I thought body diodes were an artifact of the fabrication of mosfets. A quick search proves fruitful, and the additional information may bear on your problem directly, namely that a Schottky Barrier diode may be added to the circuit to isolate the body diode: Google cache, since the page seems to be gone at the moment: http://www.google.com/search?q=cache:ObhFNaMHhfwJ:www.richieburnett.co.uk/sstate3.html+mosfet+body+diode&hl=en&gl=us&ct=clnk&cd=3 Search for body-diode. The relevant bit is included in this email since the link is likely to break up for many: ---------- 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!) ---------- On 12/1/06, David VanHorn wrote: > I'm having a real pain here with a design, and it would all go away if I > could find low Rdson, low threshold N or P channel enhancement mode mosfets > either without body diodes, or with the body diode Vf above 5V. > > Is there any hope? > > -- > Feel the power of the dark side! Atmel AVR > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Moving in southeast Michigan? Buy my house: http://ubasics.com/house/ Interested in electronics? Check out the projects at http://ubasics.com Building your own house? Check out http://ubasics.com/home/ -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist