I need to lower a dc tension from 450 volt to 350 in a simple way,also without feedback or precision bat without waste too much power( no serie regulation): so i think a chopper.
Somebody con indicate me a circuit very simple ...with LM723 or other
Tanks
Current? Fault tolerance & what kinds?
Hello,
Max 16 Ampere
Fault no problem,no problem with heatskin or other...dimension etc.
Regards
MAX
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Can we switch on the negative side? ie: Is there any other connection to the minus side of the input we need to worry about?
+IN ------------+------------+---------------- +Out ! ! --- --- ^ --- ! ! +--)))-------+---------------- -Out ! !!- !!- !!- +-- I sense \\ / \\ !-IN ------------+
For the control circuit, I would consider a constant off time current mode control built from a LM339. Such a controller is cycle by cycle current limited and easy to make stable.
The controller would normally need 3 of the 4 comparators. I would use the 4th section to pick off the I-sense and detect it spiking up to well above the current limit. With a shorted output, the 4th comparator would yank down the voltage reference and extend the off time.
Extend off time like this: Va ---/\\/\\-----+----+----- to 4th \\ ! Normal / --- Isense \\ --- comparitor ! ! ---!-\\ ! GND Off timing ! >---+ comparitor ---!+/ +----------!+\\ ! ! >--- To MOSFET driver --- --!-/ --- ! GND
Decrease the reference voltage by using Va as its source.
lso
er
rcitato
Thanks for the answer. No problem to switch on negative. Could you explane me a little the principle of control of Ton Toff time?
Thanks again MAX PS is there a web pagew where is it possible to see something of similar?
,also
other
nver
to citato
If you assume that the current in the inductor never stops, the Ton and Toff requirements do not depend on the exact value of the inductor. The average voltage on the inductor is zero so the output is the duty cycle times the input.
If it wasn't for issues like overloading and start up inrush, you could just use something like an LM555 to set the duty cycle of the MOSFET.
When the transistor is on, the current in the inductor rises linearly (nearly perfectly). This makes it fairly easy to use the current change as a control on the on time.
When the transistor is off, you don't have any such easy thing to use so making it simply how long it takes a capacitor to be charged is an easy way to set that time.
Converters done this way don't run at a fixed frequency. They have a longer on time when loaded.
The LM339 is a comparator that is fairly low cost. If you load one of its sections with a capacitor of at least 470p, it will work at a low quality op-amp. This op-amp can be used to look at the output voltage and control the current peak value. You need very high values of resistors because the voltage is high.
+Vout ----/\\/\\----+----/\\/\\---!!---+--/\\/\\--Vcc ! ! GND ----/\\/\\-----+--!-\\ ! Vpeak ! >-------+--+---- To current-Vout ---/\\/\\----+---!+/ ! comparator ! ---
+Vref ---/\\/\\----+ --- ! ! --- GND --- ! GNDIn not very strict cases, the resistor to the Vref can be two resistors one to the Vcc of the LM339 and the other to ground. The LM78XX style regulators are within a few %.
Vcc -----/\\/\\--- ! Isense ----/\\/\\---+-+-----!-\\ ! ! >
--- -!+/ --- ! ! ! GND ! ! Vpeak ---/\\/\\----------+ ! GND ---/\\/\\-----------
The resistors on the Isense assure that the Vpeak can go low enough to keep the transistor off in the no load case.
A small capacitor is needed because there is a spike on the current sense just as the transistor turns on.
I suggest you down load LTspice4 from linear's web site and try the design out before you commit it to real parts.
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