Optimizing the performance of the zero-burden battery run-down test setup

Two years ago I added a post here to “Watt’s Up?” titled:  “Zero-burden ammeter improves battery run-down and charge management testing of battery-powered devices” (click here to review). In this post I talk about how our N6781A 20V, 3A 20W SMU (and now our N6785A 20V, 8A, 80W as well) can be used in a zero-burden ammeter mode to provide accurate current measurement without introducing any voltage drop. Together with the independent DVM voltage measurement input they can be used to simultaneously log the voltage and current when performing a battery run-down test on a battery powered device. This is a very useful test to perform for gaining valuable insights on evaluating and optimizing battery life. This can also be used to evaluate the charging process as well, when using rechargeable batteries. The key thing is zero-burden current measurement is critical for obtaining accurate results as impedance and corresponding voltage drop when using a current shunt influences test results. For reference the N678xA SMUs are used in either the N6705B DC Power Analyzer mainframe or N6700 series Modular Power System mainframe.

There are a few considerations for getting optimum performance when using the N678xA SMU’s in zero-burden current measurement mode. The primary one is the way the wiring is set up between the DUT, its battery, and the N678xA SMU. In Figure 1 below I rearranged the diagram depicting the setup in my original blog posting to better illustrate the actual physical setup for optimum performance.

Figure 1: Battery run-down setup for optimum performance

Note that this makes things practical from the perspective that the DUT and its battery do not have to be located right at the N678xA SMU.  However it is important that the DUT and battery need to be kept close together in order to minimize wiring length and associated impedance between them. Not only does the wiring contribute resistance, but its inductance can prevent operating the N678xA at a higher bandwidth setting for improved transient voltage response. The reason for this is illustrated in Figure 2.

Figure 2: Load impedance seen across N678xA SMU output for battery run-down setup

The load impedance the N678xA SMU sees across its output is the summation of the series connection of the DUT’s battery input port (primarily capacitive), the battery (series resistance and capacitance), and the jumper wire between the DUT and battery (inductive). The N678xA SMUs have multiple bandwidth compensation modes. They can be operated in their default low bandwidth mode, which provides stable operation for most any load impedance condition. However to get the most optimum voltage transient response it is better to operate N678xA SMUs in one of its higher bandwidth settings. In order to operate in one of the higher bandwidth settings, the N678xA SMUs need to see primarily capacitive loading across its remote sense point for fast and stable operation. This means the jumper wire between the DUT and battery must be kept short to minimize its inductance. Often this is all that is needed. If this is not enough then adding a small capacitor of around 10 microfarads, across the remote sense point, will provide sufficient capacitive loading for fast and stable operation. Additional things that should be done include:

• Place remote sense connections as close to the DUT and battery as practical
• Use twisted pair wiring; one pair for the force leads and a second pair for the remote sense leads, for the connections from the N678xA SMU to the DUT and its battery

By following these best practices you will get the optimum performance from your battery run-down test setup!

Open sense lead detection, additional protection for remote voltage sensing

A higher level of voltage accuracy is usually always needed for powering electronic devices under test (DUTs). Many devices provide guaranteed specifications for operating at minimum, nominal, and maximum voltages, so the voltage needs to accurate as to not require unacceptable amounts of guard banding of the voltage settings.

One very significant factor that affects the accuracy of the voltage at the DUT is the voltage drop in the wiring between the output terminals of the power supply and the actual DUT fixture, due to wiring’s inherent resistance, as shown in Figure 1.

 A standard feature of most all system DC power supplies is remote voltage sensing. Instead of the voltage being regulated at the output terminals of the DC power supply’s output terminal, it is instead sensed and regulated at the DUT itself, compensating for the voltage drop in the wiring. Additional details of this are documented in an earlier posting: “Use remote sense to regulate voltage at your load”

While remote voltage sensing addresses the problem of voltage drop in wiring affecting the voltage accuracy at the DUT, it then raises the concern of what happens if one of the sense lines becomes disconnected. Will the DC power supply voltage climb up to it maximum potential causing my DUT to be damaged?  Although this is a very legitimate concern, often the voltage is usually kept within a reasonable range of the setting by a feature referred to as “open sense lead protection”. A deeper dive on the issue of open sense lines and open sense lead protection are discussed at our posting: “What happens if remote sense leads open?”

