Voltage Regulator - Page 1
Forum section: electronics / beginners

author: Mihnea
author_rank: Newbie
timestamp: October 22, 2017, 03:54:34 pm
content: Hi,This is my first post. I am a big fan of the blog and I thought someone could help me.I am trying to make a voltage regulator with discrete components as a challenge.Bassically I have a problem with the current protection. It supposed to limmit the current at 400mA but it doesn't. I traced down that the excess current come from Q11 and from the emitter of Q10, but I don't know why.Can someone give me a hint?I atached the schematic in LtSpice and the plots with a short and with a 1Meg load. If you need more details please ask.Thanks.

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: October 23, 2017, 12:22:50 am
content: Look HEREhttps://www.eevblog.com/forum/chat/the-state-of-regulators-for-linear-power-supplies-a-rant-(!)for info on discrete regulators

author: Cliff Matthews
author_rank: Unknown
timestamp: October 23, 2017, 12:34:10 am
content: That URL's a bit hard to paste? OK, try this onehttps://tinyurl.com/ya6ltufw

author: orolo
author_rank: Frequent Contributor
timestamp: October 23, 2017, 03:03:18 pm
content: In case of overcurrent, R8 makes Q11 to turn on. Q11 will turn on hard, supplying through Q10 all the current that is required. Moreover, Q11 will be able to do that without turning Q1 off, since its Vce can accomodate for both to work.Maybe place a resistor (1K or so) at Q10 collector, to make sure that Q11 turning on pulls the base of Q1 down, and at the same time avoding excessive current at Q10?

author: capt bullshot
author_rank: Super Contributor
timestamp: October 23, 2017, 03:38:44 pm
content: Yes, orolo explains the reason. There's nothing that limits the current flow through Q10, Q11 in case of output shorted. LTspice doesn't know about the limits of the transistors, in reality they'll just release the smoke.BTW for a "roll your own" regulator this circuit appears too complex. It looks like you've copied the internal schematics of an operational amplifier or a regulator. You should be able to build the regulator with fewer transistors, you won't build all these current mirrors in a discrete design, that's typical for ICs where current mirrors are easier than resistors for biasing.I'd try something with a two transistor differential amplifier, a PNP (maybe darlington) output stage, and an additional transistor for current limiting.

author: bson
author_rank: Unknown
timestamp: October 24, 2017, 07:06:57 am
content: I'd place a resistor across B-E on Q1. Otherwise if Q1 is off the feedback is floating and Q6-Q7 operates in open loop. It will capacitively couple the feedback through that junction and possibly oscillate in a real circuit if unloaded. Just 10k should be sufficient.Re current limiting: if Q11 throttles Q1, then Q6-Q7 will drive Q1 harder to make up for it since the throttling sits inside the feedback loop. Instead, pull the feedback up to rail with a PNP; this will make the LTP reduce the output.

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: October 24, 2017, 08:23:42 am
content: Other unrelated point is that if C2 is 470nF this seems a bit on the large side for a HF rolloff capacitor. I'd check this isn't causing voltage overshoots when the load reduces.Also as said, way overcomplicated. Five or six transistors tops for a fixed voltage output. It's when you want a variable output that goes smoothly down to zero that you start to need more complex circuits.

author: Kalvin
author_rank: Super Contributor
timestamp: October 24, 2017, 08:33:30 am
content: Simulate also the load transients 50% load to 100% load and back, no load to full load, capacitive loads, inductive loads etc. It easy to design a power supply which is stable with stable loads, it is more difficult to design a power supply which behaves well with transient loads without overshoot, ringing, oscillation etc. unwanted features.

author: bd139
author_rank: Super Contributor
timestamp: October 24, 2017, 10:02:42 am
content: Agree with Kalvin.IanMacDonald: agree as well. Simple diff amp is actually surprisingly good here. I've built 6 transistor versions of this which provide better than 1% load regulation and better than 1% line regulation as well with no fancy current sources and have foldback limiting. The transistors should be in an array, read CA3096, CA3028 or HEF3096 ICs for example if you want to go discrete.A couple of useful references:1. HP 6206B power supply service manual:http://literature.cdn.keysight.com/litweb/pdf/06206-90002.pdf- this contains design rationale and test procedures to measure transient response etc.2. Art of electronics chapter 9, which is the sample chapter:https://artofelectronics.net/wp-content/uploads/2016/02/AoE3_chapter9.pdf- this covers the evolution of a linear power supply, protection etc as well.With respect to the original schematic, it looks like the regulator maybe bypassed in current limit scenario via the PNP transistor before the regulator.Best bet is isolate that part and simplify and simulate standalone.