Comparison of asemi Schottky diode and ultrafast recovery diode in switching power supply

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The switching power supply consists of a high-frequency magnetic core 、 High frequency capacitors 、 High back pressure high power transistor 、 Power rectifier diode and control circuit are composed of the main components . Rectifier diode is the key component , Because of its maximum power consumption , About 30%, Therefore, the rectifier diode is required to have small forward voltage drop under high speed and large current 、 Short switching time . that ASEMI Which is the better comparison between Schottky diode and ultrafast recovery diode in switching power supply ？

actually , There are two kinds of widely used rectifier diodes ： One is ultrafast recovery diode , The other is Schottky barrier diode （SBD）. Generally speaking , The performance of Schottky diode is better than that of ultrafast recovery diode . Because of its low voltage and high current 、 low power consumption 、 High speed switch, etc , In high frequency rectifier 、 Used as rectifying and freewheeling elements in switching circuits and protection circuits , It can greatly reduce power consumption , Improve circuit efficiency and frequency , Reduce circuit noise . therefore , In the low pressure range , Schottky diode replaces ultrafast recovery diode . However , Because of the metal barrier ,SBD Unable to withstand higher voltage . therefore , In a large voltage range , Ultrafast recovery diode is essential .

 

Next , Reverse recovery time trr、 The reverse voltage and the forward voltage are compared , As shown below .

1、 Reverse recovery time trr contrast

PN The forward or reverse characteristics of the junction depend on the height of the barrier in the junction , Or to put it another way , Depends on the carrier distribution in the junction . Because a certain barrier height corresponds to a certain carrier distribution . Under positive bias , The barrier height is very low , There are many carriers in the junction , Well above the equilibrium value ; During reverse bias , There are few carriers in the junction , Below equilibrium , therefore PN The knot is positive and partial . When steering is reverse biased , There must be a process similar to capacitor discharge , This is called the reverse recovery process . The time required to go through this process is the reverse recovery time trr. Obviously, this has something to do with the longevity of the surplus ethnic minorities . Ultrafast recovery diode N The region is usually doped with gold , To reduce minority carrier lifetime and obtain shorter reverse recovery time .

Schottky diode SBD The forward current is mainly injected from semiconductor to metal “ heat ” Electronics , A few hole injection currents are negligible . After the bias voltage is reversed , In principle, electrons injected into the metal will also return to the semiconductor , But it still requires the electrons to maintain enough energy to exceed the barrier . Under positive bias , The energy of the electrons injected into the metal under the electron bias is one barrier height higher than the Fermi level of the metal , This part of the energy disappears into the metal in a very short time due to collision .

therefore , After applying the reverse bias , These hot electrons can only return to semiconductors within this order of magnitude , So Schottky diodes don't actually exist PN Minority carrier storage effect of junctions , It mainly determines that the reverse recovery time is determined externally , It is determined by the circuit , It is not determined by the internal electronic processes related to the conduction mechanism . therefore , Reverse recovery time ratio of Schottky diode PN The knot is much shorter . Schottky diodes have characteristics similar to PN Differential capacitance effect of junction , It has a certain impact on the reverse recovery time . To reduce the differential capacitance , The effective area of Schottky diode shall be reduced as much as possible , The doping concentration of the corresponding epitaxial layer is low .

2、 Reverse voltage comparison

Generally speaking , Metal - Semiconductor contacts are commonly used to form Schottky barriers . However , Because the metal contacts the semiconductor at the contact interface SiO2 layer , The contact resistance and surface density of States increased significantly , Thus, the device performance is greatly reduced . To solve this problem , A new technology , Metal silicide - Silicon contact barrier process , It is used to form a very reliable and repeatable Schottky barrier . At the same time, new technologies such as protective ring structure are adopted , The reverse characteristic of Schottky diode is greatly improved , Present ideal volt ampere characteristics . However , Due to the metal barrier , The barrier layer is relatively thin . therefore , It cannot withstand higher voltage . Ultrafast recovery diode has high substrate resistivity 、 Thickness of epitaxial layer 、 Deep junction diffusion , The withstand voltage capacity can reach several thousand volts , This is also the reason why the ultrafast recovery diode cannot be completely replaced .

3、 Forward pressure drop comparison

The special structure of Schottky diode determines its small body resistance , The forward voltage drop is much lower than that of ultrafast recovery diode , Therefore, the power consumption is low . In production practice , Ultrafast recovery diode VF and trr There are contradictions in the improvement of . in other words , When VF As low as possible , its trr Must rise . Again , If trr Improve ,VF It will also rise . The reduction of power consumption is disadvantageous , Usually only a compromise can be made between the two , This is also a disadvantage of ultrafast recovery diode . Schottky diode SBD, Because of the contact with the golden half , Determines its trr It's very small . Under normal circumstances , improve VF Will not make trr Get worse . therefore , Schottky barrier diodes help reduce power consumption .

 

As a rectifier diode , Schottky diode is relative to PN The junction ultrafast recovery diode has the following advantages ：

1、 Schottky diodes are diodes made of potential barriers formed by the contact between metals and semiconductors . It is different from diffusive PN Junction diode . The forward pressure drop is only PN Half knot , Therefore, the power consumption can be reduced by half .

2、 Schottky diodes use most of the carrier devices , No, PN Transient recovery characteristics of minority carrier injection and storage in junction diode barrier , So that the recovery characteristics of both are obvious under the same application conditions . The difference is , The recovery time is approximately 1:50. because SBD The recovery time is much shorter , Its switching speed is PN Double the junction diode . The shorter the diode recovery time , The smaller the average power loss .

in summary , In switching power supply , Schottky diode is more ideal than ultrafast recovery diode , Its power consumption can be reduced by more than half , Further improve the efficiency of power supply . therefore , Schottky diode is essential for switching power supply . Widely used in voltage regulators 、 rectifier 、 inverter 、UPS etc. , It can also be used as a fast clamping diode .