7 Replies Latest reply: Oct 10, 2012 10:12 PM by leoglh2001

Basic knowledge about power test

What is Power? How many kinds are they and how to calculate it accurately?

• 1. Re: Basic knowledge about power test

Power is the rate at which Energy (the ability to do Work) is used, in electrics/electronics this unit is typcally the Watt, which is taken to be Current multiplied by Voltage.

• 2. Re: Basic knowledge about power test

You reach the conception of Power.

Power, there are transient power, apparent power, active power, re-active power, we can use oscilloscope to calculate the square of Power waveform by using integration.

Apparent power = UI, unit: VA;

Active Power =UI*cosθ,unit: Watt;

Re-active Power =UI*sinθ, unit:Var

Power Factor= cosθ

• 3. Re: Basic knowledge about power test

Add some information about Active Power.

It is always calculated as following,

• 4. Re: Basic knowledge about power test

Using LeCroy oscilloscope, we can test power as shown in below picture,

Remarks:

1 The yellow waveform is voltage waveform;

2 The blue waveform is current waveform;

3 The orange waveform is power waveform.

For LeCroy WaveSurfer series oscilloscope, they can test all these power parameters automatically by selecting a PMA2 option.

• 5. Re: Basic knowledge about power test

One of the most difficult in power measurements is in-circuit dynamic saturation voltage across the switching device.  To understand why, we need to look at the waveform itself.

Before the device turns on, it will have hundreds or even thousands of volts across it.  When saturated, this voltage drops to only a few hundred mV.  An accurate saturation voltage measurement requires a resolution of less than 100 mV.  The measurement system must be able to handle the off state voltage and still resolve to 100 mV.  The requires linearity, overdrive recovery, AC compensation and signal to noise performance of  250 PPM - far beyond what the analog front end of an 8 bit DSO can provide.

To effectively make this measurement, an external high performance differential amplifier is required.

• 6. Re: Basic knowledge about power test

We can draw a list of what items should be tested when we test power,

1 Apparent power = UI,

2 Active Power =UI*cosθ

3 Re-active Power

4 Power Factor

5 Gate Drive Voltage

6 AC voltage

7 Rail voltage

8 Output voltage

9 SOA(Safe Operation Area)

10 Dynamic on-resistance

11 Conduction losses

12 Rush

13 Ripple

14 Saturation voltage

15 Deskew

16 Control loop stability

Can you find any other items that need to test when we deal with Power Test?

• 7. Re: Basic knowledge about power test

Ripple measurement:

Ripple is a very small signal that exist in power supply, it will harm many sensitive operation parts in a automatic system. Such as in a train, process control and defense industry.

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