FEATURE TEST & MEASUREMENT
With a little bit of thought you can get a lot more performance and functionality from the power supply that you’re using. Howard Peat, distribution sales manager, field sales engineer at Microlease, and Martin Dinmore, distribution field engineer at Keysight Technologies, examine ways to improve your operation and measurement capabilities
IMPROVING YOUR measurement capabilities
TIP 1: USE REMOTE SENSING TO COMPENSATE FOR LOAD-LEAD EFFECTS When a power supply leaves the factory, its regulation sense terminals are usually connected to the output terminals. This limits the supply’s voltage regulation abilities, even with very short leads. The longer the leads and the higher the wire gauge, the worse the regulation becomes. It gets even worse if relays are used to connect power to the load. Remote sensing, where the sense
terminals of the power supply’s internal feedback amplifier connect directly to the load, lets the power supply regulate its output at the load terminals rather than at its own output terminals. It is implemented by disconnecting local sense leads from the output terminals, then using twisted two-wire shielded cable to connect the power supply sensing terminals to the sense points on the load.
TIP 2: INCREASE SAFETY WITH REMOTE DISABLE FEATURE Remote disable offers a safe way to shut down a power supply in response to some particular operating condition or to protect system operators, for example when a cabinet door is opened unexpectedly or a panic button is activated. Implementation uses either remote
inhibit (RI - an input to the power supply that disables the output when the RI terminal is pulled low) or a discrete fault indicator (DFI) that provides a signal when the power supply detects a user-defined fault. DFI and RI can be daisy-chained without limits so that a fault in any supply disables all supplies in the system.
TIP 3: ELIMINATE NOISE FROM LOW-LEVEL MEASUREMENTS It is easier to eliminate noise than to filter it. Starting with a low-noise supply is naturally a great way to keep noise out of your measurements. Switch-mode supplies can be used successfully if their specifications include a low (<20mA) common-mode current. The next areas to consider are
36 JULY/AUGUST 2016 | INSTRUMENTATION
connections between power supplies and the device under test. Here, conducted noise is minimised by eliminating ground loops, ideally providing only one connection to ground. In rack systems the DC distribution paths need to be separate from other conductive paths that carry ground currents. Radiated pick-up is reduced by using
twisted shielded conductors for the output and remote sense leads. Cable shields should only be connected to ground at one end. Lower common-mode noise current is
achieved by equalizing the impedance to ground from plus and minus output terminals. It is also important to equalize the DUT’s impedance to ground from its plus and minus input terminals. Voltage spikes from the DUT can
be prevented by adding a bypass capacitor close to the load, which offers low impedance at the highest testing frequencies.
TIP 4: USE DOWN PROGRAMMING TO INCREASE TEST SPEED Power supply output capacitors discharge slowly under light or no load conditions. This becomes problematic when performing tests at varying voltage levels, since slow discharge means slower tests. To ameliorate this, down programming circuits in power supplies rapidly decrease the output voltage and considerably lowering discharge times. Two types of down programming circuits
are significant. In the first, an FET is placed across the output terminals. When
Keysight 66319D dual output mobile communications DC source provides two outputs for testing both the main battery power and the battery charger circuitry
The B2901A from Keysight is a precision source / measure unit featuring 1-channel with 100 fA/100 nV measurement resolution and 210 V, 3 A DC/10.5 A pulse source capabilities
the output voltage is higher than the programmed value, the FET activates and discharges the output capacitor. The FET can sink currents between 10 - 20% of the supply’s output current rating, resulting in slight degradation of the down programming current near zero volts. Alternatively, the down programmer is placed between the power supply’s positive terminal and a negative source, which pulls the output completely down with no degradation near zero. Some power supplies can
sink currents equal to their full output current rating, enabling them to be used as
either a programmable source or load.
TIP 5: SIMPLIFY SETUP WITH AUTORANGING POWER SUPPLIES With bench and rack space at a premium, being able to produce a wide range of voltage and currents with one power supply is beneficial, for example allowing DC/DC converters to be tested under several voltage and current combinations at around the same power level. A basic DC power supply has a rectangular output, having a maximum voltage (Vmax) and current setting (Imax) with a single maximum power point (Pmax = Vmax × Imax). More advanced power supplies have multi- range outputs. Today’s autoranging outputs satisfy many different voltage and current combinations and eliminate the need for many power supplies.
TIP 6: CONNECT POWER SUPPLIES IN SERIES OR PARALLEL FOR HIGHER OUTPUT Connecting two or more power supplies in series provides higher voltages. However,
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