The dielectric withstand test has been part of power supply manufacturing for decades. It’s one of those tests that everyone knows they need to do, but not everyone gets right. Apply too much voltage and you damage good units. Set the leakage threshold too tight and you get false failures that slow down the line. Use AC when you should use DC and the Y-capacitor current drowns out the real signal.
The core idea is simple: apply a voltage much higher than normal operating conditions between isolated circuits — primary to secondary, input to ground, output to ground — and measure how much current leaks through the insulation. If the insulation holds and the leakage stays below the limit, the unit passes. If the current spikes, there’s a breakdown path that needs to be found and fixed.
Why dielectric withstand testing matters
Safety standards around the world require it. IEC 60950-1, IEC 62368-1, and IEC 61010-1 all mandate dielectric withstand testing as part of the safety certification process. But beyond compliance, the test catches real manufacturing defects that other tests miss.
Consider a power supply where a screw has been over-tightened and cracked the PCB near a mounting hole. The crack is invisible under visual inspection and doesn’t affect functionality because no traces are broken. But the crack creates a creepage path that, over time, can carbonize and short the primary voltage to ground. A dielectric withstand test catches this immediately because the leakage current through the damaged insulation exceeds the limit.
Other defects that hipot testing identifies include pinched wires in the transformer winding, insufficient clearance between the primary-side and secondary-side traces on the PCB, contamination from flux residue that creates conductive paths under high voltage, and damaged optocouplers or isolation transformers that have reduced breakdown voltage.
AC versus DC: which test method to use
This is the most common question about dielectric withstand testing, and the answer depends on what you’re testing and why.
AC hipot testing applies an alternating voltage — typically 1,500 V to 5,000 V at 50 or 60 Hz — between conductive parts and ground or between primary and secondary circuits. The advantage is that it stresses the insulation in both directions, which is closer to real-world AC operating conditions. The downside is that Y-capacitors, which are present in most switching power supplies for EMI filtering, pass AC current continuously. That capacitive current can mask a genuine leakage problem or trip the tester’s current limit prematurely.
DC hipot testing solves the Y-capacitor problem. When you apply DC test voltage, the capacitors charge up and then block further current. After the initial charging transient, the measured current drops to whatever is leaking through the insulation itself. This gives a cleaner reading and makes it easier to distinguish between capacitive charging and genuine insulation breakdown.
The trade-off is that DC testing doesn’t stress insulation the same way AC does. Most safety standards allow you to use a DC test voltage equal to the peak of the AC test voltage — 2,120 VDC for a 1,500 VAC test, which is √2 times the RMS value plus a margin. You also need to ramp the voltage slowly to avoid charging current spikes, and you must discharge the unit afterward, because the stored charge can persist for minutes.
For production testing of power supplies and adapters, many manufacturers have moved to DC hipot as the primary method. It reduces false failures caused by Y-capacitor current, and the test itself is safer for the operator because there’s no continuous AC current present. The automatic power supply hi-pot and final test system from Jettest supports both AC and DC modes, with programmable ramp rates and current trip thresholds.
Setting the test voltage and duration
The test voltage depends on the product’s insulation class and the applicable safety standard. For a power supply with a 240 VAC input, the typical production hipot test applies:
– 1,500 VAC or 2,120 VDC between primary and secondary
– 1,500 VAC or 2,120 VDC between primary and ground
– 500 VAC between secondary and ground (for SELV circuits)
These values come from IEC 60950-1 and IEC 62368-1, which are the governing safety standards for IT equipment and audio/video products. Industrial equipment falls under IEC 61010-1, which specifies similar levels.
The standard test duration is one minute for type testing and one to two seconds for production-line testing. The shorter duration at the same voltage level is permitted when the test equipment can ramp quickly and the current limit is set correctly. In automated production, a ramp time of 0.5 to 2 seconds followed by a dwell of 1 to 2 seconds at full voltage is common.
