Switching power supply test items

A good switching power supply must comply with all functional specifications, protection characteristics, safety regulations (such as UL, CSA, VDE, DEMKO, SEMKO, Great Wall, etc., voltage resistance, flame retardancy, leakage current, grounding and other safety specifications), electromagnetic compatibility (such as FCC, CE, etc., conducted and radiated interference), reliability (such as aging life test), and other specific requirements.

Switching power supplies include the following types:

AC-DC: for personal use, home use, office use, industrial use (computers, peripherals, fax machines, chargers)

DC-DC: such as portable products (mobile phones, laptops, cameras, secondary power supplies for communication switches)

DC-AC: such as car converter (12V～115/230V), communication switch ringing signal power supply

AC-AC: such as AC power transformer, inverter, UPS uninterruptible power supply

The design, manufacturing and quality management of switching power supplies require precise electronic instruments to simulate the various characteristics (i.e., various specifications) of the power supply during actual operation and verify whether it can pass. Switching power supplies have many different structures (single output, multiple outputs, positive and negative polarity, etc.) and combinations of output voltage, current, and power, so flexible and diverse test instruments are needed to meet the needs of many different specifications.

Electrical Specifications Test

When verifying the quality of a power supply, the following are general functional test items, with detailed descriptions as follows:

*Functions testing:

Output voltage adjustment (Hold-on Voltage Adjust)

Line Regulation

Load Regulation

Combined Regulation

Output Ripple & Noise (RARD)

Input Power and Efficiency

Dynamic or transient response

Power Good/Fail Time

Set-Up and Hold-Up time

*Protections test:

Over Voltage Protection (OVP)

Short circuit protection (Short)

Over Current Protection (OCP)

Over Power Protection (OPP)

*Safety specification test:

Input current, leakage current, etc.

· Withstand voltage insulation: Power input to ground, power output to ground; circuit board lines must have a safe distance.

Temperature and flame retardancy: Components must meet flame retardancy safety specifications, and the operating temperature must be within the safety specifications.

·Case grounding: must be below 0.1 ohm to avoid the risk of electric shock due to leakage.

Transformer output characteristics: open circuit, short circuit and maximum volt-ampere (VA) output

Abnormal test: cooling fan stops, voltage selection switch is set incorrectly

*Electromagnetic Compliance test:

The power supply must comply with CISPR 22, CLASS B conduction and radiation with a 4dB margin. The power supply must be tested under the following three load conditions:

Each output is unloaded, each output is 50% loaded, each output is 100% loaded.

Conducted interference/immunity: Conducted interference/immunity through power lines

·Radiation interference/immunity: Radiation interference/immunity via magnetic field

*Reliability test:

Aging life test: high temperature (about 50-60 degrees) and long time (about 8-24 hours) full load test.

*Other tests:

·ESD: Electrostatic Discharge (caused by direct contact or intermittent discharge from a person or object) Under 2-15KV ESD pulse,

Each surface area of ​​the object under test should be subjected to 20 consecutive electrostatic discharge tests. The output of the power supply must continue to work without generating glitch.

Direct ESD contact should not cause overshoot or undershoot, which is beyond the voltage regulation range, and overvoltage protection (OVP), overcurrent protection (OCP), etc. In addition, when the ESD discharge voltage is as high as 25KV, it should not cause component failure.

EFT: Electrical Fast Transient or burst A series of switching noises are conducted through power lines or I/O lines (caused by the power supply or inside the building).

Surge: High-energy transient noise interference (caused by flickering of lights) passing through the power line.

VD/I: Dips and Interrupts: Power supply voltage drops or interruptions (caused by faults or errors in the power distribution system, such as power supply overload or air switch tripping)

Inrush: The impact of the power supply on the power supply system.

General Functions Testing

Output voltage adjustment:

When manufacturing a switching power supply, the first test step is to adjust the output voltage to within the specification range. This step is required to ensure that the subsequent specifications are met. Usually, when adjusting the output voltage, the input AC voltage is set to a normal value (115Vac or 230Vac), and the output current is set to a normal value or full load current. Then, a digital voltmeter is used to measure the output voltage of the power supply and its potentiometer (VR) is adjusted until the voltage reading is within the required range.

Power Regulation:

Power regulation is defined as the ability of a power supply to provide a stable output voltage when the input voltage changes. This test is used to verify that the power supply is in the worst power supply voltage environment, such as in the summer noon (due to high temperature, the power demand is the highest) when the power supply voltage is the lowest; and in the winter night (due to low temperature, the power demand is the lowest) when the power supply voltage is the highest. In the above two extremes, the stability of the power supply output meets the required specifications.

To accurately measure the power regulation, the following equipment is required:

A power supply that can provide variable voltage capability, at least the lowest to highest input voltage range of the power supply under test.

A RMS AC voltmeter to measure the input power voltage. Many digital power meters can accurately measure VAW PF.

A precision DC voltmeter with a regulation rate at least ten times higher than the object under test, generally using a 5-digit or higher precision digital meter.

A variable electronic load connected to the output of the device under test.

