Tuesday, September 30, 2014

How Do I Properly Wire My Output?

Hi everyone,















Monday, September 29, 2014

Properly sequence multiple power inputs to protect your DUT







Wednesday, September 17, 2014

Simulating battery contact bounce, part 1

One test commonly done during design validation of handheld battery powered devices is to evaluate their ability to withstand a short loss of battery power due to being bumped and the contacts momentarily bouncing open, and either remain operating or have sufficient time to handle a shutdown gracefully. The duration of a contact bounce can typically range anywhere from under a millisecond to up to 100 milliseconds long.

To simulate battery contact bouncing one may consider programming a voltage drop out on a reasonably fast power supply with arbitrary waveform capabilities, like several of the N675xA, N676xA, or N678xA series modules used in the N6700 series Modular DC Power System or N6705B DC Power Analyzer mainframe, shown in Figure 1. It is a simple matter to program a voltage dropout of specified duration. As an example a voltage dropout was programmed in Figure 2 on an N6781A SMU module using the companion 14585A software.

 Figure 1: N6700 series and N6705B mainframes and modules

Figure 2: Programming a voltage drop out using the N6705B and N6781A SMU module

While a voltage dropout is fine for many applications, like automotive, in many situations it does not work well for simulating battery contact bounce. The reason for this is there is one key difference to note about a voltage dropout versus a battery contact bounce. During a voltage dropout the source impedance remains low. During a battery contact bounce the source impedance is an open circuit. However, a DC source having the ability to generate a fast voltage dropout is a result of it being able to pull its output voltage down quickly. This is due to its ability to sink current as well as source current. The problem with this is, for many battery powered devices, this effectively short-circuits the battery input terminals, more than likely causing the device to instantly shut down by discharging any carry-over storage and/or disrupting the battery power management system. As one example consider a mobile device having 50 microfarads of input capacitance and draws 4 milliamps of standby current. This capacitance would provide more than adequate carryover for a 20 millisecond battery contact bounce. However, if a voltage dropout is used to simulate battery contact bounce, it immediately discharges the mobile device’s input capacitance and pulls the battery input voltage down to zero, as shown by the red voltage trace in Figure 3. The yellow trace is the corresponding current drain. Note the large peaks of current drawn that discharge and recharge the DUT’s input capacitor.

 Figure 3: Voltage dropout applied to DUT immediately pulls voltage down to zero

One effective solution for preventing the DC source from shorting out the battery input is to add a DC blocking diode in series with the battery input, so that current cannot flow back out, creating high impedance during the dropout. This is illustrated in Figure 4.

Figure 4: Blocking diode added between SMU and DUT

One thing to note here is the diode’s forward voltage drop needs to be compensated for. Usually the best way to do this just program the DC source with the additional voltage needed to offset the diode’s voltage drop. The result of this is shown in Figure 5. As shown by the red trace the voltage holds up relatively well during the contact bounce period. Because the N6781A SMU has an auxiliary voltage measurement input it is able to directly measure the voltage at the DUT, on the other side of the blocking diode, instead of the output voltage of the N6781A. As seen by the yellow current trace there is no longer a large peak of current discharging the capacitor due to the action of the blocking diode.

 Figure 5: Blocking diode prevents voltage dropout from discharging DUT 

Now you should have a much better appreciation of the differences between creating a voltage dropout and simulating battery contact bounce! And as can be seen a blocking diode is a rather effective means of simulating battery contact bounce using a voltage dropout. Stay tuned for my second part on additional ways of simulating battery contact bounce on an upcoming posting.

Wednesday, September 10, 2014

How do I protect my DUT against my power supply sense lines becoming disconnected, misconnected, or shorted?

The remote sense lines are a vital part of any good system power supply. As shown in Figure 1, by using a second, separate pair of leads for sensing, the output voltage is now regulated right at the DUT rather than at the output terminals on the power supply. Any voltage drops in the force leads are compensated for; assuring the highest possible voltage accuracy is achieved right at the DUT.

Figure 1: Remotely sensing and regulating output voltage at the DUT

Of course for this to work correctly the sense leads need to have a good connection at the DUT. However, what if the sense leads become disconnected, misconnected, or shorted?

One might think if one or both of the sense leads became disconnected, the sensed voltage would then become zero, causing the output voltage on the force leads to climb up out of control until the over voltage protect (OVP) trips. This turns out not to be the case, as a co-contributor here, Gary had pointed out in a previous posting “What happens if remote sense leads open?” (Click here to review). Basically a passive protection mechanism called sense protect maintains a backup connection between the sense line and corresponding output terminal inside the power supply in the event of a sense line becoming disconnected.

While sense protect is an indispensable feature to help protect your DUT by preventing runaway over-voltage, if a sense lead is open the voltage at your DUT is still not as accurate as it should be due to uncompensated voltage drops in the force leads. This can lead to miscalibrated DUTs and you would not even know that it is happening. To address this some system power supplies include an active open sense lead fault detection system. As one example our 663xx Mobile Communications DC Sources check the sense lead connections during each output enable and will issue a fault protect and shut down the output if one or both sense leads become disconnected. It will also let you know which of the sense leads are disconnected. It can be enabled and disabled as needed. I had written about this in a previous posting “Open sense lead detection, additional protection for remote voltage sensing” (Click here to review).

Taking sense protection further, we have incorporated a system we refer to as sense fault detect (SFD) in our N6900A and N7900A Advanced Power System (APS). It can be enabled or disabled. When enabled it continually monitors the sense lead connections at all times. If it detects a sense fault it sets a corresponding bit in the questionable status group register as well as turn on status annunciator on the front panel to alert the user, but does not disable the output. Through the expression signal routing system a “smart trigger” can be configured as shown in Figure 2 to provide a protect shutdown on the event of a sense fault detection.  In all, sense fault detect on APS provides a higher level of protection and flexibility.

Figure 2: Configuring a custom opens sense fault protect on the N6900/N7900 APS

What happens if the sense leads become shorted? Unlike open sense leads, in this case the output voltage can rise uncontrolled. The safeguard for this relies on the over voltage protect system. The same thing happens if the sense leads are reversed. The power supply will think the output voltage is too low and keep increasing the output voltage in an attempt to correct it. Again the safeguard for this relies on the over voltage protect system. The N6900/N7900 APS does actually distinguish the difference when the sense leads are reversed by generating a negative OVP (OV-) fault, giving the user more insight on what the fault is to better help in rectifying the problem.

Remote voltage sensing provides a great benefit by being able to accurately control the voltage right at the DUT. Along with the appropriate safeguards against sense lead misconnections you get all the benefit without any of the corresponding risks!

Remote sense protect and sense fault detect were just two of many topics about in my seminar “Protect your device against power related damage during test” I gave last month. As it was recorded it is available on demand if you have interest in learning more about this topic. You can access the sign up from the following link: (Click here for description and to register)