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Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
I'd like to try to refrain from using a non self-resonant topology because of the lower switching losses and I'd like the heater to work ok without fiddling some pot all the time :)
As it looks now, you will not get any benefits from that kind of feedback, and you are actually pretty much hard switching at this point according to steve.
Using a vco is only currently known way to get out of trouble.
Registered Member #538
Joined: Sun Feb 18 2007, 08:33PM
Location: Finland
Posts: 181
Firkragg wrote ...
I'd like to try to refrain from using a non self-resonant topology because of the lower switching losses and I'd like the heater to work ok without fiddling some pot all the time :)
As it looks now, you will not get any benefits from that kind of feedback, and you are actually pretty much hard switching at this point according to steve.
Using a vco is only currently known way to get out of trouble.
Whats the reason for the hard switching? I havent really checked it because I'm running without an isolation transformer and the FETs have stayed pretty cool so I have assumed I'm soft-switching.
Heres the waveform over the GDT primary while "normal" operation: and
And the waveform goes all mashed up when at curie point:
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
Do a simple test for me.
Wind another CT if you dont have one. Load it with a few ohms... this is for measuring your inverter's output current. Check the phase relationship between your gate drive signal and the inverter current. This will tell you what phase angle you are switching at. My intuition is saying its gonna be hard switching, likely by 90 degrees, but it really depends on the R component (your workpiece) of the LC tank. So when i say it "wont work" its not that it wont function, but just that it isnt optimal.
Your bridge is likely running cool because you are only running enough power to heat up a bolt. I mean, my little half bridge on a CPU heatsink did that just fine, while hard switching a mostly inductive load. My heatsink never got warm at 400W input power. It had no trouble heating up 1/2" diameter bolts to orange hot temps (and reaching curie temp as the input power would drop off). So im not surprised that your setup isnt getting warm. When i built my LCLR test rig, i was pushing 1500W continuously, and even with some hard switching it didnt warm up much, but my L-match eventually melted on me (saturated the ferrite core, me thinks).
Registered Member #538
Joined: Sun Feb 18 2007, 08:33PM
Location: Finland
Posts: 181
Steve Ward wrote ...
Do a simple test for me.
Wind another CT if you dont have one. Load it with a few ohms... this is for measuring your inverter's output current. Check the phase relationship between your gate drive signal and the inverter current. This will tell you what phase angle you are switching at. My intuition is saying its gonna be hard switching, likely by 90 degrees, but it really depends on the R component (your workpiece) of the LC tank. So when i say it "wont work" its not that it wont function, but just that it isnt optimal.
Your bridge is likely running cool because you are only running enough power to heat up a bolt. I mean, my little half bridge on a CPU heatsink did that just fine, while hard switching a mostly inductive load. My heatsink never got warm at 400W input power. It had no trouble heating up 1/2" diameter bolts to orange hot temps (and reaching curie temp as the input power would drop off). So im not surprised that your setup isnt getting warm. When i built my LCLR test rig, i was pushing 1500W continuously, and even with some hard switching it didnt warm up much, but my L-match eventually melted on me (saturated the ferrite core, me thinks).
Could I just compare the output from the feedback CT to the gate drive signal?
My judgement about soft/hardswitching could have well been wrong, I was just comparing the amount of heat generated compared to my SSTC endeavours when everything wasnt working properly and was hard switching and the bridge was heating up like mad but it prolly was something else (improper gate drive for example) wrong then too.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
I don't see why direct feedback wouldn't work. You should be able to excite the tank circuit resonance with direct feedback from the inverter output current. If you ignore the DC block capacitor, the circuit (of two L's and one C) only has one resonant mode that draws any current. (there is another mode but it's a zero, not a pole)
If you take the DC block cap into account, the circuit has two resonant modes that are poles, like a DRSSTC. You just have to be careful not to excite the wrong one. I imagine the unwanted mode is at a very different frequency and has a much lower gain, though.
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
Could I just compare the output from the feedback CT to the gate drive signal?
No, because those 2 *will* be in phase (since that CT drives your gate drivers) and this gives no new information. You need to know the inverter current, not the tank circuit current phase.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
I don't see why direct feedback wouldn't work. You should be able to excite the tank circuit resonance with direct feedback from the inverter output current. If you ignore the DC block capacitor, the circuit (of two L's and one C) only has one resonant mode that draws any current. (there is another mode but it's a zero, not a pole)
Yeah, I'm confused as well.
I'l try to reformulate it:
When I use SG3525 to tune it, wich frequency I actually need to go for?
If I manually tuned it exactly for a frequency of work LC and ignored the L match, I would still get 90 degre phase shift on that inductance? Or is that a part where I'm missing something?
Matching network and work LC together are indeed two coupled LC's as steve said, but since coupling is direct with k = 1 they could be figured as a single resonant circuit with frequency somewhat different that of work LC alone.
So I would expect the thing to be driven at that frequency, and not the one of work LC, in order to get ZCS.
This surely reduces power troughput, but when we need more we just reduce the matching inductor.
So I expected adding a current transformer for feedback directly on the inverter output to run the circuit at that frequency and attain ZCS, same as if it was hand-tuned by a VCO.
Putting a CT on the work LC just looks lieka wrong way to go.
Anyway, i first stuck the CT where Dago has it (between the L and C of the tank circuit). This works "OK" as i thought. Depending on the load resistance, the phase changes between inverter current and LC current. With light loads (100 ohms) the current is very much in phase, but it climbs to dangerously high levels (which is why i suspect this thread ends with "current limiter"). With greater loads (<30 ohms) the inverter current drops off where it should be, but the phase of the LC current isnt the same as the inverter current. The inverter commutates before the current crosses zero, which is really the worst situation (switching late is always easier on the devices). The lower the load resistance, the worse it becomes, eventually the 2 currents look 90 degrees out of phase meaning you are switching at the peaks of the inverter's current. On the plus side, the inverter current is lesser now...
Next i tried to place the CT on the output of the inverter. As i suspected, it oscillates with the blocking C and Lmatch at a very low frequency. I believe this is because Lmatch > Ltank, or rather that Ltank makes the Ctank look much less significant. In fact, shorting out the LCtank *increases* the frequency that this resonates at (which is of course still far too low). I even made Cblock 200uF to try and stop this oscillation, but it just oscillates lower now. Of course, making Ltank an open circuit (or effectively huge) produces oscillations at the "right" frequency of Lmatch with Ctank. Making Ltank slightly larger than Lmatch still doesnt fix the situation.
So there you have it, the simulation backed up what i originally thought would happen. So you *can* run it with the CT on the tank LC, but its not as efficient as it could be. You may be better off running from a VCO as Firkragg said. You should be able to tune it for the right frequency and get it to work well. You will still probably want a current limiter employed here. My idea was to have something like my SSTC OCD circuit (with CT on the output of the inverter) and watch the peaks. If it trips, then have it wait for like a second (since this likely means there is no work-piece in place) and then have it try again. This way you keep the duty cycle at these high currents very low.
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Self-resonant LCLR induction heater current limiter
