Transistor and MOSFET Substitutes: Check Gate Drive, RDS(on), and Thermal Limits
A customer sent a small appliance board BOM last week. His usual AO3400A was out of stock and he had a reel of Si2302, in SOT-23, and asked if they were drop-in replacements. We tested both on the same board. At no load, they matched. With a 12 V motor, the su
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A customer sent a small appliance board BOM last week. His usual AO3400A was out of stock and he had a reel of Si2302, in SOT-23, and asked if they were drop-in replacements. We tested both on the same board. At no load, they matched. With a 12 V motor, the substitute ran hotter and produced worse spikes on the freewheel node.
An identical package only solves soldering. The AO3400A reaches its published RDS(on) near a 4.5 V gate drive. A board driven directly from a 3.3 V GPIO does not reach that condition, so effective resistance is much higher. Si2302 also differs in voltage rating, on-resistance and gate charge. It can look fine at light load; heating and efficiency loss appear as current rises.
First compare the board's gate voltage. Datasheets list RDS(on) at several VGS values. Matching 30 V VDS is not enough if the substitute needs 4.5 V while the board supplies 3.3 V. Gate charge matters too: a larger Qg slows switching and increases driver loss. Check SOA, avalanche capability and thermal resistance as well. Relay and solenoid turn-off sends energy back into the device; a SOT-23 has limited copper area for cooling.
For parts such as MMBT3904, hFE is only one item. Look at VCE(sat), parasitic capacitance and breakdown voltage. Check the manufacturer suffix, marking and tape orientation; the same part number can differ in assembly details.
Before choosing a substitute, send the schematic, gate-drive voltage, load and thermal conditions. Common MOSFETs can be reviewed at https://www.szkwic.com/en/c/diodes-transistors-mosfets.html . For a cross-check, send the original and proposed part with the BOM to https://www.szkwic.com/en/bom.html .
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