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Registered Member #3637
Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
Hey all.
Been fooling around with my manual tune SG3525/ KA3525 induction heater. I've been trying to figure out what would work as a gate driver, and have already tried using two different types of half bridge drivers.
Neither worked correctly.
The low side of the drivers would work fine. The high side, however, wouldn't. I don't know what's wrong with these chips. I've tried following the schematics given on their datasheets as well as possible. Grenadier recommended using a 100 ohm resistor as a bootstrap, and a 10 uF capacitor for the bootstrap cap. Used a regular UF diode for the bootstrap diode.
Still no worky, with either the FAN7392, or the FAN7382.
I'm considering just using two of these, instead, but I wish I wouldn't have to.
Registered Member #1875
Joined: Sun Dec 21 2008, 06:36PM
Location:
Posts: 635
I would avoid the second one you linked as it is fairly slow.
I think you need to look at how bootstrap works. I see a potential problem with the values you/grenadier has chosen, especially for high frequencies. Hint: RC time constant. Also, using an UF diode will worsen this problem due to its larger voltage drop vs a schottky, though it should be fine to leave in your design once you've worked the rest out. Using a tantalum bootstrap capacitor is recommended, as well. As for the optimal value, well I can't say without knowing your switch's gate capacitance. The bootstrap resistor value depends on your diode's peak current rating (assuming your low-side IGBT's peak current rating is higher!).
And of course I assume HIN and LIN are receiving complementary signals?
Registered Member #51
Joined: Thu Feb 09 2006, 04:17AM
Location:
Posts: 263
I don't think those things ever work. Maybe if you are using them under just right conditions (low noise, same ground reference for power and logic, etc) you can get them to work but I would not count on it.
I prefer a GDT and two low side drivers to those finicky things. Plus, if those fail you could have a breakdown between your high voltage and control electronics. There is a reason why GDT's are used and those are often not.
Registered Member #3637
Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
ScotchTapeLord wrote ...
I would avoid the second one you linked as it is fairly slow.
I think you need to look at how bootstrap works. I see a potential problem with the values you/grenadier has chosen, especially for high frequencies. Hint: RC time constant. Also, using an UF diode will worsen this problem due to its larger voltage drop vs a schottky, though it should be fine to leave in your design once you've worked the rest out. Using a tantalum bootstrap capacitor is recommended, as well. As for the optimal value, well I can't say without knowing your switch's gate capacitance. The bootstrap resistor value depends on your diode's peak current rating (assuming your low-side IGBT's peak current rating is higher!).
And of course I assume HIN and LIN are receiving complementary signals?
That's the IGBT I'm using. And the HIN and LIN are inverted signals, One is high, while the other is low. It switches back and forth at the tuned frequency.
I don't really know much about bootstrapping, or why the high side even has problems to begin with. I'm considering just using the low side of the drivers and leaving the high side's unconnected, would that work?
And I really don't want to use GDT. Inductive spikes just add another problem and make more parts needed to the design to protect against back EMF. I don't want blown mosfets or dead IC's.
Registered Member #1875
Joined: Sun Dec 21 2008, 06:36PM
Location:
Posts: 635
Can you measure any voltage at all directly from the High side output pin?
As for leaving the high sides disconnected... it will certainly prevent you from killing your FETs... but that's because they won't be doing anything.
As for a GDT, it really shouldn't be a problem if you design it properly, use appropriate gate resistance, and put zeners/TVS across g-s. The same measures should be taken with high side driving, as well.
Registered Member #3637
Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
ScotchTapeLord wrote ...
Can you measure any voltage at all directly from the High side output pin?
As for leaving the high sides disconnected... it will certainly prevent you from killing your FETs... but that's because they won't be doing anything.
As for a GDT, it really shouldn't be a problem if you design it properly, use appropriate gate resistance, and put zeners/TVS across g-s. The same measures should be taken with high side driving, as well.
And yes, there is very very low voltage coming out of the high side. It's like a half a*sed square wave, less than a volt. And I meant, using two High-Low Gate drivers, using just the low sides. Two of them.
Like, say, the SG3525 outputs two signals. They're inverted, one is on while the other is off right? Take signal one, and put it in INPUT LOW for one of the FAN7382 or FAN7392. Take the output LOW from the gate driver and send it to IGBT one.
Take signal two, and do the same thing, except to Gate Driver 2. And Sending the output to IGBT two.
Make sense?
It's just using the Low side aspect of the gate driver, not the high side.
Also, why does High side even exist? What is it's purpose?
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Inducktion wrote ...
Also, why does High side even exist? What is it's purpose?
The high side is isolated and insulated for 300, 600, 800, or 1200 volts.
Example: If you had a 600Vdc power input you wanted a full bridge to chop. Well 2 of 4 transistors would be Gates close to ground. While the other 2 of 4 transistors would have there Gates close to the 600Vdc when there diagonally swithcing. Otherwise your 3.3v or 5v logic would get blown through by 600V.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi grenadier
Also, why does High side even exist? What is it's purpose?
Out of switching devices in a half-bridge, the lower one has it's source on ground. This is the low side device. The high side one is the device with it's drain on bridge supply voltage. Our control electronics's ground is, in this case, referenced to the bridge ground.
A low side gate driver is just a buffer-amplifier. It can be connected from the logic circuit to the bottom mosfet without problems. For the high side device, this isn't the case, because it's source isn't connected to ground all the time - it goes from0 to full supply voltage depending on whether the lower device is on or off! You can't connect anything from the control circuitry to it's gate directly.
To solve this problem a high side driver is used. It's basically a low side driver, with a level shifter on it's input which decpuples it from the control circuitry so it's 'ground' can swing from 0 to supply voltage. It's power supply is the bootstrap cap, which gets charged only at times the bottom device is on.
All this is complicated circuitry and I've seen many people troubling with it so far. I've never used those chips anyway because they're expensive and I wanted my control circuits to be isolated :P
And I really don't want to use GDT. Inductive spikes just add another problem and make more parts needed to the design to protect against back EMF. I don't want blown mosfets or dead IC's.
I'm not sure why're you getting so delusional with the gdt's. What voltage spikes? You can drive smaller devices (IRFP460's or like at 200khz) with sg3525 directly. For bigger devices, you an add bipolar totem poles on the outputs or use gate drive chips for boost. If you're using sg3525 directly or amplified with bjt's, you can use higher supply voltage and a step-down gdt for higher current drive. Remember to put schottky diodes across the driver outputs, whichever you use.
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High-low side Gate Drive IC's have issues hm?
