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Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi,
I'm also not sure how reliable the circuit would be if mag amp is in series with mosfet section output - it would probably be more reasonable to use it in series with flyback primary. But then you will have much larger current that will toast your ferrite even more.
If you are using toroidal cores for your mag-amp I'd expect them to heat up in such application a lot anyway, most likely by eddy currents than anything else. They are just too small, and they also show very high inductance with low energy storage when not saturated which might disrupt the oscillator. I'd try out EI cores with a little air gap or perhaps even AM radio ferrite rods just to see how they do. Do your cores heat up when fully saturated (full power throughput) as well?
I can't offer much more advice than that though. Only application where I saw a mag amp to sometimes be used is ATX power supplies, but those are weird ones with single winding on a toroidal core and I couldn't really explain how the whole circuit works, I'd be glad if someone does.
Regarding PWM-ing the royer oscillator, by standard it's done by a buck converter driving the actual dc link inductor of the royer - with current control. I'm not sure but I think you could even get away with using just a ne 555 or like and a P mosfet, without feedback or current control because you don't have a capacitor loading the inductor.
Registered Member #2941
Joined: Fri Jun 25 2010, 08:08AM
Location:
Posts: 143
I agree with you ,those small ferrite are simply to small ,,a lot of smp use some big ring made of compress iron dust (or at list that's how they look like)they are also made to be non conductive,I could try with that. I will make further test when I have some time.. for what regarding pwm-ing the royer oscillator,replacing the rf chock by a buck converter look like a good idea but than I loose some of the benefice of the royer : compact ,and very powerful. I want to use the oscillator to produce high voltage on a rc plane (not joking),I already made a full pwm with flyback and I believe it give me around 150 w of power but I need more,and I need to be able to control the output power . considering the resonance frequency of the oscillator I might be able to control it's power by having a mosfet on the gate of both mosfet witch would switch off once a wile to limit the power,preventing the tank from receiving exitation,and this could be pwm...I don't know if that's reasonable...
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
cedric wrote ...
Ash Small wrote ...
Could you use an IGBT on the power supply? Would this not give a sort of PWM, at least limiting the current?
I am not sure to understand your comment, you want to use a igbt instead of the mosfet ? or before it?
Use an IGBT as well as the MOSFETs in the ZVS circuit but put the IGBT between the power supply and the ZVS.
In your first post you said you didn't want to use an SMPS, but an IGBT with PWM would achieve the same thing.
I suggested IGBT because it would operate at much lower frequency but higher power.
Maybe this won't be suitable for your application, but it is one way to regulate the current in a ZVS circuit. (basically switching the circuit on and off at a relatively low frequency with PWM to control current)
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
This is how Royer oscillators are controlled in applications such as CCFL lamp ballasts: the DC link is PWMed with a MOSFET. It is effectively a buck converter: the DC link inductor needed by the Royer is also the filter inductor for the converter.
The two frequencies don't need to be the same, except in so far as the inductor current should be continuous for efficient operation of the ZVS, so you can't run the buck converter too low.
The CCFL ballast controller chips that I've seen run the buck converter at twice the frequency of the Royer. Or half, or something.
Registered Member #2941
Joined: Fri Jun 25 2010, 08:08AM
Location:
Posts: 143
Steve McConner wrote ...
This is how Royer oscillators are controlled in applications such as CCFL lamp ballasts: the DC link is PWMed with a MOSFET. It is effectively a buck converter: the DC link inductor needed by the Royer is also the filter inductor for the converter.
The two frequencies don't need to be the same, except in so far as the inductor current should be continuous for efficient operation of the ZVS, so you can't run the buck converter too low.
The CCFL ballast controller chips that I've seen run the buck converter at twice the frequency of the Royer. Or half, or something.
I was wondering: what is the max (reasonable )power of a buck converter using only one switch,is it just enough to use big igbt to get 1000 w out of it or is there some efficiency issue?I don't think I ever sow a smp arc welder using only one brick + ferrite ...
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
That's because an arc welder needs isolation, so it has to use a transformer. The transformer equivalent of the buck converter is the forward converter, and I've seen welders that use this.
There is no limit to the power of a buck converter, they can be made up into the 100s of kilowatts.
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mag amplifier and zvs drivers
