A charger with all three protections listed on its specification returned from a customer with a swollen battery case. The over-voltage threshold had been set above the level the device would accept, so the protection existed, engaged late, and did not protect the thing that mattered.
OVP, OCP and OTP: what each one is for
Three protections, three different things protected: the device, the charger itself and the customer’s environment.
The arrangements look similar on a datasheet and behave differently in practice, because each has a threshold, a response and a recovery condition, and the three are chosen for different failure modes. Reading them as one feature is how a design ends up with three protections and one gap.
The comparison below treats each one through its three parameters, which is also the form the verification takes.
OVP: protecting the device being charged
The threshold has to sit below what the device will tolerate.
Over-voltage protection protects the load, so its threshold is set against the device specification rather than against the charger’s capability. A threshold above the device’s tolerance protects the charger and not the phone. The response may be a shutdown with a latch, or a fold-back to a lower level, and the recovery condition decides whether the customer has to unplug the product to reset it.
OCP: protecting the charger and the cable
The threshold protects the source, and it has to respect the cable.
Over-current protection limits what the charger delivers into a fault or an excessive load, which protects the switching stage, the connector and the cable. Its threshold has to sit above the normal operating current, including the transient drawn when a device starts charging, or it will trip on a legitimate load. On a multi-port product the per-port limit interacts with the sharing rule, and both have to be verified together.
OTP: protecting against heat
The threshold is a derating point as much as a shutdown point.
Over-temperature protection usually works in two stages: a first threshold where the design reduces output, and a second where it shuts down. The first is a specification item because the customer experiences it as a slower charge on a warm day; the second is a safety behaviour. Both need their trigger, response and recovery recorded, and the measurements have to be taken in the shipping enclosure at the stated ambient.
| Protection | What it protects | Threshold set against | Recovery question |
|---|---|---|---|
| OVP | The device being charged | The device tolerance | Latched or automatic |
| OCP | The charger, connector and cable | Normal current plus transient | Automatic with hysteresis |
| OTP | The product and its surroundings | The thermal design and the ambient | Derate first, shut down second |
How the three interact
Two interactions decide whether the design behaves sensibly.
An OCP trip heats the switching stage, which can bring the product towards its OTP threshold, so the recovery behaviour of one depends on the state of the other. And on a multi-port product, the sharing rule changes the current each port sees, which changes both the OCP margin and the thermal load. Testing them separately on a single port describes a product that does not exist.
How to verify all three
Three tests, each at the boundary rather than in the middle.
The OVP test drives the output towards the threshold and records where the response occurs; the OCP test loads the port to the trip point and records the trip and the recovery; the OTP test raises the ambient until the derating point and then until the shutdown point, recording both. Each is performed in the shipping enclosure with a stated load and input, and each produces a number rather than a confirmation.
What belongs in production testing
One protection verified on every unit, two verified on samples.
The functional output check on every unit verifies the regulation that the protections protect; the OCP and OTP behaviours are usually verified on a defined sample because triggering them stresses the product. Whichever split is chosen, the plan states it, with the limits and the frequency, so that the behaviour is verified somewhere rather than assumed everywhere. The plan records which protection is checked where, and the record is what a review reads.
FAQ
Why did the protection not save the device?
Usually because the threshold was set against the charger’s capability rather than the device’s tolerance. Over-voltage protection is there to protect the load, so its threshold has to sit below what the device will accept. Ask what the load will accept and set the threshold below it, because over-voltage protection exists to protect the device rather than to describe the charger.
Should OCP be latched?
An automatic recovery with hysteresis suits a transient overload, and a latch suits a fault that should not restart unattended. The choice belongs in the specification along with the recovery condition, because it decides what the customer has to do. Ask which behaviour the customer will meet, because a latched unit stays off until it is unplugged while an automatic recovery restarts.
Is derating the same as OTP?
Derating is the first stage of the thermal protection: the design reduces output at a threshold and shuts down at a higher one. Both thresholds are specification items, and both are measured in the enclosure at a stated ambient. Ask for both thresholds, because the derating point and the shutdown point are separate specification items and both are measured in the enclosure.
How often should the protections be tested in production?
Triggering them on every unit stresses the product, so they are usually verified on a defined sample or during characterisation. The regulation behaviour that they protect is verified on every unit. Triggering them on every unit stresses the product, so the regulation behaviour they protect is verified on every unit instead. and the sample size and the frequency are stated in the plan.
Do the protections affect certification?
They affect the fault conditions the standard requires to be assessed, and the thermal behaviour of the product. The evidence is the measurement for each protection together with the construction review. The evidence is the measurement for each protection together with the construction review, because the standard assesses fault conditions. Ask which fault conditions the standard requires to be assessed.
Next step
Send the output range, the port layout and the markets to Wecent Group through the contact page. The reply states the threshold, response and recovery for each protection, with the measurements taken in the shipping enclosure. Related reading: GaN charger applications, charger quality control and OEM and ODM charger manufacturing. External references: USB Power Delivery specification, IEC standards, IEC Webstore, UL standards and certification, TUV testing services and Intertek testing services.
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