Electronics > Projects, Designs, and Technical Stuff
Driving (AC) mosfet switch directly from MPU using GDT
BrianHG:
--- Quote from: T3sl4co1l on October 23, 2019, 11:05:25 am ---Brian: would be easier to just FWB rectify the gate signal. :P
There's no 50Hz flicker, it's 100Hz, and you can't get rid of that without energy storage. You could make a VFD where AC comes in, gets rectified to DC, then is output as either infrequently changing DC (i.e., swapping polarities to reduce sputtering of the filament; low frequency AC, full duty cycle square wave), or high frequency AC (a few kHz will not be visible to the eye under any condition). The DC supply could be varied (with a buck converter, or since efficiency really doesn't matter here given the incandescent light and apparently thermal application, a linear regulator), varying output. In the higher frequency AC case, duty cycle could also be varied.
I once did this for strings of LEDs:
https://www.seventransistorlabs.com/tmoranwms/Elec_LEDs.gif
Tim
--- End quote ---
What I think the OP is trying to do, and not clearly explaining to us all, without a power transformer, drive a 12v lamp from 240v mains, pwm-ing the middle of the source sine wave so that it appears to look on average like 12v AC square wave 'power wise' to the filament. If the op sticks with the 120v lamp, he can remove the flicker only if he wants the lamp to operate around 10% brightness...
This means the bulb will still shine full brightness since when the source AC waveform is between that tiny period where it's within +/-12v, the mosfet is full on. While the AC waveform is larger than +/-12v, the MCU chops up that power making it on average to appear to be a regulated + or - 12v over time.
With such a short off time, the bulb will appear to be truly DC powered. This project will also have the advantage of maintaining a true high power factor.
The op needs the high speed on-off mosfet drive to really make tiny pulses during the almost 300v peaks. He will also need a soft power-up cycle as a 12v halogen bulb drives massive current when the filament is cold.
The 4x mosfet circuit I posted protects the one side off mosfet since a transformer drive cant keep both mosfets on at one time unless the OP want to loose half of his power.
If they were fast enough, (I know they are for 1-2Khz, they turn off slow, it's been awhile since I used them.) I would recommend using 2 photovoltaic optocouplers to drive the mosfet gates instead of a gate transformer. Also, 2-4ma from the MCU will run the photovoltaic fine.
As for our eyes, filament flicker isn't too bad, as well as for many cameras, but when used for high quality photographing/filming or special sensitive situations, this project will make the bulbs effectively appear DC powered.
The final cheapest solution means a bridge rectifier and 1 mosfet and the MCU driving the gate of 1 mosfet, though, the OP will be touching the mains with his MCU directly and I think that what's her trying to avoid.
T3sl4co1l:
--- Quote from: beduino on October 23, 2019, 10:38:49 pm ---IRF840 datasheet says Ciss=832pF Input Capacitance @ VDS25V f =1MHz VGS=0 , 1664pF capacitance for two mosfets in AC switch.
--- End quote ---
I pulled out Qg specifically, because Ciss underestimates it by typically about a factor of 4.
Which... hm, you've already got to be careful here, because there's the original, created by International Rectifier:
https://www.datasheets360.com/pdf/-653557743118012551
which says 63nC max Qg and 1300pF typ Ciss. The Fairchild (dated 2002) and the Vishay/Siliconix (2016) datasheets have very similar spec. The ST (2002) version however shows significant improvement with 39nC max Qg and 832pF typ Ciss:
http://www.alphacron.de/download/hardware/IRF840.pdf
So it matters which one you're buying. Not all manufacturers make exact copies of a multi-sourced part!
(FWIW, Vishay is the current owner of the original IR product line, including this part.)
If we use the average capacitance calculation, we get Ceff = (63nC) / (10V) = 6.3nF. Note this is max, so it's not directly comparable to typ Ciss. The Fairchild datasheet gives 42nC typ., or 4.2nF equivalent, 3.5 times the small-signal Ciss figure.
What's going on?
The gate capacitance charges as normal, and is in fact around 1.2nF, and doesn't vary much with voltage by itself. The stinker is the G-D capacitance, which because the drain voltage has a huge swing (up to 500V), this capacitance is hugely amplified (Miller effect). The only reasonable resolution is to look at the charge required to move the gate from 0 to 10V, and divide by that voltage swing to turn it into an average capacitance.
That's the justification for the Qg calculation. :-+
So if the gate looks like 4.2nF (or up to 6.3nF), and it needs to charge in a tiny fraction of 600kHz (1.6us), what resistance is required?
If we say 1/20th of a cycle maximum, or 80ns, in two time constants (t = 2RC), we need a resistance of about 10Ω maximum.
We likewise need to deliver a peak current of (10V) / (10Ω) = 1A, preferably even more to drive it faster. Per transistor, so two in parallel also.
With a ~2:1 current ratio on your drive transformer, that's a demand of over four amperes from the poor ATtiny!
