Design Con 2015

When is a multichannel power supply more convenient than a single-channel power supply?

-May 28, 2013

Power supplies have long been considered among the basic essentials on every EE’s and circuit designer’s benchtop. As a result, a dizzying array of them is available to the instrument buyer, many of them optimized for specific applications. One common option is the inclusion of multiple outputs. In addition, multichannel units with high output accuracy are available at increasingly economical prices. But given that no equipment budget can afford every possible “bell and whistle” that test hardware vendors offer, it makes sense to ask which applications really demand a multichannel power supply and which ones don’t.

Are you creating a device that has both digital and analog circuitry or bipolar circuitry? If so, a multichannel supply is the more convenient power source. Triple-channel power supplies typically have two higher voltage channels for analog circuits (to power multi-voltage circuits or to create bipolar power supplies for testing bipolar analog circuits) and a third channel intended to power a digital circuit. The voltage for this third channel is typically 10V or less (for testing digital circuits operating at 5V or less). Pay close attention to how the power supply’s vendor specifies that channel. Some supplies have fixed (i.e., non-programmable) voltage output channels, which are often 5V channels. However, if the application requires testing digital circuitry that operates at 3.3V or 1.8V, a programmable third channel is essential.

Does your DUT require individual isolated power supply sections? If so, you will either need to configure multiple isolated supplies (which can be both expensive and less convenient to operate) or buy a multichannel supply. However, be aware that multichannel units can either have isolated outputs or they can have output channels that are tied to a common point on their low side. When the channels are connected with the same common point, they aren’t suitable for powering circuits that are isolated from each other. For example, medical monitoring devices have circuits that are in direct contact with patients. Those circuits use a reference point that is isolated from the earth ground-based circuitry on the power line side of the device. This is also true for devices that use opto-isolators to create separate, independent common reference points for different analog circuits or analog and digital circuits. For these applications (Figure 1), look for multichannel supplies with isolated outputs.   


Figure 1: Multichannel supply with isolated outputs

Do you need to be able to power circuits up and down in a specific sequence? A multichannel supply can do this much more simply than a set of individual supplies. Digital board designs often have circuits that operate at different voltages. When testing these circuits with external power supplies, it is important to power the circuits in the correct order to avoid stressing and damaging the low voltage circuits. When choosing a multichannel supply for this kind of application, make sure it allows independent control of each channel so that the channels can be powered on in a desired sequence. For example, as shown in Figure 2, channel 1 could output 1.2V to power a microprocessor core and FPGA circuit block, then channel 2 could power up 1.8V DDR memory circuits, and, finally, channel 3 could output 3.3V for an I/O circuit. When testing is complete, the channels can be powered down in the opposite sequence to de-energize the circuits, starting with the high-voltage circuit.


Figure 2. A multichannel supply with independent control of each channel allows channels to be powered on in a desired sequence.

Do you occasionally require sourcing more voltage or current than what the individual channels can provide? Some multichannel power supplies allow channels to be combined in series or in parallel to extend the ranges of the output voltage and the output current. In some cases, a multichannel supply might allow connecting two channels in series to increase voltage output, but not allow combining channels in parallel. This condition occurs if the two channels are not isolated, and one channel is set up to output a negative voltage relative to the other. For the maximum flexibility of the total voltage and current that can be delivered, look for a multichannel power supply with isolated output channels.

Do you need tracking functionality?
During circuit development, it’s essential to confirm the circuit operates within its performance specifications over its defined voltage operating range. Multichannel supplies with tracking functionality offer a convenient way to test a bipolar circuit by linking both channels (a positive-configured output and a negative-configured output) so they change synchronously with each other (Figure 3). Although some multichannel power supplies can track only with both channels outputting voltages of the same magnitude, newer offerings, such as the Keithley models 2220-30-1 and 2230-30-1, allow tracking with a variable ratio between the two channels.


Figure 3. Multichannel supplies with tracking functionality offer a convenient way to test a bipolar circuit by linking both channels.

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References
Keithley’s Application Note, Ensuring that Power Supply Performance Meets Your Requirements.

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