How to find SMT equivalents of TH parts?

Question

A lot of times hobbyist circuits are designed with thru-hole components since they're easier to solder. What approach do you take when attempting to find the SMT equivalent of a TH part?

For example, I'm trying to find the cheapest SMT diode that is equivalent to the extremely popular 1N4001. In my attempt to do this, I went to digi-key, chose the correct product category, and started using filters for things like current, voltage, etc. I ended up finding the Diodes Incorporated S1J-13-F.

1N4001 Specs:

Voltage - DC Reverse (Vr) (Max) 50V
Current - Average Rectified (Io)    1A
Voltage - Forward (Vf) (Max) @ If   1V @ 1A
Speed   Standard Recovery >500ns, > 200mA (Io)
Reverse Recovery Time (trr) -
Current - Reverse Leakage @ Vr  5µA @ 50V
Capacitance @ Vr, F 15pF @ 4V, 1MHz
Thermal Resistance  100°C/W Ja
Operating Temperature - Junction    -65°C ~ 150°C

S1J-13-F Specs:

Voltage - DC Reverse (Vr) (Max) 600V
Current - Average Rectified (Io)    1A
Voltage - Forward (Vf) (Max) @ If   1.1V @ 1A
Speed   Standard Recovery >500ns, > 200mA (Io)
Reverse Recovery Time (trr) 3µs
Current - Reverse Leakage @ Vr  5µA @ 600V
Capacitance @ Vr, F 10pF @ 4V, 1MHz
Thermal Resistance  30°C/W Jl
Operating Temperature - Junction    -65°C ~ 150°C

Since the specs for both seem comparable, can I assume this SMT device will "just work" when I put it in a circuit with a 1N4001 shown?

Answer 1

TL;DR: The part you've picked is adequate for all but the most humongous relays (at least to 1A coil current and coil voltage of 24V or more).

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As Andy says, the peak current is just the maximum relay coil current (maximum input voltage divided by minimum coil resistance (use the minimum 25°C coil resistance, adjust it for the lowest ambient temperature possible, and use the maximum supply voltage possible). Or just use the nominal and leave plenty of safety margin. The PIV rating is just the maximum power supply voltage (you should include some margin for transients). The current only flows briefly when the relay switches.

Reverse recovery time is never given on modern 1N4001 datasheets, but it's actually about 5usec, so the SMT part is a bit faster, but in the same league. 1N4001 to 1N4004 tend to be the exact same die so if you were getting away with a 1N4001 in a circuit with a 48V coil relay, it might not work as well with a part that is made closer to the line (such as a Schottky diode).

It's possible you could use something like a cheaper and smaller LL4148 (200mA 75V with a 6ns recovery) but you'd have to analyze the circuit to ensure it was adequate (if the relay is less than about 48V and 200mA it should be fine).

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Going from the specific to the general, as a rule, I try to determine the necessary requirements (not what the through-hole part happens to have) including safety margins and allowance for transients and do a parametric search from there (including price and maybe quantity in stock to pick popular parts). Then consider common parts available overseas if it is an item to be made in large quantity (the part numbers and suppliers may be different). If it's only (say) one to one thousand pieces, a search on Digikey or Mouser will suffice.

Even if the exact same die is used in an SMT package, the thermal properties of the SMT part will be quite different (usually, not always, quite a bit worse) so you should have some understanding of the real requirements- substituting parts is really engineering design work. The SMT parts also tend to have thermal properties that are much more dependent on PCB layout than through-hole parts.