Hello:
After going through the data sheet - it’s not clear. I’m looking to find out what is the maximum steady-state current with a 12VDC supply?
Assume a good optimized board layout with 2oz copper.
Thanks for any help, or references.
Gary
Greetings,
The overcurrent detection function may actuate at currents as low as 2.7A, so I’d consider that a ceiling on the maximum current one could rely on the part to carry. Thermal considerations may impact this, but that will be application dependent.
Well, a bad datasheet, maybe something lost in the translation. Can’t rely on it for anything that makes sense. They should outright state it, instead of some silliness range from 2.7 to 9.
Thanks for your help.
Hi Gary,
One could make arguments both ways. If one intends to use overcurrent detection as an indication of an actual overcurrent event, you might want to know that it may trip over a quite broad range, and is, therefore, not something to be relied upon for downstream protection.
Regarding the actual operating conditions within which a device can be operated, looking at the maximum and minimums are the key specs, unlike the front page marketing blurbs one tends to see first.
For devices such as these, the max steady-state current depends on several factors, but it comes down to either the minimum overcurrent value or the combination of the maximum ambient air temperature you expect to encounter and the actual thermal impedance of your circuit board. Depending on the specifics of the latter specifications, both of which can vary greatly from one application to the next, the maximum current may be even less than the minimum overcurrent value of 2.7A.
To determine the maximum steady-state current, based on the maximum junction temperature, Tj, rather than the overcurrent spec, one has to consider the following:
The maximum junction temperature, Tj, of the BV1HD090FJ-CE2 is 150°C, so you must subtract your max expected operating temperature from 150°C to find the maximum increase in temperature allowed within the device. One then has to consider the thermal impedance of the circuit board and the maximum on-resistance of the device at the maximum junction temperature.
For example, assuming your circuit board has a thermal impedance of 86.9°C/W, as described in Note 3 on page 4 and 5 of the datasheet (2-layer board, 2oz copper, 114.3mm x 76.2mm), assuming you have a maximum operating temperature of 60°C (a relatively moderate assumption), and reading from the datasheet on page 7 that the maximum on-resistance of the device may be as high as 215mΩ, one can determine the maximum steady-state current.
With a max operating temperature of 60°C, the maximum Tj rise is:
Tj rise max = 150°C - 60°C = 90°C
With thermal impedance of 86.9°C/W, the maximum allowed power dissipation in the device while not exceeding Tj-max of 150°C is:
Pd-max = 90°C / 86.9°C/W = 1.036 W
With a maximum on-resistance of 215mΩ, the maximum current can be calculated as:
Imax = √(1.036 W / 0.215 Ω) = 2.195 A
So, even with a relatively low max ambient temperature of 60°C and a pretty decent thermal impedance of 86.9°C/W, the maximum current is only about 2.2A before exceeding Tjmax. So, because of all of the variables involved, it would be impractical for them to publish a hard number for max current, but this shows why it is important to actually calculate it for any given application.
Thanks David, thorough answer.
However, my point was, if you look at many of the SSR’s in the database, they list a specific maximum current. For example, look at the Coto CT/CS128. A great part that I have used. The first graph on page 3 - and arguably one of the most important - is output current vs ambient temperature. Now this device is also in a DIP/SOIC packgage without external pads for dissipation. Rohm went out of their way not to publish a similar graph, instead choosing to smoke and mirror behind “overcurrent detection”. in fact, I have never seen an SSR that did not publish output current graph, this is the first. So if this is a new product from Rohm, maybe they should rethink their data sheet and be more user realistic. Sorry about the rant, my 0.25c worth…for what it’s worth.
Very valid points, Gary.
One might argue that they do give you the data necessary to figure it out, but a nice graph of max load current vs. temperature, like that given in the Coto datasheet, would be a very useful rule-of-thumb guide. Such a graph allows the designer to know, at a glance, whether the part is within the ballpark of what is needed and doesn’t force the designer to perform multiple calculations to figure that out.