But it cannot deliver anywhere near that current, because the primary circuit has about 280 total ohms. It can deliver about 14mA short circuit.
What could we achieve, then?
This in turn suggests that Fsw should be about as many times slower as the gate drive is. So, if we can't do 4A but we can do 14mA, we should run at a frequency 4/(0.014) times lower, or about 21kHz. And again, lower would be preferable, to save on switching losses.
Or we can add a gate driver IC, but we need to power it at some point. If it's wired directly to the transistors, it needs an isolated supply of 9 to 15V.
--- Quote ---However, there is no current limiting resistor on GDT secondary, only turns of copper wire, so its resistance should be quite low - cheap YATO multimeter says: R: 1.3 Ohm ::)
--- End quote ---
Yikes! Realize what kind of an assumption this is -- if the secondary had a source resistance of 1.3Ω period, then it must be that, no matter what resistance the primary circuit has, you can draw many times more power from the secondary. You've made a huge power amplifier! That's a very fishy conclusion, so the premise probably was in error. :D
The equivalent circuit for a transformer may look odd. It's a chunk of wire, right? Well, that chunk of wire is supposed to look like a large AC impedance (that's what the turns around the core does). So it doesn't factor into our circuit. Rather, the primary and secondary sides are connected by the ratio. This looks more obvious if we draw it with no transformer at all -- but to do that, we have to scale all the values from one side, by the ratio, so that they are equivalent to what's on the other side.
Say we scale up the primary side. Assuming 1:2 turns ratio, the primary voltage shows up on the secondary side as double, and the primary current shows up as half. 2 * 1/2 = 1, because energy is conserved of course. This means that the primary resistances appear to go up by a factor of 2*2 or 4. So the MCU and two 100Ω resistors looks like 8V logic (instead of 4V), with ~160Ω equivalent pin resistances, and 400Ω series resistors. A total of almost 1kΩ!
This is why the gate drive will be, needless to say, a bit disappointing. :(
*Technically, any nonideal transformer can be replaced by an ideal transformer (if needed for 1:1 isolation) and three inductors, if you don't mind that one of them may be negative. Obviously, negative inductances are hard to come by, and this is just an equivalent circuit. But it's nice to know that equivalent circuits don't stop working just because a number comes out unrealistically. :)
--- Quote from: beduino on October 23, 2019, 10:54:56 pm ---Now calulated something like average current needed to provide charge for 10Vgs of those IRF840 @ 600kHz and it looks liek this if I do not missed something-I've assumed half of the time charging, second half discharging gate by GDT secondary:
--- Code: ---(%i11) F: 600000.0;
(%o11) 600000.0
(%i12) T: 1.0/F;
(%o12) 1.666666666666667e-6
(%i13) T2: T/2;
(%o13) 8.333333333333334e-7
(%i14) t: R*C;
(%o14) 2.1632e-9
(%i15) T2/t;
(%o15) 385.2317554240631
(%i16) Q/T2;
(%o16) 0.019968
--- End code ---
Looks like something about ~20mA average current.
Probably by lowering down this switching frequency a few times will help get enougth current for full open/close AC mosfet switch? :-\
--- End quote ---
I'm not sure what numbers you used here. Qg(tot) * Fsw does equal supply current for a gate driver IC. The peak current is circa T2/t times higher though.
Tim
beduino:
--- Quote from: T3sl4co1l on October 24, 2019, 01:44:42 am ---This is why the gate drive will be, needless to say, a bit disappointing. :(
--- End quote ---
Looks like your calculations are right for 600kHz, since I've just tried to connect to GDT secondary with only one zener 12V on AC mosfet switch gates and no resistance drop - cheap YATO multimeter says infinity resistance on all ranges, while when connected this AC mosfet switch gate to 4VDC we have ~3.8 Ohm resistance on mosfets drains, so ~1.9 Ohm per each IRF840 in series, so AC mosfet switch itself is fine after GDT test 8)
So, for the moment this is the only good news - we have working AC mosfet switch with DC Vgs >:D
beduino:
--- Quote from: BrianHG on October 23, 2019, 11:18:37 pm --- The op needs the high speed on-off mosfet drive to really make tiny pulses during the almost 300v peaks. He will also need a soft power-up cycle as a 12v halogen bulb drives massive current when the filament is cold.
--- End quote ---
I've infared light bulb 230VAC 80W-100W like shown below, and it is used in winter days to warm up my custom made sleeping capsule where warm air is prowided in aluminium pipes, so temperature controll needed and light flickering even at 100Hz can disturb my dreams, not mention that I'd like to experiment with different freqencies to study how it affect sleeping quality or maybe it also affects water molecules in air, so that is why I might be interested in high frequency switching, but now I'm shocked it doesn't work with pure MPU pins output @ 600kHz switching frequency, so maybe external transistors needed in totem pole configuration I've already used in other project to be able use lower resistors on GDT primary ;)
Simon:
light flickering at 100Hz disturbs your dreams? the light does not flicker.
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version