Two chargers rated at the same output behaved differently in a warm room. One held its rating to a stated ambient and then reduced output gradually; the other reduced output earlier, without saying so, and the customer measured the difference. The rating was identical; the derating curve was not.
Charger derating: what the curve has to state
Five items: the rated output and the ambient it applies to, the threshold where the design reduces, the shape of the reduction, the recovery condition and the load condition the curve describes.
A single derating temperature is not enough, because the behaviour above the threshold is what the customer experiences. A curve that steps down sharply at the threshold behaves differently from one that reduces gradually, and both are defensible provided they are stated.
The items are ordered so that a specification can be tested: each one is a measurement or a written condition, and together they describe the product’s behaviour across the range it will be used in.
Item 1: the rated output and its ambient
The rating only exists at a stated ambient.
A charger rated at its full output at 25 degrees and one rated at the same output at a higher ambient are different products, and the difference is what a market with warm conditions notices. The specification should state the ambient the rating applies to, and whether the figure is the continuous output or a peak. Without the ambient, the rating describes a laboratory rather than a product.
Item 2: the threshold
The point where the design begins to reduce is a design decision with a number.
The threshold follows from the thermal design: the point at which a component or the surface approaches its limit. Where the threshold is set conservatively the product is safe and appears weaker on a warm day; where it is set aggressively the product holds its rating longer and runs closer to the limit. Either way, the number belongs in the specification, because it is the point the customer will find.
Item 3: the shape of the reduction
A step, a ramp, or a stepped curve, and each behaves differently in use.
A gradual reduction is rarely noticed unless the customer is measuring; a step is visible because the charge rate changes suddenly. The shape also interacts with the device being charged, since a device negotiating an adjustable profile may renegotiate when the available output falls. The curve should show the output at several ambient points rather than a single reduction figure.
Item 4: the recovery condition
What has to happen before the output returns.
Recovery may be automatic once the temperature falls, or require the unit to be unplugged, and the difference is a customer experience. The condition should be stated with the hysteresis: how far the temperature has to fall before the output returns, and whether it returns fully. A design that recovers at the same temperature it reduced at will oscillate, which the customer experiences as an unstable charge.
Item 5: the load condition the curve describes
Single port, multi-port, or both.
The thermal state depends on how many ports are active, so a derating curve measured with one port loaded describes a different product from one measured with three. On a multi-port charger, the curve should state the condition, and ideally show both the single-port and the simultaneous condition, because the simultaneous case is where the interior is warmest.
| Item | What to state | How it is measured | Common omission |
|---|---|---|---|
| Rated output | The output and the ambient it applies to | Full load at that ambient | The ambient |
| Threshold | The temperature where reduction begins | Temperature sweep at full load | A threshold left unstated |
| Shape | The output at several ambient points | A curve across the range | A single reduction figure |
| Recovery | The condition and the hysteresis | Reduction and return measured | An oscillating threshold |
| Load condition | Single port, multi-port or both | Both conditions measured | Only the easiest condition |
Where the curve comes from
Three design inputs, and each one can be changed.
The thermal path, because it sets how quickly heat leaves; the efficiency at full load, because it sets how much heat there is; and the thermal limits of the components nearest the source. A design that needs a higher threshold has to change one of the three, which is why the derating curve is a design output rather than a marketing choice.
How to verify it in production
Two checks, and neither requires an ambient chamber on the line.
The production test verifies the output at a defined load and the functional behaviour of the thermal protection, while the curve itself is established during characterisation on a sample. The production plan records which parameter is checked per unit and which is checked per build, so the curve’s integrity is maintained by the sample data rather than assumed from the design.
What to ask a supplier for
Four items, and each is a figure rather than a phrase.
The rated output with its ambient; the threshold temperature; the output at two or three points above the threshold; and the recovery condition with its hysteresis. Ask for the load condition the figures were measured under, because a curve taken with one port active describes the easiest case. With those four, the derating behaviour can be compared with another platform and stated on the datasheet.
FAQ
Why does my charger charge more slowly in summer?
Because the design reduces output above a threshold temperature to stay inside its thermal limits. The behaviour is designed, and the threshold with the curve above it should be stated so the customer can predict it. The design reduces output above a threshold temperature to stay inside its thermal limits, and the threshold with the curve above it should be stated.
Is a higher derating threshold always better?
A higher threshold means the product holds its rating longer and runs closer to its thermal limit. Whether that is better depends on the market’s ambient profile and on the margin the design carries, which is why the threshold follows the thermal design. A higher threshold means the product holds its rating longer and runs closer to its thermal limit, so the threshold follows the thermal design.
What is hysteresis for?
It prevents the output from returning at the same temperature it reduced at, which would produce an oscillation the customer experiences as an unstable charge. The recovery condition states how far the temperature has to fall first. Hysteresis prevents the output from returning at the same temperature it reduced at, which would produce an oscillation the customer experiences as unstable.
Does the curve change with a second port?
It does, because several active ports produce the warmest interior. A curve measured with one port loaded describes the single-port condition, and the multi-port condition should be stated separately. Several active ports produce the warmest interior, so a curve measured with one port loaded describes the single-port condition and the multi-port condition is stated separately rather than assumed. Ask for both curves with the conditions stated.
Next step
Send the output, the port layout and the market ambient profile to Wecent Group through the contact page. The reply provides the rated output with its ambient, the threshold, the curve above it and the recovery condition for both single and multi-port use. Related reading: GaN charger applications, charger quality control and OEM and ODM charger manufacturing. External references: IEC standards, IEC Webstore, EU energy-efficient products, USB Power Delivery specification, UL standards and certification and TUV testing services.
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