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what do you think of these bricks?

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LAN_403

Reaching

Fri Aug 11 2006, 08:31PM

Registered Member #76 Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458

i build everything exactly ,with resistors etc, but it doesnt work properly and i dont want to toast my bricks so i made my own driver, and it works and i can slap myself around if something bad happens. okay, the disadvantage is that i must use 2 different supply voltages, but who cares.

next problem is that im a total noob in smps so i want to use a normal transformer, selfwound of course for 4x12 and 4x40volts, a lot easier for me than a smps, thats nothing for me.i experimented a bit but i only get crap and i wanted it to be regulated but it have to be isolated and i have no parts here to build it like i want and im out of money, so >normal transformer

Steve Ward

Fri Aug 11 2006, 09:12PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Oh, i just noticed something with my schematic. While it technically is the right schematic for what i drive my CM300 IGBTs with, there are some things that might not allow it to work CW. Resistors R2 and R3 might be too big for CW operation as they would limit the current provided to the gate driver circuit. Also, i wanted to mention i used a 9.4V zener to clamp the negative voltage, so the real drive on my bricks is -9V to +30V right now, not -15V as i might have stated before. So the DC input would be lower accordingly.

Reaching, im not quite sure how your circuit:

could work with 40VDC applied across the half-bridge while only using a 15V drive from the gate driver chip (from what i can figure, it shouldnt be able to operate both fets, one or the other or both would be ON the whole time). And umm, your 30V TVS is shorting out any negative gate voltage, unless you used a bi-directional TVS, but you didnt indicate that in your schematic.

Plasmaniac

Sat Aug 12 2006, 09:40AM

Registered Member #206 Joined: Sat Feb 18 2006, 03:17PM
Location:
Posts: 72

Firkragg wrote ...

Using separate supplies for all 4 drivers is the point.
Bridges cannot work without some kind of isolated gatedrive.

You just doubled the trouble by tapping the power supply again for each board, that's 8 separate windings you need to wind on some toroidal transformer or etc.

Eh, you dont need to make 4 separate supplies. 2x2 IGBTs are connected anyway, so you need just 2 separate PSUs for your brick drivers. I'm a noob when it comes to build a DRSSTC but I always try to save as many parts as I can, so I'd use just 2 separated PSUs, one for the electronics and one brick driver pair and another one for the other pair of brick drivers. Also, I'd make just 2 ~25V windings in total and a voltage doubler like this:

You can use a 7815 and 7915 to get the Voltages you need for your drivers and electronics (+-15 and +30?).

Reaching

Sat Aug 12 2006, 12:25PM

Registered Member #76 Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458

meh, i can definately say thats way too high for me, negative voltage, positive voltage,umg, smps and everything like this. mhh, omg, have to try some normal gdt drivers or so,

okay, update, gdt tests ended in crap, now i got my high side driver version working, but why should it not work steve, i think its fine, here are a few pics

(the brick driver fits perfectly on my bricks)

(on testing)

(gatewaveform on 50khz measured directly on the brick, v/div 5 volt 20ys/div input voltage 15 volt driver ic and 15 volt bridge)

(same but with 100khz input signal)

from the pics and the longtime test for 15 minutes im sure that it works, drawing around 0,2A 15 volts (okay have to make some tests with 30volts or so on the bridge but why should that cause trouble?

Steve Conner

Sat Aug 12 2006, 05:25PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Plasmaniac, you need at least 3 separate power supplies. Sure, the collectors of the top two IGBTs are connected together, but the gate drive has to be referenced to the emitters, which are connected to the RF outputs.

You can use a common power supply for the two bottom gate drivers, but like most people, I used 4 completely isolated supplies and drive signals. It's not really that much more hassle and you don't have to worry about the L*di/dt voltages that tend to appear between things that are supposed to be "connected together" in power electronics.

I also used a string of zeners instead of resistors, to regulate the supply to 36V and split it into +24 and -12. I see other people use higher voltages, but I think +24 is enough: I ran my OLTC gate driver at that voltage and ended up putting about 3000-4000A peak through each 600A brick with no problems.

Steve Ward

Sat Aug 12 2006, 05:26PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

OK, yes, your driver will work fine at 15V, but at about 20V, i predict the "sh*t will hit the fan". The reason you dont see a problem is probably because you arent too familiar with the workings of a P-ch mosfet. For a Pch to be off, you must have 0V or a positive G-S voltage. To turn them ON, the gate has to be lower than 0V relative to the source (typically -5V to turn it on). Now, your gate driver can apply 15V of total "swing" to both of the gates. Tell me, with 40V across the 2 sources of the bridge, how do you plan to turn both fets off at any point in time with only a 15V swing?

My driver avoids this problem in a sneaky way, by capacitively coupling the drive to the Pch fet, but not the Nch. Due to the self-biasing (provided by the resistor and diode across the G-S), the capacitive coupling only pulls the mosfets gate negative, and then pushes it back up to 0V.

Eh, you dont need to make 4 separate supplies. 2x2 IGBTs are connected anyway, so you need just 2 separate PSUs for your brick drivers.

Unless the IGBTs share a common emitter, they MUST have isolated power for the gate drivers. So, i suppose if you wanted, you could use one supply for the "bottom" 2 IGBTs. Id worry about ground loop issues (which could be a real problem). The top IGBTs have a common collector, but their emitter voltage is going up and down between 0V and Vin, so they each need their own isolation.

Reaching

Sat Aug 12 2006, 06:56PM

Registered Member #76 Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458

yeah steve youre right, im not familiar in p channel, its the first time ever i used one. but its no problem to add this cap,resistor,diode to my current circuit, than it should work or not?

okay, made a few changes in my shematic and now everything works fine even with more than 15 volt applied to the bridge,

heres a pic of the gate waveform with 22volt applied to the bridge and 12 volt applied to the driver ic, same as above, 50khz 20ys/div 5volt/div

hehe, now everything is right,hope so

Steve Conner

Sat Aug 12 2006, 10:26PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

That waveform looks great. You still ought to have some sort of resistor or zener string across your two electrolytic caps, though.

williamn

Sun Aug 13 2006, 02:18AM

Registered Member #55 Joined: Thu Feb 09 2006, 04:56AM
Location:
Posts: 149

If you know how to build a simple SSTC you can build a SMPS. There is no scary Voodoo

Here is a pic of one I built about six months ago, nothing fancy, no PWM or anything just free running at 30khz. One of the output windings can charge a 6800uf capacitor to 45VDC in about 2.5 seconds I have beat on this thing hard and it works fine in my medium DRSSTC. I have since moved on to seperate onboard PWM SMPS for each gatedrive card, but a simple one as above will do.

Reaching

Sun Aug 13 2006, 08:02AM

Registered Member #76 Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458

yeah, i build several small smps but the results were always dissapointing and i dont know what i do wrong or if something is wrong. i found a program to calculate the turn ratio for smps transformers and build a transformer for 4x15volt out, . i used the halfbridge converter with a simple 555 driving 2 mosfets at 100khz (to prevent inducing voltage spikes etc in my 50khz drive signal). i use a double E core with about 5x5cm and 1 cm thickness , that should be enough for 100watts

the only thing i dont understand, the powersupply charges the cap without load to about 20volts, with a 1k! load the voltage drops to 12 volt, and with a small motor connected the voltage drops to 11 volt drawing 200mA thats only about 2 watts! this is really dissapointing

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