Schottky special features

Schottky diode is a special type of diode characterized by low voltage drop and fast switching speed. This means that it can perform fast switching operations at high frequencies and achieve efficient power management at low voltages.

The special features of Schottky are as follows:

In general design, we need to leave a certain margin

For example, VR only uses less than 80% of its rated value (which can be controlled to less than 50% in special cases), while IF uses less than 40% of its rated value. In a single ended flyback (FLY-BACK) switching power supply, assume a product with an input voltage Vi (max) of 350VDC, an output voltage Vo of 5V, and a current Io of 1A. According to the calculation formula, it is required that the reverse voltage VR and forward current IF of the rectifier diode meet the following conditions:

VR≥2VI×NS/NP

IF≥2IO/(1-θmax)

Among them, NS/NP is the transformer secondary, and the primary turn ratio θ max is the maximum duty cycle

Assuming NS/NP=1/70, θ max=0.35

VR ≥ 2 × 350/20=10.7 (V)

IF≥2×1/(1-0.35)=3(A)

In this way, we can refer to the use of MDD Schottky SS34B (SMB package 3A40V) products. However, the heat dissipation of surface mount devices is usually limited, and the current of rectifier devices is greatly affected by temperature. As the junction temperature of the chip increases, the current capacity decreases. Therefore, temperature must be considered when selecting SS34. In practical selection and application, there is a risk of temperature rise and current attenuation. At the same time, if the load is a capacitive load, there will also be mutual inductance superposition between transformers. Therefore, sufficient margin must be left in the selection of current and voltage to ensure the normal operation of the product and extend its service life. Therefore, it is recommended to choose SS56B (SMB package 5A60V) or SS56C (SMC package 5A60V) products.

Attention: The heat dissipation and installation form of power Schottky diodes should be clear about the product's temperature requirements, usage environment, and adopted heat dissipation methods. Power devices should be installed in locations that are easy to ventilate and dissipate heat to improve product reliability. Thermal grease should be added between the Schottky diodes of plug-in types such as TO-220 and TO-220F and the heat sink to ensure good contact between the diode and the heat sink. For products with surface mount packaging, it is recommended to increase the area and thickness of the copper foil at the soldering position as much as possible to facilitate better heat dissipation, reduce the temperature during power device operation, and improve product reliability.

 2. Correct selection of RC compensation network RC buffer for Schottky diode

Due to the leakage inductance of high-frequency transformers and the junction capacitance of the diodes forming a resonant circuit at the cutoff, it can cause instantaneous overvoltage oscillation. Therefore, it is necessary to install an RC buffer in the power output to protect the safety of the transistor. In addition, RC networks can also reduce output noise, decrease diode heat consumption, and improve product efficiency and reliability.

The principle of selecting a buffer is to ensure that it is effective while minimizing losses as much as possible. Here is the reference formula.

R=√ (Li/Cj)/n, where Li is the leakage inductance of the transformer (μ H)

CJ is the junction capacitance (PF) of the diode

N is the ratio of primary to secondary turns (NP/NS)

The capacitance C can be arbitrarily taken between 0.01 and 0.1 μ F, and the specific value will be determined experimentally.

If Vo=5V, optional R=5.1 Ω, 0.5W, C=0.01 μ F

Pipes with smaller IR have lower heat consumption, so in the same situation, pipes with smaller IR can be chosen.

When designing PCBs, it is important to keep the pipes and heat sinks as far away as possible from heat sensitive components such as electrolytic capacitors. To increase the lifespan of the product.

The welding pad for welding pipes should be large enough, welded firmly, and avoid desoldering caused by thermal stress.

Once a Schottky diode is selected, it should undergo simulation experiments to measure its temperature rise and operating waveform under the worst input and output conditions of the product, and confirm that all indicators do not exceed their limit parameters.

The above is the special feature of Schottky.