Power supply - some questions - Page 1
Forum section: electronics / projects

author: anrio
author_rank: Newbie
timestamp: February 15, 2016, 03:35:19 pm
content: Hi everyone!This is my first post on this forum, so I want to say hello.I want to build power supply which satisfy my requiements:CC: 0 - 2ACV: 0 - 20Vwith accuracy 10mV, 10mA (ultimately: 1 mA; 1mV)This is my first serious PSU project and I have a few question about it:1. Stability1.1 I know that I have to have sufficient gain and phase margin to prevent oscillation, but at what frequency gain must be 0dB? 1Hz?; 10Hz?; 0.0001Hz?1.2 How about capacitive load? Is there any possibility to 'isolate' it? Does 'professional' PSU has maximum value of load capacitance?1.3 I've found that Zobel network highly improving stability. Any hints how should I calculate value of R and C?1.4 I've found as well that C connected in parallel with feedback resistor in op-amp also improve stability. Why? How should I calculate value of C?1.5 Is there any possibility to eliminate minimum output capacitance value?2. LTspice2.1 How can I check in simulation if my circuit is stable?[Over time I will be adding more questions.]Thanks in advance.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 15, 2016, 07:41:43 pm
content: The 0 dB break point should be somewhere in the kHz range for a supply using a emitter follower. It can be close to 1 MHz for a fast low drop regulator.A good regulator can be stable with any load capacitance, but regulation can get rather poor (lots of ringing) with large low ESR caps (e.g. 10 mF 1 mOhms ESR). So there usually is no hard limit for the maximum capacitance, but regulation just gets worse if the capacitance is to high. Not so good supplies might begin to oscillate for large low loss capacitive loads. The critical loads are something like 10-100 µF and something like 10 mF with very low ESR/loss (e.g. < 1 mOhm) , so it might be hard to find such a capacitor in real life. Some professional supplies specify a maximum, but not many.The Zobel network can stabilize, and are essential to get a supply stable at any load. A good starting point is looking at the output impedance to find the frequency where damping is most critical - this gives the RC product. Often the resistor in comparable to the maximum output impedance. Much higher values don't help much.The capacitor in the feedback network adds a lead part to the loop. This helps to prevent a impedance with very close to 90 deg phase shift (which is an ideal inductor like behaviour and thus causes ringing with a capacitive load). As a rule of thumb the capacitor should increase the feedback gain by about a factor of 10, so there often is more than just a capacitor. The capacitor should start working at about a frequency of 1/10-/100 the frequency of the maximum in output impedance.A regulator without any output capacitor (or RC network) will have a very hard time to be stable with every load. It will also be not practical, alone from EMI considerations. The regulator itself tends to behave like an inductance, so there needs to be something to take over at high frequencies. But if the regulator is rather fast, the output capacitor can be small - e.g. 100 nF low ESR plus 1 µF with 0.1 Ohms in series. The overall output behavior can also act like a capacitance, especially with a emitter follower type regulator changing from CV to CC. So it's not only the real capacitor that is important.To check for stability, the standard is looking at the loop gain. I prefer looking at the output impedance: if the phase shift is less than +-90 degree the regulator should be stable with any passive load impedance (or in rare cases may not be stable with any load, usually spice will have trouble finding the DC point if this happens). The second step is than looking at the impulse response, to check for hidden long time constants and possible large signal instabilities - here something like a 10 mA -1 A - 10 mA pulse and one to current limiting (excess current flows trough reverse diodes) are the two cases to test.

author: uncle_bob
author_rank: Unknown
timestamp: February 15, 2016, 08:57:17 pm
content: HiOk, I'm assuming this is a bench supply.A proper bench supply should:1) Run in a stable fashion into any practical load. I would expect a proper design to have unlimited tolerance for capacitive load. In practice that does not include 100's of Farads. It does include a few thousand microfarads.2) Maintain stability both in constant current and constant voltage mode if advertised as such.3) Have a sufficiently low intrinsic output impedance (constant voltage) that it does not do odd things when loads are abruptly applied or removed. (the corollary would be high output impedance in CC mode, that is a bit rare)4) Should not do anything odd when turned on or turned off (no spike to max output etc).5) Should be safe and properly cooled at any of it's rated output conditions.6) Current measurement should not destroy the output impedance of the design.7) Have a low enough ripple / noise density / whatever to be useful in the intended applications. This limit will be enormously different in low noise audio than driving a bunch of logic.Meet it's stated accuracy / stability / repeatability specs or if not stated meet reasonable ones.9) Be compact and easy to lug around / easy to set up / easy to monitor.Most home built designs miss on at least one of those points. Many of mine have failed on multiple points.Bob

author: Alfons
author_rank: Regular Contributor
timestamp: February 15, 2016, 09:35:05 pm
content: The design of a power supply is one of the most demanding exercises.

