Power supply designers are faced with a variety of challenges throughout the design phase of a switching power supply. The challenges differ from verifying the emitted current harmonics compliance to international standard, improving power factor, troubleshooting noise, and improving efficiency. This article suggests how to make most out of oscilloscopes to get desired results faster and easier.

What is power application?
Power supply designers use oscilloscopes to measure and observe the voltage and current behavior on different circuitry parts of a switching power supply. Measurements are performed from the AC input circuitry, switching circuitry and DC output circuitry. The usefulness of the oscilloscope for this purpose has created specific measurements that perform post calculations within the oscilloscope. Designers no longer need to manually transfer data for post calculation themselves. A switching power supply basically has four blocks. The blocks are categorized as rectifier circuit, switching circuit, output filter circuit and the control circuit (Figure 1).

The Agilent InfiniiVision 3000 X-series oscilloscope�s power application software provides analysis for these main blocks. Block A, the rectifier circuitry usually uses a bridge rectifier to convert an AC signal to a DC signal. Analyses associated to this block are:

* Current Harmonics � This analysis provides current harmonics emission checking standard EN 61000-3-2 (Class A, B, C and D) in graphical display.

* Power Quality - This analysis measures the power factor, real power, apparent power, reactive power, crest factor and phase angle.

* Inrush Current � This analysis measures the peak inrush current when the power supply is first turned on.

Block B, the switching circuitry is the core of the switching power supply. It�s a DC to DC converter that uses switching devices like MOSFET and IGBT. Analyses associated with this block are:

* Switching Loss � This analysis characterizes the power loss of the switching devices. This parameter is important as it affects the efficiency of the power supply.

* Modulation � This analysis provides visual insight of the PWM signal changes in plot of duty cycle over time. User can also select other format of frequency, period, positive pulse or negative pulse over time plots.

* Slew Rate � This analysis measures the dV/dt and dI/dt (rate at which the voltage or current changes at switching)

Block C, the output filter circuitry is a DC to DC converter that reduces the noise of the signal to be output. Analyses associated with this block are:

* Output Ripple � This analysis measures the ripple of the output voltage.

* Transient Response � This analysis measures the time needed for the voltage to stabilize after a load change.

* Turn On/Turn Off � This analysis measures the time it takes for the power supply to turn on and turn off.

* Efficiency � This analysis measures the overall efficiency of the power supply by measuring the output power over the input power. This requires a 4 channel oscilloscope.

How can the 3000 X-series oscilloscope�s power application help make measurements faster and easier? Usually when performing a measurement, one would refer to a user guide for connection guide either in hard or soft copy. However, sometimes these documents are nowhere to be found and need to be searched or downloaded. This can happen when using common or shared instruments in a lab. With the DSO-X 3000 series oscilloscope, one has little to worry about. The connection diagram for all the power application analysis is displayed on screen when the user enters the Signals menu. This is also available for the deskew menu. Refer to Figures 2 and 3.

The 3000 X-series oscilloscope�s power application provides an auto setup feature that performs the setup of signals by automatically setting the vertical and horizontal orientations. It will maximize the vertical scale on screen while using user settings to determine the horizontal settings. In addition to that it also adds labels to the signal to clearly identify the voltage or current waveform. By using the auto setup, the user gets consistent waveform setups every time. This is available under the Signal�s menu in Figure 3. Generally, after performing auto setup user can go back to the main menu and press the Apply soft key to perform the measurements. However if user wanted to make some change after the auto setup they can do so before pressing the Apply soft key.

The auto setup is different than performing an auto scale only as auto scale would display all channels waveform nicely on screen but lack the maximization of the waveform�s vertical display of each channel. It is important to maximize the vertical display of the waveform for better measurement resolution. Refer to Figures 4 and 5 for the comparison of power quality measurement on the same signal source using auto scale and auto setup. The waveforms are adjusted to have five cycles.