*The test steps are as follows: After the power supply under test is warmed up and stabilized under normal input voltage and load conditions, measure and record its output voltage values ​​at low input voltage (Min), normal input voltage (Normal), and high input voltage (Max).

The power regulation rate is usually expressed as the percentage of the output voltage deviation caused by the input voltage change under a normal fixed load, as shown in the following formula:

V0(max)-V0(min) / V0(normal)

The power regulation rate can also be expressed in the following way: when the input voltage changes, the deviation of the output voltage must be within the specified upper and lower limits, that is, within the absolute values ​​of the upper and lower limits of the output voltage.

Load Regulation:

Load regulation is defined as the ability of a switching power supply to provide a stable output voltage when the output load current changes. This test is used to verify whether the power supply meets the required specifications under the worst load environment, such as when the PC has the least number of peripheral cards and the hard disk is not in operation (due to the least load, the power demand is the smallest) and the load current is the lowest; and when the PC has the most peripheral cards and the hard disk is in operation (due to the most load, the power demand is the largest) and the load current is the highest.

*The equipment and connections required are similar to those for line regulation, the only difference being that a precision ammeter is required in series with the output of the power supply under test.

The test steps are as follows: After the power supply under test is heated and stabilized under normal input voltage and load conditions, measure the output voltage value under normal load, and then measure and record its output voltage values ​​(Vmax and Vmin respectively) under light load (Min) and heavy load (Max). The load regulation rate is usually the percentage of the output voltage deviation rate caused by the load current change under normal fixed input voltage, as shown in the following formula:

V0(max)-V0(min) / V0(normal)

Load regulation can also be expressed as follows: When the output load current changes, the output voltage deviation must be within the specified range.

Within the upper and lower voltage limits, that is, within the absolute values ​​of the upper and lower limits of the output voltage.

Comprehensive adjustment rate:

The comprehensive regulation is defined as the ability of a power supply to provide a stable output voltage when the input voltage and output load current change. This is a combination of the power regulation and load regulation. This test is a combination of the above power regulation and load regulation, which can provide a more accurate performance verification of the power supply under changing input voltage and load conditions. The comprehensive regulation is expressed in the following way: when the input voltage and output load current change, the output voltage deviation must be within the specified upper and lower voltage limits (that is, within the upper and lower absolute values ​​of the output voltage) or within a certain percentage limit.

Output noise (PARD):

Output noise (PARD) refers to the voltage value of the periodic and random deviation on the average DC output voltage when the input voltage and output load current remain unchanged. Output noise refers to all the unwanted AC and noise parts on the DC output voltage after voltage regulation and filtering (including low-frequency 50/60Hz power frequency multiplication signals, high-frequency switching signals above 20 KHz and their harmonics, and other random signals), usually expressed in mVp-p peak-to-peak voltage. The specifications of general switching power supplies are within 1% of the output DC output voltage as the output noise specification, and its bandwidth is 20Hz to 20MHz (or other higher bandwidths such as 100MHz, etc.). The worst conditions when the switching power supply is actually working (such as the maximum output load current, the lowest input power voltage, etc.). If the power supply is in a harsh environment, the output DC voltage plus the output instantaneous voltage after noise can still maintain a stable output voltage that does not exceed the output high and low voltage limits. Otherwise, the power supply voltage may exceed or be lower than the power supply voltage that the logic circuit (such as TTL circuit) can withstand, causing malfunction and further causing a freeze.

For example, for a 5V output, the output noise is required to be within 50mV (this includes all other changes such as power regulation, load regulation, dynamic load, etc., and the output instantaneous voltage should be between 4.75V and 5.25V to avoid malfunction of the TTL logic circuit). When measuring output noise, the PARD of the electronic load must be lower than the PARD value of the power supply to be measured, so as not to affect the measurement of output noise. At the same time, the measurement circuit must have good isolation processing and impedance matching. In order to avoid unnecessary interference, ringing and standing waves on the wire, dual coaxial cables are generally used with 50Ω at their ends, and differential measurement methods are used (to avoid noise current in the ground loop) to obtain correct measurement results.

Input power and efficiency:

The input power of a power supply is defined by the following formula:

True Power = Pav(watt) = V1 Ai dt = Vrms x Arms x Power Factor

That is, it is the integral value of the product of the input voltage and current in one cycle. It should be noted that Watt≠VrmsArms but Watt=VrmsArmsxP.F., where PF is the power factor. Usually the power factor of a power supply is around 0.6-0.7, and a high-power power supply with a power factor corrector usually has a power factor greater than 0.95. When the input current waveform and the voltage waveform are exactly the same, the power factor is 1, and depending on the degree of difference, the power factor is between 1 and 0.

The efficiency of a power supply is defined as:

ΣVout x lout / True Power (watts)

It is the ratio of the total output DC power to the input power. Usually, the efficiency of a power supply for a personal computer is about 65% to 80%. The efficiency provides verification of the correct operation of the power supply. If the efficiency exceeds the specified range, it means that there is a problem with the design or component material. If the efficiency is too low, it will increase heat dissipation and affect its service life. Due to the recent attention to environmental protection and

Energy consumption is becoming more and more important. For example, the Energy Star requirement for computer power supplies is: when the AC input power is 30Wrms, the efficiency must be above 60% (that is, the DC output power must be higher than 18W at this time); and for ATX switching power supplies, the input power should not exceed 5W in the DC Disable state. Therefore, the AC power test instrument needs to be both accurate and have a wide range to meet the needs of this test.

