When it comes to high-frequency switching applications, the quality of the Schottky diode used as the secondary side rectifier is more important than ever. This is because power supplies that switch at high frequencies are sensitive to diode parameters such as current handling capability.
Schottky diodes are the most commonly used due to their low forward voltage drop, which improves the overall performance of the power supply. However, as frequencies increase, the selection of a suitable Schottky diode becomes a challenging process. Let's break down the key considerations and parameters to keep in mind when selecting a Schottky diode for power supplies with high switching frequencies.
Reverse Recovery Time (TRR)
Reverse recovery time is the most significant parameter when choosing a Schottky diode for power supplies with high switching frequencies. It is the time it takes for the minority carrier to escape from the pn junction once the diode is turned off. A shorter reverse recovery time is desirable as it allows the diode to bounce back before the next switching cycle, improving the overall performance.
To minimize TRR, you should look for Schottky diodes with a small minority carrier lifetime and a suitable design that reduces minority carrier storage at the pn junction.
Forward Voltage Drop (VF)
Forward voltage drop is the minimum voltage required for the diode to conduct electricity. A lower VF is better as it improves the overall performance of the power supply.
When selecting a Schottky diode, you should look for ones with a minimum forward voltage drop of 0.3V for power supplies with high switching frequencies.
Current Handling Capability
Current handling capability is the ability of the diode to handle high currents without failing without conducting. This parameter is crucial when selecting a Schottky diode for applications that require high-frequency switching as peak currents can be quite high in some cases.
When choosing a Schottky diode, you should select ones that have a better current handling capability than the maximum expected current rating of your design.
On-Resistance (RDS(ON))
On-resistance is the resistance between the anode and cathode when it is switching. It has a significant impact the efficiency of the power supply as higher RDS(ON) results in greater power loss.
You should look for Schottky diodes with a small on-resistance, which is typically expressed in milli-ohms. The on-resistance is temperature-dependent, so it is essential to find a compromise between RDS(ON) and junction to lead inductance.
Junction Temperature (TJ)
Junction temperature is the temperature inside the diode where it is in operation. High junction temperature can lead to increased MTBF of the diode.
When selecting a Schottky diode, you should choose ones that have a good heat dissipation which translates to a upper limit temperature rating.
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Slope resistance is the resistance of the diode when it is in operation and a bias voltage is applied. It has a profound impact in minimizing power loss in the diode of a power supply with high-frequency switching.
When selecting a Schottky diode, you should look for ones with a low slope resistance of less than 0.5 ohms to reduce power losses.
Package Type and Design
The package type and design of the Schottky diode also have a significant impact in high-frequency switching applications. For example, a Schottky diode sits on a copper plate increases system reliability.
Select the right combination of package and design for your application that requires high-frequency switching.
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