In order to better understand how the Rohde & Schwarz HMP4040.04 stacks up with the competition, I like to begin my RoadTests by first looking at what the market has to offer and comparing on-paper specifications. This will give us an idea of how competitive the product is on specifications and pricing.
Surveying the Market … What Market?
The Rohde & Schwarz HMP4040.04 is a four-channel, 384W programmable DC power supply. Knowing this, my initial instinct was to scour the Newark website and a number of other suppliers to find power supplies of a hybrid-linear design with four channels and a comparable level of power. Rather unexpectedly, there are not many four-channel power supplies on the market at all! As a result, I broadened my criteria to cover all power supplies from well-known brands that had three or four channels so we had something to compare with. I excluded any specialty and modular power supplies owing to their extremely high cost and flexible specifications.
Please note – because of the summary nature of this market survey, it’s not possible to include every single product nor compare every single specification. On-paper specifications do not capture every performance aspect of a product and it is taken on good faith that the manufacturers are honest with their specifications. While this comparison is provided for informational purposes in good faith, I cannot be held responsible for any errors or omissions. Before purchasing, please do your own research to confirm the suitability of the equipment for your needs.
In the following comparison tables which features power supplies sorted by price, the rightmost column always represents the R&S HMP4040.04’s specifications.
With such a broad approach to a market survey, we find that the power supplies vary significantly in price and capabilities. At the low-end of the market (<US$1200), the total power outputs are relatively limited with some of the most basic supplies not offering four-wire connections for sensing and compensating voltage drops and some units sporting very basic user displays. This category is not really what the HMP4040.04 is aimed at, instead, R&S offer their NGE103 which has some of the benefits of the HMP-series but with more modern LCD interface and interfaces.
The majority of the supplies offer three channels, but the third channel is not “equal” to the other two – so it would be fairer to refer to these supplies as “2+1”. The only four rail supply in this table turns out to have severe restrictions on the third and fourth rails, so it’s more like “2+1+1” as these are “auxiliary” outputs.
The only exceptions seem to be the ELC ALR3206T with a beefy 400W total output over a “2+1” rail configuration which appears to be a purely switch-mode solution based on its low weight. Unfortunately, such solutions tend to have downsides with regulation performance and noise – the lack of full specifications and lesser known nature of the brand leaves some uncertainty especially as ripple voltage could be improved with large filter capacitors, at the cost of current limiting performance and potentially risking your device-under-test (DUT).
The Aim-TTi MX100TP is noteworthy a three-rail supply which is probably the first to even approach what the HMP4040.04 has to offer, with a rating of 315W total in three equal 105W rails. It is about half the price of the HMP4040.04, with internal series-parallel configuration and better connectivity. On the downside, the voltage and current is divided into fixed sub-ranges (35V/3A or 16V/6A) has lower programming/readback resolution, slightly poorer regulation, longer command processing time. In this price range, R&S also has a lower-cost HMC8043.02 that offers 4-wire sensing and three equal channels, it is a 100W unit only.
Moving into the higher end units (>US$1200), surprisingly, the situation is just as mixed with a plethora of relatively lower-powered units from bigger names commanding significant sums of money despite having rather limited connectivity in some cases and simple user display.
Of this bunch, the Aim-TTi PL303QMT-P and the QL355TP actually seem to offer less power and a more restrictive “2+1” configuration than the MX100TP, in return for slightly improved performance. The beefier bigger-brother MX180TP has a relatively comparable 378W rating to the 384W rating of the HMP4040.04, but is still a “2+1” configuration, instead with a pair of beefier 180W channels. Unfortunately, this is still in a fixed sub-range configuration – the downsides of this is discussed in the next section.
This doesn’t leave many other contenders – the Keithley 2230G-30-6 and B&K 9132B are basically carbon-copies of each other, which are also “2+1” configurations with slightly different rail configurations and relatively similar performance specifications across most metrics. Finally, the Keysight E36313A offers just 160W over a “2+1” configuration, again limiting flexibility. As a result, the R&S HMP4040.04 is a rather unique offering, despite its higher price.
