Schottky Diode Package Selection For Power Paths
A Schottky diode is chosen for two properties that a ordinary silicon rectifier cannot match: a low forward voltage drop at moderate current, and a junction that stores almost no charge when it switches. Those properties come from the metal semiconductor junction itself, and they are the same in every package. What differs between packages is how well the device can get rid of its heat, how it is assembled, and how much board area it occupies.
This article compares the package families that appear in power paths, and it sets out how to choose between them without losing the electrical advantage that made the Schottky diode attractive in the first place.
What The Schottky Junction Offers
A conventional rectifier has a forward drop of roughly 0.7 volts, and it stores charge in the junction that has to be removed before the device blocks. A Schottky junction has a forward drop closer to 0.4 volts at typical currents and stores essentially no minority charge, so the reverse recovery time is negligible. In a low voltage supply, where the output may be only 3.3 or 5 volts, that difference in forward drop is a meaningful fraction of the total loss.
The trade is a higher reverse leakage current and a lower reverse breakdown voltage. A Schottky junction is normally used below about 100 volts, and its leakage rises sharply with temperature. In a power path, that leakage has to be included in the standby current budget, because a device that is nominally off can still conduct enough to drain a battery.
The Three Package Families
At the small end sits a surface mount body such as the A-405 outline, which is roughly a few millimetres in each direction and is placed by machine with the rest of the board. It suits portable equipment, where the board area and the assembly method matter more than the thermal capability.
In the middle is the classic axial package, with a cylindrical body and two leads, such as the DO-41 outline. It is easy to handle, easy to inspect and easy to replace, and it has a better thermal path through its leads than a small surface mount body. At the large end sits a bigger axial outline such as DO-201AD, which carries a higher average current and has the best thermal path of the three through its thick leads and its larger body.
Forward Voltage Drop And Loss
The forward voltage drop is quoted at a specified current and temperature, and it rises as the current rises. The conduction loss of the diode is the product of that drop and the current, so a device chosen at its rated current may dissipate more than the thermal design allows. The figure should be read at the actual operating current and at the highest junction temperature, because the drop falls as the junction warms, which is a rare case where a warmer device is slightly more efficient.
The package affects the measurement indirectly. Two devices with the same die in different packages have the same drop at a given current when both are at the same temperature, but the device with the better thermal path runs cooler and therefore has a slightly higher drop. Comparing packages on a datasheet value without checking the test conditions is a frequent source of confusion.
Reverse Recovery Time And Switching
Because the Schottky junction stores almost no minority charge, its reverse recovery time is very short, and a datasheet may quote a figure of a few nanoseconds that is dominated by the package and the test circuit rather than by the junction. In a switching converter this is the property that reduces the switching loss and the noise generated at each transition.
The package still contributes. The lead inductance of an axial part is larger than that of a small surface mount body, and that inductance rings with the junction capacitance at each transition. In a high frequency converter the ringing can be significant, so a surface mount part with a short current loop is often the quieter choice even when the thermal argument favours the larger body.
Thermal Resistance And Mounting
The thermal resistance from junction to ambient decides how much current a package can carry before the junction reaches its limit. A small surface mount body relies on the copper pads to carry heat away, so the pad area is part of the thermal design. An axial part loses heat through its leads into the board and through the body to the air, and a larger body has a lower junction to ambient resistance for the same reason.
Mounting therefore changes the rating. A surface mount device soldered to a generous copper area with thermal vias will carry more current than the same device on minimum pads. An axial device mounted with short leads close to the board behaves better than one standing on long leads in still air. When a design is marginal, the practical fix is usually more copper rather than a larger package, because the copper is cheap and the package change may force a layout revision.
Choosing For A New Design
Start from the current and the duty cycle, then the allowable temperature rise, and only then the package. A reverse polarity protection diode that carries the full load current continuously is a thermal problem, and it needs a package with a low thermal resistance and enough copper to match. A clamping diode that conducts only during a transient can be much smaller, because its average dissipation is low.
The assembly method is the second filter. A surface mount part suits an automated line where the reflow profile is already defined, while an axial part suits a design that is hand assembled or that must be serviceable in the field. The soldering considerations for both routes are covered in the comparison of leaded and lead free processes and in the causes of component shift during reflow, and the copper sizing follows a trace width calculation against the allowable temperature rise.
Process Control and Verification
On a design of this kind, forward voltage drop is the item that decides how the rest of the board is arranged. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.
FAQ
Should a Schottky diode be derated for leakage at high temperature? Yes. Reverse leakage rises steeply with junction temperature, and in a battery powered product the leakage of a nominally off device can dominate the standby current.
Is a surface mount Schottky always worse thermally than an axial one? Not always. A surface mount device on a generous copper area with thermal vias can outperform an axial part in still air, because the board becomes part of the heat sink.
Why does the forward drop differ between two parts with the same rating? The drop depends on the die area and the current density. Two devices rated for the same current can have different drops, and the difference shows up as a difference in conduction loss.
