High current, low headroom linear regulator. - Page 1
Forum section: electronics / projects

author: CJay
author_rank: Super Contributor
timestamp: March 22, 2017, 05:27:38 pm
content: How would you rectify and regulate the output of a toroidal 8-0-8 transformer that's capable of in excess of 70 amps to 13.8VDC and be able to supply, say, 60 amps to an attached load?I've got two of the transformers (they weigh a *LOT*) and I could put them in series but that means I waste a ridiculous amount of power as heat as well as probably putting my back out...

author: ovnr
author_rank: Frequent Contributor
timestamp: March 22, 2017, 05:47:55 pm
content: Rectification: I'd probably go for someLT4320ideal diode controllers and some ultra-low-Rds(on) transistors. You could do 60A with less than 5W loss.Regulation: A 60A linear reg is a bit exciting, but all you really need to do is scale up a smaller regulator circuit. You will need a rather hefty heatsink, however.

author: Zero999
author_rank: Super Contributor
timestamp: March 22, 2017, 05:56:17 pm
content: Quote from: CJay on March 22, 2017, 05:27:38 pmThis isn't possible without wasting crazy amounts of power. The peak output voltage of 8VAC is 11.3V, so using full bridge rectifier, with two diodes, wouldn't give a high enough voltage. The peak voltage of 16VAC is 22.6V, so you'd have to waste a lot of power dropping it down to 13.8V. A low drop-out regulator will not be required. A TL431 and an array of Darlington transistors on a gigantic heat sink aught to suffice.I wouldn't do it. I'd get a switched mode power supply to do that.13.8V is a fairly common voltage for automotive applications and is widely available.

author: mariush
author_rank: Super Contributor
timestamp: March 22, 2017, 06:11:15 pm
content: Using something cheap and easy to buy ... maybe 10+ LT1084/LM1084 linear regulators each with a resistor of around 0.2 ohm on the output (to balance the current)Check the LT1083/LT1084 datasheet, they have an example showing two lt1083 (the 7.5a version of lt1084) in parallel, for a 15a power supply.You'll need a bunch of bridge rectifiers as well, it wouldn't be a good idea to use a single one for 60 amps of current... not only because you'd have [ 2 x voltage drop x 60 A ] watts dissipated on a single rectifier, but using multiple will mean lower voltage drop on each diode of the rectifier.Maybe something like GBJ5006 or GBJ5010 (50a rectifiers easy to heatsink)

author: MagicSmoker
author_rank: Super Contributor
timestamp: March 22, 2017, 06:20:32 pm
content: You could use phase-angle regulation on the primary (using a triac or back-to-back SCRs) and a full wave rectifier feeding an LC filter on the secondary. This is essentially a mains-frequency buck converter, so relatively high efficiency (especially since regulation is on the primary side) but terrible transient response (loop bandwidth will need to be less than 10-15Hz).

author: Kleinstein
author_rank: Super Contributor
timestamp: March 22, 2017, 06:26:19 pm
content: 8 V AC is to little to give you 13.x V DC. With both windings in series there is a problem getting enough current: with a simple rectifier and filter 70 A AC current would lead to about 35-40 A of DC current only. So to come close to 60 A, you would need a kind of PFC. Doing it passive, with an inductor is heavy (might be as much as the transformer) - the good thing is that PFC would reduce the voltage and need less filter caps.Still getting a suitable switched mode supply is much easier. !3.8 V is common.