Even with open sense lead protection and the voltage being kept within a reasonable range of the setting, this can be a concern for some customers who are relying on a high level of DC voltage accuracy at the DUT for test and calibration purposes. One categorical example of this is battery powered devices, where ADC circuits that need to precisely monitor the battery input voltage have to be accurately calibrated. If the voltage from the DC power supply has significant error, the DUT will be miss-calibrated.

One issue with open sense lead protection is it is a passive protection mechanism. It is simply a back up that takes over when a sense line is open. There is no way of knowing the sense lead is open. No error flag is set or fault condition tripped. The voltage being read back is the same as that is being regulated by the voltage sensing error amplifier, which is the same as the set voltage, so all looks fine from a read-back perspective. This is where open sense lead detection takes over. Open sense lead detection is a system that actively checks to see if the sense lines are doing their job. If not it lets the test system know there is a fault.

Open sense detection is not a common feature for most system DC power supplies. As one example we do employ it in our 663xx series Mobile Communications DC Sources as these are used for powering, testing and calibrating battery powered wireless devices. In the case of an open sense line condition it generates a fault condition and it keeps the output of the DC source powered down. It also provides status information on which of the sense lines are open as well.

What happens if remote sense leads open?

Remote sense is a feature on many power supplies that allows the power supply to regulate its output voltage right at your load (“remotely”) instead of at the power supply output terminals. Use remote sense when you want to compensate for load lead voltage drop caused by load current flowing in your load leads. This is accomplished by using a pair of remote sense leads that are in addition to your load leads. See an example in Figure 1. The power supply uses the voltage across the remote sense lead terminals to sense (measure) the voltage at the load terminals and regulate the voltage directly across the load by adjusting the output terminal voltage. Refer to this post I wrote last year on remote sense:
http://powersupplyblog.tm.agilent.com/2011/08/use-remote-sense-to-regulate-voltage-at.html

Remote sense leads could be accidentally left open, or once connected, one or both leads could inadvertently become open. I have had users of our power supplies testing very expensive devices under test (DUTs) ask me what would happen to the output voltage if a sense lead wired in a system opened; they were worried about subjecting their very expensive DUT to excessive voltage.

To understand why this is an important consideration, it is necessary to better understand the role of the sense leads. To regulate its output voltage, a power supply uses internal circuitry that acts as a feedback loop. The voltage is set to a particular value and the feedback loop monitors (measures) the voltage across the sense terminals and compares it to the setting. If it is too low, the loop circuitry increases the output voltage. If it is too high, the loop circuitry decreases the output voltage. So the actions of this loop result in the output voltage settling (being regulated) at a value such that the sense lead voltage equals the voltage set point.

If one or both of the sense leads is open, the feedback loop is broken and incorrect voltage information is sent to the loop. With an open sense lead, the sense voltage is typically near zero. The loop thinks the output voltage is too low and responds by increasing the output voltage. But this does not result in a corresponding increase in the sense lead voltage since the wire is broken so the loop increases the output voltage more. This continues until the value is increased to the maximum amount possible, which is usually somewhat higher than the maximum rated voltage of the power supply and very much beyond the desired set point. This could easily damage the DUT!

The scenario described in the previous paragraph is what would happen if no action was taken to prevent a runaway output voltage due to an open sense lead. Agilent power supplies have an internal circuit, called open sense protection, that prevents the output voltage from rising significantly above the set voltage if one or both of the remote sense leads is open. In fact, with one or both sense leads open, the output voltage of most Agilent power supplies will rise only 1 or 2 percent above the setting. Additionally, some Agilent power supplies can detect an open sense lead and respond by shutting down the output and alerting the user by changing a bit in a status register.

Note that this open sense protect circuitry is in addition to and independent from the over-voltage protection (OVP) circuitry common on most Agilent power supplies. OVP is a setting that is separate from the output voltage setting. If the actual output voltage exceeds the OVP setting, the OVP will shut down the output to protect the DUT.