High precision power current testing technology in the field of electronic power

1、 Firstly

Power conversion efficiency testing of power regulators and inverters? High precision power (current and voltage) testing is required in various aspects of power electronics, such as motor efficiency testing and reactor loss testing.

2、 Regarding the testing method of current

The current test of a power analyzer is generally conducted through direct measurement(image1(a))Compared to the current sensor method(image1(b))One of them is to proceed.

Below, we will introduce their respective characteristics.

image1 Direct measurement method(a)And the current sensor method(b)

1Direct measurement method

The direct measurement method is to directly connect the test line of the test object to the current terminal of the power analyzer for testing. This method has a relatively simple testing principle and has been used since the past because the power meter itself has the advantage of being able to test current. However, connecting the current test line to the current input terminal of the power analyzer and directly inputting the current into the test circuit may have the following defects.

(1)During testing, the state of the test object is different from the actual running state.

(2)The impedance of the test line used will increase losses.

(3)Wiring room and wiringGNDGenerate capacitance between them, increasing high-frequency leakage.

For example, as mentioned above(2), using5mlongAWG6The impedance of the test line and wiring is approximately6.5mΩIf the current of the test object is30AThe loss caused by wiring impedance is5.85WJust5.85WIn terms of this value, it cannot be determined whether it is large or small, but based on the power value of the test object, this loss cannot be ignored.

In addition, the direct measurement method generally uses shunt resistors to test the current. This method of shunt resistance has the following drawbacks.

(1)Current passing through a shunt resistor will generate current2Joule heating of the power ratio. If these heat are calculated into the loss of the measuring device, it is due to the change in the resistance value of the shunt resistor caused by self heating, which affects the accuracy of the test.

(2)To suppress Joule heating, a shunt resistor with a smaller resistance value can be chosen. However, shunt resistors with smaller resistance values cannot ignore very few induced components, which can deteriorate the frequency characteristics.

All of these are important reasons that affect the accuracy of current and power testing, especially when testing high currents.

image2 The self heating of shunt resistors

in the figure2It explains20AThe current passes through2mΩThe phenomenon of self heating when the shunt resistor is used. For comparison, connect our company's rated wires on the wiring50AThe current sensorCT6862The shunt resistor heats up due to Joule heating, causing the temperature to rise to50The degree of temperature. On the other hand, the current sensor is not affected by Joule heating and has almost no heating phenomenon on its own. The loss of the measuring device and the temperature characteristics of the sensor itself have almost no impact on the testing accuracy.

Based on the above discussion, the direct measurement method is used in the standby power testing of electronic devicesLEDIn terms of power consumption testing for lighting, due to the relatively small influence of Joule heating from shunt resistors, when testing small currents(1Adegree)Time is very effective.

2Current sensor method

Current sensor method. It is a method of connecting a current sensor to the wiring of the measured object, and inputting the output signal (current or voltage) of the sensor into a power analyzer for current testing. By using a current sensor, the state of the tested object during testing is the same as the actual operating state. And it generates very little heat during high current, which has no impact on the accuracy of the test. Current sensors are commonly used in the field of power electronics. in the figure3, represents the range of high-precision testing and frequency range that can be achieved by both direct testing and current sensor methods. It should be noted that this does not mean that each method cannot test the parts outside the range represented in the graph.

image3 The range and frequency range of current values that can be accurately measured using direct measurement and current sensor methods.

(It doesn't mean that the parts outside the range in the picture cannot be tested)

3、 High precision testing of power using current sensors

As mentioned earlier, it generally exceedsUsed under current conditions. The method of current sensor, like linear measurement, is not without defects. In order to conduct high-precision current testing, there are also several points that need to be noted. In this chapter, we will explain the precautions for using current sensors for high-precision power testing.

1Selection of suitable current sensors

The prerequisite for conducting high-precision and highly repetitive power testing using current sensors is to choose a suitable current sensor. As a specific benchmark for selection, let's first give the following two examples:

(1)The rated current value of the current sensor matches the current value of the test object.

