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Registered Member #75
Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711
Yeah, I have modded my PSU in such a way that it basically runs on full duty cycle all the time, so when there is no load the voltage rises quite a bit.
I have not tried designing a SMPS myself, at the moment I am trying to learn all about them by modding existing ones. But I am sure EVR and Steve Conner can help you if you have specific questions.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Circuit is not self-resonant, its yust normal astable. Anything put thre (transistor, voltage stabiliser...) will suffer ohms law in same way as resistor does. Used transistors but11A. Driver just eats too much current for base, at few tens of mA resistor has to disipate lots of power.
I was suprised by driver staying cool and stable with large loads, its just resistor that suffers, not the transistor. When replaced by mosfet (this would be solution to overheating resistor) it started to heat up very fast...
I looked for solution long and only seems to add some external supply to circuit (then it stops to be smll and simple SMPS)... It may be possible to use transformer with feedback and use it as power source, and that wasnt near my hand...
Tried also to power oscillator using high inductance choke but it needs really to be huge, tried capacitor as load but without sucess.
Last thing was using capacitor to limit current directly from mains, rectified and filtered but that made circuit unstable and humming, transistor overheated fast in humming transitions...
Registered Member #75
Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711
Have you tried reverse-engineering a normal PC PSU? I have not read about this anywhere, but from what I have seen, the main current loop driving the power transformer also has a two turn winding on the gate drive transformer. Apparently when the supply switches on, one of the transistors becomes active (by chance) and then the induced kick in this winding turns off the conducting transistor and turns on the other one. This is what drives the PSU as a simple blocking oscillator, until the TL494 takes over.
I hope it is OK to intentionally double post when it is for a completely different topic: I keep blowing bipolars in my SMPSs, but I only stock FETs, so I cannot replace them. So I was wondering if I could replace a BJT by a FET without any other extensice modifications? I suppose the driver circuit would not mind if the switch is not drawing any current, but I am not so sure about the voltage rising to the right level, and more importantly, falling to zero again for switching off. But with minimal modifications of a zener diode and a gate discharge resistor, could it work? I am sure Steve knows this!
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Have you tried reverse-engineering a normal PC PSU? I have not read about this anywhere, but from what I have seen, the main current loop driving the power transformer also has a two turn winding on the gate drive transformer. Apparently when the supply switches on, one of the transistors becomes active (by chance) and then the induced kick in this winding turns off the conducting transistor and turns on the other one.
Old SMPS have used technique to power up driver by 'kick', this one seems like that. Power is applied and transistors themselves have to give starting pulses and power up TL494. That kind of SMPS usually need power to be applied quickly or transistors would turn on too slow and starting voltage pulse would be too weak.
This actually doesnt exist any more today as standby voltage is needed and it is provided by small back up SMPS (also solved turnon problem).
<Some smaller sistems use voltage divider and zener stabiliser to give IC enough voltage for starting, or entire driver is just moved to high voltage side.
I hope it is OK to intentionally double post when it is for a completely different topic: I keep blowing bipolars in my SMPSs, but I only stock FETs, so I cannot replace them. So I was wondering if I could replace a BJT by a FET without any other extensice modifications? I suppose the driver circuit would not mind if the switch is not drawing any current, but I am not so sure about the voltage rising to the right level, and more importantly, falling to zero again for switching off. But with minimal modifications of a zener diode and a gate discharge resistor, could it work? I am sure Steve knows this!
Driver transformer for BJT is wound for low voltage and higher current, for turning on bipolars propertly. Mosfets would just stay barely opet at 2-3 volts that gdt gives, they would start to dissipate huge amount of power and blow up.
If you remake gate driver circuit (transformer and few conponents) its no problem using MOSFETS. Once made for mosfet you can replace them with IGBT and have huge power output (until you saturate transformer...)
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The SMPS modding thread (aka. The "Ask Dr. Steve" Thread)
:-@
