I. Why is the voltage withstand test necessary?
Withstand voltage test (commonly known as hipot test in the industry, which is the abbreviation of High Potential Test) is the most basic and core testing item in the safety certification of electronic and electrical products. Its essence is to verify the "minimum tolerance ability" of insulating materials with instant high voltage to ensure that when the product is in normal use or encounters instant overvoltage (such as lightning strikes and power surges), the insulation will not break down, resulting in the risk of electric shock.
1. Basic security verification: The final assessment of the insulation "defense line"
Insulating materials are the safety walls that isolate live parts from accessible parts in electronic devices, such as the plastic casing of power adapters, the insulating layer of enameled wires in transformers, and the green solder mask on PCB boards. The hipot test verifies whether this wall can withstand extreme conditions by applying a high voltage far exceeding the normal operating voltage (usually twice the operating voltage plus 1000V) to both sides of the insulation:
- During normal operation, the insulation only needs to withstand the operating voltage of 220V/110V.
- Instantaneous over - voltages (such as lightning induction and power switching surges) may be as high as over 1000V. The 1000V baseline for the hipot test is precisely set based on this actual risk.
More importantly, the hipot test is "non-destructive" — as long as the insulation is qualified, high voltage will not cause permanent damage to it; only when there is a defect in the insulation will an abnormal current be "triggered" by the high voltage and thus detected.
2. The detector for manufacturing defects: Identify hidden problems in production
Minor mistakes in the production process may lead to hidden bombs that cause insulation failure. These problems are difficult to detect through visual inspection or routine tests (such as resistance testing), but the hipot test can accurately identify them.
Winding process defects: When a new operator winds a transformer, if the winding spacing is too small, it will result in insufficient "electrical clearance" (the shortest air distance between the live part and the grounded part).
Welding error: During wave soldering, if the solder joints are too large and overflow into the insulation area, it will reduce the "creepage distance" (the shortest distance along the insulation surface).
Insulation material defects: Pinholes in injection-molded parts and scratches on enameled wires. These minor defects allow current to find loopholes under high voltage.
For example, a power supply factory once had a situation where a new operator failed to maintain a sufficient distance when winding the wire, causing the electrical clearance between the primary side of the transformer and the iron core to shrink from the specified 3mm to 1mm. During the hipot test, the current instantly exceeded the limit, and this batch of products with the risk of electric shock was intercepted in time.
3. Insulation verification after environmental stress: The "final gatekeeper" of type tests
Products will encounter various environmental stresses during their life cycle: humidity test (simulating a high - humidity environment, where the insulation resistance of insulating materials decreases after absorbing moisture), vibration test (simulating transportation bumps, where component displacement may change the electrical clearance), and failure test (simulating overload, where insulation ages due to overheating). These stresses may quietly "weaken" the insulation performance, but it is imperceptible to the naked eye. The hipot test is precisely the key to verifying whether these stresses have caused insulation degradation:
- For the product after being in a damp state, if the insulation absorbs moisture, the leakage current will increase significantly during the hipot test.
- For the product after vibration, if the displacement of components causes the insulation gap to shrink, the hipot test will trigger a breakdown alarm.
In short, the hipot test is the final judge of the type test—only by passing it can the product be confirmed to remain safe in extreme environments.
II. Setting of current limit for hipot test: The art of balancing "false positives" and "missing detections"
The core of the hipot test is to monitor the leakage current on both sides of the insulation. If the current exceeds the limit, it indicates that there are defects in the insulation. However, setting the limit is a "balancing act" - it cannot be too low (otherwise, qualified products will be judged as unqualified), nor can it be too high (otherwise, real insulation breakdowns will be missed).
1. Empirical method based on samples: Define the reasonable range with data
The most commonly used method for setting the limit value is to first test 10 - 20 qualified samples, record their hipot currents, calculate the average value, and then increase it by 10% - 20% as the limit value.
- Why choose the slightly higher than the average value? Because there are slight differences in the capacitance of each product (such as filter capacitors and Y capacitors), and the leakage current will have individual fluctuations. If the limit value is equal to the average value, about 1/3 of the qualified products may be misjudged due to normal fluctuations.
- For example, the average hipot current of a mobile phone charger sample is 0.5 mA, and the limit is set at 0.6 mA. This not only covers individual differences but also does not let defective products with a sudden increase in current slip through.
2. Formula derivation method: From capacitor characteristics to current prediction
If the line leakage current of the product (the leakage current during normal operation, mainly from the Y capacitor) is known, the hipot test current can be predicted through a formula:
\[ I_{\text{hipot}} = 2 \times I_{\text{line}} \]
The logic of "2" in the formula is: during normal operation, the leakage current only comes from the Y-capacitor of one line (L or N); while during the hipot test, high voltage is applied to both L and N simultaneously, and the leakage current comes from the capacitors of both lines - so it is twice the line leakage current.
