Lab/bench power supplies - Pure Linear vs SMPS + post regulation - Page 1
Forum section: electronics / general

author: jpb
author_rank: Super Contributor
timestamp: June 18, 2016, 04:28:28 pm
content: I've been looking at bench power supplies with a view to getting a second one. My existing one (a TTi PL320QMD) is pure linear and I am willing to put up with weight/size/inefficiency to get the robustness and low ripple and noise but the R&S (Hameg) HMC8043 caught my eye - this is a SMPS with post regulation.There is a review here:https://www.elektormagazine.com/articles/RS-HMC8043-PSUI have compared it with a Keysight E3613A and TTi PL303QMD in the attached page of my notebook.Part of the comparison is no surprise (linear supplies are bigger and heavier and less flexible) but there are a couple of surprises for me:1. The rms ripple on the Hameg is only 450uV rms and 4mV p-p which is very similar to the linear supplies.2. The key and major difference is the transient response. The Hameg is 1msecs while the two linear supplies are around 50 usecs so the Hameg is 20 times slower!Is this a fundamental difference -> you cannot get the response time down if you're using SMPS and post-regulating it ?And, does it matter that much? (The review implies that 1 ms of a 1V overshoot might be enough to destroy sensitive electronics.)

author: uncle_bob
author_rank: Unknown
timestamp: June 18, 2016, 06:25:58 pm
content: HiI have seen the turn off overshoot issue on a number of supplies. It is *not* unique to a switcher + linear approach. It also has noting at all to do with the response time of the supply. It simply is an aberration in the operation during power down. You probably will find that it is dependent on the load current and the output voltage setting. It is not something I would want in a supply. I have seen *much* worse on supplies I have actually used.Bob

author: Marco
author_rank: Super Contributor
timestamp: June 18, 2016, 07:22:58 pm
content: A linear supply is an edge case of SMPS + linear, so obviously it can get the same response timeLooking at the scope shot the response to increased current is about the same as to decreased current, so the switcher doesn't even seem to play a part ... the slow response is because of the linear part.

author: jpb
author_rank: Super Contributor
timestamp: June 18, 2016, 08:10:05 pm
content: Quote from: uncle_bob on June 18, 2016, 06:25:58 pmYes, you're right, I was confusing the two. I should experiment with my existing supply (which is pure linear) to see how that behaves on turn-off.It is one thing that spec sheets don't give, the behaviour at mains failure.

author: uncle_bob
author_rank: Unknown
timestamp: June 18, 2016, 08:38:43 pm
content: Quote from: jpb on June 18, 2016, 08:10:05 pmHiYou will find that a lot of supplies out there have odd turn on and turn off behavior. Some digital units always come up with the output disconnected for this reason. We all poke the power button to turn a circuit on and off. Very few of us (YES I'm included) check our supplies to see if that will be a problem. I have a pair of supplies about 3 feet from me and a nice digital scope about 2 feet away. Have I ever checked them -- not ever. Do I poke the power button? Yes I most certainly do.Bob

author: jpb
author_rank: Super Contributor
timestamp: June 18, 2016, 09:14:48 pm
content: I just dug out a 10 ohm power resistor and attached my scope.Triggering was a problem - I set it to trigger on the spike (just above the 1.8V I set). The spikes are rather larger than I'd hope but they last only a few tens of microseconds.I attach a couple of screen shots.With 1.8V set the peaks are nearly 3.5V and the troughs near to zero.

author: jpb
author_rank: Super Contributor
timestamp: June 18, 2016, 09:29:44 pm
content: Measuring turn-off spikes seems to be like trying to measure Fractals!What you measure depends on the time base set. Setting the time base to 1 usec leads to spikes between -2V and +6V (for a nominal 1.8V) as shown in the attached screen shot.I suppose I'm essentially measuring inductive effects in the cables and so on. I've not really given it much thought before but it is difficult to say what is intrinsic to the power supply and what is just a measure of the crude setup I have.Certainly the power supply itself appears to smoothly turn off and not produce any control peaks like were shown in the review of the R&S HMC8043.

author: tautech
author_rank: Super Contributor
timestamp: June 18, 2016, 09:35:08 pm
content: There's various ways that on/off spikes are managed, some PSU's have output switching and many users think this is the only way to ensure spike aren't a problem.However a cheap little SMPS that I get is only mains switched and doesn't suffer from any of these off/on ailments.https://www.eevblog.com/forum/testgear/cheap-variable-switching-power-supply/msg666082/#msg666082I'd never call it a quality supply, ripple is not low but when connected to real circuitry with adequate bulk capacitance onboard ripple is rarely an issue.You just have to check on/off's with a DSO and Single shot capture to be sure.

author: tautech
author_rank: Super Contributor
timestamp: June 18, 2016, 09:38:20 pm
content: Quote from: jpb on June 18, 2016, 09:29:44 pmUse a falling edge trigger.

author: uncle_bob
author_rank: Unknown
timestamp: June 18, 2016, 09:41:28 pm
content: HiThere are a couple of basic things to check:No (or light) load vs Full loadLow end of the range vs high end of the voltage rangePower on vs power offThat's eight combinations and it covers most of what you need to look at. A banana to BNC adapter right on the supply output can be helpful when trying to work out what is what. I have seen some supplies that "need" an external 0.1 uf cap to take care of narrow spikes. That's pretty easy to mount right on the supply.Bob