Four items decide whether surge protection is real or nominal: the level the product has to withstand, the paths the energy can take, how the protection coordinates with the fuse, and what happens after the protection has done its job once.
Charger surge protection: the four items to specify
The withstand level, the differential and common-mode paths, the coordination with the input fuse, and the condition of the protection element after an event.
The four are ordered because each depends on the previous one: the level decides the element, the paths decide how many elements are needed, the coordination decides which one fails first, and the post-event condition decides whether the product still works.
Specifying them turns a part list into a behaviour, and each has a measurement behind it rather than a datasheet claim.
Item 1: the withstand level
State the level the product has to survive, and the waveform that goes with it.
A voltage figure alone is not a requirement: the test waveform, the repetition and the source impedance all change what the protection has to do. The level should also be tied to the market, because different regions apply different transient environments to mains-connected equipment. Once the level is stated, the element and its rating follow from it rather than from habit.
Item 2: the paths the energy can take
Differential mode and common mode are two different problems.
Differential-mode energy arrives between the mains conductors and is handled near the input; common-mode energy arrives between the conductors and earth and can travel through the product towards the output, which is where damage to the low-voltage side appears. A design that protects only the differential path can pass an input test and still fail when the energy takes the other route. The RFQ should ask which paths are protected and with which elements.
Item 3: coordination with the fuse and the rest of the input stage
Two protection devices in the same circuit have to be coordinated, or one saves the other.
The surge element and the input fuse have different response times and different failure modes. Where the coordination is wrong, a surge event can open the fuse without the element absorbing the energy, or degrade the element without the fuse noticing. The verification is a sequence of events: after the specified number of surges, the product still works and the fuse has not opened.
Item 4: the condition after an event
Some elements degrade each time they act.
Metal-oxide elements absorb energy by changing state, and the change accumulates; a product protected by an element that has degraded can fail on a later, smaller event. The specification should state how many events the design is expected to survive, and the verification should show the condition of the element afterwards. That is the difference between protection for one event and protection for the product’s life.
| Item | What to state | Verification | Common gap |
|---|---|---|---|
| Withstand level | The level with its waveform and repetition | The surge test result | A voltage with no waveform |
| Paths | Differential and common mode | Tests on each path | Only the differential path |
| Coordination | Response times and failure order | The fuse intact after the sequence | One device saving the other |
| Post-event condition | Events to survive and element state | The element measured afterwards | Protection assumed for life |
How the protection interacts with the rest of the design
Three interactions that belong in the same conversation.
The input filter, because the protection element and the filter components share the same node and influence each other; the isolation barrier, because common-mode energy stresses it; and the thermal design, because a protection element that has absorbed energy is a warm component near other warm components. Each interaction is a design decision rather than a component choice.
What to ask for in the specification response
Four answers, and each is short.
The withstand level with the waveform; the elements used on each path; the coordination rule with the fuse; and the number of events the design is expected to survive with the evidence behind it. A supplier that answers all four is describing a designed protection; one that names a component is describing a placement.
Where surge protection is usually specified
Three places, and the third is the one that matters to the customer.
The safety evidence for the market, where the level and the test are recorded; the incoming stage of the design, where the paths and the coordination are fixed; and the product claim, where the protection is either stated accurately or left out. Claiming a protection level without the test behind it is the version that creates a query, and the query usually arrives with a failed unit attached.
FAQ
Is a varistor enough for surge protection?
It is one element on one path. Common-mode energy and the coordination with the input fuse are separate questions, and a design that answers only the first can pass an input test and still damage the low-voltage side. Common-mode energy and the coordination with the input fuse are separate questions, so a design answering only the first can pass an input test and still damage the low-voltage side.
How is the withstand level chosen?
From the transient environment the product will see and the market it is sold into, expressed as a level with its waveform and repetition. A voltage figure without those conditions cannot be tested or compared. Express the level with its waveform and repetition, because a voltage figure without those conditions cannot be tested or compared between two designs at all.
Does protection degrade?
Some elements do, and the degradation accumulates with each event. The specification states how many events the design is expected to survive, and the verification shows the condition of the element afterwards. State how many events the design is expected to survive, and show the condition of the element afterwards, because the degradation accumulates with each event rather than appearing at once.
Should the packaging mention surge protection?
Only as far as the evidence supports. A stated level with the test behind it is a useful claim; a level quoted from the element datasheet is a claim about a component rather than about the product. A stated level with the test behind it is a useful claim, while a level quoted from the element datasheet describes a component rather than the product.
Next step
Send the markets, the input arrangement and the required protection level to Wecent Group through the contact page. The reply states the withstand level, the elements on each path, the coordination rule and the events the design is expected to survive. Related reading: GaN charger applications, charger quality control and OEM and ODM charger manufacturing. External references: IEC standards, IEC Webstore, EU Low Voltage Directive, UL standards and certification, TUV testing services and Intertek testing services.
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