For example, if the line leakage current of a certain product is 0.3 mA, then the hipot test current is approximately 0.6 mA, and the limit value can be set to 0.7 mA (a 17% increase).
3. The core logic of the limits: neither "wronging" qualified products nor "letting go" of potential hazards
The setting of the current limit must meet two conditions:
Lower limit: Higher than the maximum leakage current of all qualified products (to avoid misjudgment);
Upper limit: Below the minimum current at the time of insulation breakdown (to avoid missed detection).
For example, if the insulation breakdown current of a certain product is 5mA and the limit value is set to 2mA - when the current exceeds 2mA, it indicates that the insulation is close to breakdown and must be intercepted; if the limit value is set to 4mA, an alarm may not be triggered until the current reaches 5mA. By this time, the insulation has broken down and the product is scrapped.
III. Selection of AC and DC test voltages: Technical paths with respective advantages and disadvantages
The hipot test can select either alternating current (AC) or direct current (DC) voltage. The difference between the two stems from the voltage characteristics, which directly affects the test results.
1. AC voltage: The practical challenge of peak voltage
The essence of alternating voltage is a sine wave, and its peak voltage is √2 times (approximately 1.414 times) the root mean square (rms) value. This means that when you apply a test voltage of 1500Vac, the insulation actually withstands a peak voltage of 2121V (1500 × 1.414).
- Why use the effective value? Because the effective value is an indicator to measure the work - doing ability of AC voltage and complies with the unified regulations of safety standards.
- However, the withstand capability of insulation is for peak voltage. If the insulation can only withstand a peak voltage of 2000V, a 1500Vac test will directly break it down.
2. DC voltage: Advantages of lower leakage limits and "gradual warning"
If you choose DC testing, you need to set the voltage to √2 times the RMS value of the AC voltage (i.e., the equivalent peak voltage). For example, 1500Vac corresponds to 2121Vdc. Two major advantages of DC testing:
Lower leakage current limit: Under direct current (DC), the leakage current of insulation mainly comes from the insulation resistance of the material (rather than capacitance). Therefore, the limit can be set lower (for example, if the alternating current (AC) limit is 1 mA, the DC limit can be set to 0.5 mA). This means that DC testing can detect more subtle insulation defects (such as pinholes on the verge of failure).
Warning function of gradual voltage: The voltage of the DC tester gradually rises from 0 to the specified value, and the operator can observe the change of current with voltage.
- If the current increases linearly (proportional to the voltage), it indicates that the insulation is normal.
- If the current suddenly jumps (for example, when the voltage rises to 1000V, the current jumps from 0.2mA to 1mA), it indicates that there is a hidden danger in the insulation (such as cracks). The test can be stopped in advance to avoid breakdown.
However, DC testing also has its drawbacks:
Higher cost: The circuit of DC high-voltage power supply is more complex (requiring rectification and filtering), and its price is 10% - 20% higher than that of AC testers.
Discharging is necessary: Direct current will charge the capacitors (such as filter capacitors) in the product, and the residual voltage after testing may be as high as several thousand volts. If the product is not discharged in time, the operator may get an electric shock when touching it.
3. Common problems with AC and DC: Solutions to the issues of Y capacitors and false judgments
A major "pain point" in AC testing is the interference of the Y capacitor. The Y capacitor is a safety capacitor connected across L/N to the ground, used to suppress EMI (electromagnetic interference). AC can pass through a capacitor. If the capacitance of the Y capacitor is large (e.g., 0.1 μF), the leakage current during hipot testing will increase significantly (Formula: \( I = 2πfCU \), where f is 50 Hz, C is 0.1 μF, and U is 1500 V. Calculation yields I ≈ 47 mA), far exceeding the normal limit (e.g., 1 mA), causing the tester to misjudge insulation failure.
There are two solutions:
Disconnect the Y capacitor: If permitted by safety standards (e.g., IEC 60950), disconnect the Y capacitor during testing to eliminate interference.
Switch to DC testing: DC cannot pass through the capacitor, so the Y capacitor will not generate current, which completely solves the problem of misjudgment.
IV. Implementation method of hipot test: The key details of accurately applying voltage
The effectiveness of the hipot test depends on the location where the voltage is applied and the connection method - incorrect connections can lead to "false positive" or "false negative" results.