The sensitivity of waveforms capture when not maximizing the vertical scale causes a bigger variation on real power value as shown in Figure 4. The real power values using auto setup is of lower value as shown in Figure 5. This is a good sign of measurement accuracy. Not all automated setups are created equal, generic setups like auto scale is intended for visual assistance. For meaningful automated setups, use the dedicated auto setup feature in power application as it is intended to assist designers to get reliable results faster and easier.

The other advantage of the automated setup is the auto deskew. Deskew compensates the delay differences between the voltage and current probes. This is using the U1880A deskew fixture shown in Figure 6. Each probe has a different delay from probe tip to the oscilloscope due to different cable length. Current probes usually have larger delays than voltage probes.

It is important to perform deskew when measuring high frequency signals, an example for switching loss. The delay different between voltage and current can impact the measurement of power loss (Figure 7). On the left, P1 waveform is the multiplication of V and I signal, the area below P1 is the power. On the right is the same V and I signal but I (Skew) signal is skewed a little to the left. This skew has caused the P2 waveform shape to be wider and the area below P2 to be larger than P1 indirectly increasing the power value. This example shows that skewing can affect the accuracy of the power measurement.

At a press of a button, the automated deskew is performed accurately and consistently, thus making setups fast and easy. This feature is exclusively available on the 3000 X-series oscilloscope from Agilent only. Refer to Figure 8 below on a completed auto deskew operation. The menu shows the result of the skew value set to channel 2 (Current) with value of -60.0ps

Getting more
Some power supply measurement can be performed manually, for example turn on time, turn off time or transient response. The 3000 X-series oscilloscope provides these measurements with some guided steps and measurement capability. The advantage in using these built-in measurements is the consistency in measurement. Once the settings are provided, the cursor positions for measuring are placed consistently based on calculations. It is useful to setup consistent measurement setups when comparing results with different parties. This can reduce chances for dispute and ambiguity. Figure 9 shows a sample of the automated cursor placing start and end time for measuring turn on time. The start time is defined as the time when the input AC voltage first rises to 10% of its maximum amplitude; the left most cursor and the end time is defined as the time when the output DC voltage rises to 90% of its maximum amplitude; the right most cursor.

The 3000 X-series oscilloscope comes with a built-in 20MHz waveform generator. This hybrid of waveform generator and oscilloscope in the same box does have an advantage in this power measurement application. The Power Supply Rejection Ratio (PSRR) measurement is where these two entities work together and shine. Note that a separate waveform generator and oscilloscope do not provide the same capability in plotting frequency domain. This is due to the fact that the frequency change in the waveform generator and oscilloscope data capturing is not synchronous for logging in frequency domain. However, they could provide verification in time domain.

PSRR is a measurement to determine the ability of a circuit to reject/filter incoming ripples. The equation below shows the ratio of the output ripple over the input ripple. The unit is in dB.

This measurement is usually performed using network analyzers or gain/phase meter, but these dedicated instruments are expensive. Refer to Figure 10 for the setup of a PSRR test using the 3000 X-series from Agilent.

Compared to a network analyzer, it is difficult for this solution to provide better than -60dB due to the oscilloscope�s higher noise floor and lower vertical sensitivity. However, this cost effective solution does provide good value for simple verification. Refer to Figure 11 for an example screen shot of a PSRR analysis in action.

Summary
The 3000 X-series oscilloscope�s power application has useful features to help power supply designers be more productive. First, the online connection diagram and auto setup feature are very handy for speedy and reliable measurement setup. Second, the guided measurement for turn on time provides consistent placed cursor positions for repeatable measurement every time based on settings and calculations. Lastly, the new PSRR analysis provides great value utilizing the built-in waveform generator for a one box integrated cost effective solution. All these features are provided to assist designers to do what they do best; pushing technological limits.

About the author
Soo Beng Teh is a software engineer at Agilent Technologies' Oscilloscopes Product Division. He started his career in Agilent in 2002 as a test engineer working on power supplies, and later worked on oscilloscopes R&D. Soo Beng is currently a software engineer for the High Volume Scopes R&D team, and power application feature lead. He earned his Bachelor�s degree in Electrical and Electronics Engineering from the University Science of Malaysia.