Technical Articles & Engineering Insights
OBC and DC-DC 3D Models and Drawings: What to Request First
The CAD package to request from an OBC or DC-DC supplier, the checks to run once the model is in your assembly, and how to compare integrated and separate units by measurement rather than opinion.
On a space-constrained platform, the choice between an integrated power unit and separate OBC and DC-DC units is settled on paper long before a sample is ordered. The datasheet gives a weight, an outline and a mounting sketch - enough to shortlist, not enough to commit. The number that decides the program is not the size of the box. It is the size of the box plus everything that must exist around it.
Why a Dimension Table Is Not an Envelope
A datasheet dimension describes the housing. The space a charger actually consumes includes:
• Connector shells and their mating plugs. A receptacle protruding 20 mm needs 20 mm, plus the plug body, plus the latch, plus room for a hand to reach the latch.
• Cable bend radius. For fixed power cable routing, four times the outer diameter is a common minimum; a cable that flexes needs six or more. A 10 mm cable leaving perpendicular to a bulkhead can need 40-60 mm of clear space that appears in no dimension.
• Mounting feet and fastener access. The bracket is part of the envelope, and so is the socket and extension that tighten its bolts.
• Cooling intake and exhaust. An air-cooled unit needs unobstructed inlet area, and an exhaust path that leads somewhere other than into the battery pack.
• Service path. If the harness cannot be disconnected without dropping the unit, a 4.3 kg component has become a 4.3 kg component plus a lift, a stand and a second pair of hands.
None of that appears in a table with three numbers in it. And there is one further item that is not a volume at all yet decides the enclosure just as firmly: orientation against splash. A connector facing upward is a cup. The unit fits, it passes every electrical test, it occupies exactly the volume the drawing promised - and it collects water for eight years. The same unit rotated 90 degrees is fine, at identical cost and identical schedule. The only variable is whether somebody checked the mounting direction in three dimensions before the bracket was cut.
The File Package to Request
Ask for this set explicitly. Suppliers who have supported vehicle programs have it on hand; the request itself is a useful filter.
Deliverable | Format | Stage | What it settles |
Envelope model | STEP AP214/AP242, solid | Concept | Whether the unit enters the bay |
Full detail model | STEP, solid, not surface | Prototype | Interference, harness routing, bracket design |
Outline drawing | PDF, DWG on request | Prototype | Mounting pattern, datums, tolerances, connector positions |
Pin definition table | PDF or XLSX | Prototype | Harness build and electrical integration |
CAN matrix or DBC | DBC or documented matrix | Prototype | Communication integration |
Weight and centre of gravity | Drawing note or sheet | Concept | Bracket loads, vibration analysis |
Thermal data | Heat rejection at rated and derated load, airflow requirement | Concept | Duct sizing, recirculation risk |
Revision-to-date-code map | Document | Series | Tracing a field unit back to its geometry |
Two points on this list. Ask for a solid model, not a surface model: surface geometry imports but cannot produce clearance volumes or mass properties, and it fails at the moment you need it. And ask for the envelope model separately from the detail model - an envelope with the internals stripped is often releasable under a lighter agreement, and it is enough to answer the packaging question.
Six Checks to Run in Your Own Assembly
Load the model into the vehicle assembly and work through these in order.
• Plug insertion and removal travel. Sweep the plug body and latch through the full insertion and release motion, not just the receptacle position. A connector that fits but cannot be unplugged in situ is a service problem disguised as a packaging success.
• Cable bend radius and harness route. Model the harness as a swept solid at minimum bend radius. This is the most common source of "it fit on the bench" surprises.
• Tool clearance on fasteners. Add the socket and ratchet extension to the assembly, not only the bolt head.
• Service access. Can a technician disconnect HV and CAN with the unit in place, with the vehicle on the ground rather than on a lift?
• Air path. Check inlet free area against the supplier's stated airflow requirement, and trace where the exhaust goes. Recirculation inside a closed compartment looks fine in a static model and fails in a thermal chamber.
• HV clearance and creepage. These are two different measurements: clearance is the shortest path through air, creepage is the shortest path across an insulating surface. Check both from the HV terminals to adjacent metal, to the battery enclosure, and to any surface that sees spray or condensation. On light vehicle platforms, note where the DC threshold sits - many HV safety frameworks place it at 60 V DC, so a 48 V platform sits below the HV rule set while 72 V and 96 V platforms cross it. That changes the clearance you are required to hold, and it changes which of your two architecture options is easier to validate.
Then check the mounting load path: weight, centre of gravity and the vibration profile of the mounting location combined. A 6.5 kg unit cantilevered off a thin panel behaves differently from the same unit on a cross-member.
Units, Datum, Revision
These three cause more delay than any geometry problem, because they produce a model that looks correct.
• Units. Millimetres against inches announces itself at 25.4x scale - unless someone already scaled the import to "look right" without touching the annotations. Then it is invisible and wrong.
