PSU vs specialist hardware for low voltage testing | add2

PSU vs specialist hardware for low voltage testing

We often get asked “why can’t I use a power supply to do this test?”. This discussion attempts to highlight the key differences between power supply units (PSUs) and specialist low voltage test equipment, such as add2’s low voltage test unit LVTGO-VBS.

Output sinking and sourcing

Power supplies that can both source and sink output current are very rare and tend to be only available in very high-end PSUs.

As a general rule PSUs only source current from the positive pin and do not actively drive the negative pin downwards, pulling current into the PSU.

Additionally, PSUs may offer one or the other option, i.e. either ‘sourcing’ or ‘sinking and sourcing’ but not both on the same piece of hardware.

The LVTGO-VBS has a software controlled hardware configuration that allows the output stage to change between ‘sourcing only’ or ‘sinking and sourcing’.  It can also be added to the output of a sourcing only power supply to turn it into a ‘sinking and sourcing’ controlled output.

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Slew rate

Slew rate controlled power supply units are very hard to find. It is almost impossible to find a PSU with both controlled rise and fall times.

Power supplies traditionally offer very slow slew rates – in excess of 50ms on lower cost units – and on more expensive units this may drop to low numbers of ms.

The slew rate of the LVTGO-VBS is constant and configurable (depending on variant), offering three slew rate speeds with the option of allowing the vehicle impedance to define the fall time of the output or the test unit.

The fastest hardware slew rate on the LVTGO-VBS available is of the order of 3μs rise or fall time, but can also be configured to 10μs or 500μs for certain waveform types. Slower rates are additionally offered via the software interface, with rates as slow as a few days then being achievable.

The LVTGO-VBS can also be fitted to the output of a slow response power supply and with sufficient headroom can offer fast output response times.

Transient response

In addition to poor power supplies’ output slew rate, they may also suffer from poor transient response characteristics – thus when a high current, very fast pulse is taken from the output of a PSU, there will then be a small dip in the voltage until the control circuit responds.

The LVTGO-VBS has a controlled output that offers stepped load response in the order of 500μs on the standard units and better than 10μs on the microCUT models.

The LVTGO-VBS can also be fitted to the output of a slow response power supply and with sufficient headroom can offer fast output response times.

Noise levels

The majority of D.C. power supply units available are of the switched-mode converter type, which are chosen for efficiency, size and cost.

These output types tends to inject noise pulses in the order of a few hundred KHz on the output when switching occurs inside the unit.

The LVTGO-VBS can be driven from a battery if necessary and is based on a sophisticated, high performance linear design stage which does not add switching noise.

Linear vs switched Mode

Using a switched mode PSU to feed the LVTGO-VBS is possible, as the LVTGO-VBS hardware tends to clean up the switching signals, provided there is sufficient headroom.

LVTGO-VBS units with integrated PSUs can offer the best of both worlds by minimising power loss at the same time as achieving all the capabilities of the test equipment range. Special software controls the input voltage to minimise losses.

Waveform generation

This area is one of the weakest for off-the-shelf power supply units. While some offer basic pre-programmed waveform shapes such as cranking waveforms, they do not offer a wide enough range of waveforms to meet the many standards that are now in place to support automotive testing.

In addition, playing a constant waveform out of a power supply does not provide any means of ‘windowing in’ on areas of susceptibility, and certainly does not allow the more sophisticated methods offered by the LVTGO-VBS units.

The LVTGO-VBS offers complex, yet easy to use, pseudo randomisation techniques to allow you to window in onto certain areas of susceptibility.

The LVTGO-VBS offers a wide range of waveforms designed for meeting standards, along with the ability to use captured data from other sources, such as oscilloscopes, as CSV data.

It is also rare for power supplies to offer external control (say via a BNC socket) to modulate the output voltage via a separate arbitrary waveform generator, and of course you then need both of these pieces of equipment too.

The LVTGO can also be purchased with an external BNC option allowing any 0-10V signal to control the LVTGO output as desired.

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