TL783 100V DC linear PSU questions - Page 1
Forum section: electronics / beginners

author: sw2022
author_rank: Contributor
timestamp: April 02, 2022, 09:07:16 am
content: Hi,Hope this beginner question on linear power supplies is not too basic. I read recently about the TL783 which offers what I believe is the highest DC voltage output of the available 3 terminal regulators. I wanted an isolated output PSU for vacuum tube breadboarding, primarily for low current grid bias and a 0-100V supply based on this regulator seems ideal. I’d like to see what this reg can do. I would not rule out using the PSU for other higher current applications and at this point realised I was out of my depth trying to work out the maximum viable current drain, as dissipation here is potentially high given the voltage span. I’ve built linear reg circuits for lower voltages to about 5A with input voltages up to about 20v higher than output, as an amateur building one offs just attaching the reg to an over engineered heat sink without deep consideration of thermal issues has worked nicely in the past.So, the datasheet shows the reg used with a 125V supply and regulating down to a minimum of 1.25V. The thing is capable of 500mA, but at that sort voltage differential that’s about 50W dissipation, wow! While common sense dictates that a TO220 could never handle that even with liquid cooling, how to work out what current IS viable. I did some reading on this and got lost in the detail about junction temp specs, package dissipation, heat sink thermal transfer etc. The datasheet does not give me anything simple in terms of Watts on my reading at least.Could anybody suggest what current the TO220 version might be able to supply at say 125V max voltage differential? I don’t mind a very big heat sink. I guess there is a maximum useful size here though. Any clarification as to how this is worked out in practice at an understandable level would also be really helpful. Comments appreciatedSteve

author: Kleinstein
author_rank: Super Contributor
timestamp: April 02, 2022, 09:30:52 am
content: With good cooling 50 W is about the maximum one can get from a TO220 case. Usually one should stay lower, more like 30 W.With a relatively high voltage the more relevant limit is likely the SOA. Modern regulator should include SOA protection, so that the current is limited to significant lower levels when the votlage drop is high.This may cause the regulator to never come up with some loads, that are more like a constant current load and thus take quite some current already at a low voltage.The Ti Datasheet give 100-250 mA as the short time (30 ms) current limit for a 125 V drop in the specifications. With rising temperature this limit may drop somewhat.For the usable power, a parameter to chose is the temperature of the heat sink. A heat sink temperature of some 50-70 C is a reasonable compromise. Dending on the part (internal temperature max.) this means something like 40-70% of the rated power. For the details it is than to calculate a few alternative configurations (e.g. large heat sink + 1 transistor or smaller heat sink + 2 transistors in parallel and than decide what fits better). If in doubt a somewhat larger heat sink does not hurt, but a blown transistor may cause damage. Specs are often somewhat optimistic: e.g. free air flow, SMD parts with huge copper area, perfectly flat / matching to heat sink. Especially Ptot ratings for MOSFETs in a TO220 are often inflated and not realistic.

author: Terry Bites
author_rank: Super Contributor
timestamp: April 02, 2022, 09:51:59 am
content: What ever linear system you use the dissipation will be comparable. (Vi-Vo)/Io watts.MJE13005?

author: Ian.M
author_rank: Super Contributor
timestamp: April 02, 2022, 11:44:29 am
content: Maybe, but like most bipolar power transistors, secondary breakdown limits its high voltage performance to a small fraction of its max. rated dissipation. Seehttps://www.onsemi.com/pdf/datasheet/mje13005-d.pdfpage 6.At 125V Vce its good for a max. of only 200mA @25 deg C, dropping to 170mA @70 deg C.

author: sw2022
author_rank: Contributor
timestamp: April 02, 2022, 01:03:41 pm
content: Thanks. I had no idea a TO220 could potentially even get close to 50W . It sounds that about 250-300mA might be possible based on package dissipation then.Steve

author: Terry Bites
author_rank: Super Contributor
timestamp: April 02, 2022, 03:06:27 pm
content: Its freezing today- get it built, save on gas!