• Datum. A model with its origin at a corner, dimensioned from a centre line, agrees with no drawing ever produced. Confirm the datum before you dimension a bracket from it, not after the bracket reaches the fabricator.
• Revision. Model, drawing and datasheet at three different revision letters is normal in a supply chain. Know which one you designed around, and put the revision letter in the filename rather than in the email subject.
Comparing an Integrated Unit Against Two Separate Units
Architecture comparisons in prose tend to list the same trade-offs: fewer components against easier service, less wiring against more thermal coupling. All true, and none of it decides a specific vehicle. What a bay-constrained platform needs is a measured comparison inside its own geometry.
Take a power-matched set, so that only the architecture differs. On a 96 V platform, that means a 3.3 kW OBC (PO-AA-3D-96) plus a 1 kW DC-DC converter (PD-BC-1A-96Y) on one side, and the 3.3 kW OBC + 1 kW DC-DC unit on the same 96 V platform on the other. Same charging power, same auxiliary power, same battery voltage - so every difference you measure belongs to the architecture.
Build both as assemblies, then fill in this worksheet:
Measure | Record as | Why it decides the architecture |
Enclosed volume, connectors and plugs included | litres, read from the CAD solid | The number the bay was sized from is usually the housing alone |
Mounting points and bracket volume | count, plus bracket volume in litres | Two units mean two brackets and two sets of fastener access |
HV and LV connector count | count per option | Every connector is a harness end, a crimp and a sealing test |
Harness length and number of runs | metres, and number of separate runs | Separate units often cannot share a run |
Cooling duct length and free inlet area | mm of duct, cm² of free area | Two heat sources in one bay need either one duct or two |
Assembly time on the line | minutes | Every extra mount is takt time |
Time to replace one failed function | hours, including access | The row most often skipped - see below |
The last row is the one that gets skipped, and on a two-in-one unit it is the interesting one: a failed auxiliary converter takes charging down with it, because they share a housing and usually a control board. Whether that matters is not a component question - it depends on how your fleet handles a vehicle that cannot charge for a week.
For the qualitative side - thermal coupling, CAN architecture, platform flexibility - see OBC + DC-DC 2-in-1 vs separate units. Use that article to frame the question and this worksheet to answer it.
Higher power changes the numbers but not the method: the 6.6 kW standalone charger compares against the 6.6 kW OBC + 1.5 kW DC-DC unit on the same worksheet.
When the Supplier Will Not Share Native CAD
Some refusals are legitimate. Ask for the next best thing:
• a simplified envelope solid, internals removed, external interfaces intact;
• a 2D drawing with critical dimensions, connector positions and the mounting pattern;
• a defined keep-out volume the unit will not exceed in any build.
Any of these places a component in an assembly. A supplier who will not provide even a keep-out box is telling you how the rest of the program will go.
Delivering Files That Are Too Large to Email
A full detail model with internal structure runs to tens of megabytes - past most corporate mail limits, and often filtered outright. A file that was "sent three times" has usually never arrived at all, and the delay is spent on the wrong side of the exchange.
What works: a cloud link with an expiry date, the file name and size stated in the email body, one archive per request with the revision letter in its name, and a reply that confirms the revision received. "I received it" and "I received revision C" are different pieces of information, and only the second one protects you.
Frequently Asked Questions
Do I need native CAD, or is a STEP file enough?
A STEP file is enough for packaging, interference and bracket design - that is what the format is for. Native CAD matters only if you intend to modify the supplier's geometry, which is rarely the case and rarely permitted. Format matters far less than the model being a solid, at the correct scale, with a stated datum.
What if the 3D model and the drawing disagree?
Treat the drawing as the contractual document and the model as the design aid, then resolve the discrepancy in writing before cutting a bracket. One millimetre in a mounting hole is rework; five millimetres in connector height is a redesign. Ask which document will be inspected at incoming quality control.
Should I order a sample before the CAD review is complete?
Not if the sample choice depends on which unit fits. Order after the envelope check, and use the sample to confirm the electrical and thermal behaviour a model cannot show. Reversing that order is how a program ends up with a sample that is technically excellent and physically impossible.
Conclusion
The mechanical question is settled with a solid model, a drawing at a known revision, and an interference check in your own assembly - not with a dimension table. Request the envelope early, run the six checks, fill in the worksheet with a power-matched set, and confirm the revision you received in writing.
Designing a bay-constrained platform? Send your battery voltage, target power and available envelope to our engineering team and we will match the layout work to the platform.
About Echelon Charger
Echelon Energy Technology specializes in onboard chargers, DC/DC converters, and integrated EV power solutions for commercial and specialty electric vehicles.
For technical discussions, product selection, or OEM/ODM cooperation:
✉️ Email: nancy@tici-energy.com
💬 WhatsApp: +86 18114665311
For step files, outline drawings, pin definition tables and CAN matrix or DBC files, use our contact page.
