A customer connected a laptop and a phone and found that the laptop charged at half its usual rate. Nothing was faulty: the sharing rule had reduced the primary port to keep the total inside the budget, and the rule had never been written down for anyone to read.
Charger power sharing: what the rule has to define
Four elements: the total budget, the allocation per port, the behaviour when a device negotiates an adjustable profile, and the fallback when the input adapter is smaller than the design assumes.
The rule is a table rather than a sentence, because the behaviour changes with each combination of ports in use. Writing the table is what turns a firmware behaviour into a specification that can be tested, explained and sold.
The four elements are ordered because each constrains the next: the budget sets the allocation, the allocation interacts with the negotiated profiles, and the fallback decides what the product does when the input cannot deliver the budget.
Element 1: the total budget
The budget is set by the input and by the thermal limit, not by the sum of the port ratings.
A three-port charger with generous per-port figures cannot deliver all of them at once unless the input can supply the total and the interior can dissipate it. The budget is therefore the lower of the two: the input available after conversion losses, and the thermal limit at the stated ambient. Stating the budget as a figure with its condition is the first line of the rule.
Element 2: the allocation per port
Priority or equal division, and which ports receive the priority.
Priority allocation gives one port a higher share when several are active, which suits a product where the laptop is the primary device. Equal division spreads the budget, which suits a family charger where no port is more important. Both are legitimate, and the choice should follow the use case rather than the firmware default, because the customer experiences the allocation directly.
Element 3: the interaction with negotiated profiles
An adjustable profile and a sharing rule have to be designed together.
Where a device negotiates an adjustable level inside a range, the sharing rule has to define what happens to that level when a second device connects: whether the range is reduced, whether the level is held and the current limited, or whether the port falls back to a fixed profile. Each behaviour is a different customer experience, and each has to appear in the table.
Element 4: the fallback for a smaller adapter
The product will be used with adapters other than the one specified.
The rule should state what the charger does when the input cannot supply the budget: which ports remain active, what output each delivers, and whether the product indicates the reduced condition. Without that line, the fallback is whatever the firmware happens to do, which is the origin of most support queries in this category.
| Element | What to state | Where it appears | Common omission |
|---|---|---|---|
| Total budget | A figure with its input and ambient condition | The specification and the packaging | A sum of port ratings |
| Allocation | Priority or equal division, per combination | The sharing table | Left to the firmware default |
| Profile interaction | What happens to an adjustable level | The sharing table and the test plan | Designed separately from the rule |
| Fallback | Behaviour with a smaller adapter | The specification and the manual | Undefined behaviour |
What the rule means for the test plan
Each combination in the table is a test position, and the limits come from characterisation.
The characterisation data has to cover every combination the rule allows, because it is the source of the limits used in production. The production test then verifies the positions that a unit can realistically differ on, plus a defined sample of the rest. A plan that tests only the single-port case confirms the easiest condition and leaves the rule unverified.
How to write the rule for customers
One short table, and one sentence for the fallback.
The useful version is a table with the port combinations in rows and the output in columns, plus a sentence stating the adapter required and what happens with a smaller one. That is enough for a customer to predict the product’s behaviour, and it is short enough to appear on a product page. Anything longer is usually a specification document rather than a customer explanation.
Where sharing rules go wrong
Three patterns, and each is visible before the first order.
A budget that exceeds what the input can supply; an adjustable profile that is reduced in a way the packaging does not mention; and a fallback that depends on the adapter the customer happens to own. Each is a specification gap rather than a design fault, and each generates the same kind of query.
FAQ
Why does my laptop charge slower when my phone is connected?
Because the sharing rule reallocates the budget while both ports are active. The behaviour is designed, and it should be stated as a table so the customer can predict it rather than discover it. The rule reallocates the budget while both ports are active, and it should be stated as a table so the customer can predict the behaviour rather than discover it.
Does the rule affect certification?
It affects the behaviour that is tested and the thermal condition, because several active ports produce the warmest interior. The safety and electromagnetic evidence still follows the product and the market. The rule affects the behaviour that is tested and the thermal condition, because several active ports produce the warmest interior, while the safety evidence follows the market rather than the rule.
Can the rule be changed after launch?
A firmware change can alter the allocation, and it changes the product’s behaviour, so the packaging text and the test limits are updated with it. A quiet change produces a discrepancy between what the customer bought and what the unit does. A firmware change can alter the allocation, and it changes the product’s behaviour, so the packaging text and the test limits are updated with it.
Should the packaging state the rule?
At least in summary form, with the adapter requirement. The table is the clearest version, and it is also what the support team will end up quoting anyway. State it in summary form with the adapter requirement, because the table is the clearest version and it is what the support team will end up quoting to customers anyway. State the table on the pack rather than only in the manual.
What if the customer uses a different adapter?
The fallback line in the rule covers it: which ports stay active, what each delivers, and whether the product indicates the reduced state. Leaving it undefined means the behaviour varies with the adapter rather than with the product. The fallback line covers which ports stay active, what each delivers, and whether the product indicates the reduced state when the adapter changes.
Next step
Send the port layout, the required adapter and the use case to Wecent Group through the contact page. The reply provides the sharing table, the budget with its conditions, the fallback behaviour and the test positions that verify it. Related reading: GaN charger applications, OEM and ODM charger manufacturing and charger quality control. External references: USB Power Delivery specification, USB-IF compliance programme, IEC standards, UL standards and certification, TUV testing services and Intertek testing services.
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