Hi,
I made an attempt to add a switch to turn off the current flow through
R8 once the flyback starts up.
http://www.nekrom.com/rocketresearch/new/flyback/flyback3-switch.jpg
How can I turn M1 off once the flyback has started up, or should I use
another type of switch for this?
cheers,
Jamie
Geez, this is getting awfully complicated for a circuit that only has
to deliver 1 watt or so. ;-)
Is M1 a depletion-mode mosfet? If not, you won't ever get much current
through it...and it looks like it's turned upside down: a P channel
device would normally have its source and substrate tied to a more
positive terminal than the drain...and there's more than that wrong
there, I'm afraid.
Actually, what I see in the data sheet for the LT1615 is that the Vcc
current is 1uA max with the /shutdown at 0; the /shutdown pin takes it
out of shutdown by by the time it reaches 1 volt, and the current
into /shutdown at that point appears to be less than 10uA. Sounds
like the simulation is wrong. But at some point you need to get some
parts and play with it on the bench; that's how you find out how it
really works.
Back to the switch to shut off the bootstrap bias current: if the
LT1615 really is so low current, I'd say it's not worth worrying
about. If you need even 20uA, at 100V that's 4.7 megohms. At 400
volts, it becomes 80uA, and about 32 milliwatts dissipation. If you
do need to switch it off, I'd be inclined to use an NPN transistor
rated for 500V or so. (Seems like that starts to get big...I know of
some SOT23 parts rated to 350 or 400V, but I'm not sure you'll get 500
in a small package). Then I'd tie the base to a zener to ground, and
a high-value resistor to the +supply input. The emitter supplies
current to wherever the resistor did. The collector connects to the
+supply input, either directly or through a resistor that will limit
the maximum current--i.e. the same value or a bit lower than you'd
have used without the switch. If the zener is a lower voltage than
the aux supply (C5 in that latest jpg schematic), the transistor
emitter will be pulled up by that supply and turn the transistor off.
You could do the same with a mosfet, but the gate threshold voltage is
not as sharply defined as the base-emitter voltage of a bipolar
transistor, so you give up margins elsewhere if you use a mosfet. But
in that suggestion, beware of the zener voltage at very low
currents...it may be quite a bit less than the rated voltage of the
part. 10V zeners have a very sharp knee, but 5.6 volt zeners have a
very "soft" knee.
Are you going to be building a lot of these, or what? I'm trying to
get a sense for why you're going through all this; I'm sure it's a
good learning experience for you, but I'm not convinced it's the most
practical way to make a little low-power supply.
Cheers,
Tom