Grande Landa,
Com certeza, os FETs já tem um diodo desse tipo internamente, e a essa tensão tá tudo certo.
Estes dias o Sir Jorge me enviou um arquivo texto que explica melhor este "defeito de fabricação", como diz o mestre Fabio. Desde o começo do meu aprendizado nesta área (acabou de fazer 1 ano), Sir Jorge bate nesta tecla. Nem precisa dizer que o Smile tem um diodo rápido em anti-paralelo com o dreno-source de cada mosfet
Espero que seja útil.
Abraços,
Rudolf
Beware the MOSFET body-diode !
Those familiar with MOSFETs will know that the fabrication process results in a built-in anti-parallel diode between the source and drain terminals of the device. This is often referred to as the "body-diode." Referring to any MOSFET data sheet will reveal specifications for this intrinsic diode.
At first it appears that the internal body-diodes are a bonus since they provide the desired free-wheel diode function for free. This is often the case in many power electronics applications where they provide the function of the free-wheel diodes with ease. Sadly it is not the case in this application. The MOSFET body-diode is a side effect of the fabrication process and is not a particularly good diode. The same design criteria for good MOSFET characteristics do not produce the best body-diode characteristics. The design of a MOSFET is always a compromise, and it is the characteristics of the body-diode that suffer.
When compared to discrete high speed diodes, the body-diode's reverse recovery time is very long. This means that the diode takes a long time to turn off when the current flowing through it changes direction. As explained previously, this leads to a shoot-through condition when the opposing switch is turned-on. For this reason the body-diodes are clearly not suitable for free-wheel diode duty in this application and should be isolated.
The body-diode is isolated by means of a Schottky Barrier diode connected in series with the MOSFET drain lead. (Schottky diodes operate due to majority carrier conduction, and therefore do not exhibit any significant reverse recovery time. Essentially, they turn off immediately when the current tries to change direction.) This prevents current from flowing through the MOSFET body-diode and forward biasing it. If the body-diode is never forward biased it does not exhibit a reverse recovery problem.
An external fast recovery diode is then connected across the pair to provide the necessary path for the free-wheel current.
The use of external free-wheel diodes gives the designer greater choice in the characteristics of this critical component. It also removes a source of dissipation from the MOSFETs, since the free-wheel current no longer enters onto the MOSFET die.
Although dedicated fast-recovery diodes are much faster than the MOSFET body-diodes, they still have a finite reverse recovery time.
This is typically several tens of nanoseconds. Any remaining reverse-recovery problems can usually be solved by slowing down the turn-on of the switches. This allows longer for the free-wheel diodes to recover, and reduces the peak reverse recovery current. (The recovery is also softer and results in less radiated interference
too!)
