MrAl you are saving me here...
The spice modelling has the limitation that it wont converge unless all isolated nets have a ground... in a real world environment I recognise this is not a limitation. In the latest incarnation I have a situation where I used an IR2183 which connects the Mosfet drive circuitry directly to the low voltage circuit via a shared ground (see above) - the IC has a level shifter in it to handle the high side and doesn't need a special supply on the output... I think I understand now I must make sure the 320DC (rectified Australian mains) must not have the same ground as the low voltage circuit or, indeed, any ground (floating) and be completely isolated from the rest of the power supply circuit... the only connection being via the main switch mode transformer and its secondary...
Considering the IR2183 is connected to the gates of the mosfets and also to the TL494 (no isolation) do I dispense with the IR2183 in this regard.... or... how can I use the IR2183 ensuring I still have isolation? optocoupler/ gate drive transformer or is there another more elegant solution to this...
Kind regards
Simon
Hi Simon,
I just happened to notice that your feedback circuit isnt quite right yet. The feedback for this kind of regulation can not be linear, it actually must be non linear or you wont be able to regulate the input to the LT1083 properly. With a linear feedback the voltage across the LT will rise as the output rises and this would cause more dissipation than needed in the LT1083. You can check this in the simulation. It cant be turned down either or else the LT1083 wont get enough differential voltage near the low end of the adjustment range. The way it is supposed to be is the voltage across the LT is to stay somewhat constant, at say 2 or 3 volts. The network you are using in your previous post is made up entirely of resistors and that means it must be a linear network, so we have to fix this.
Your second problem is the isolation between the line and the output. You want isolation there as you indicated.
Taking both problems into account, it looks like the best solution might be to use an opto coupler for the feedback. The opto coupler will provide the needed non linear feedback because of the way we can connect it to the circuit, and also at the same time provide isolation between the LT section and the TL494 chip (and associated driver chip and MOSFETS and line). This means you wont have to use opto couplers on the output of the TL494 either which sometimes introduces an imbalance in output transformer primary currents.
The idea then is to connect the opto coupler input across the IN and OUT of the LT1083, possibly with an included series 1N4148 diode. The opto is to conduct when the input output gets to around 2 or 3 volts but not conduct much otherwise. This is the needed non linear function.
The output of the opto goes between the TL494 power +Vcc pin and the feedback input 1IN+ on your schematic, also with say a 1k resistor from 1IN+ to ground. The other input 1IN- goes to a reference voltage of maybe 2 volts developed from the reference of the TL494.
The operation then starts out by pumping up the input to the LT494 little by little with no feedback signal. As the input rises and the LT1083 output rises, the LT1083 starts to put out some current into the load, and this causes more of a voltage drop across the LT1083. Eventually the LT1083 starts to regulate, and this causes its input impedance to rise which means the input voltage rises yet the output doesnt rise anymore. This eventually causes more of a voltage drop across the LT1083 and that causes the opto coupler internal LED to conduct. When that LED conducts, the output transistor starts to conduct. When the output transistor starts to conduct the feedback on the input to the TL494 starts to rise. When the LED conducts enough that means the voltage across the LT1083 must have reached a particular level such as 2 or 3 volts, and since the opto transistor is conducting that eventually causes enough feedback to the TL494 to cause it to start to cut back the PWM pattern duty cycle, and this of course causes the input to the LT1083 to stop rising. This mechanism keeps 2 or 3 volts across the LT1083 at all times.
This isnt the way it is usually done however, as normally just a transistor is used here instead of an opto isolator chip. The difference is that the transistor can respond extremely fast when the input (to the LT1083) changes, but the opto coupler responds just a little bit slower. This means you'll have to check for stability and if it seems unstable you'll have to try a faster opto coupler.