Power supplies are normally regarded as simple animals and indeed they are – but 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, distill a lifetime’s experience of power supplies and power measurements into six quick, easy-to-apply tips
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.
Figure 1: A down programming circuit with an FET across the output terminals
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 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 equalising the impedance to ground from plus and minus output terminals. It is also important to equalise 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 the output voltage is higher than the programmed value, the FET activates and discharges the output capacitor. The FET can sink currents between 10 and 20 per cent 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.
Figure 2: A down programmer situated between power supply’s positive output and a negative source
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, it is important to avoid exceeding the floating voltage rating of any of the supplies or subjecting any of the power supplies to negative voltages. Each power supply should be programmed independently to deliver an equal fraction of the total output voltage and limit current to the maximum that the load can safely handle.
Connecting multiple power supplies in parallel provides higher currents, but again there are limitations. One unit must operate in constant voltage (CV) mode and the rest in constant current (CC) mode. The output load must draw enough current to keep CC unit(s) in CC mode.
In modern power supplies outputs can be grouped to create a single output with higher current and power capability.
Figure 3: Autoranging output characteristics
Tip 7: Simplify battery drain analysis with analysis tools
To adequately specify the power source for devices that exhibit pulsed and dynamic current loading, it is necessary to evaluate both the peak and DC average current draws.
A typical approach is to use an oscilloscope to monitor a shunt or a current probe, but it is simpler and cheaper to use a power supply with built-in measurement capabilities. Units such as the Keysight 66300 mobile communications DC source store up to 4,096 data points at sample intervals from 15µs to 31,200s. Like oscilloscopes, they acquire pre- and post-trigger buffer data by crossing a user-set threshold.
Device characterisation software works with DC sources that have battery emulation capabilities to accurately test designs for mobile, short-range radio, and wireless LAN access devices. Tests are facilitated by dynamic current characterisation, data logging and complementary cumulative distribution function (CCDF) measurements.
Tip 8: Characterise inrush current with an AC power source/analyser
Characterising inrush current versus turn-on phase can uncover component stresses, test whether a product produces AC mains disturbances that interact with other products, and help designers to select proper fuses and circuit breakers.
Traditionally this involves an AC source with programmable phase capability and an output trigger port, a digital oscilloscope, and a current probe. Advanced AC power source/analysers with built-in generation, current waveform digitisation, peak current measurement and synchronisation capabilities can perform inrush current characterisation without cabling and synchronising separate instruments. Similar analysers are available for DC measurements.
Tip 9: Use a power supply to measure DUT supply current
Accurately measuring DUT supply currents above 10A is beyond the range of typical DMMs in ammeter mode. One solution is to use an external shunt and the DMM’s voltage mode. Using the power supply itself is better. Many supplies boast an accurate measurement system, including a shunt and can be started via a single command to the power supply. With typical accuracy around ±0.5 per cent or better at full output levels, the advantages of using power sources to measure high currents is clear. Using them to measure low currents may not be so straightforward. Nevertheless, a power supply with multiple range readback caters for most requirements, offering full scale accuracy of 0.04 per cent + 15 µA at low range (100 mA) or 0.04 per cent + 160 µA at high range (3A).
Tip 10: Create DC power waveforms with list mode
Instead of using a DAC or arbitrary waveform generator to drive a power supply for creating DC power waveforms, there may be advantages to using a single power supply with list mode. List mode enables complex sequences of output changes to be generated with rapid and precise timing, which can be synchronised with internal or external signals. Complex DC power waveforms can be produced including pulse trains, ramps, staircases, low frequency sinewaves with DC offset, arbitrary voltage and current waveforms. Once a list of commands is stored in the power supply, the entire list is executed by a single command. Example applications include power supply rejection ratio test, simulating automotive crank profiles, and generating pulse dropouts.