Help choosing components for a power supply - Page 1
Forum section: electronics / beginners

author: derationalize
author_rank: Newbie
timestamp: January 30, 2020, 03:18:56 pm
content: Hello all! I would like some help choosing the transistor and component values for my project. I am building a linear regulated power supply. I plan on using several of these in isolation at 9vdc and several with an 18vdc output housed in the same project box. This is for powering audio equipment. The reason I decided not to purchase a device that already does this is because my application requires at least 4 seperate 18v outputs at 500mA. The best available commercial option is the VooDoo labs PedalPower ISO 5, but it is too big in one dimension to fit in the space I have available; therefore I must construct my own. I watched Dave's video on the "capacitance multiplier" to remove power supply ripple and thought it would be a great idea to implement that into the design. I presume I would need a transistor designed for power electronics for this application. Does anyone have any recommendations? Any advice on part selection, component values, or design improvemnts would be greatly appreciated. This is my first project involving anything power related so I'm trying to be safe and not blow something up.

author: MarkF
author_rank: Super Contributor
timestamp: January 30, 2020, 04:18:54 pm
content: That is NOT the correct way to connect a bypass transistor.

author: MarkF
author_rank: Super Contributor
timestamp: January 30, 2020, 04:29:29 pm
content: However, why not use fixed regulators? You can get fixed 9V and 18V regulators. Fewer parts.https://www.digikey.com/product-detail/en/on-semiconductor/MC7809CTG/MC7809CTGOS-ND/1481254https://www.digikey.com/product-detail/en/on-semiconductor/MC7818CTG/MC7818CTGOS-ND/1481308

author: Vovk_Z
author_rank: Super Contributor
timestamp: January 30, 2020, 08:50:12 pm
content: Quote from: MarkF on January 30, 2020, 04:18:54 pmTS is talking about capasitance multiplyer not about bypass transistor.

author: Vovk_Z
author_rank: Super Contributor
timestamp: January 30, 2020, 08:52:48 pm
content: TS forget to add a capasitor after diode bridge. Even if you use capasitance multyplier you still need bulc filter cap after diode bridge before the active filter (about 1000 uF for 500 mA).

author: Vovk_Z
author_rank: Super Contributor
timestamp: January 30, 2020, 09:03:33 pm
content: Q1 have to be rated >50..60 V CE voltage (it depends on Tr1 secondary voltage), and about an order larger then load collector current (>3..5 A) to everything was ok with SOA.You need something like BD139, but a liitle bit more powerful (to it was reliably).BD909 or BD911 should do that work.

author: Jwillis
author_rank: Super Contributor
timestamp: January 31, 2020, 07:17:45 am
content: Quote from: Vovk_Z on January 30, 2020, 09:03:33 pmThis question doesn't really apply to the component request of the OPDaves video mentions the use of a Mosfet and BJT For the "capacitance multiplier" . Is there ant reason an IGBT couldn't be used? And can these circuits be used in parallel for high currents and voltages?

author: Vovk_Z
author_rank: Super Contributor
timestamp: January 31, 2020, 03:52:45 pm
content: Quote from: Jwillis on January 31, 2020, 07:17:45 amWhat are you talking about? What question?IGBT type can be used (but I don't see a reason to use it). They work like mosfets.There are powerful mosfets (or bjts) for high currents (with TO247 case for example). I don't see a reason to make "parallel active filter/capacitance multiplier".

author: David Hess
author_rank: Super Contributor
timestamp: February 01, 2020, 02:22:56 am
content: Quote from: Jwillis on January 31, 2020, 07:17:45 amA MOSFET or IGBT could be used but a bipolar transistor has a much better cost per die area which is proportional to power dissipation which is what matters.A capacitance multiplier before a linear regulator acts as a preregulator to increase the ripple rejection of the regulator. If increased ripple rejection is required, it is probably better to use a higher performance regulator. The example I like to use is shown below which places the integrated regulator inside the feedback loop of a precision operational amplifier.

author: derationalize
author_rank: Newbie
timestamp: February 01, 2020, 03:26:15 am
content: LM109 is $17 a piece. LM317 is 0.58 cents on Mouser. I figured I could just order a quantity of them and have some jellybean components for future projects. I have to keep the cost of this project down lest it would not be worth the effort.I have a few TIP41CG transistors sitting in a scrap bin. If I recall those can handle quite a bit. Is that a bit overkill? Also, is a 4800uF electrolytic big enough for the main cap? Should I use any polyfilm caps in that circuit? Should I worry about using cheap carbon film resistors?

author: David Hess
author_rank: Super Contributor
timestamp: February 01, 2020, 03:49:48 am
content: Quote from: derationalize on February 01, 2020, 03:26:15 amThose specific parts are not required. It works fine with an LM317, a PNP transistor and perhaps a couple cheap diodes replacing the JFET, and your favorite operational amplifier. Last time I did it with a 7805 regulator and LT1007 operational amplifier but I had to add external frequency compensation networks.QuoteTIP4CG? TIP4?Carbon film resistors are fine for the preregulator. They may be acceptable for the output divider depending on the requirements for output voltage accuracy.The series preregulator could be implemented with a power depletion mode MOSFET with the gate tied to the output of the regulator.

author: Jwillis
author_rank: Super Contributor
timestamp: February 01, 2020, 10:13:35 am
content: Quote from: David Hess on February 01, 2020, 02:22:56 amI'm thinking in terms of very high currents (20 A to 100A) and high voltages (50V to 100V) for very specialized industrial applications.

author: David Hess
author_rank: Super Contributor
timestamp: February 03, 2020, 03:16:01 am
content: Quote from: Jwillis on February 01, 2020, 10:13:35 amBipolar transistor still have a cost advantage even when extras for drivers are required to supply the base current. More of them have to be used in parallel but more devices should be used anyway to lower case to heat sink thermal resistance. Their drop in current gain at high currents is not a problem because multiple devices are needed to handle the power dissipation anyway.MOSFETs and IGBTs will still require drivers to handle their high gate capacitance. There is no advantage to using IGBTs instead of MOSFETs unless they are available with higher power ratings.