One thing that catches manufacturers off guard: when using DC voltage for production testing, the ramp rate matters. If the test equipment applies voltage too quickly, the charging current into the DUT’s internal capacitance can exceed the trip threshold and cause a false failure. A good rule of thumb is to set the ramp to at least 0.5 seconds per 1,000 V of test voltage, which gives the capacitance time to charge without tripping the overcurrent detection.
Understanding and avoiding false failures
False failures are the hidden cost of dielectric withstand testing. Every time a good unit fails the hipot test, it has to be pulled off the line, retested, inspected, and either cleared or scrapped. That takes time, creates a bottleneck, and adds cost.
The most common cause of false failures in AC hipot testing is the Y-capacitor current. A typical switching power supply has two or three Y-capacitors between primary and ground, each rated at 2.2 nF to 4.7 nF. At 1,500 VAC and 60 Hz, the total capacitive current can reach 3 to 5 mA. If the test equipment’s trip threshold is set at 5 mA, the unit will fail even when the insulation is perfect. The solution is to either switch to DC testing or set the trip limit high enough to account for the known capacitive current.
Another source of false failures is poor test fixture design. Long test leads, unshielded connections, and contamination on the fixture’s insulation surface all contribute to leakage current that the hipot tester can’t distinguish from a genuine fault. Regular cleaning of test fixtures, using guarded test connections, and keeping leads as short as practical all reduce the noise floor of the measurement.
Arc detection also deserves attention. Some hipot testers use a separate arc detection circuit that triggers on fast current transients. While this is useful for catching insulation defects that don’t produce sustained leakage, the arc threshold can be sensitive enough to trigger on relay switching noise or static discharge. When this happens repeatedly, check whether the arc detect setting is appropriate for the product type.
Environmental conditions matter too. High humidity increases surface leakage on PCB assemblies, especially if there’s flux residue present. The same unit that passes hipot testing at 30% relative humidity might fail at 70%. This is why production hipot testing should be done in a controlled environment or after a bake-out cycle that removes moisture.
Integrating hipot testing into automated production
Standalone hipot testing works for low-volume manufacturing, but once production scales past a few thousand units per day, manual testing becomes a bottleneck. Each unit has to be connected, the test initiated, the result recorded, and the unit disconnected — all by a human operator who may get distracted or fatigued.
Automated hipot testing solves this by embedding the dielectric withstand test into the production line’s normal material flow. In a fully automatic test and burn-in line from Jettest, the hipot test is one station among several — after ICT/FCT and before final functional test. The unit moves through each station on a conveyor, with automatic pin insertion and extraction. The test results are logged to the MES, and failed units are automatically routed to a reject bin for analysis.
The key advantage of this approach is consistency. Every unit gets the same test voltage, the same ramp rate, the same dwell time, and the same pass/fail criteria. There’s no operator variability. And because the data is captured electronically, quality engineers can trend hipot test results over time to detect drift in insulation quality before it becomes a compliance issue.
The hipot test as part of a complete safety test strategy
Many manufacturers think of hipot testing as a standalone gate — either the unit passes or it doesn’t. But the most effective quality programs treat it as one data point in a multi-layered safety verification process. Ground bond testing, insulation resistance measurement, and hipot testing all check different aspects of electrical safety.
Ground bond testing verifies that the protective earth connection can carry fault current without overheating. Insulation resistance testing checks the DC resistance of the insulation at a lower voltage, typically 500 V or 1,000 V. Hipot testing verifies that the insulation can withstand the worst-case overvoltage condition without breaking down. Each test catches different types of defects, and together they provide comprehensive safety coverage.
Nailing the three basics
Dielectric withstand testing is a mandatory gate in power supply manufacturing. Getting it right means choosing the right test method for your product, setting parameters that match the standard and the real behavior of the unit under test, and integrating the test into a production flow that doesn’t trade quality for throughput. When those three things are in place, hipot testing catches real defects without slowing down the line or creating unnecessary rework.
