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Registered Member #58280
Joined: Sat Jan 09 2016, 06:48AM
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Posts: 43
Does the litz wire help? I've always assumed that the primary has so great losses, what you save in the secondary doesn't make much difference. Plus, you're going to loose a good amount of RF energy heating up that PVC former.
I was unsure about the RFC too. The RFC blocks AC and passes DC. Therefore, the inductance of the choke has to present an much higher impedance to AC than the load resistance. I'm not sure what's a minimum acceptable value, Richie and Uzzors2k use 56/47 uH, like you said, but Reaching uses a 470uH, which is an order of magnitude difference.
One thing I do know that it's best to use a powered iron core that's rated for these frequencies, so type 2 is usually recommended. It has a very low AL value, so 470uH isn't that easy, I bought some 3" diameter T300 size cores that I have to wind ~200 or so turns to get 470uH. About 70 turns for 47uH though, which is more manageable.
I personally like to use mica capacitors or film capacitors, like PP or polystyrene.
For the DC blocking cap, greater capacitance can pass more current. So it doesn't hurt to have a higher value, but it probably won't make a difference.
That has an awfully high Rds(ON) @ .98 . You're going to have very high I2R losses.
Registered Member #3114
Joined: Sat Aug 14 2010, 08:33AM
Location:
Posts: 608
Chris_Knight wrote ...
Does the litz wire help? I've always assumed that the primary has so great losses, what you save in the secondary doesn't make much difference. Plus, you're going to loose a good amount of RF energy heating up that PVC former.
I was unsure about the RFC too. The RFC blocks AC and passes DC. Therefore, the inductance of the choke has to present an much higher impedance to AC than the load resistance. I'm not sure what's a minimum acceptable value, Richie and Uzzors2k use 56/47 uH, like you said, but Reaching uses a 470uH, which is an order of magnitude difference.
One thing I do know that it's best to use a powered iron core that's rated for these frequencies, so type 2 is usually recommended. It has a very low AL value, so 470uH isn't that easy, I bought some 3" diameter T300 size cores that I have to wind ~200 or so turns to get 470uH. About 70 turns for 47uH though, which is more manageable.
I personally like to use mica capacitors or film capacitors, like PP or polystyrene.
For the DC blocking cap, greater capacitance can pass more current. So it doesn't hurt to have a higher value, but it probably won't make a difference.
That has an awfully high Rds(ON) @ .98 . You're going to have very high I2R losses.
Litz makes a world of difference when it comes to HF and skin effect. The primary will also be litz and be comprised of 19 strands of 32 awg wire.
From what I have been reading and understand is that the Rds on goes out the window compared to the capacitance of the gate. If the device cannot turn on/off fast enough the Rds doesnt matter.
I am just trying to get my cart all full so I dont have to pay twice for shipping on something I forgot to get
edit: litz also reduces the amount of heating due to the lower resistance of the wire at RF
Registered Member #58280
Joined: Sat Jan 09 2016, 06:48AM
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Posts: 43
Well I mean you want to balance both gate capacitance and Rds(ON). Both matter. Plus, your chosen transistor doesn't even have that low a gate capacitance. Total gate charge is 72nC, compared the 43 of the old IRF630, which has half the Rds(ON)! Plus, the IRF630 has less than half the switching times. The main advantage of that FQA11N90_F109 is the much higher Vds, but that really is pointless unless you're planning to run your coil at 700v Vcc or something. So I don't get it, unless I'm missing something here.
I'm not arguing that litz is better at RF. My argument is that in Class E sstc work, your savings in using litz are heavily outweighed by your losses elsewhere in the system. But I may be wrong. It's certainly worthwhile to try!
Registered Member #33
Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971
Chris_Knight wrote ...
Well I mean you want to balance both gate capacitance and Rds(ON). Both matter. Plus, your chosen transistor doesn't even have that low a gate capacitance. Total gate charge is 72nC, compared the 43 of the old IRF630, which has half the Rds(ON)! Plus, the IRF630 has less than half the switching times. The main advantage of that FQA11N90_F109 is the much higher Vds, but that really is pointless unless you're planning to run your coil at 700v Vcc or something. So I don't get it, unless I'm missing something here.
In class E amplifiers, the peak drain voltage is close to three times the supply voltage ( ), so the IRFP630 should not be used with supply voltages higher than approximately 60 V DC. The FQA11N90 could theoretically be used with supply voltages up to around 300 V DC, though some more safety margin is probably a good idea.
Registered Member #3114
Joined: Sat Aug 14 2010, 08:33AM
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Posts: 608
Yeah like wolfram said, when tuned properly the voltage is around 3.5 times the Vcc. Well how about this guy this is what Eastern Voltage uses in their coils.
Registered Member #3114
Joined: Sat Aug 14 2010, 08:33AM
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Posts: 608
sorry for the double post wanted to include scope shot.
Yellow: IXDD614 output looks bad.. not sure why. blue: drain. MOSFET gets hot fast
I have 440pf of shunt capacitance, 100nf DC blocking and 6 turns on the primary 18awg wire. 30uH for my RF choke. Its wound on #2 iron powder good between 2-8Mhz. Would more inductance help ? I couldnt find any information on choosing a value
edit: the coil also make a high pitch noise, like a screaming sound although its not really loud. I thought 3Mhz plasma was silent
Registered Member #55219
Joined: Tue Jun 09 2015, 11:21PM
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Posts: 80
I'd say start adjusting your coupling, from what I understand it's fairly critical with class E. You may also need more shunt capacitance.. Go to Steve Wards link I posted and compare scope shots, he explains how he tuned his. Yet he is not using a choke or blocking cap.. so .. I think it's Thevenin's theorem that plays a role in his setup (equivalent impedance's).
So did you go fixed osc. or feedback driven? that may help as well, try direct/coupled feedback?
Registered Member #3114
Joined: Sat Aug 14 2010, 08:33AM
Location:
Posts: 608
jdub1581hv wrote ...
I'd say start adjusting your coupling, from what I understand it's fairly critical with class E. You may also need more shunt capacitance.. Go to Steve Wards link I posted and compare scope shots, he explains how he tuned his. Yet he is not using a choke or blocking cap.. so .. I think it's Thevenin's theorem that plays a role in his setup (equivalent impedance's).
So did you go fixed osc. or feedback driven? that may help as well, try direct/coupled feedback?
fixed oscillation. ~2.88MHz.
switched out FET and got much better results ie. less heating. Tried mains and forgot to up my shunt capacitance which im sure cause a huge voltage spike killing the fet and shoring my IXDD frying both of the chips I had.
Ordering new parts..better ones?!? What would cause the IXDD614 to get burning hot olny after ~10 sec operation. I noticed that when the Fres. was below 1MHz the chip ran cool but and more then that and it got HOT fast. Im getting the TO-263-5 okg so I can mount it to the heatsink. Best results were about 1.5cm discharge with 36V unknown amp draw.
Registered Member #33
Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971
What MOSFET are you using? At these frequencies, it doesn't take a huge amount of gate capacitance before the gate driver power dissipation becomes significant.
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Class E? Class E...
