Siglent SPD1168X power supply - slow CC response time (>200ms) - Page 1
Forum section: electronics / testgear

author: jimminyjosh
author_rank: Newbie
timestamp: June 11, 2019, 02:15:15 pm
content: Hi All,I have a the Siglent SPD1168X, and I measured a very slow CC response (>200ms). The specs suggested something much faster. I emailed the Siglent people the data below and they say there's a tradeoff between fast CC response and fast CV response. So one or the other will be slow. Does that sound plausible, or can anyone explain why that might be? Do all power supplies have this issue?My set up was I connected a 50 Ohm resistor to the output leads and set the SPD1168X to put out 5V at 0.003A (to force it to go into CC mode). Attached is a pic of the output. What happened was the unit puts out a 5V for ~150ms before it even starts to ramp down the voltage to meet the maximum current requirement.I'd been hoping the CC mode would respond on the order of microseconds or better.Thanks,JoshP.S. Apparently this issue also affects the SPD3-series power supplies except they're tuned for faster CC and slower CV response.

author: BillB
author_rank: Unknown
timestamp: June 11, 2019, 04:42:10 pm
content: I think only one control loop can have priority during cross-over. See this article:https://www.electronicdesign.com/power/getting-your-priorities-straight-managing-overshoots

author: jimminyjosh
author_rank: Newbie
timestamp: June 11, 2019, 05:49:21 pm
content: Thanks for the article reference. The article makes it sound like the notion of overshoot and voltage/current priority to apply on a microsecond time scale, not the milliseconds that I'm seeing. Is that right?Also, in the scope pic I sent, note that the output voltage is a rock-steady 5v for > 100ms while the current is way over the limit. It appears that the system does not even try to limit the current for > 100ms. I imagine there's some subtlety going on there that I don't get.Given that the power supply is digitally measuring the voltage and current (for the digital display), it should know within micro or nanoseconds when it has overshot and be able to theoretically compensate quickly, right?Josh

author: plurn
author_rank: Regular Contributor
timestamp: June 11, 2019, 06:10:02 pm
content: I'll assume you mean SPD1168X.With my similar SPD1305X and a 38.78 Ohm resistor it starts to restrict current at about 7ms. With a 0.011 Ohm resistor (pretty much shorted), I was getting about 2ms for it to start restricting current (if I interpreted it correctly).See my posthttps://www.eevblog.com/forum/suggestions/suggestion-for-an-eevblog-video-lab-psu-_current_-limit-overshoot/That also has some links to information about "tradeoff between fast CC response and fast CV response" so what Siglent says does sound plausible. BillB linked to one of them.My take on that is that everyone here complains about voltage overshoot for power supplies and almost no-one mentions current overshoot, so Siglent uses voltage priority mode to ensure no voltage overshoot. It is what everyone here asks for so its our fault.As for my concern about whether such huge current overshoot is normal for most power supplies - don't know - I didn't get much response to my post.As to why it is >200ms and if that is normal - I don't know sorry. I don't think it is normal based on my tests. But maybe we did something differently, eg using the channel 'on' button - or connecting the resistor while power was live. I used the channel on button. Or perhaps different voltage settings change the result.

author: BillB
author_rank: Unknown
timestamp: June 11, 2019, 06:45:10 pm
content: Hmmm. I replicated Josh's setup on my 1168X and 3303X-E, load connected using channel on button.The first image on the 1168X matches what you are seeing. The next two show the 3303X-E, the first set to 3mA while the second set to 10mA (my 3303X-E is hacked to 1mA resolution, so wanted to make sure). The 3303X-E is behaving like I would expect, beginning to limit within a few ms.

author: jimminyjosh
author_rank: Newbie
timestamp: June 11, 2019, 07:05:35 pm
content: Hi BillB,Thanks for checking. The Siglent folks said that the faster response of the 3303X-E (the SPD3 series) comes at the cost of slower CV response and gave the example that the voltage ramp up is much slower on the SPD3303X-E than the SPD1168X. I'm still wonder why these tradeoffs and effects happen at such large timescales - maybe it just costs more to have a circuit that manages voltage and current quicker.Josh

author: plurn
author_rank: Regular Contributor
timestamp: June 11, 2019, 07:36:56 pm
content: I have replicated this too now on SPD1305X, using a 46.78Ohm resistor.5V 0.003A:The thing is it gets worse the lower you have the voltage set. At 1V 0.003A it varies but can be over 1 second and you can see it on the display of the power supply:It is shorter at higher voltage settings. Capacitance at output would be part of this.