Cooling an SMA - Page 1
Forum section: electronics / projects

author: Opticalworm
author_rank: Contributor
timestamp: February 07, 2011, 04:21:05 pm
content: Hi all,I'm currently designing a zener regulator using (0BZG05C8V2http://www.farnell.com/datasheets/69919.pdf) and i'm hoping to dissipate around 3 watt. Does any one know of a practical way to cool down an SMA (DO-214AC)?

author: armandas
author_rank: Frequent Contributor
timestamp: February 07, 2011, 05:56:39 pm
content: If you check your datasheet, you will see that 3W dissipation is an absolute maximum for this diode. Plus, with a junction-lead thermal resistance of 30 K/W you will have a hard time finding practical way to cool it. Why don't you try and find a part in a larger package?

author: Zad
author_rank: Super Contributor
timestamp: February 07, 2011, 06:39:34 pm
content: Put it on the base of an NPN transistor and use it as a shunt?http://www.circuitstoday.com/zener-controlled-transistor-voltage-regulators

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: February 08, 2011, 06:15:28 pm
content: The only way you'd get anywhere near 3W without it unsoldering itself would be on an aluminium clad board, or with a lot of forced air. If you really need to dissipate that much, spread it over several series devices, or use a heatsinkable package.

author: Unknown
author_rank: Unknown
timestamp: February 08, 2011, 07:26:46 pm
content: Funny how they specify a junction-ambient thermal resistance of < 30 K/W for dissipating 3W, but the thermal resistance between junction and lead is already 30 K/W, so you wouldn't be able to meet the spec even if you soldered it to a huge heat sink. Guess they use forced air to achieve that.I agree with the others: use some other device (eg. transistor, or regulator) to dissipate the power, or get a zener in a package with a lower thermal resistance.

author: Afrotechmods
author_rank: Unknown
timestamp: February 10, 2011, 10:49:51 pm
content: 3 watts into an SMA package is really, really high. I would strongly recommend you switch to an SMC diode, or try out Zad's idea. Either way, you still have 3 watts to dissipate, so you'll need 2-3 square inches of copper plane to act as a heatsink.

author: Opticalworm
author_rank: Contributor
timestamp: February 14, 2011, 12:28:37 pm
content: Thank you all.Sorry Zad I'm afraid that my design has to use Zener diodes has mean to limit voltage, this is due to the nature Intrinsically safe equipment which sees simple components as infallible, but thank you for your suggestion.Thank mikeselectricstuff, I will use several Zeners in parallel to share that power and hopefully reduce the power the zenner sees.

author: Zad
author_rank: Super Contributor
timestamp: February 14, 2011, 03:27:03 pm
content: Sounds to me like a VERY narrow failure mode analysis!

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: February 14, 2011, 04:48:28 pm
content: Quote from: Opticalworm on February 14, 2011, 12:28:37 pmI've done some IS stuff in the past - increasing rating by paralleling isn't very practical, as you can't guarantee that the load will be spread evenly between parallel zeners - as well as tolerance issues there is the possibility of one or more failing open, increasing dissipation in the others.They have to be lower-voltage ones connected in series. To get the required fault coverage you may need 2 or 3 seperate parallel strings. See if you can reduce the worst-case power going in, or split it into seperate zones, as this will make things a lot easier all round. 3W is a lot of power in the IS world.Using a device with a higher inherent power rating is likely to save you a lot of grief.

author: Opticalworm
author_rank: Contributor
timestamp: February 17, 2011, 12:26:34 am
content: QuoteThanks mikeselectricstuff IS is harder than what I originally though and on top of that I now realise that I was going on about it wrong. I was trying to use the Zener as a mean to regulate the system voltage to (Say 30v @400mA+Iz).I Now know that I should really be using the zener (or TVS) to short circuit the supply when it goes above the normal voltage operation . I'm a little fuzzy as too how I would go about picking and matching the correct TVS diode to a Fuse. But I've post the question into a seperate Posthttps://www.eevblog.com/forum/index.php?topic=2545.0

author: Frant
author_rank: Regular Contributor
timestamp: February 17, 2011, 10:59:16 pm
content: Quote from: mikeselectricstuff on February 14, 2011, 04:48:28 pmYou are quite right, but the power rating is not always the most reliable parameter, especially for diodes. It all depends on the way it is measured and presented in the datasheet. Doing an IS project myself, I ran into a similar trouble with Zener diodes. Common types of diodes in SMD packages (e.g. SMA) are poorly designed in terms of power dissipation. Thus theirrealRthJA is very high and the manufacturers tend to publish "optimistic" thermal data obtained under unrealistic or not clearly defined conditions. Fortunately, there are some newer diode and Zener diode types that can dissipate more power than the older ones of the similar size.

author: cyberfish
author_rank: Regular Contributor
timestamp: February 19, 2011, 09:41:50 am
content: If you want to parallel diodes/transistors, you'll also need to make sure they have positive temperature coefficient - resistance goes up when temperature goes up.If resistance goes down when temperature goes up, if one diode takes slightly more load and heats up, the resistance will decrease, and it will take even more current, which causes it to heat up more, and take even more current...Eventually it will take all the current, and pop. Then it goes to the next one, etc.

author: Frant
author_rank: Regular Contributor
timestamp: February 19, 2011, 06:05:18 pm
content: Quote from: cyberfish on February 19, 2011, 09:41:50 am"Current sharing" by paralleling components of individual rating lesser than the required rating is not possible in circuits upon which the intrinsic safety (IS) depends. If an infallible shunt safety assembly is to be built, at least two parallel paths of diodes or Zener diodes must exist. Each of the used diodes or Zener diodes must be rated to carry at least 1.5 times the total current and to dissipate at least 1.5 times the power in normal operation. Faults must also be taken into account according to the IS standard.