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what do you think of these bricks?

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LAN_403

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Thu Aug 10 2006, 06:28PM

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

my parts arrived and i build up the driver and tested it, but there are a few problems.
here my test condition:
i use the brick igbt driver from steve ward, the circuit is correct and with the same parts steve used in his shematic, i tested with a signal generator (squarewave 50khz 5volt out) and with a voltage between 15 and 30volt. if i set the voltage to around 15 volts i get no output (measured between the emitter and base connections without a brick installed) i only get a positive voltage of around 3 volts. if i set the voltage to about 20volts i get a weak squarewave falling to 0 in about 1 or 2 seconds, in place with a heavy current draw of over 1 A and the 10R resistor on the p and n channel mosfets is smoking. if i vary the voltage between 0 and 30 volts very fast i get a nice squarewave , ..what do i wrong??????

Steve Ward

Thu Aug 10 2006, 07:49PM

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

Your gate driver chip probably has some undervoltage lock out, but i dont know if that would explain it. You have the polarity of the voltages correct? My gate drivers never seemed to care how slowly i brought up the voltage, or even if there was not enough voltage on the gate driver chip, it would just shut down.

Are you sure you wire the P-channel fet in right? The source connects to the +30V, the drain connects to the IGBT gate (through the resistor). Its easy to flip the drain and source if you arent paying attention, or used to working with P-channel stuff.

Steve Conner

Fri Aug 11 2006, 11:03AM

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

It's probably a bad idea to try testing the circuit with a continuous drive signal from a signal generator. If you try to drive a brick continuously, the power dissipation will be huge and things will smoke. These drivers are really only designed to work at DRSSTC duties (ie, a 300us burst followed by a 10ms rest)

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Fri Aug 11 2006, 12:00PM

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

but steve, its a very important circuit and i dont like the thought that it only works for short burst lenghts or so. 1 failure in one of 4 brick drivers and the whole bridge can get damaged.

btw, i got the driver working, i wired the p channel wrong, okay never made anything with p channel mosfets so steve ward was right, . but is it normal that the squarewave output goes back to zero voltage after about 10 seconds of runtime in continious mode?. maybe i should test it with an interrupter in place to simulate the drsstc mode

whats about this driver?

this is what ive build and tested and it works just fine, i can push the big bricks up to 170khz with a relatively good squarewave, on 50khz the squarewave is perfect, and it runs continious without problems. now the difficulty is to get a single supply voltage for this driver, can i use a capacitive divider like steve did or wil it result in a output going zero after a few seconds? i want the driver to be bombproof so if its difficult to divide 40volt properly to drive the ucc driver ic than iÂ´ll go with 2 supply voltages instead

Marko

Fri Aug 11 2006, 03:00PM

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

It's not a problem to divide voltages at all, and there's actually no sense to use a bunch of separate supply voltages (wich could trouble you because of higher supply inductance, you would again need decoupling and etc..)

It will be completely OK with etc. 1000uF and 2200uF caps, if you are really willing to be 1V precise then use 3 1000uF caps or similar.

You can use much smaller caps if you are using SMPS supply or have good initial filtering.

Capacitive divider does not cause 'draining after 10 secs' problem you are talking about, unless you wired something wrong.

Caps recharge with each cycle and it must work forever in CW if it is done right.

Running CW your drive mosfet's may dissipate a bit more heat but they are pretty beefy and hard to damage.

You must just care not to terribly overheat them if you aren't using heatsink...

Reaching

Fri Aug 11 2006, 04:32PM

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

okay, but iÂ´ll stay away from a capacitive divider or so, i experimented a bit with it and it seems that they cannot supply enough current to be on the safe side, the voltage drop is to high and the caps recharge too slow,no sorry no way iÂ´ll use 2 supply voltages instead,.

now to another problem, everyone uses a seperate powersupply for one high side driver, is this important or can i use the same powersupply for all four driver pcbs?

Marko

Fri Aug 11 2006, 05:01PM

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

now to another problem, everyone uses a seperate powersupply for one high side driver, is this important or can i use the same powersupply for all four driver pcbs?

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.

That's why most people use a SMPS there, simply because small ferrite transformers are easier to wind.

Electrolytic caps themselves probably won't produce enough peak current anyway, and you are going to need somedecoupling caps (tantalum or ceramic) as close as posible to mosfets.
Using a tapped supply won't help there.

I also didn't understand the thing about the voltage drop you are talking about, but surely I'l let you be with your own design.

Reaching

Fri Aug 11 2006, 05:53PM

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

i only build the driver from steve ward and had trouble with it , cause i had the p channel mosfet wired wrong. after correcting the failure the circuit worked but after some seconds the output dropped to 0 volt ,dont know why, i checked everything but the circuit is exactly what steve build. i experimented with the capacitive voltage divider and it seemed to me that the divider caps cannot recharge fast enough to supply the driver ic though the voltage dropped and output goes zero,

i supplied the driver with a normal 0-30volt powersupply, no such weird smps

Steve Ward

Fri Aug 11 2006, 06:50PM

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

can i use a capacitive divider like steve did or wil it result in a output going zero after a few seconds?

Its NOT a CAPACITIVE DIVIDER as this is a DC input voltage! Its a resistive divider with filtering. If you left out the 1k resistors across the caps, it will cause the voltage to not distribute evenly, and eventually one of the voltage sections will just drop to 0V.. I left the 1k off the bottom cap for the -15V supply because the gate driver will draw about that much current as a 1k resistor anyway.

Anyway, ive tested this driver in CW mode driving CM600s. The gate resistor did eventually burn up (only 1/2W) but without the gate resistor, it ran CW just fine. You have to have proper mosfet heatsinking if you want to run CW, as the fets will have to dissipate a few W probably depending on the load. Be sure your gate resistor is very beefy!

Marko

Fri Aug 11 2006, 08:07PM

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

There seems to be misunderstanding around here.

'Capacitive divider' (or whatever you call it) gets AC component from the halfbridge since both caps are discharged with each cycle (but it is supplied with DC)
Since drive isn't symetric your resistive divider (equalising reisstors?) are used to 'drain' bad oltage distribution on the caps.

As you already guessed reaching's problem is probably because of leaving out the resistors (?).

(or maybe I should just shut up

)

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