ULN2003A Thermal Calculations - Page 1
Forum section: electronics / projects

author: hobbyelectronics
author_rank: Regular Contributor
timestamp: November 08, 2021, 05:02:53 pm
content: Hello,I just need some clarification as to the Max power that can be used by a ULN2003A Darlington Driver IC.The part in Question is a Diodes Inc, ULN2003AS16-13. (SOIC Package with 63 °C/W for θJA)The ambient temp of where the PCB is located will be 70 Deg. C. Max worse case. So I am using 70 Deg.C. for TA.I need to drive six 12V relays (3x 360R and 3x 720R Coils) so 3x 17mA and 3x 33mALooking at the data sheet it seems that at 70 Deg. C. Ambient and driving all relays on 100% of the time with a 100% duty cycle it would seem that the Max current would be just under 200mA total.So going on this should be ok.Any suggestions of calculations with regard to this would be helpful, but looking at the calculations I have done it should be ok.Thanks

author: 2N3055
author_rank: Super Contributor
timestamp: November 08, 2021, 05:59:16 pm
content: Looking at the datasheet for ULN2003A from TI for D package (SOIC), you are more than fine.ULNs have maximum 500mA per transistor one (single) at a time (current limited) or total 500 mA simultaneous for the whole chip (thermally limited).That is at 100% duty cycle and 70°C ambient. You are comfortably inside SOA.Make sure to connect COM from ULN2003 to V+ to have protection diodes functional.

author: DavidAlfa
author_rank: Super Contributor
timestamp: November 08, 2021, 06:01:34 pm
content: For a darlington ulm2003 the power dissipation is 0.7Vce x Ic.So 0.7x(3x0.017 + 3x0.033) = 100mw (Perfectly fine).If 0.7Vce is a problem, get the uln2003v12, uses mosfets instead darlingtons, they have 2-3ohm RDS on.Taking 3ohms as Rds, the power for each Channel would be I²xR, so 3x(0.017²x3) + 3x(0.033²x3)=12mw, much lower, but doesn't worth the extra cost for driving simple relays.I had to use them because 0.7Vce caused issues.

author: hobbyelectronics
author_rank: Regular Contributor
timestamp: November 08, 2021, 06:34:32 pm
content: Hi,Thank you both for a quick reply. That is really helpful.I didn't know about the FET version, about the same price in lager quantities so as a drop in replacement is good to know.This also depends what I can get hold off as I need around 12K pieces.Looking at the Diodes Inc. Data sheets for the graph attached suggests that the max current for driving 6 channels at 100% Duty at 70 Deg. c ambient is just below 200mA.Maybe I am reading this wrong, but that is also more than the 150mA total I need for the 6 relays.Thanks. I really do like the look of the FET version. I have thought about this in the past, but never searched for one. And they do 4 channel version which is nice.Thanks

author: DavidAlfa
author_rank: Super Contributor
timestamp: November 08, 2021, 09:00:01 pm
content: That current rating is for each channel, and it's caused by thermal limitations.At 500mA each channel dissipates 0.35W, which is not a lot, but multiply that by 7 and you get 2.45W, now that's a lot of heat for a small chip.BTW mosfet version has lower current capabilities, from memory I think it was 140mA per channel.Some quick math and you get why: 2ohm RDS at 0.14A make 0.4W of heat.Both versions have their benefits, but you must take in count the application details, it's not always possible to replace them.

author: bdunham7
author_rank: Super Contributor
timestamp: November 08, 2021, 09:21:10 pm
content: Quote from: hobbyelectronics on November 08, 2021, 06:34:32 pmThat's per output. Your VCEwill be less than 1V at your current levels, so less than 150mW power and less than 10C temperature rise. They're rated for 150C, so I think you're fine. I used the PDIP version in a project recently with currents similar to yours but not 100% duty cycle and they didn't even get noticeably warm.