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Technology / Technical guide
Buck, Boost & DC/DC Conversion Concepts
A DC/DC stage changes the relationship between input and output voltage through switching and energy exchange. Buck and boost describe different basic topologies. Their familiar duty-ratio equations are ideal steady-state relationships, not instructions for setting an actual wind controller.
Use the voltage ratio only with its model conditions.
Inductor volt-second balance explains the ideal average ratio; it does not guarantee regulation, efficiency or unlimited gain.
01 / Duty ratio
Describe time allocation rather than an operating command.
For a conceptual switch with period Ts, duty ratio D is its on-time divided by Ts.
For a conceptual switch with period Ts, duty ratio D is its on-time divided by Ts. It is dimensionless. This definition says how a model partitions one cycle; it does not identify a practical device’s permitted settings.
The voltage ratio also depends on topology and conduction mode. The same D in two different circuits need not produce the same output. Input voltage, output voltage and the chosen switch’s state definition must remain explicit.
02 / Inductor balance
Explain why a steady waveform cannot climb forever.
For an ideal inductor, voltage determines the rate of current change.
For an ideal inductor, voltage determines the rate of current change. In periodic steady state the current returns to its starting value after one cycle, so the cycle integral of inductor voltage is zero.
Continuous conduction mode, or CCM, means the relevant inductor current does not fall to zero during the cycle in the stated basic model. A zero-current interval changes the balance structure and can change the simple voltage-ratio relationship.
03 / Buck relation
Follow the ideal step-down balance.
For an ideal basic buck in steady-state CCM, the inductor sees Vin − Vout during the conceptual on interval and −Vout during the other interval.
For an ideal basic buck in steady-state CCM, the inductor sees Vin − Vout during the conceptual on interval and −Vout during the other interval. Zero average inductor voltage gives Vout = D Vin.
TI’s topology explanation identifies the buck’s step-down role and CCM waveforms. The ideal result omits switch and diode drops, resistive losses and finite operating constraints. It is an average relationship rather than a claim of zero output ripple.
04 / Boost relation
Follow the ideal step-up balance.
For an ideal basic boost in steady-state CCM, the corresponding inductor voltages are Vin and Vin − Vout.
For an ideal basic boost in steady-state CCM, the corresponding inductor voltages are Vin and Vin − Vout. Their cycle balance gives Vout = Vin/(1 − D), for the defined ideal model with 0 < D < 1.
In a hypothetical normalised comparison at D = 0.25, the ideal buck ratio is 0.25 and the ideal boost ratio is about 1.33. These are mathematical examples, not suggested duty settings or MARSTON voltage ratios.
05 / Practical limits
Do not interpret an ideal denominator as unlimited voltage.
As D approaches one, the ideal boost expression grows, but the real circuit cannot supply arbitrary gain.
As D approaches one, the ideal boost expression grows, but the real circuit cannot supply arbitrary gain. Losses, current demand, allowable device conditions and finite time intervals prevent that interpretation. Output power must also be supplied at the input. In the separate ideal lossless average-power model, Vin Iin = Vout Iout. A higher output voltage therefore corresponds to a lower output current for the same transferred power. Voltage gain does not create additional energy.
Buck-boost names cover different circuit arrangements, including differing polarity and control roles. The name does not establish isolation, reverse-energy capability or a battery interface. An ideal voltage ratio is only one part of a complete conversion description.
06 / Scope boundary
Separate conversion physics from supported service.
A regulated converter additionally needs an implemented control relationship and an applicable operating envelope.
A regulated converter additionally needs an implemented control relationship and an applicable operating envelope. A basic ratio alone does not establish transient behaviour or stability; the control-principles guide explains another part of that picture.
No MARSTON buck, boost or buck-boost topology, duty limits or storage compatibility is confirmed here. The page provides theoretical balance equations without component selection, controller tuning or operating instructions.
Continue the review with PWM Switching Frequency, Carrier & Fundamental.
Continue the review with LC Filter Resonance: Natural Frequency and Damping.
Continue the review with DC Capacitor Ripple Current, ESR & Frequency Dependence.
General background: Texas Instruments: buck, boost and CCM topology principles. These references explain industry concepts; they are separate from the applicable MARSTON product and project documents.
Key concepts and conditions.
Read each quantity with its reference, units and stated conditions. A general relation does not establish a model-specific value.
| Concept / quantity | Meaning / relation | Conditions / limits |
|---|---|---|
| Duty ratio | D = ton/Ts | Defined conceptual switch and cycle. |
| Steady state | Net inductor volt-seconds are zero | Periodic current returns to its initial value. |
| Ideal buck | Vout/Vin = D | Basic topology, CCM and negligible losses. |
| Ideal boost | Vout/Vin = 1/(1 − D) | Basic topology, CCM and 0 < D < 1. |
| Actual output | Envelope, losses and implemented regulation | Not determined by the ideal ratio alone. |
Engineering questions.
Does D alone define a converter’s voltage?
No. Topology, input and conduction conditions also matter.
Does CCM mean perfectly constant current?
No. Current can ripple without reaching zero.
Can the ideal boost formula predict infinite practical voltage?
No. Real losses and operating constraints invalidate that extrapolation.
Are the D = 0.25 ratios suggested settings?
No. They are hypothetical normalised arithmetic.
Does buck-boost always mean the same output polarity?
No. Several arrangements use that broad family name.
Does a DC/DC concept establish battery compatibility?
No. Actual interfaces and supported functions are separate.
Technical enquiry
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A DC/DC stage changes the relationship between input and output voltage through switching and energy exchange. Share the model and applicable operating or design information for an engineering discussion.