Speaking of high frequency transformer stuff...
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From: Tim Williams on 20 Feb 2010 17:36 How would you tackle the problem(?) of 1MHz, 100V common mode on transformers or whatever coupling you're using? Think high side drive. Is an ordinary gate drive transformer suitable, does it need special construction or drive? Or would it be better to use a big fat CM choke to bring things down to earth? I don't think optical coupling would be very useful. I've already had poor results with 6N136's at 20kHz. I know there are fancier parts available, too, some with built in gate drivers. I've seen it before where fast edges will couple through the pri-sec capacitance of a GDT, even when driven hard. Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
From: Yzordderrex on 20 Feb 2010 19:16 On Feb 20, 5:36 pm, "Tim Williams" <tmoran...(a)charter.net> wrote: > How would you tackle the problem(?) of 1MHz, 100V common mode on > transformers or whatever coupling you're using? Think high side drive. Is > an ordinary gate drive transformer suitable, does it need special > construction or drive? Or would it be better to use a big fat CM choke to > bring things down to earth? > > I don't think optical coupling would be very useful. I've already had poor > results with 6N136's at 20kHz. I know there are fancier parts available, > too, some with built in gate drivers. I've seen it before where fast edges > will couple through the pri-sec capacitance of a GDT, even when driven hard. > > Tim > > -- > Deep Friar: a very philosophical monk. > Website:http://webpages.charter.net/dawill/tmoranwms Hi Tim, Is the 1MHZ on the primary side - and getting into the secondary? Sorry, I work better with drawings. For gate drive transformer for inverters you normally want very good coupling and low capacitance. These contradict each other so that the design is a compromise. I've used some of the HP-Agilent-Avago opto-driver parts and I think they're rated at near 10kv/us. I don't do large inverters anymore, but when I did I always crossed my fingers when releasing to production and it has seldom failed me. regards, Bob
From: John Larkin on 20 Feb 2010 20:26 On Sat, 20 Feb 2010 16:36:42 -0600, "Tim Williams" <tmoranwms(a)charter.net> wrote: >How would you tackle the problem(?) of 1MHz, 100V common mode on >transformers or whatever coupling you're using? Think high side drive. Is >an ordinary gate drive transformer suitable, does it need special >construction or drive? Or would it be better to use a big fat CM choke to >bring things down to earth? > >I don't think optical coupling would be very useful. I've already had poor >results with 6N136's at 20kHz. I know there are fancier parts available, >too, some with built in gate drivers. I've seen it before where fast edges >will couple through the pri-sec capacitance of a GDT, even when driven hard. > >Tim One nice combo is a dc/dc converter to make 10 volts or whatever, and a fast logic coupler to get the gate drive signal up there, and then a proper gate driver chip floating with everything else. Several people also make nice gate drivers with built-in isolators. Avago, IR, like that. I've made my own transmission-line gate-drive transformers for short-pulse stuff, like 100 ns pulse widths/400 volt swing/5 ns risetimes. Longer pulses and high duty cycles become problematic. Extreme case: use fiber optics. Megavolt isolation! John
From: Tim Williams on 20 Feb 2010 22:08 "Yzordderrex" <powersupplyguy(a)netzero.net> wrote in message news:f3539b69-33b0-4181-b6e7-a9fb6a0bed59(a)q21g2000yqm.googlegroups.com... > Is the 1MHZ on the primary side - and getting into the secondary? > Sorry, I work better with drawings. Ok. Picture the average half bridge MOSFET circuit: http://webpages.charter.net/dawill/tmoranwms/Elec_IndBridge.gif At 1MHz, the FETs need local gate drivers (e.g. TC4420), so plant some inbetween the GDT and FETs. Add a DC-DC converter to power the drivers. Now, the GDT can be a higher impedance, since it only has to drive the TC4420's, not gates directly, which helps. But now the problem is the pri-sec capacitance on the GDT and DC-DC conv. secondaries. The current flowing through that capacitance has to go somewhere. If it goes through the windings, you get coupling, which can lead to feedback and self destruction (I've seen the feedback before). The windings can be shielded, but that makes more stuff in the transformer = more leakage inductance. A tradeoff as usual. Note that leakage in the DC-DC converter isn't a big deal -- I could filter and regulate that locally. But the GDT has to be fast and square (<50ns t_p and t_r), with no bouncing to upset the driver. The big question I'm wondering is, is it feasible to shield the transformers and sink away the dV/dt, without sacrificing GDT bandwidth, or is an optocoupler suitable (I'll have to check if there are any fast enough), or is the whacky idea of a three-wire common mode choke feasible? One thing in favor of transformers is the high frequency. I'm not interested in anything under 200kHz, so the maximum pulse width is 0.25us, and a 5V pulse would carry a maximum flux of 1.25 uWb*t. A medium or low-mu ferrite would work okay (like mix #43, not too lossy at 1MHz), and a typical 1" toroid will easily handle that in just one turn. And let's say I put on three turns of coax for this transformer: how much capacitance is there in 4" of RG174? 30pF/ft, or 10pF for the whole thing. I'm expecting edges of 200V and maybe 20ns, or dV/dt = 10V/ns, or peak currents of only 100mA. That's a lot for a signal level, but I could drive this transformer with a FET driver (TC4427 or whatever), which would happily sink a current like that. So it's not looking too bad. > For gate drive transformer for > inverters you normally want very good coupling and low capacitance. > These contradict each other so that the design is a compromise. I've > used some of the HP-Agilent-Avago opto-driver parts and I think > they're rated at near 10kv/us. That number is meaningless in at least one case. For the 6N136, they spec 10V/ns, by applying a 10V, 1ns ramp. Sure, at 10V/ns, the peak current might be high, but with only ten volts difference, there simply aren't enough coulombs delivered to have any effect. So it's a lie of a rating. One optocoupled gate drive I've made suffered from unreliable behavior with 320V/0.1us edges. The datasheet doesn't say anything about external shielding, but I had to put some on myself, which helped a little bit (it still chatters if you bring your finger near it!). Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
From: Tim Williams on 20 Feb 2010 22:26
"John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message news:1g21o5t1kho6kdag86of0ku49ft20j4bm4(a)4ax.com... > One nice combo is a dc/dc converter to make 10 volts or whatever, and > a fast logic coupler to get the gate drive signal up there, and then a > proper gate driver chip floating with everything else. Yup, that's the plan. But I'm not sure about transformer (and what design to use) or opto anything. > Several people also make nice gate drivers with built-in isolators. > Avago, IR, like that. The HCPL-9000 does look pretty nice (and it's specified for 15kV/us at *1kV* common mode). The datasheet is suspicously quiet on what a "symmetric magnetic coupling barrier" is, but later on it states supply current is drawn in 2.5ns gulps. I wonder if the output state is stored in a flip-flop, and I wonder what its initial state is. > I've made my own transmission-line gate-drive transformers for > short-pulse stuff, like 100 ns pulse widths/400 volt swing/5 ns > risetimes. Longer pulses and high duty cycles become problematic. > > Extreme case: use fiber optics. Megavolt isolation! Perfectly acceptible If I were building a "valve house". ;-) Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms |