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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Hi all, I was having some problems with the slow turn-on RD combo on the gate of my MOSFET (which was kind of working) in my quasi-resonant inverter, with the purpose to decrease the peak current resulting from the resonant capacitor discharging right into the FET.
Today I designed a new way of limiting the current which is shown below:
The inductor limits the peak capacitor discharging current, and during the flyback phase, the leftover energy is dissipated in the diodes (they don't really heat much). It consists of 3 turns on a computer PSU yellow core.
The problem is, the core will get very hot after a while. I don't really understand this, the core is designed to operate as a DC choke and the frequency is also right. It's kind of funny to see a core like this overheat from just 3 turns around it.
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
If the core is getting hot and the diodes don't really heat much, then I'd guess the majority of power is actually being dissipated as core loss in the iron-powder core.
With only 3 turns on the iron-powder core, the inductance is probably not enough to limit the current ripple, hence the excessive heating.
Remember iron-powder cores in SMPSUs are designed to smooth current and don't like high peak-to-peak current ripple. That causes lots of core losses. As it happens the type-26 material used in cheap PC SMPSUs is the most lossy grade of iron-powder. Most iron-powder cores are also prone to thermal ageing so if it is getting hot, the binder material will degrade and the properties of the core change!
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
If the Quasi-resonant converter is set up properly, then the switch should turn on when it's anti-parallel diode is already conducting and there is no voltage across it. If this is the case, then there should be no current spike through the switch at turn-on, that is how it's designed to work...
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Yeah but this is a dynamic-load multipurpose HV experimenter's PSU. It has variable power and secondary current limiting control, and the flyback spike amplitude and duration changes greatly depending on these settings and load (arcs for now), so it's impossible to get it to turn on always with 0 current.
Edit: Just got an idea - air cored inductor! With 1uH it shouldn't be that big...
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Flyback inverter: Smoking inductor