author: uncle_bob
author_rank: Unknown
timestamp: February 15, 2016, 10:08:19 pm
content: Quote from: Alfons on February 15, 2016, 09:35:05 pmHi..... and it seems so *simple* at first !!!!The one thing that can be said about it - you can get your hands on most of the performance with fairly simple gear. Until you get to stability, most of the concepts are directly out of what you have learned in basic DC circuit training. I think this is what makes it such a popular first project.The list of what you need to do does go on and on. Looking at my list, I now see a couple of things I left out !!!Bob

author: anrio
author_rank: Newbie
timestamp: February 16, 2016, 03:23:43 pm
content: Thanks for all replies.Yea, at first glance making bench psu seems to be piece of cake.Nevertheless I've made some progress:This is power supply's response without any load:For this circuit:QuoteI tried to make roll-of around 300kHzQuoteSo myvoltageloop is stable, isn't it?QuoteI've measured output resistance based on:http://www.diyaudio.com/forums/software-tools/199462-whats-best-way-show-output-impedance-ltspice.html#post2810236Result:But I still don't know "where damping is most critical".QuoteAlso is ~10 Ohm value low enough for output impedance?QuoteCould you explain where should I put this capacitor?I know that current loop hasn't stable yet but for now I want to make sure that I'm going in the right direction.Most of my work was about tweaking value of C3 C1 and C8(in voltage feedback loop), but it was rather brainless.EDIT:Or should I try to compensate current loop alongside with voltage loop?Also; how to speed up response? My goal is to make it <1ms. Is that possible?

author: dom0
author_rank: Super Contributor
timestamp: February 16, 2016, 03:42:14 pm
content: QuoteFourty years ago <50 µs was the usual spec of lab-grade supplies.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 16, 2016, 04:01:21 pm
content: A 10 ohms output impedance is way to high for voltage regulation and too low for current regulation. One should get something like a maximum output impedance of about 0.1 - 0.5 Ohms (e.g. about 0.5 to 2 times the value of the shunt) in a frequency range somewhere around 20-500 kHz. To lower frequencies the impedance should go down to something like 1 mOhm at 100 Hz and lower at 1 Hz. At high frequencies its essentially the impedance of the output capacitor.The circuit should have a resistor between the OPs and transistors. Real OPs don't like driving a capacitive load. There usually also need to be diodes to select the minimum from the two controlling OPs. The capacitor C8 is too large and you may not want to have one at that place at all. The capacitor C3 would need a series resistor (e.g. 1-10 K range) or alternatively a resistor between the divider and the OP. The OP to amplify the current signal is rather slow - this should be a faster one. A rather slow response of the current regulation is the weak point of this type of regulator anyway. The value of R7 is rather high. I would start with the voltage regulation only.The response of a variable setpoint / reference value is not that important. The two interesting curves are loop gain (response to an additional voltage between the output and R1/C3) and output impedance (current source at the output).

author: uncle_bob
author_rank: Unknown
timestamp: February 16, 2016, 11:03:55 pm
content: HiThere are a *lot* of people who make power supplies commercially. They have been doing it for many decades. Rather than trying to invent the power supply from scratch .... how about spending some quality time with Google? Do a search for power supply schematics, manuals, and spec sheets. They will give you a very good idea of what to aim for.A true CV / CC supply is not as easy to find as a normal supply with current limit. You may need to dig a bit for one of those.Bob

author: Audioguru
author_rank: Super Contributor
timestamp: February 17, 2016, 01:34:03 am
content: You have no frequency compensation in your circuit that wrongly uses a comparator instead of an opamp.An opamp has frequency compensation capacitor inside that cuts the gain of frequencies higher than a few Hz so that at the frequency where phase shift in the amplifier causes the negative feedback to become positive feedback its gain is less than 1.