But How Many Watts Can I Actually Use?
I mentioned earlier that many of the competing supplies have a “fixed sub-range” configuration which can limit the flexibility of the power supply. To better illustrate this, I’ve decided to examine the Aim-TTi QL355TP’s channel configuration.
The fixed sub-ranges on each channel are 5A up to 15V, then 3A up to 35V. While each channel is 105W on paper, the only place where this is reached is at 35V. When operating at any other voltage, you won’t be able to deliver 105W. If you’re unlucky, you’ll need 16V and only have 48W at your disposal.
As a “2+1” configuration, you can really only series/parallel two of the three rails. But as each of the rails have their own limitations, you end up with this sort of “operating area” map.
In yellow is the operating points which can be achieved with one channel – identical to the chart above. Once you put two channels in parallel, you can cover the blue and purple areas. Once you put two channels in series, you can cover the purple and green areas.
The blue line represents what an “ideal” supply that delivers 210W (2x105W) would be able to achieve. Notice there are areas marked “unattainable” – this is operating area that cannot be achieved purely due to the fixed sub-range nature of the supply. The actual power deliverable is shown by the orange line – notice how this varies across the operating range.
As a power supply marketed with a 228W output, assuming you wanted 16V, then you’d only have 96W available to use!
The R&S HMP4040.04 power supply channels are “continuous” power limited channels with a maximum current limit. The power delivery is shown in the graph above – below 16V, the current is limited to 10A. But above 16V, the current is limited as a function of the output power (maintaining 160W). As a result, on a single channel, as long as you are operating at 16V or above, you can obtain the full 160W. Below this, it is current limited but the sheer number of equal channels comes in to make this less limiting than it would otherwise be.
Creating a map for the R&S HMP4040.04 that covers all four channels actually took some thinking because of the many operating combinations availed by the channels. Notice how much of the area under the “ideal” 384W curve (in grey) is shaded? This shows just how in almost any case, it is possible to get 384W out of the supply with only a small gap between around 32V and 38V and below 9.6V. This means that the “watts” you paid for can be used more “often” and is why having equal “beefy” rails is important.
When it comes to power supplies, the term “channel” is used very loosely. While normally referring to independent floating outputs, many of the supplies on the market feature dissimilar capabilities between channels. If you are in the market for a four-channel power supply, guess what? There aren’t many choices at all. If you’re willing to settle for three, there are a few more choices, but many of them only possess two equal channels with an “auxiliary” channel that has additional limitations (e.g. fixed voltages, limited current, limited protection).
But even more than this, the total power rating and channel power rating is also something that can catch buyers unaware. Some supplies have “discrete” ranges of voltage/current limits which must be respected which can leave you unable to use all of the “rating” depending on your operating voltage.
As a result, while the R&S HMP4040.04 was the most expensive supply in the round-up, it is the only true four-rail power supply I could find. Unlike other supplies, the four rails on the supply are equal in capability which is important for flexibility in parallel/series operation and maximising the use of the power rating. Also, unlike many of the supplies in the round-up, the voltage/current ranges for each rail are “continuous” rather than in fixed sub-ranges which further improves flexibility. While you do pay for the 160W per channel and 384W total, you’re much more likely to be able to use it more often. It also saves space, as it is only marginally larger than some of the lesser alternatives and of similar weight.
The downside compared to the other supplies mainly lie in the monochrome LCD which is somewhat outmoded by the newer colour LCDs, limited default connectivity of USB/LAN (relegating GPIB and RS-232 as options) and lack of internal series/parallel operation which can result in cable clutter.
While the price of the HMP4040.04 may seem high, considering that the unit is truly four identical channels which are much more flexible in regards to channel load balance, the price doesn’t seem as bad once you realise that for quite a few configurations, the third rail of “2+1” style supplies might not be useful at all. Add to this the Hameg Instruments heritage of this design, the Rohde & Schwarz reputation backed by a three-year warranty, and it’s clear that the HMP4040.04 does have a distinct position within the market that few supplies could match.
This blog is a part of the Rohde & Schwarz HMP4040.04 Programmable Power Supply RoadTest Review.