Hello guys.
I'm currently designing the power stage for a Class D amplifer. My primary choice at the moment are the IRFB4020pbf power MOSFET.
Concerning this device, even when internal body diode specs are relatively good, they obviously look poor when compared to discrete ultrafast soft-recovery diodes or schottky rectifiers.
I was wondering about disabling the internal diode by means of a series and an antiparallel diode. I'm aware that conduction loses would be increased and also strait inductances but it could pay off in term of relaxing MOSFET current stress and minimizing switching loses... at least at first sight. I saw a couple of designs around the web and it called my attention that the antiparallel diode is usually a ultrafast diode and not a schottky as I would have expected (because of they not presenting reverse recovery issues and having smaller Vf). Would it be because there is not too much availability of schottky rectifiers with high reverse voltage capability?
Anyway, if any of you have tried this topology, comments on what resulted would be very welcome!
Thanks!
Pablo
I'm currently designing the power stage for a Class D amplifer. My primary choice at the moment are the IRFB4020pbf power MOSFET.
Concerning this device, even when internal body diode specs are relatively good, they obviously look poor when compared to discrete ultrafast soft-recovery diodes or schottky rectifiers.
I was wondering about disabling the internal diode by means of a series and an antiparallel diode. I'm aware that conduction loses would be increased and also strait inductances but it could pay off in term of relaxing MOSFET current stress and minimizing switching loses... at least at first sight. I saw a couple of designs around the web and it called my attention that the antiparallel diode is usually a ultrafast diode and not a schottky as I would have expected (because of they not presenting reverse recovery issues and having smaller Vf). Would it be because there is not too much availability of schottky rectifiers with high reverse voltage capability?
Anyway, if any of you have tried this topology, comments on what resulted would be very welcome!
Thanks!
Pablo
I have used seires schottky and anti parallel Fred with mosfets to get rid of body diode issues which are posing reliability problems.
Losses are not that high, only slightly more, but again what you get with this approach is bullet proof reliability from the drastic effects of freewheeling current.
Have a look at this combo device
http://ixdev.ixys.com/DataSheet/16fd3bdd-f163-4cf6-b758-0fc143100898.pdf
Losses are not that high, only slightly more, but again what you get with this approach is bullet proof reliability from the drastic effects of freewheeling current.
Have a look at this combo device
http://ixdev.ixys.com/DataSheet/16fd3bdd-f163-4cf6-b758-0fc143100898.pdf
It will be hard to find a Si Schottky with a 200V rating, and it is likely that its voltage drop will be comparable to the MOSFET. You can look at SiC or GaAs Schottky diodes, but their voltage drop is likely to be even higher.
The voltage drop of the external anti-parallel diode should be substantially less than that of the internal body diode.
The voltage drop of the external anti-parallel diode should be substantially less than that of the internal body diode.
Lars Clausen said:
Nice to see you too, Lars !!
No, I never tried that open-loop test, in my close-loop design the THD is not effected by using series schottkys, but one thing I have achieved is added reliability with this scheme.
My amp runs with rails @ +/- 185VDC and target application is professional use..........so i always focus for reliability first, i donot care whether the THD is 0.1% or 0.001%, because it hardly matters in sound reinforcement systems..........but Yes, it does matter in fancy amps
Thanks Workhorse for your comments. The amplifier I am designing will work in +- 75. Then the impact of freewheeling currents will be quite smaller than in your case, but it would be quite interesting to compare both performances with and without series schottkys. Sadly I dont have the time to build both prototypes so I guess I'll have to pick one in advance. I'll run simulations as accurate as possible and decide on this basis...
Tekko and Dave
Just consider that the current has to leave the channel and enter the parallell diode in a matter of nano seconds if it is going to give an improvement on reverse recovery. If the parallell diode has 0,2V lower drop and the stray inductance and internal inductance inside the MOSFET/diode is 10nH you can easily calculate that it takes 20A of current 1 microsecond to "move" to the other diode. Has anyone here seen an improvement on EMI, efficiency or THD by doing this?
Just consider that the current has to leave the channel and enter the parallell diode in a matter of nano seconds if it is going to give an improvement on reverse recovery. If the parallell diode has 0,2V lower drop and the stray inductance and internal inductance inside the MOSFET/diode is 10nH you can easily calculate that it takes 20A of current 1 microsecond to "move" to the other diode. Has anyone here seen an improvement on EMI, efficiency or THD by doing this?
Pafi
The body diode only conducts after the channel has been turned off if the current is flowing from source to drain (i.e. freewheeling). Immediately when the channel is turned off the current will flow through the body diode. This is of course a simplification as the body diode is actually the channel itself when not turned on. In order for the current to move to an external diode which is connected in series a certain time is needed. In a class d amplifier it has to happen within the dead time which is usually very short. So if the voltage drop is even half of the body diode it will take much longer than the dead time itself hence giving no improvement in reverse behaviour.
I am missing something?
The body diode only conducts after the channel has been turned off if the current is flowing from source to drain (i.e. freewheeling). Immediately when the channel is turned off the current will flow through the body diode. This is of course a simplification as the body diode is actually the channel itself when not turned on. In order for the current to move to an external diode which is connected in series a certain time is needed. In a class d amplifier it has to happen within the dead time which is usually very short. So if the voltage drop is even half of the body diode it will take much longer than the dead time itself hence giving no improvement in reverse behaviour.
I am missing something?
Has anyone ever tried to use a very small series resistors in the drain path ? This should also help to reduce reverse-recovery stress. It does however not speed-up anything but it would help to reduce the current-peak. If dimensioned properly it would not give more conduction losses than a series diode.
Regards
Charles
Regards
Charles
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