Schottky diode doesn't conduct current / General Science and Electronics / Forums

Forums

4hv.org :: Forums :: General Science and Electronics

« Previous topic | Next topic »

Schottky diode doesn't conduct current

Move Thread

LAN_403

Tonic

Sat Jul 12 2008, 04:26PM

Registered Member #528 Joined: Fri Feb 16 2007, 10:32PM
Location: Warsaw, Poland
Posts: 166

I'm using MBR1045 10A 45V Schottky diodes for blocking current coming out from MOSFET's built-in free-wheeling diode. The problem is that the current from positive DC bus doesn't go to drain through diode and I have no idea why. The voltage drop across it is 220mV. It conducts in only one way, to drain (checked with resistance meter). I was testing with 34V DC. Is there something I don't know?

Another question regarding Schottky diodes - since they have low reverse voltage, wouldn't a situation, where voltage across Schottky is higher than reverse voltage and allow the current from primary coil to pass through it, occur? Or perhaps it's the outer fast diode (not the one inside transistor, but that we add) takes current and prevents from situation I described?

Ken M.

Sun Jul 13 2008, 01:24AM

Registered Member #618 Joined: Sat Mar 31 2007, 04:15AM
Location: Us-Great Lakes
Posts: 628

If understood correctly, the diode we add is a transorb, and from a guess I say it means transiant abosrber.

Tonic

Sun Jul 13 2008, 08:25PM

Registered Member #528 Joined: Fri Feb 16 2007, 10:32PM
Location: Warsaw, Poland
Posts: 166

No, it's a ordinary fast diode, like this one on scheme (from Richie's site):

Bodydiod

Luckily, I solved it - surprisingly I had add more solder to track area between Schottky's cathode and MOSFET's drain. Very weird, the solder was looking pretty fine. And stupid, too

Marko

Sun Jul 13 2008, 08:47PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Tonic, what circuit are you building?

In really majority of uses those external diodes are absolutely unneeded. All that needs to be assured is that mosfet drives inductive load and it's internal diode will be more than fine.

Those external diodes of appropriate rating are going to be very expensive.

Tonic

Sun Jul 13 2008, 09:19PM

Registered Member #528 Joined: Fri Feb 16 2007, 10:32PM
Location: Warsaw, Poland
Posts: 166

Marko wrote ...

Tonic, what circuit are you building?

To be exact, it's a completed half-bridge, which you can see in my another topic on Tesla coil forum section.

wrote ...

In really majority of uses those external diodes are absolutely unneeded. All that needs to be assured is that mosfet drives inductive load and it's internal diode will be more than fine.

Well, it's certainly true, but I've done it for few reasons: I'm a n00b and I have a tendency to destroy components because of my knowledge and experience. It doesn't hurt (or it hurts?) and the half-bridge is supposed to be redesigned in future for larger SSTCs, perhaps DRSSTC, too, reverse current could destroy transistor - in case I'd forgot to plug inductive load, for example.

And TO-220 diodes add a lot to cool factory

Yeah, they are pretty expensive, about 1$ each piece :(

Tom540

Wed Jul 16 2008, 08:56PM

Banned on 3/17/2009.
Registered Member #487 Joined: Sun Jul 09 2006, 01:22AM
Location:
Posts: 617

I don't think you need those diodes. I stopped using them a long time ago when they were the only things blowing.

GeordieBoy

Wed Jul 16 2008, 09:23PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

I have explained this briefly before, but will mention it again here:

The Schottky barrier diodes are required to block the MOSFETs intrinsic body-drain diode only when the inverter is trying to drive the TC resonator at a frequency _BELOW_ it's natural resonant frequency. Under these conditions the load current appears capacitive and leads the inverter output voltage. It changes direction _BEFORE_ the voltage changes polarity and this causes forced reverse recovery of the free-wheel diodes. Under these conditions the slow body-drain diodes of the MOSFETs are completely inadequate to withstand the full DC bus voltage being applied in reverse when heavily in forward conduction. The results are horrendous shoot-through currents, bus over-voltage spikes, high RFI and un-explained device failures.

If the inverter is always tuned _ABOVE_ the resonant frequency of the TC then the load current appears inductive and always _LAGS_ the applied voltage. Under these conditions ZVS occurs at MOSFET turn-on and the free-wheel diodes do not see a forced reverse-recovery. Under these conditions the MOSFET body-drain diodes are perfectly adequate.

Most cases in traditional Power Electronics involve sourcing a square voltage waveform to an inductive load (such as a transformer, motor, or choke to smooth current ripple) Therefore you rarely see MOSFET body diodes bypassed with the arrangement shown, except for in some DC motor speed controllers.

I hope this clears up any mystery regarding body-drain diode bypassing for SSTC and RF power applications.

-Richie,

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob