Friday, November 19, 2010

Power Supply Performance when using Remote Sense

In test systems the cabling path impedance between power supply and DUT can be significant. Meters of cabling combined with switch relays can equate to large voltage drop during times of high current consumption by the DUT. Its probably no secret to you that the method to compensate for this is to use the power supply's remote sensing capabilities. But what you may not know is the effect that remote sense has on the output performance of a supply. For most test applications this degradation to performance can be ignored. When it  becomes a factor is when a DUT makes large and fast load changes during operation. An example of this would be communication equipment when it goes into a transmit state.
When in remote sense mode the power supply is a feedback control system designed to control a voltage at the DUT. The figure below represents the voltage control system in block diagram form.


"Vprog" represents the desired voltage at the DUT. The summing amplifier compares the desired output voltage with the voltage at the DUT as measured by the sense amplifier. Any difference is amplified and applied as a correction signal to the power control circuit block. This block adjusts the power supply output until the measured output voltage equals the desired output, compensating for any external voltage drop. As shown in the block diagram the load and lead impedance become part of the voltage control system.
This system provides great control of the average DC voltage at the DUT. However, the accuracy to which the instantaneous voltage can be controlled for sharp load changes depends on several factors, namely:
1. The resistance and inductance of the cable connecting the power supply and DUT.
2. The input impedance of the DUT.
3. The amplitude, rise and fall times of the load change or current pulse.
4. The bandwidth of the power supply control loop.
5. The voltage slew rate and transient response capability of the power supply.
The final result is that while the average voltage may be ideal, the instantaneous value may be
less than ideal. If the instantaneous voltage drops too low it can cause your DUT to go into a reset mode which ruins your test. There are three common ways to compensate for this: minimize path impedance, add filtering to the DUT, and use high performance supplies.
To lower path impedance (both R and L) shorten wire lengths when possible, use larger gauge wire, make good connections, use twisted pair, pay attention to contact resistance, and try to avoid hot switching. A second solution is to place a large electrolytic capacitor across the terminals of the DUT right at the test fixture.When the load goes through a sharp change the capacitor will charge or discharge to maintain a constant voltage giving the power supply time to catch up. This method does have drawbacks though such as longer settling times when a voltage change is made and distorted current measurements during sharp load transients due to the charging and discharging of the cap. Finally the best way is to just buy high performance power supplies with high control loop bandwidth and fast transient response (to see a post on the transient response spec click here). Look for transient response specs < 100 us.

Click here for more info on Agilent N6700 series of high performance power supplies

1 comment:

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