SMPS INFO - Page 1
Forum section: electronics / beginners

author: max_pg
author_rank: Newbie
timestamp: December 10, 2024, 04:18:49 am
content: Hello, I'm trying to build my own power supply for a soldering station, which has a power of 100W.I've read a lot of literature, but some things remain unclear, so I'm writing here in the hope that someone can help. The requirements for this power supply are as follows:Input voltage 180VAC-265VAC,output voltage 24VDC and current from 0A to 4A, the station will dynamically draw current from the power supply, due to the fact that it has its own PWM regulator in it (that is, 4A current will be drawn from the power supply, then it rests for a while and some current is drawn again).1. I'm wondering what topology to use for the power supply. I've settled on a few, for certain reasons.a.) Forward convertor with 2 transistors-----this option is good in my opinion because it will withstand peaks from the network better and the stress on the transistors will be less. There are disadvantages here that this type requires a minimum load to work well and does not behave well with changes in the input voltage within wide limits.b.) Flyback convertor------it is easier to implement, but there is more stress on the diode in the secondary and usually works in current mode control.2. I wonder which control method would be better for my conditions (changes in the input voltage and changes in the load from 0A to 4A).Can someone explain when it is better to use voltage control and when current control, that is, under what conditions is one method and the other more suitable, respectively?Thanks.

author: inse
author_rank: Super Contributor
timestamp: December 10, 2024, 06:17:11 am
content: Those open frame power supplies are so cheap and a 230V power supply isn’t a good project for a beginner.Just my recommendation though

author: Andy Chee
author_rank: Super Contributor
timestamp: December 10, 2024, 07:28:26 am
content: For heating a soldering iron element, I would consider a phase controlled Triac chopper on the low voltage AC secondary, and use a traditional low frequency 50/60Hz mains transformer.No need for a forward or flyback.Sure, the thing might end up heavier and bigger. But such weight can be helpful for securing the iron stand.

author: soldar
author_rank: Super Contributor
timestamp: December 10, 2024, 08:50:20 am
content: Quote from: inse on December 10, 2024, 06:17:11 amI agree with this. A 24V, 4A PSU can be had for a very good price.If the load is self regulating and does not require precise voltage regulation then i would consider just a transformer followed by rectifier bridge and capacitor.

author: inse
author_rank: Super Contributor
timestamp: December 10, 2024, 09:01:54 am
content: Transformer, though straightforward approach, is problematic with the required input voltage

author: Nominal Animal
author_rank: Super Contributor
timestamp: December 10, 2024, 09:11:09 am
content: I'm only a hobbyist, but I'd use Mean WellRPS-120-24(21€ plus taxes and shipping fromtme.eu;PDF datasheet) as the 24V DC supply. It is EMI class B for both grounded and ungrounded operation (class I and II), has very low leakage (2×MOPP; see ANSI/AAMI ES60601-1 and IEC/BS EN/EN60601-1), and even has a separate 12V/0.5A fan supply (two-pin JST PHR-2). I'd do the power PWM via low-side switching using N-channel power MOSFETs, maybe IRFI4321PBF (about2.7€ in singles at Mouser in TO223-3). I'd drive the power MOSFET gate from a 10V DC rail (linearly regulated from the 12V), using a logic-level N-channel MOSFET. For the IRFI4321 the total gate charge is about 100nC, so at say 100mA draw from the 10V rail we can estimate the switching time to be on the order of one microsecond. On-state resistance is under 0.017Ω, so the MOSFET shouldn't generate too much heat –– a bit over a quarter of a watt when passing 4A current, about 20°C temperature rise above ambient ––, but during switching it will briefly be in the linear region and generate much more heat; the switching rate will dictate whether you need a heatsink for IRFI4321.If I had more experience, I might do somewhat along the lines outlined by Andy Chee, and use a toroidal transformer from mains AC and so on.

author: max_pg
author_rank: Newbie
timestamp: December 10, 2024, 04:02:53 pm
content: Thanks for these answers.I'm still wondering if anyone can answer the specific questions I've described?

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: December 10, 2024, 09:27:16 pm
content: Hello max_pg,Quote from: max_pg on December 10, 2024, 04:02:53 pmTable 4gives you the preferred topology depending on input voltage and output power.And the difference between voltage and current control mode can be foundhere.Check also table 1 at the bottom of page 31.Part 2of the AN features the design equations for each topology.

author: max_pg
author_rank: Newbie
timestamp: December 11, 2024, 09:32:49 pm
content: I'm considering making an active PFC + Half Bridge converter, because in this way the entire power supply will work in a very light mode and the amount of heat released in the device case will be quite small.According to calculations, I can do without radiators on the key elements.This is not the case with other types of converters. They have more losses and stress on the elements.What do you think about this?

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: December 11, 2024, 11:41:34 pm
content: Sincerly, my opinion is close to what @inse stated ealier :Quote from: inse on December 10, 2024, 06:17:11 amI've seen PFC going bersek and output voltage raised to about 10 kV (nominal voltage was 390 VDC).Nice arcing indeed. Some vaporized metal spill out. So the board becomes conductive. 15 parts in vicinity were all dead and had to be replaced.If you are interested in power electronics, start by designing low voltage DC-DC converters, stepper or BDLC motor drivers, energy harverster circuits and so on.Devising switchers for mains voltage input is a step further.