Dynamic load or transient load

A power supply with a constant voltage output has a feedback control loop in its design, which can continuously maintain a stable output voltage. Since the feedback control loop actually has a certain bandwidth, it limits the power supply's response to changes in load current. If the phase shift between the input and output of the control loop exceeds 180 degrees when the gain (Unity Gain) is 1, the output of the power supply will become unstable, out of control, or oscillate. In fact, the load of the power supply during operation is

The load current also changes dynamically, rather than remaining constant (such as hard disk, floppy drive, CPU or RAM operation, etc.), so dynamic load testing is extremely important for power supplies. Programmable electronic loads can be used to simulate the worst load conditions when the power supply is actually working, such as the slope and cycle of the load current rising and falling rapidly. If the power supply can still maintain a stable output voltage without overshoot or undershoot under the bad load conditions, otherwise it will cause the output voltage of the power supply to exceed the load component (such as the output instantaneous voltage of the TTL circuit should be between 4.75V and 5.25V, so as not to cause the TTL logic circuit to malfunction) and malfunction, further causing a freeze.

Power Good/Fail Time (Power Good, Power Fail or Pok)

The power good signal, referred to as PGS (Power Good Signal or Pok High), is a signal sent by the power supply to the computer system. When its output voltage is stable, it notifies the computer system so that the boot process can be performed. The power failure signal (Power Fail or Pok Low) is the power supply indicating that its output voltage has not reached or dropped beyond a normal working state. The above is usually represented by the logic change of a "PGS" or "Pok" signal. When the logic is "1 or High", it indicates that the power is good (Power Good), and when the logic is "0 or Low", it indicates that the power fails (Power Fail). Please refer to the timing diagram in Figure 5:

The power good time of a power supply is the time from when its output voltage stabilizes to when the PGS signal changes from 0 to 1, and the general value is between 100ms and 2000ms. The power failure time of a power supply is the time from when the PGS signal changes from 1 to 0 to when its output voltage falls below the voltage regulation range, and the general value is more than 1ms. The electronic load can directly measure the power good and power failure times, and can set upper and lower limits as a judgment of whether it is qualified.

Set-Up Time and Hold-Up Time

The startup time is the time from when the power supply is connected to the input until the output voltage rises to within the voltage regulation range. For example, for a 5V output power supply, the startup time is the time from when the power supply is turned on until the output voltage reaches 4.75V.

The hold time is the time from when the power supply is cut off to when the output voltage drops out of the regulation range. For example, for a 5V output power supply, the hold time is the time from when the power supply is turned off to when the output voltage drops below 4.75V.

The general value is 17ms or 20ms or above to avoid being affected by the power company's power supply being shortened by half a week or a week.

other

Power Up delay: +5/3.3V rise time (the time from 10% to 90% voltage)

·Remote ON/OFF Control: Remote ON/OFF control

Fan Speed ​​Control/Monitor: Cooling fan speed "control" and "monitor"

Protection function test

Overvoltage protection (OVP) test

When the output voltage of the power supply exceeds its maximum limit voltage, its output will be shut down to avoid damaging the circuit components of the load. This is called overvoltage protection. The overvoltage protection test is used to verify that the power supply will not produce abnormal output high voltage when the above abnormal conditions occur (when the feedback control circuit or parts inside the power supply are damaged).

Overvoltage protection is particularly important for some voltage-sensitive loads, such as CPU, memory, logic circuit, etc., because if these valuable components are operated at too high a voltage and exceed their rated value, they will be permanently damaged, resulting in heavy losses. When the power supply is overvoltage, its output voltage waveform is shown in Figure 7.

Short circuit protection test

When the output of the power supply is short-circuited, the power supply should limit its output current or shut down its output to avoid damage. The short-circuit protection test verifies whether the power supply can respond correctly when the output is short-circuited (which may be caused by incorrect wiring connection or short-circuit of the components or parts of the power supply).

Over current protection OCP test

When the output current of the power supply exceeds the rated value, the power supply should limit its output current or shut down its output to avoid damage due to excessive load current. If the internal parts of the power supply are damaged and cause a larger load current than normal, the power supply should also shut down or limit its output to avoid damage or danger. The overcurrent protection test verifies whether the power supply can respond correctly when any of the above conditions occur.

Over power protection OPP test

When the output power of the power supply (which can be a single output or multiple outputs) exceeds the rating, the power supply should limit its output power or shut down its output to avoid damage or danger due to excessive load power. If the internal parts of the power supply are damaged and cause a larger load power than normal, the power supply should also shut down or limit its output to avoid damage. The overpower protection test verifies whether the power supply can respond correctly when any of the above conditions occur. This test usually includes power limiting protection for two or more groups of output power, so it is slightly different from the above single output protection test (OVP, OCP, Short, etc.).