1n4142 Diode replacement from digikey

Thread Starter

inwo

Joined Nov 7, 2013
2,419
Glass diode (1n4142) is across 100 ohm resistor between output of smps ic (uc3842) to gate of mosfet. (2sk1808)
Looking for sub from DigiKey.
 

Thread Starter

inwo

Joined Nov 7, 2013
2,419
Looked up 4148. Have a bunch of them.
Never knew they were 4a pulse.
So that's good news.
Thanks again.
 

ian field

Joined Oct 27, 2012
6,539
Looked up 4148. Have a bunch of them.
Never knew they were 4a pulse.
So that's good news.
Thanks again.
AFAIK: they're not - continuous current is in the hundreds of mA, I expect the 4A duty cycle would be pretty short.

What package style is the original diode?

When I serviced PC monitors for a living; some manufacturers used a 1A 20V Shottky diode to protect the MOSFET gate - they failed short circuit much more decisively when the MOSFET punched through, once or twice they actually succeeded in protecting the chip.

Usually when the MOSFET fails short circuit, it blows open the current sensing resistor in the source lead. The gate invariably shorts as well and it tries to dump 320V on the chip output pin. The fast silicon diode most manufacturers fit usually fail to protect against that - 1A 20V Shottky diodes have been known to succeed.
 

ian field

Joined Oct 27, 2012
6,539
I have always used the In4148 in that application without any problems.
Max.
In what application?

When used as a gate protection diode in a 3842 SMPSU - the diode literally does nothing, and you hope it never needs to.

When the MOSFET shorts it usually blows the source resistor and the gate rams whatever's left of the reservoir charge into the 3842 output pin.

A 4148 in that position would probably be blown apart physically. Most manufacturers use a common fast 1A silicon diode, it usually gives no protection whatsoever when the MOSFET fails - a few manufacturers used a 1A 20V Shottkty barrier diode, they fail SC very decisively and have once or twice been known to protect the chip.
 

ian field

Joined Oct 27, 2012
6,539
THE OP's.
Max.
A 4148 in that application would be entirely irrelevant.

As I've already mentioned, it serves no function at all in normal operation.

When the MOSFET fails S/C; it rips the source resistor open and dumps whatever remains of the reservoir charge on the 3842 output pin.

You hope that diode will also fail S/C and protect the 3842 - a 4148 will certainly fail S/C, but not long enough to outlast the reservoir charge.
 

ian field

Joined Oct 27, 2012
6,539
I had originally acquired this information from Tahmid's blog: Using the high-low side driver IR2110 - explanation and plenty of example circuits
To quote: D2 and D3 discharge the gate capacitances of the MOSFET quickly, bypassing the gate resistors, reducing the turn off time. R1 (10R) and R2 (10R) are the gate current-limiting resistors.
Consequentially using it for a couple of recent applications.
Max.
I don't remember the TS mentioning anything about an IR2110 - the diode under discussion is between a UC3842 and the MOSFET.
 
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