Mosfet gate drive transformer...is ok? - Page 2 - diyAudio
Go Back   Home > Forums > Amplifiers > Power Supplies

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 28th September 2012, 06:19 AM   #11
diyAudio Member
 
Join Date: Dec 2001
Location: South Sweden
Quote:
Originally Posted by Terry Given View Post
Osvaldo,
yep. they really do work. perhaps the biggest disadvantage is that if and when it goes bang, the trail of destruction can reach back into the isolated side. of course thats also true when you have a low-side switch thats directly driven.

like most things: done properly they work well. done badly they dont. for all values of "they".

you make a great point about matched drive paths. for any type of half-bridge power stage this is CRUCIAL. I cannot emphasise that enough, and you win todays internet for pointing it out. Because half-bridges have a built-in self-destruct mode (turning high- and low-side switches on together). Interlock is your friend (IGBTs are a good way to find this out, as they dont like turning off).

the diagonal half-bridge is a great topology for several reasons, one of which being that its immune to this failure mode (its the operating mode). and because nothing turns on until both switches are on, but it turns off when the first switch does, it can tolerate huge variations in gate drive timing with essentially no change in performance. I once designed a 2MHz DHB converter that drove the high-side switch from the low-side winding - it used the primary as its own level shifter, so only needed low-side drive. 24V in, using bipolar transistors, and only a few watts. we made about 300,000 or so.
The major problem with all bridges , timing is essential or you will have a big bang on the bridge where everyone meets

A question though, diagonal half-bridge, is it same as two-transistor forward? Like in the other topic we are discussing

I e it drives the transformer in an unipolar fashion and you have two catch diodes taking care of reset and any leakage inductance voltage spikes?
__________________
listen! Can you hear that ...
  Reply With Quote
Old 28th September 2012, 08:14 AM   #12
diyAudio Member
 
Join Date: Jun 2011
Quote:
Originally Posted by rikkitikkitavi View Post
I e it drives the transformer in an unipolar fashion and you have two catch diodes taking care of reset and any leakage inductance voltage spikes?
Doesn't that happen in a bridge also?
The mosfets internal diode takes care of those as in TTForward?
And if not, why not?
  Reply With Quote
Old 28th September 2012, 10:15 AM   #13
diyAudio Member
 
Join Date: Dec 2001
Location: South Sweden
yes , but they are lossy (high vf ,even more so than silicon diodes) and very slow which of course can not be accepted in a high frequency application.
__________________
listen! Can you hear that ...
  Reply With Quote
Old 28th September 2012, 10:52 AM   #14
diyAudio Member
 
Join Date: Jun 2011
I c...

Any recomendations for core reset diodes for a TTForward?

I would expect an ordinary 4148 to blow in no time - but would be fast enough at least (max 100V limited off course)? How much current exactly are we talking about here in such a fact of the matter?

Any known formulas for calculations?
  Reply With Quote
Old 28th September 2012, 11:33 AM   #15
diyAudio Member
 
Join Date: Dec 2001
Location: South Sweden
the diode catches the magnetising energy + any energy stored in leakage inductances , and they should be fairly small so powerwise not so big. But I have seen 3-400 W single switch forwards where the catch diode is bolted to the heatsink...

peak power can be.huge though , as they are fast. very fast...

I would say though that about 1-2 amps Iavg rating is sufficient. Voltage rating as for for the mosfets. But dont.take my word for it without.some simulations or experience to back it up.
__________________
listen! Can you hear that ...
  Reply With Quote
Old 29th September 2012, 03:47 PM   #16
diyAudio Member
 
Join Date: Dec 2001
Location: South Sweden
I havw simulated the circuit in Fritz schundlers report and.it works nicely but when shutting.of .drive , the leakage inductance forms a resonance with the.coupling caps , causing the powerFET to turn on a few times over a few ms., on time quite undefined.

what this means is catastrophic failue in the FET by enormous currents. I think this needs a pnp shorting out stored energy in the cap. i have a app note mentioning the problem but not why, how . see if I can dig it up.
__________________
listen! Can you hear that ...
  Reply With Quote
Old 30th September 2012, 07:28 AM   #17
diyAudio Member
 
Join Date: Mar 2008
Quote:
Originally Posted by rikkitikkitavi View Post
The major problem with all bridges , timing is essential or you will have a big bang on the bridge where everyone meets

A question though, diagonal half-bridge, is it same as two-transistor forward? Like in the other topic we are discussing

I e it drives the transformer in an unipolar fashion and you have two catch diodes taking care of reset and any leakage inductance voltage spikes?
Yes, it is. You can draw the TTForward converter as a bridge converter with two switches and two diodes. the active switches are on opposite diagonals, as are the passive switches (diodes). IOW if you replace two diagonal switches in a full bridge with diodes, you get the Diagonal Half-Bridge aka Two-Transistor Forward converter.

and yes the diodes clamp both leakage and mag current. assuming its a forward converter. a DHB flyback (works great, almost no leakage loss) has the constraint that the flyback voltage must be less than the input voltage, else all the stored (mag.) current gets returned to the input supply.
[edit: duh, forward has same constraint on Vout & turns ratio]

as for the diode current: the actaul current depends on the topology (forward or flyback), but in either case the following statement is always true:

The leakage inductance is in series with the switch(es) and the parallel combination of magnetising inductance and reflected load impedance

Therefore when the primary switch(es) turn off, the current stored in the leakage inductance is simply the switch current at turn-off (so the peak switch current). Always. every single time. CCM or DCM. the leakage is in series with the switch....