author: BrokenYugo
author_rank: Super Contributor
timestamp: April 02, 2022, 03:14:08 pm
content: Quote from: sw2022 on April 02, 2022, 09:07:16 amI think that honor goes to the LR8 at 450 volts max input, 440 out.

author: TimFox
author_rank: Super Contributor
timestamp: April 02, 2022, 04:29:22 pm
content: Quote from: Terry Bites on April 02, 2022, 09:51:59 amCorrecting your typo:PD= (Vi-Vo) xIo.

author: floobydust
author_rank: Super Contributor
timestamp: April 02, 2022, 04:42:52 pm
content: I find one trap with theTL783is the minimum load current spec of 15mA. This is high and lots of heat if you must add a dummy load resistor. At 100V out, 15mA is 1.5W as heat with a 6.8k resistor for example, if used as a light load (bias) regulator. It's an old IC from 1981 and 125V is not much use for tube gear.TheLR8I have not used but it looks much better, aside from the SMT (DPAK) package meaning low dissipation, lucky to get 1-2W, and the 20mA output current-limit is more practical.I'd have to try it for regulating B+ with an extra pass-transistor added. I've just used a zener and couple junkbox power transistors for that.

author: TimFox
author_rank: Super Contributor
timestamp: April 02, 2022, 05:29:36 pm
content: For regulating vacuum-tube power supplies, I have successfully used variations on the "Maida" circuit, first posted in National Semiconductor app notes.Here is a circuit from P Millett:http://www.pmillett.com/HV_reg.html.The gate of a high-voltage MOSFET follower is bootstrapped by a Zener (D1) to keep the voltage across the LM317 within its limits, and holding off the input voltage during start-up.(I like to run 7586 Nuvistor triodes with plate-cathode voltage around 75 V.)

author: Gyro
author_rank: Super Contributor
timestamp: April 02, 2022, 05:36:35 pm
content: Link corrected...http://www.pmillett.com/HV_reg.html

author: TimFox
author_rank: Super Contributor
timestamp: April 02, 2022, 05:50:36 pm
content: Link problem: this is weird. I copied the link from another site, and it worked. Now, when I click on the link in my post it goes to 404.However, I don't see the difference between the link you posted (which works) and the one I posted (which doesn't).

author: Ian.M
author_rank: Super Contributor
timestamp: April 02, 2022, 06:04:06 pm
content: You added a trailing full stop, and because you left it to the forum to autoconvert the URL to a link, it included your added punctuation FUBARing the link. If you need punctuation adjacent to a URL, you need to wrap the URL in BBcode url tags - seehttps://wiki.simplemachines.org/smf/urlAlso its worth noting that the SMF linkifier messes up most URLs containing embedded punctuation apart from / and . that isn't URLencoded, and using explicit url tags avoids that.

author: sw2022
author_rank: Contributor
timestamp: April 02, 2022, 06:06:27 pm
content: Thanks for the insight all, points taken. Just to clarify this isn’t for an anode voltage circuit. To easily breadboard/test a tube without setting up any other bias circuitry really needs an independent negative grid supply, replacing the old grid battery, I’d like to be able to look at tube characteristics with variable negative bias. It won’t go to -100 in practical use I think. I’ve got a 300v commercial PSU for anode volts and have made a linear 6.3v PSU for heaters. I don’t 100pc trust isolation on cheapo commercial power supplies though. If I’m going to reference V+ on the new grid supply PSU to B+ ground I want to be very sure the grid psu is truly floating. I appreciate there’s always capacitive coupling etc so that floating is is a theoretical term, but I’m thinking with a good transformer and careful construction this thing won’t go bang,Steve

author: sw2022
author_rank: Contributor
timestamp: April 02, 2022, 06:13:56 pm
content: Quote from: Terry Bites on April 02, 2022, 03:06:27 pmIt’s not a warm April here so far, this could be a useful gadget. Except for the energy price crisis thing,Steve