Linear regulator input voltage limitations.

[David_]

Very cool, you can create two inductors L1 and L2 and give them a appropiate inductance value for primary(L1) and secondary(L2) and then use the icon 'SPICE directive' and type "K L1 L2 0.98". I'll guess 0.98 is the coupling or anything in that direction.

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Oh yeah, that will link the two inductors to act as a transformer.

 

[crutschow]

LTspice is actually optimized to simulate switching regulators.

 

[David_]

You don't say, you do not have any suggestion for a model for my case do you?
I have created a schematic with a voltage source with 270VDC(207VAC * 1.4 - 20(Vin,min)) that is feeding a transformer with the three windings. W1 has its other end connected to a N-channel MOSFET and the secondary's has there diode, capacitor and load resistor.

But how to implement the PWM driver..., and the feedback which will depend on the driver.
I will try to use a signal source and manually change duty cycle and see if that works.

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There are tons of flyback ICs but no one that is driven from 230VAC mains and I have hit a wall.

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http://www.linear.com/product/LT3798
Is the best I can do but that is not what I am after at all, this demands its own thread.

 

[KlausST]

Hi,

There are tons of flyback ICs but no one that is driven from 230VAC mains and I have hit a wall.

i never used a 230V flyback IC, but i expected there are many of them, because every simple wall wart should work that way.
If i remember right, then i saw one with external resistor and shunt zener as supply voltage ... this is not much effort...

But better to rely on datasheets and application notes..

Klaus

 

[David_]

I meant in LTspice

 

[D.A.(Tony)Stewart]

One weird thing that a LM317 was doing was to change the output voltage when the input voltage changed, I had(with a pot) dialed in 3.3V while having two 9V batteries in series on the input then I added one battery to make three batteries in series on the input and I expected to LM317 to maintain 3.3V but no. Now the output was 4.1V and in that manner was the output changing... anyway.....

LDO's must have loads to ground. If you have any load to pullup from Vout, it will force the output high.

 

[crutschow]

One weird thing that a LM317 was doing was to change the output voltage when the input voltage changed, I had(with a pot) dialed in 3.3V while having two 9V batteries in series on the input then I added one battery to make three batteries in series on the input and I expected to LM317 to maintain 3.3V but no. Now the output was 4.1V and in that manner was the output changing... anyway.
....................

What were the pot and other resistor values in the circuit?

 

[David_]

Its hard to say, I ended up soldering a few resistors in series with a 100ohm 10turn pot to get a really narrow adjustment range but the fixed resistor in the divider was 1k. But It was not long ago that I desoldered that board and switched it against a fixed voltage regulator and later today I can search and find the parts used to see the value, if that can answer anything.

It was a textbook implementation otherwise, no strange ideas or connections. and both input/output decoupling.

 

[crutschow]

The fixed resistor of an LM317 (between the output and the Adj input) should be no more than 120 ohms.
Otherwise the voltage may not be correct with no load on the output.

 

[David_]

Okay, I was not really thinking about it but 1k felt reasonable. It was some time since I last actually read one of those datasheets but I will make it a point not to do the same thing again.

The circuits that was powered from that regulator did not draw much current, below 1mA I believe.
That might constitute a close to 'no-load' condition.

Regards