MOSfets in H-bridge additional anti-parallel Diodes, where ? - Page 1
Forum section: electronics / beginners

author: Hr_Satch
author_rank: Contributor
timestamp: June 09, 2025, 02:26:51 pm
content: Dear Forum,as I am in the process of developping a full H-bridge inverter:with a massive inductive Load as a Toroidal power transformer is:I am wondering where to put the additional anti-parallel diodesto protect the power mosfets intrinsic body diode ?I am going to use Fast Soft recovery MESA rectifiers.Isn't it best to put them closest to the source of Inductive spiking,so I mean near the Load, THE Transformer where the inductivity starts the counter EMF ?Since all MOSfets have their own internal body diode.So I think it is best to kill inductive spikes near the source of them ;-?Because all designs I see, they are drawn over the Mosfets DS-channel.But these have already one internally.Can some of you specialists on this subject please ellaborate a bit deeper,thanks, Harry.Furthermore, once drawn, these diodes form a full-wave bridge rectifier,used in the opposite way, it seems to me ?Is this a problem to form back an AC sinusoidal wave ? ? ? ?I hope getting a sane, intelligent, response on this

author: Zero999
author_rank: Super Contributor
timestamp: June 09, 2025, 02:29:09 pm
content: The internal body diode might be enough. If not, then add a Schottky diode in parallel with the body diode. You can even use a bridge rectifier to save space.

author: klammerj
author_rank: Newbie
timestamp: June 09, 2025, 02:45:07 pm
content: The internal body diode is a silicon diode. So some people are worried because of the reverse-recovery time.If the internal diode never turns on, that problem disappears. For this to work, the vfwd of the paralleled diode needs to be below 0.6V.Typically people use Schottky diodes (or those copackaged fetky things).https://www.hybridelectronics.com/images/cataloug/o/ON_FETKY.pdf

author: AlexRG
author_rank: Regular Contributor
timestamp: June 09, 2025, 03:27:09 pm
content: Quote from: Hr_Satch on June 09, 2025, 02:26:51 pmYou are trying to eliminate the consequences. But maybe it is better to eliminate the cause of the high-frequency ringing?What is a MOSFET channel that opens and closes? Schematically, this can be represented as a capacitor that is charged and discharged. Several "chokes" are connected to this capacitor: transistor crystal + transistor pin, transistor pin + PCB track, PCB track + load inductance. As a result, we get a serial LLLC oscillatory circuit. The resonance frequency of this circuit is determined by the type of transistor, the speed of the PWM signal and the inductance of all the "chokes".Therefore, to reduce the high-frequency ringing, you need to reduce the length of the tracks as much as possible. This is the first thing.The second, most important thing. You need to limit the closing speed and opening speed of the MOSFET. That is, so that the voltage goes to this oscillatory circuit as slowly as possible. This is done very simply - by adding a resistor to the gap of the MOSFET gate. The value of this resistor depends on the transistor gate capacitance, gate charge, and PWM frequency. Try experimentally installing different resistors from 10 to 100 Ohm. But be sure to do it for all four MOSFETs at once. For example, 10, 20, 30, 40, 50..100 Ohm. It is quite possible that at some value, the high-frequency ringing will be reduced so much that additional diodes will not be needed.

author: MariuszD
author_rank: Regular Contributor
timestamp: June 09, 2025, 08:31:56 pm
content: In low voltage MOSFET body diode is fast recovery, and can act as protection. In high voltage MOSFET body diode is slow and can destroy the bridge.

author: Hr_Satch
author_rank: Contributor
timestamp: June 11, 2025, 09:27:20 pm
content: Allright Gents,thankyou,allthrough I understand the suggestion of Shottkeys in parallel,for their lower Vf, so quicker conduct than the Si body diode.Are there Shottkeys above 200Volts PiV ? ? ?-- -- --

author: Hr_Satch
author_rank: Contributor
timestamp: June 11, 2025, 09:44:07 pm
content: I am wondering where to put the additional anti-parallel diodes :You are trying to eliminate the consequences.But maybe it is better to eliminate the cause of the high-frequency ringing?What is a MOSFET channel that opens and closes?Schematically, this can be represented as a capacitor that is charged and discharged.Several "chokes" are connected to this capacitor: transistor crystal + transistor pin, transistor pin + PCB track, PCB track + load inductance.As a result, we get a serial LLLC oscillatory circuit.Therefore, to reduce the high-frequency ringing,you need to reduce the length of the tracks as much as possible.The second, most important thing.You need to limit the closing speed and opening speed of the MOSFET.That is, so that the voltage goes to this oscillatory circuit as slowly as possible.This is done very simply - by adding a resistor to the gap of the MOSFET gate.The value of this resistor depends on the transistor gate capacitance, gate charge, and PWM frequency.Try experimentally installing different resistors from 10 to 100 Ohm.But be sure to do it for all four MOSFETs at once. For example, 10, 20, 30, 40, 50..100 Ohm.It is quite possible that at some value, the high-frequency ringing will be reduced so much that additional diodes will not be needed.Allright thanx AlexI do understand about slowing down the dI/dt changes.Because that's where inductances play at. The faster the more important they come.But than I wonder why opting for faster MOSFETS, over Power Bipolars,if their speed advantage, anyhow needs reducing ? ? ?But here I get stuck, how to keep everything short as possible, but in the end you need thick wiresgoing from the PCB with all switching MOSFETs towards the Load, an ouput transformer, . . . ? ? ?And this giant can not go onto the PCB ? ? ?So, what about applying a Negative bias of around -3V to -5V max.Maximum -5V, because when applying a 15V gate drive, most mosfets have a max of 20V gate supply.Althou, it makes things perhapse more complex.But AN's from Texas Instruments / Unitrode / etc . . . and many others suggest such Neg. biasin their datasheet ;-?I am curious what you about this approach, thanx, Harry.