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Test & measurement


Tektronix supplies a range of oscilloscopes and instruments to meet different requirements


pressed, the +5V standby voltage is turned on, enabling the switching conver ter to star t. After the +12V output is in regulation, the Power Good (PW OK) signal goes high to indicate to the load that the supply is reliable. The +5V standby voltage signal provides a


simple rising edge trigger for the acquisition of the relevant signals. Automatic measurements verify that the delay to the output voltage turn-on is <100ms, and the delay from output voltage turn-on to PW OK is in the specification range of 100 – 500ms.


Turn-off delay wiTh remoTe on/off


After the power supply’s main switch is turned off, the switching conver ter is turned off and the output voltage decreases. The power supply is specified to remain in regulation for at least 20ms after the switch is pressed. Most impor tantly, the PW OK signal is specified to fall 5 – 7ms before the +12V output voltage falls out of regulation, allowing the load time to react and shut down cleanly. As shown in Figure 2, the PW OK signal provides a falling edge trigger for the acquisition of the relevant signals. The waveform cursor measurement verifies that the PW OK pre-warning signal is operating as specified.


Verify Timing oVer mulTiple power cycles


To verify that the power supply turn-on timing remains within specifications over multiple power cycles, infinite persistence can be used to display the signal timing variations and statistics displays of automated timing measurements quantify the variations. In the setup in Figure 3,


20


Figure 4


the 50 percent point of the +5V standby voltage serves as the timing reference. The turn-on sequence is repeated 10 times and the timing variations over the 10 turn-on cycles are within a little over one per cent.


power supply rise-Time measuremenTs


In addition to the power supply sequencing, the rise-times of power supplies must be controlled to meet the specifications of some critical components in a system. Automated rise- and fall-time measurements are also made based on voltage reference points which are, by default, automatically calculated to be 10 per cent and 90 per cent of the signal amplitude of each channel. In the example in Figure 4, the rise-


times of the positive supplies and the fall-times of the negative supplies are shown in the results boxes on the right side of the display. The proliferation of power rails in today’s


systems presents a significant test and measurement challenge. To optimise for power consumption, performance and speed, even a relatively simple system can have a 12V bulk supply, a couple of 5V supplies, a 3.3V supply and a 1.8V supply. Verifying and troubleshooting the turn-on and turn- off sequence of these supplies can be more efficiently accomplished using oscilloscopes that offer more than the traditional four channels.


Tektronix www.tek.com April 2019 Instrumentation Monthly


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