The TO-220 carries up to about 40 A with mounting discipline, and the TO-247 takes over at higher currents with a larger die, more heat, and more mounting care. This guide compares the current, thermal, and mounting columns and the upgrade path.
The 40 A Boundary and Package Limits
The 40 A boundary is where the TO-220’s tab and package begin to limit the thermal path: the tab carries the heat to the heatsink, and the package’s thermal resistance and the mounting interface decide the real current. The TO-247’s larger body and tab carry more die and more heat, at the cost of a bigger footprint and a more demanding mount.
The boundary is a thermal line, not a fixed number: a TO-220 on a well-mounted heatsink with airflow can carry 40 A, and the same part on a poor mount cannot. The package comparison reads the system thermal resistance, not the package name.
TO-220: Up to 40 A With Discipline
The TO-220’s value is its established discipline: a familiar footprint, a standard mounting with torque and TIM, and a catalog of 30–40 A parts. The AMBR40250S is the 40 A, 250 V TO-220AB example, and its 2.0 °C/W junction-to-case is usable only with a properly mounted heatsink—the mounting guide owns the torque, TIM, and insulation rules.
The TO-220’s limit appears at the thermal result: at 40 A, the loss and the interface resistance set the junction, and the design that skips the mounting discipline pays at the case temperature.
TO-247: More Die, More Heat, More Mounting Care
The TO-247 carries a larger die and a larger tab, moving more heat to the heatsink—and demanding more mounting care: a bigger screw, a higher torque, and a larger TIM area. The package’s thermal resistance is lower than the TO-220’s class, but the mounting’s quality decides how much of it is kept.
The TO-247 also costs board space and adds mechanical stress, and its use is justified where the current and the thermal demand exceed the TO-220’s disciplined ceiling.
Current, Thermal, and Mounting Comparison
| Item | TO-220 | TO-247 |
|---|---|---|
| Current class | Up to ~40 A | Above 40 A |
| Tab size | Standard | Larger |
| Mounting | M3 screw, standard TIM | Larger screw, more TIM |
| Thermal ceiling | Heatsink-dependent | Higher with care |
| Footprint | Smaller | Larger |
The table is the comparison: the TO-220 wins the footprint and the familiarity, and the TO-247 wins the thermal ceiling at the cost of the mounting and the space.
Upgrade Path From TO-220 Designs
The upgrade from a TO-220 to a TO-247 design is a board and mounting change: the hole pattern, the screw, the TIM, and the heatsink are resized, and the thermal design is re-read at the new package. The upgrade is justified when the current or the thermal demand exceeds the TO-220’s disciplined ceiling, and the record—the old and new thermal measurements—is the change’s evidence.
Engineering note. The current and thermal bands are guidance, not limits—the real capability follows the heatsink, the mounting, and the ambient—and the upgrade path follows the mounting and thermal design methods.
The Thermal Reality in Practice.
At 40 A, the TO-220’s loss and the interface resistance set the junction, and the TO-247’s lower thermal class and larger tab move the ceiling higher with the mounting care. The heatsink, the TIM, and the airflow are the system’s variables, and the case-temperature measurement on the prototype is the rung’s judge. The AMBR40250S is the TO-220AB example at 40 A, and its 2.0 °C/W path is usable only with the mounted heatsink. The upgrade to the TO-247 is justified when the current and the thermal demand exceed the TO-220’s disciplined ceiling, and the production side—the larger screw, the TIM, and the inspection—is part of the change. The record—the old and new thermal measurements—is the decision’s evidence.
The Upgrade Decision in Full.
At 40 A, the TO-220’s loss and the interface resistance set the junction, and the TO-247’s lower thermal class and larger tab move the ceiling higher with the mounting care. The heatsink, the TIM, and the airflow are the system’s variables, and the case-temperature measurement on the prototype is the rung’s judge. The AMBR40250S is the TO-220AB example at 40 A, and its 2.0 °C/W path is usable only with the mounted heatsink. The upgrade to the TO-247 is justified when the current and the thermal demand exceed the TO-220’s disciplined ceiling, and the production side—the larger screw, the TIM, and the inspection—is part of the change. The record—the old and new thermal measurements—is the decision’s evidence, and the comparison closes with it.
The Rung Decision in Full.
The rung decision is read in full: the current, the thermal, the mounting, and the production are the four columns, and the measurement on the prototype closes them. The TO-220 carries up to 40 A with the mounting discipline, and the TO-247 takes over with the larger screw, the TIM, and the inspection. The AMBR40250S is the TO-220AB example, and the upgrade is justified when the demand exceeds the disciplined ceiling. The record—the old and new thermal measurements—is the decision’s evidence.
The Upgrade Record.
The upgrade’s record closes the decision: the old and new thermal measurements, the mounting and production changes, and the current and thermal justification are filed together, and the next high-current design reads the same record. The TO-220 carries up to 40 A with the mounting discipline, and the TO-247 takes over with the larger screw, the TIM, and the inspection. The AMBR40250S is the TO-220AB example, and the upgrade is justified when the demand exceeds the disciplined ceiling. The record is the decision’s evidence, and the measurement is its judge.
The Rung Decision’s Final Reading.
The rung decision’s final reading is the system: the current, the thermal, the mounting, and the production are the four columns, and the case-temperature measurement on the prototype closes them. The TO-220 wins the footprint and the familiarity, and the TO-247 wins the thermal ceiling with the mounting care. The upgrade is a board, mounting, and process change together, and the record is the evidence. The two rungs serve the high-current world at their own scales, and the system reading places each one.
The Production Reading.
The production reading completes the rung decision: the TO-220’s established footprint and mounting are a proven production step, and the TO-247’s larger screw and TIM are a new step the line must absorb. The upgrade’s cost and risk are read with the thermal gain, and the decision table’s winners hold across the production side. The record—the old and new thermal measurements, the mounting and production changes, and the justification—is the evidence, and the measurement is the judge. The two rungs serve the high-current world at their own scales, and the system reading places each one.
The Rung Record.
The rung record is the decision’s deliverable: the current, the thermal, the mounting, and the production columns, with the old and new thermal measurements, are filed together, and the next high-current design reads the same record. The TO-220 carries up to 40 A with discipline, and the TO-247 takes over with the mounting care. The upgrade is justified when the demand exceeds the disciplined ceiling, and the record is the proof the comparison closes with.
Frequently Asked Questions
Where is the 40 A boundary?
Where the TO-220’s tab and mounting begin to limit the thermal path—a thermal line, not a fixed number, set by the heatsink and the mounting quality.
When does the TO-220 work at 40 A?
With a well-mounted heatsink, TIM, torque, and airflow; the discipline is the package’s price.
What does the TO-247 buy?
A larger die and tab, a lower thermal resistance class, and a higher thermal ceiling—at the cost of a bigger footprint and a more demanding mount.
How do I upgrade from a TO-220 design?
Resize the hole pattern, screw, TIM, and heatsink, re-read the thermal design, and confirm with the old and new measurements.
Which package is right for me?
The TO-220 up to ~40 A with discipline, and the TO-247 above, where the current and the thermal demand exceed the TO-220’s ceiling.
Conclusion
The TO-220 and the TO-247 are two rungs of the tab-package ladder: the TO-220 carries up to 40 A with mounting discipline, and the TO-247 takes over with more heat and more care. Read the current, the thermal, and the mounting together, and the upgrade path follows.
Review the AMBR40250S product page on the Good-Ark site, and contact Good-Ark with your current class and mounting architecture for a package recommendation.