Three prototypes is the working number on a charger programme, and a brief that asks for one usually receives a unit that satisfies nobody. Twelve units are built at the first stage, three at the second and thirty at the third, and the quantity is set by what has to be measured rather than by what has to be shown.
Charger prototype development: the three stages
Bench, form factor and pre-production.
Each stage answers a different question, and each one produces a unit that is deliberately not the final product. A programme that treats the first stage as a sample for approval spends the second stage explaining why the enclosure changed.
Three units, not one
The stages exist because the questions cannot be answered together.
A bench unit establishes whether the circuit works; a form-factor unit establishes whether it works inside the enclosure; a pre-production unit establishes whether the line can build it the same way twice. Combining them means the thermal measurement is taken on a unit whose mechanics have not been settled.
Stage one: the bench prototype
It answers whether the topology delivers the output.
An open assembly on the intended controller, measured across the output range with a thermal image and an efficiency curve. The enclosure is not part of this stage, which is what makes the measurement clean and the unit unusable as a sample.
Stage two: the form-factor prototype
It answers whether the design survives its own enclosure.
The same circuit in the intended mechanics, measured again at the same points. The temperature rises, the efficiency falls slightly and the derating on the magnetics becomes a real number rather than a calculation. This is where the enclosure earns its second review.
Stage three: the pre-production prototype
It answers whether the line repeats the result.
Units built on production tooling with production fixtures, and the yield, the cycle time and the test failures recorded. The units are then available for the compliance submission and for the buyer’s approval, which makes this stage the one that produces the sample.
| Stage | Question answered | Typical quantity | What it produces |
|---|---|---|---|
| Bench | Does the topology deliver | Five to twelve | Measured output and thermal data |
| Form factor | Does it survive the enclosure | Three to five | Thermal data in the shipping form |
| Pre-production | Does the line repeat it | Twenty to fifty | Submission units and approval samples |
What to measure at each stage
Four measurements, repeated at every stage.
The output across the range, the efficiency at the rated load, the temperature at the hottest component and the behaviour at the extremes of input voltage. Repeating the same four at each stage is what turns three prototypes into one dataset rather than three opinions.
Where prototypes go wrong
Three patterns, and the third wastes the most time.
A bench unit measured without a thermal image; a form-factor unit built in a prototype enclosure that does not represent the moulded part; and a pre-production stage run on hand-built units. Each produces a number that cannot be compared with the stage before it, and a comparison across stages is only valid when the configuration is described as well as the result.
What to keep from each stage
Three records, and each is used later.
The measurement data with the configuration it was taken on, one unit retained from each stage, and the list of changes made between them. The retained units are what a later comparison uses when a unit from the second production run behaves differently.
Where the prototype data goes afterwards
The measurements taken at each stage are the evidence behind the specification, and they are worth keeping with the revision they describe. A file holding a temperature figure without the enclosure revision it was measured in answers a question that may no longer apply.
The retained units serve the same purpose. When a unit from a later production run behaves differently, the comparison is made against the retained prototype rather than against a description of it, and the difference is measured rather than argued. Keep the comparison sheet with the batch record. File it with the retained unit.
The third use is the one buyers discover late: a second market or a second channel usually asks for evidence that the design was characterised before tooling. The three measurement sets are that evidence. A file assembled after the request usually takes longer than the test cycle it describes, because the units and the records travel together.
FAQ
Can the bench stage be skipped?
It can where the platform already exists and its behaviour is documented. On a new topology the bench measurement is what stops an enclosure being designed around a circuit that does not meet the specification. Where the platform already exists, the bench stage reduces to confirming the documented behaviour on the new load points rather than repeating the full characterisation.
Why does the enclosure change the thermal result?
Because it removes the free air the bench unit was measured in and adds a path for heat into the plastic. The rise is normal, and the enclosure is judged on the second measurement rather than the first. Compare the two measurements taken at the same load and the same ambient, because a change in either turns the difference between them into a measurement error rather than a thermal result.
How many units does the buyer need to approve?
Enough to inspect, test to destruction and retain. Three units cover inspection and the retained reference; a submission to a laboratory needs more, which is why the pre-production stage is sized to include them. Ask for the test records for each unit as well, because a submission built on three units without records describes a build rather than a batch.
Do the prototypes have to be built on production tooling?
Only the third stage. The first two are built by hand or on soft tooling, and the pre-production stage is where the production tool and the production fixture are used. The second stage is where soft tooling earns its cost, because a unit built in a machined enclosure behaves differently from one in a hand-formed shell. The second stage is also where the enclosure material is confirmed, because the measured result depends on it.
Next step
Send the concept, the output target and the market list to Wecent Group through the contact page. The reply sets out the three stages, the quantities and the measurements each one produces. Related reading: OEM and ODM charger manufacturing, charger manufacturing services and charger quality control. External references: IEC standards, USB Implementers Forum charger specification, BSI standards, EU CE marking, EU energy efficient products and Wireless Power Consortium.
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