1. Typical scenarios of voltage application
The core of the hipot test is to apply high voltage to both sides of the insulation. There are two common scenarios:
Primary side and enclosure: Connect the high-voltage probe to the primary side (short-circuit L and N), and connect the grounding probe to the metal enclosure - to verify the insulation between the live parts and the accessible parts.
Primary side and secondary side: Connect the high-voltage probe to the primary side (short-circuit L and N), and connect the grounding probe to the secondary side (short-circuit all output terminals) - to verify the insulation between the high voltage and the low voltage (for example, the insulation between the 220V input and the 5V output of a power adapter).
2. The underlying logic of the connection method
Short-circuit L and N: Apply high voltage to both the live wire and the neutral wire simultaneously to cover all live parts and avoid overlooking the insulation defects of any line.
Short - circuit the secondary side output: Short - circuit all low - voltage terminals (such as V+ and V - of USB) to the same potential to simulate the "worst - case scenario" where all secondary terminals are grounded, ensuring that the insulation withstands the maximum voltage.
The EUT does not work: The product is not powered on during the test to avoid the interference of the working current with the monitoring of the leakage current.
3. Requirements for "voltage ramp-up" in type testing
Type tests (for certification) have strict requirements for voltage application:
1. First, apply 1/2 of the specified voltage (for example, if the specified voltage is 2000V, apply 1000V first).
2. Gradually increase to the specified voltage within 10 seconds.
3. Hold for 1 minute.
The purpose of doing this is to avoid the impact of sudden voltage changes on insulation. If brittle insulating materials (such as ceramics and glass) suddenly bear high voltage, they may crack due to "electrical stress". A gradually changing voltage is closer to the actual over - voltage (for example, the voltage rise rate of a lightning strike is about 1 kV/μs), which can more realistically simulate extreme scenarios.
V. Test duration: The time compromise between certification and production
The setting of the test duration essentially aims to balance "safety requirements" and "production efficiency". The one - minute requirement of the safety standard is "too slow" for the production line, while the few seconds needed by the production line require compensation with a higher voltage.
1. The 1 minute in the certification requirements: Simulate long-term overvoltage
Most safety standards (such as IEC 60950, UL 60950) require the duration of type tests to be 1 minute. This is to simulate the situation where the product encounters long - term overvoltage (for example, the surge after a lightning strike may last from a few seconds to dozens of seconds). A 1 - minute test can verify the stability of insulation under "continuous high voltage". If there are hidden defects in the insulation (such as aged enameled wires), it will gradually break down within 1 minute.
2. Efficient adjustment of the production line: High voltage + Short time
The production line produces thousands of products every day. Each product takes up a large number of testing stations per minute (for example, with 10 stations, only 600 products can be tested per hour). Therefore, manufacturers usually adopt a compromise solution of "high voltage + short time":
- Increase the voltage to 110% - 120% of the specified value (for example, if the specified value is 2000V, use 2200V).
- Shorten the time to 1 - 2 seconds.
For example, the specified test voltage for the power supply of a certain router is 2×220V + 1000V = 1440V. The production line conducts the test at 1584V (110%) for 2 seconds, which not only meets the production capacity requirements but also compensates for the impact of the shortened time through the high voltage.
However, it should be noted that such adjustment must be approved by the certification body. The "voltage-time characteristics" of different insulating materials vary (for example, the withstand curve of PVC insulation is "high voltage for a short time ≈ low voltage for a long time", while the curve of epoxy resin is flatter), and empirical rules cannot be applied arbitrarily.
3. Voltage-time characteristic: The "personalized curve" of insulating materials
Each insulating material has its own voltage-time withstand curve:
- PVC insulation: It can withstand high voltage for a short time (e.g., 120% of the specified value for 2 seconds), but it will break down under low voltage for a long time (e.g., 90% of the specified value for 10 minutes).
- Epoxy resin insulation: It can better withstand long - term low voltage, but high voltage for a short time may cause cracking.
Therefore, the time adjustment of the production line must be based on experimental data. For example, test 100 samples by applying 110% of the rated voltage for 2 seconds. Only when all samples pass the test and there are no problems in the subsequent reliability tests can the adjustment be promoted.
Summary
Hipot testing is not a perfunctory and formalized inspection, but a safety "umbrella" for the entire process from design to production.
- It verifies the "minimum capability" of insulation, in production, and confirms the reliability after environmental stress.investigate hidden defects
- Its current limit, AC/DC selection, and test time are all based on the trade - off of "safety logic".
- The implementation details (connection method, voltage climb) directly determine the accuracy of the detection.
For manufacturers, understanding the "underlying logic" of hipot testing is more important than mechanically implementing standards. Only in this way can they truly use it to safeguard the safety baseline of products.