Heat dissipation issues for my psu - Page 1
Forum section: electronics / beginners

author: amateur_25
author_rank: Regular Contributor
timestamp: January 22, 2012, 10:35:13 pm
content: I'm trying to build a psu based on Dave's design. However instead of 10v max I wanted more like 20vI have a 200VA @ 24v transformer already from a previous design I didn't get round to build. But I'mrunning into the dreaded heat disserpation issues at low voltages where the difference bewteen input andoutput is exceedingly high!I've been told it's impossible for me to get 100w dissipation capacity out of T0220 case style. So does this meanI would need to use surface mount and get a pcb made? There was also a a suggestion in the datasheet, you can put a resistorin between the input and output so dissipate some of the heat at the expense of a bigger inital load. In my case this isn't practical is it?

author: ejeffrey
author_rank: Super Contributor
timestamp: January 22, 2012, 11:00:26 pm
content: Very few packages, surface mount or otherwise, can handle 100 watts per device. If you want that you need to parallel multiple devices.A resistor works if you have a fixed output voltage. You pick your resistor so that at maximum current draw the resistor drops just enough voltage to keep the input above dropout. It is no good on an adjustable supply as it will seriously limit the available current at the top end of the voltage range.If you really want the full 200 VA out of your supply you may be better off with a switch-mode tracking pre-regulator followed by a linear regulator. That should keep the heat under control over the whole operating range.

author: Psi
author_rank: Super Contributor
timestamp: January 22, 2012, 11:18:11 pm
content: This is why linear powersupplys (like Dave's one) typically don't have a wide output voltage range at high current.There's just too much heat to get rid of.Normally if you need to get rid of 100W in a linear power supply you design it to have an external transistor/FET. Something like a TO-3 on a massive headsink.But the LT3080 looks to have been designed to use parallel chips instead. There is no mention of using an external transistor that i could see.I cant see it being practice to parallel enough of them together to get rid of 100W.

author: IanB
author_rank: Super Contributor
timestamp: January 22, 2012, 11:21:57 pm
content: The thermal resistance junction to case of the LM317 (for example) in the TO-220 package is 4°C/W. The SMT packages have much worse numbers than this.Since the maximum junction temperature is 125°C, if you were somehow able to keep the case at 25°C (unlikely) you would have a delta of 100°C to work with, equal to 25 W. In practice, you will be limited to something more like 15 W with realistic heat sinking arrangements.To dissipate more than 15 W you would need to use pass transistors in parallel, or adjust the design in ways suggested by ejeffrey.(Edit: The LT3080 has a thermal resistance in the TO-220 case style of 3°C/W. So you are still limited to something like 20 W heat dissipation per device.)

author: amateur_25
author_rank: Regular Contributor
timestamp: January 22, 2012, 11:30:57 pm
content: sorry you guys are talking in jargon I never ever heard of. I think I have once again bitten off more than I can chew. First of all could you explain what the heck is a tracking regulator???Secondly could you explain your "pass transistors in parallel" with a link or schematic pleaseThanks guys I appreciate your help. Even if I have to stick to batteries / dc adapter I have learnt something here.

author: IanB
author_rank: Super Contributor
timestamp: January 22, 2012, 11:35:15 pm
content: Look at the datasheets and application notes for the devices you are interested in. You will find typical circuits and examples inside them with explanations of how to handle higher currents than one device alone can support.

author: amspire
author_rank: Super Contributor
timestamp: January 22, 2012, 11:37:55 pm
content: With a 24V 200VA transformer, you could make a linear supply up to 25V 5A. It is getting to the limit of what is practical with a linear supply, but it could be done, particularly if you can live with a cooling fan. Linear supplies have the habit of growing in size very quickly as the power increases as you need a bigger transformer, bigger capacitors, bigger heatsink, and then a case strong enough to hold all the components.The regulator IC Dave is using has limited power capability. With a 1A limit, and choosing a maximim regulator temperature of 100degC, you are limited to about 10V maximum output.You can parallel them at the costs of adding a small resistor in the outputs which degrades regulation slightly.There are ways to piggyback power transistor on regulator IC's to boost the current, but there is a point where it is probably better to go to a standard transistor regulator configuration with separate voltage sense leads connected to the output. The designs are pretty basic except for the work in getting the frequency compensation just right for stability with difficult loads. That was the one big issues Dave wanted to avoid. You can always look at the designs of HP supplies for a good working circuit. If I was going that way, I would add another small 5W mains transformer to separately power the regulator circuit as it leads to a better supply.Richard

author: Unknown
author_rank: Unknown
timestamp: January 22, 2012, 11:43:26 pm
content: Efficiency of a linear regulator depends on the difference between the input and the output voltage, since a linear regulator works by converting excess voltage to heat. So if the input voltage is 32 V (rectified DC from a 24 VAC transformer), and the output voltage is 5 V, you will have to dissipate 27 W at 1A of output current. The idea of a tracking pre-regulator is to bring down the input voltage via a switching regulator, which is much more efficient, to something like the output voltage + 3 V and this way reduce the dissipation of the switching regulator.Another way often employed by commercial designs is tap switching, were multiple transformer taps are switched depending on the output voltage. This is another (coarse) way of reducing the input voltage of the regulator.The idea of pass transistors in parallel is to add an external pass transistor to the regulator (see LM317 datasheet for an example), and put multiple in parallel to spread the current across multiple devices. Note that you will need emitter resistors to prevent thermal runaway. Any decent electronics text about transistors should explain this.If you have trouble following the dissipation numbers IanB quoted, do some research on thermal heat sink calculations. Dave did a video on it for example.HP also published a power supply design handbook (AN-90B) which is available as PDF online and which may contain some helpful hints.

author: Mechatrommer
author_rank: Super Contributor
timestamp: January 23, 2012, 12:05:21 am
content: Quote from: amateur_25 on January 22, 2012, 11:30:57 pmrecently came acrossCurrent Boosted LM317/LM337 Regulatoryou could outrun me in just a few steps

author: amateur_25
author_rank: Regular Contributor
timestamp: January 23, 2012, 12:22:51 am
content: Is the tracking on the regulator to keep paralled voltage regulators balanced?

author: Kevin.D
author_rank: Frequent Contributor
timestamp: January 24, 2012, 01:48:07 pm
content: Hi Amet .I faced Similar probs with my linear supply I built years ago .It was a 0-30v at 7 amps out . It just to much heat for the regulator todissipate and is also a complete waste of power when you want low V out at high current and your V in is 30V + or something.I rewound the secondary of my transformer and bought out 3 taps.That's the simplest and most efficient way in my opinion ,I wouldjust rewind your secondary and havea centre tap in your case , you could then select taps manually with a V Out Range switch or automatically switch between transformer taps using a relay when V out goes over or under 50% .