(2)The frequency range that can be tested by a current sensor should include all frequency components of the tested object's current.

And, above2On the basis of points:

(3)The testing accuracy of the frequency range covered by the current sensor should be above the requirements of the test object.

(4)The main causes of errors in current sensors, such as output interference, temperature characteristics, conductor position, external magnetic field, magnetic field, common mode voltage, etc., must be based on specified criteria or even smaller.

So, great attention should be paid to the selection of current sensors. Especially regarding(3)The accuracy of a typical current sensor is defined asDCor50/60HzThe characteristics of other frequency ranges are often easily overlooked. It is necessary to pay attention. In order to conduct high-precision current testing using sensors on power analyzers, it is important to note that the current sensor must have sufficient performance.

2Including the overall integration of current sensor power testing system

In order to conduct high-precision power testing using current sensors, as mentioned in the previous chapter, it is necessary not only to select suitable current sensors but also to develop a power testing system that includes current sensors. That is to say, no matter how high the testing accuracy of the current sensor is, if the output signal of the sensor cannot be transmitted to the power analyzer normally. So it is also impossible to conduct high-precision current testing.

image4 General power testing system

in the figure4A general power testing system including a current sensor is displayed. And, as mentioned earlier, there are current output signals and voltage output signals for current sensors. Generally speaking. Current sensors are more widely used than voltage sensors, and this discussion is based on the premise of using current sensors.

The condition for correctly transmitting the signal from the current sensor to the power analyzer.

(1)The power supply of the sensor should have good power quality.GNDThe acquisition method should be appropriate.

(2)Between wiring and wiringGNDThe binding capacity between them should be small and have strong anti-interference ability.

(3)The frequency characteristics of the current input part of the power analyzer are good, with less heat generation and insulation performance（CMRRHigh, low leakage).

In addition, it has high anti-interference performance,GNDAppropriate acquisition methods and other conditions that can be listed.

Generally speaking. The current situation is that the current sensor, power supply for the sensor, and power analyzer are all from different manufacturers. The types of testing wires and wiring methods are all carried out according to the customer's instructions. In this situation, meeting all the above conditions, the output signal of the current sensor is transmitted normally to the power analyzer, ensuring the actual high-precision test current value. For manufacturers of current sensors. Manufacturer of power analyzer. Moreover, for manufacturers of power supplies for sensors, it is undoubtedly very difficult.

On the other hand, our company has been developing current sensors and power analyzers with our own technology since the past. All elements of the power testing system can be completed by a single company. It is also the manufacturer of all related products that can be achieved in the world.

Our company's power testing system has the following characteristics.

(1)The current sensor is a voltage output type that can calibrate the full frequency band accuracy listed.

(2)Use a voltage output type current sensor as the exclusive current input of the power analyzer, so that the output voltage level of the sensor matches the input voltage level of the current input part of the power analyzer to the fullest extent possible.

(3)The power analyzer is equipped with a power supply for sensors, and the quality of the power supply to the sensors must be standardized by our company when specifying the accuracy. Moreover, our company's power analyzer, sensor power supply, andGNDAt the same time, in addition to the important reason of grounding circuit, repeated evaluations and improvements have been made to improve testing accuracy and repeatability.

(4)Using shielded wire as the output circuit of the sensor to prevent interference while adjusting the gain to counteract the small voltage drop caused by the circuit. And our company combines current sensors and power analyzers to evaluate testing accuracy and conduct anti-interference tests both internally and through third-party certification agencies. in the figure5 It also displays our company's current sensor(CT6862TheCT6863、9709TheCT6841TheCT6843、3274)And power analyzer(PW6001)The anti-interference of the combined power testing system is tested in a scenario conducted by a third-party certification agency. That's why, in order to achieve the best overall matching of the system, while designing each constituent element, the adaptability of the constituent elements in the entire system was also evaluated.