SMPS and mains voltage? - Page 1
Forum section: electronics / beginners

author: Gecko
author_rank: Contributor
timestamp: August 17, 2010, 11:39:47 am
content: Hi all,I'm just trying to get a little bit into SMPS. Dave's video inspired me ;-)I read very much at allaboutcircuits.com and ecircuitcenter.com (I can really recommend these sites), and spiced some schematics.But in all schematics I've found so far the input voltage is much lower than mains voltage (230V here in Europe). And if I look in the datasheets of commercial switching regulators, they also have only a maximum input voltage of about 100V. So I took apart an old mobile phone wall plug and extracted the schematic. I found out that it was an "ringing choke converter", with no switching regulator at all.Are there any other techniques to get from 230V down to say 5V? How do variable Bench supplies work - They can even get near 0V, I can't imagine that it is all one PWM with ultra-short pulses.Or do they go the "old" way of first transforming it down with a "normal" transformer to about 40V?

author: Unknown
author_rank: Unknown
timestamp: August 17, 2010, 12:36:49 pm
content: High power SMPSs (like in a computer) indeed use a transformer, but not exactly a 'normal' one. They rectify the AC to DC, PWM the DC (creating a pulse train), through a much smaller transformer (switching frequency >> 50/60Hz), and filter the result to something resembling DC. The voltage at the secondary side of the transformer is used for feedback, and this feedback is isolated through an opto-coupler. This is to achieve galvanic separation between mains and the low voltage side, otherwise you would have to build everything with mains-rated isolation and clearances. Because of the transformer, you don't need a ridiculously low duty cycle for low voltages. You can find schematics for these on the net (I wouldn't recommend to clone these without thorough understanding, since the layout tends to be very critical, because of the high dI/dt).Most low-power (<=3A or so) bench supplies are linear or linear with a switching pre-regulator (to take care of the bulk of the dissipation), but there are fully switching power supplies, especially the high-power ones. I believe it's not a simple exercise to design these, most SMPS designs only allow a fairly limited duty cycle range. Plus it's hard to get the noise down low enough. You can find service manuals with schematics for some older ones at the Agilent website, they usually have a good description of the circuit and the theory of operation.

author: Gecko
author_rank: Contributor
timestamp: August 17, 2010, 03:13:56 pm
content: Thanks for the answer :-)I think I've found some circuits that do what you meant. I think I have seen them before too, but I always misunderstood the "off-line" as "not connected to the mains" but in reality it had just the opposite meaning ^^Just now I'm looking at a circuit from a Linear Technology Application note (AN25http://cds.linear.com/docs/Application%20Note/an25fa.pdf). Here is the schematic (figure 6 in the document):http://img39.imageshack.us/img39/1374/smps1.pngI got a little bit stuck with the startup circuit. I know what it should do, but I don't understand what Q2 is or does. The arrows next to it and the open collector are irritating me. Is it really a phototransistor? I can't imagine that.The description to the circuit is also not very helpful regarding this.

author: Simon
author_rank: Unknown
timestamp: August 17, 2010, 04:31:44 pm
content: what ever you do DO NOT try and build something involving the mains until your familiar with 40V input SMPS circuits

author: Unknown
author_rank: Unknown
timestamp: August 17, 2010, 08:18:40 pm
content: This is the kind I was talking about. For mains operation, just imagine a bridge rectifier and smoothing capacitor at the input (plus different component values for 320V vs. -48V, and active PFC for many commercial designs).The function is described in the second paragraph on page 5, I believe. The BE junction is reversed biased (remember -48V is up, and ground is positive), so it acts like a zener diode. Not sure why they don't use a real zener, probably to keep the BOM small and because stability and max. current is not critical. Using a real 7V zener should give the same result, although this solution might be temperature compensated (tempco of Q2 reverse of Q3?). I don't think the arrows are intended to indicate light, they're just there to point out that it's a 7V zener, though I agree it's confusing.

author: Gecko
author_rank: Contributor
timestamp: August 18, 2010, 12:50:13 am
content: Yes I read the description too, but had my problems with the where it says Q2 is "zener connected". Googling for "Zener connected transistor" got me only to thousands of Zener diodes that where connected to a transistor, which wasn't very helpful. Then I tried to simulate it in LTSPice but couldn't notice any special behaviour.But your keywords "reverse biased BE junction" helped. Thanks for thatI found a few articles regarding this topic. Really interesting application.Since the title of the Application Note is "Switching Regulators for Poets", the reverse biased npn BJT could perhaps be understood as a poetic version of a zener diodeI've now tried it on a piece of breadboard and it worked. So I think the Spice model lacks at this point.