boost inductor.
- Interleaving
- Running several converters in parallel with shifted switching so that their combined capacity rises.
Buck-Boost and Flyback PV Interfaces FAQ
When should I choose a buck-boost for a PV interface?
Only when the MPP voltage range and the output range overlap so that neither a buck nor a boost can regulate in every condition, and when inverted polarity is acceptable. The lecture ranks it last because both its input and output currents are chopped, so both ports need heavy filtering.
Why do the buck-boost and flyback case studies give identical numbers?
The flyback case uses a turns ratio of ten into 360 V, so n times the MPP voltage is 370 V against 360 V, the same proportion as 37 V against 36 V in the buck-boost case. Since the flyback also uses the buck-boost inductor and capacitor equations, duty, inductance and capacitance match.
What limits the flyback to low power?
Design limits of the single-switch transformer topology keep it practical mainly below about 500 W, which suits module-level converters. Higher capacity is reached by interleaving several flybacks in parallel, the approach used in commercial microinverters.
Why does a larger turns ratio lower the flyback duty ratio?
The transformer multiplies the voltage gain by n, so for a fixed input and output voltage more of the step-up comes from the windings and less from the switching. In the duty formula, the output voltage divided by the output voltage plus n times the MPP voltage, a larger n enlarges the denominator, so the switch needs less on-time.
Exam move
Build a four-row comparison table, buck, boost, buck-boost and flyback, with the selection rule, duty formula, input-capacitor form and main drawback in each row, and rewrite it from memory until it is exact. Practise one buck-boost and one flyback design, and check the lecture case studies with the correct formulas so the slide slips cannot mislead you in the exam.
Remember that the flyback simply adds the turns ratio to the buck-boost model.
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