A theoretical question about boost converter switch body diode current..

[hiphei]

I don't know why i started thinking this in the first place, but eventually it has started bothering me more and more because i haven't been able to confirm this from any book i have my self or find the proper answer even with google..

If there is basic textbook boost converter circuit (input cap, inductor, switch, and freewheeling diode going out from their junction to output cap).. The switch is most often mosfet and they have body diodes.. Is there a real principal function (can one say like that?) with a body diode in such a circuit..? If i think how it works, i can see no function with it except being in non conductional state when required..thus having some junctional charge and contributing to some switching losses..

Even though the circuit resembles crt television line output stage a lot, it still functions very differently; in line output the switch, which is typically bjt, has parallel flyback diode oriented like the body diode in boost converter..collector is connected to transformer primary and also to load which is deflection coil and resonating cap..while the beam sweep, the energy is 'charged' to primary circuit magnetics, but during the beam flyback the switch opens, and the resonating 'flyback' capacitor sets in and gets the beam quickly back (during a half of cycle)..but since current and voltage lag each other, the diode is needed for the 1/4 of the cycle..or half of the flyback..for the deflection current to run that way..

But i can not see any such functional duty for the switch body diode even though some parasitic currents can flow there..

But somehow my brain haunts me that there may be something else going on there..i just can't find it out or conclude it out.. Any real knowledge out there?

 

[std_match]

In a standard boost converter, the MOSFET body diode has no function and it never conducts.
Other switch elements can be used, but a MOSFET is often the best choice for low voltage applications.

 

[KlausST]

Hi,

the body diode is a side effect of a MOSFET. It is not built in intentionally.
It is across S-D and enables current flow in the opposite direction than usual FET current.

In many circuits this diode is not used at all.

However in half bridge configuration the diode of the opposite MOSFET may work as a voltage limiter feeding inductive currents back to the supply lines.
This may work for motor control .. with rather low switching frequency. But at higher switching frequencies this body diode is simply too slow. It usually has a rather high "reverse recovery time".
For higher switching frequencies one does not want the body diode to become conductive at all. To prevent this often a fast (Schottky) diode is externally connected in parallel.
The low forward voltage of the Schottky diode also helps to reduce conduction loss.

Klaus

 

[hiphei]

Thank You, gentlemen, for the facts!! All the other information was easily available either from my brain, old books or online, but even though i tried, i was unable to find single straight out cold centence "In a standard boost converter, the MOSFET body diode has no function and it never conducts.".. although now it sounds totally too familiar, so it MUST BE somewhere in my old materials still.. well.. a repetition is a mother of learning.. It's also funny that while this is easily seen and concluded on how the circuit works..it's sometimes too difficult to trust my own conclusions unless i get some confirmation..but thank You!!

 

[BradtheRad]

If not in boost converters then we could find one of the other switched-coil converter where the inductor wants to pull current from anywhere as soon as current gets shut off abruptly. It's a characteristic of inductors.

Or we could find some arrangement of adjoining load or other circuitry which results in the body diode turning in case of great emf imbalance.

 

[KlausST]

Hi,

boost converters with synchronous rectifiers.

Klaus