In a flyback the secondary diode is off, so there is no reflected load and the leakage inductance current is the peak magnetising current + any DC offset if its in CCM. which is of course the peak switch current at turn-off.

In a forward converter the secondary diode is on, so the leakage inductance current is the peak magnetising current + the reflected output inductor current. which is of course the peak switch current at turn-off.

and lastly, with a relay catch diode - same thing. the relay diode current is whatever the relay current was just before it turned off. I dont know why, but I could never figure this out when I was a technician, or studying for my degree. but about 1 month into my first real job, I asked one of the technicians about it, and got told in no uncertain terms how it worked. with an exasperated "how much work is this idiot going to make for me" tone of shouting. That got through my thick head. I must have fitted thousands of 1N4007 catch diodes to 30mA 12V relays and coin counters over the years......

these questions can also be answered using one of Maxwells equations. which is simply "current flows in loops". draw the circuit, explicitly showing ground and power connections, then trace the loop the current flows in.

The clamp diode duty cycle is pretty low though - with a DCM flyback its Ton*(L_leak/Lmag) because the clamp voltage is the supply, and the switch current at turn-off is only magnetising current. so if the leakage is 2% of the mag current, the diode conduction time will be about 2% of the on time.

so it has to be a fast diode.

with a CCM flyback that goes up because of the stored (magnetising) current. And in the forward converter it has a similar increase due to the reflected output inductor current.

regardless of topology, the diode conduction time is always given by:
Tdiode*Vsupply = L_leakage*I_switch_peak

because the diode conduction time is small, you can use quite a small diode. In a flyback with 3% leakage and Ipeak = 4A I use 1A 35ns diodes for my leakage clamp (same reasoning applies). I got all carried away and did a transient thermal analysis, but simple calculations work pretty well.

for two clamp diodes, each diode dissipates about 0.5*I_switch_peak*Vdiode*Tdiode*Fsmps watts. if you want you can use (Vf + I*Rdiode) instead of Vdiode, but if you use Vdiode at I_switch_peak then you've got a bit of margin.

just make sure the diode Ifsm >> I_switch_peak. you can happily use a diode with Iavg = I_switch_peak/2 and ignore pretty much all calculations.

Last edited by Terry Given; 30th September 2012 at 07:57 AM.
  Reply With Quote
Old 12th October 2012, 12:57 PM   #18
diyAudio Member
 
Join Date: Dec 2001
Location: South Sweden
I have fiddled around a bit with some ferrite toroids used in common mode filters, they have fairly high Al, in the range 8-10000 nH/n^2 (at low flux , even very low)
Would they be suitable for a good GDT or is the material to lossy ?

in general, these came out of a broken ATX supply, CE marked (chinese equipment) so I have no more data.
__________________
listen! Can you hear that ...
  Reply With Quote
Old 12th October 2012, 01:21 PM   #19
diyAudio Member
 
Join Date: Mar 2008
Rikkitikkitavi,

high perm cores are often used for GD transformers, precisely because they are high perm. which increases the (parasitic) magnetising inductance, thereby reducing the (parasitic) magnetising current.

As always you need to make sure the core doesnt saturate - and Bsat varies with temperature, anywhere from quite a bit to a whole lot. And as you suggest, you also have to keep the losses under control.

its easy if you know the exact material - read the datasheet. harder for a random core.

you can easily check saturation at high temperature with a heat gun and a splat test (fairly large cap charged to some V, "splatted" across the winding while you measure V & I. If V is pretty constant then dI/dt = current slope = V/L, and you'll see it saturate quite clearly).

losses are harder to measure though. if you build a diagonal half-bridge driver stage and run it at just under 50% duty cycle, with no load on the secondary (or even a secondary) then you will make the mag current ramp up then back down, repeatedly. bypass the supply with a reasonable sized cap, then measure the DC power drawn by the primary driver. that'll give you a reasonable idea of the losses, and you can also measure the temperature rise.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Gate drive transformer...no v.us value eem2am Power Supplies 2 13th December 2011 01:09 PM
Need SMPS Gate Drive Transformer infinia Power Supplies 3 10th March 2009 03:51 PM
Gate Drive Transformer microsim444 Everything Else 1 2nd December 2008 12:52 PM
design gate drive power mosfet in class d yanuarbob5150 Class D 3 7th May 2008 07:52 PM


New To Site? Need Help?

All times are GMT. The time now is 08:40 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2