boylesg
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The clue here is that the schematic "Figure 1" only adjusts up to about half the voltage you put into it. To apply a similar principle to yours, you have to put around 36V in to it from your charge pump.
Incidentally, the current limiting part of the regulator prevents it passing more than about 3A. You can remove Q15 and short out R7 as there is no chance of you ever reaching a current as high as that. I'm not familiar with the simulator but if X1 is a 7.5W lamp used as a load, it is never going to work, the diodes in the charge pump will light up brighter than the lamp !
Brian.
Well we can get rid of that bit then - might improve matters.R8 and C4 form a low-pass filter at the input with a pole at 12.5 Hz. After that filter, the regulator circuit begins.
The problem is with the boost converters is that I could not figure out how to invert it to boost -12V to -18V and I could not find any examples of an inverted boost converter.I think a fundamental stumbling block here is you are using a very inefficient voltage doubler and following it by a very inefficient voltage regulator. The 'goodness' of regulation in that schematic is very poor and you might find it has poor temperature stability and it's output voltage changes according to the light level falling on the LED. The R8/C4 statement is only partially true but in any case is irrelevant to the circuit.
As mentioned elsewhere, a much simpler circuit using an inductor will do the job better and would be many times more efficient.
Brian.
I didn't think ambient light had any effect on LEDs. It will ultimately be in a closed box at any rate. I could always put a couple of series rectifier diodes in place of the LED if ambient light is a problem for it. I heavy duty rectifier diode with a Vf of 1V and a lighter diode with a Vf of 0.7V might do the trick.light level falling on the LED
I assume you would prefer to start with +12V rather than -12V although the schematic remains basically the same. Google for MC34063A it will turn up several manufacturers data sheets, the TI and OnSemi ones are best (and curiously almost word-for-word identical). Download the data sheet then go to this web site : **broken link removed** and enter the following in the boxes:
Vin = 12V
Vout = -18V
Iout = 200mA
Vripple = 10mV
Fmin = 20KHz
Then click on "calculate", it will draw the schematic and tell you all the values to use. It will give about five times the current output of your present design with lower ripple and will be fully stabilized and overload protected. All in 9 Components and none of them are expensive. Where are you located? I may be able to help you find a supplier.
Brian.
High-efficiency step-down switching regulators for positive voltages are very common, however negative step-down switching regulators (negative voltage in, negative voltage out, common ground) are not as well known, even though they are often needed. Although they are not difficult to set up, literature on how to build them is rather scarce. This article analyzes the architecture and detailed operation of the negative buck topology. It will also discuss actual circuit implementations for the topology, from a system perspective down to the building of the needed circuit blocks, and include examples on how to build a voltage translator circuit, a key block in implementing a negative buck regulator using readily available boost ICs.
Both are storing energy but they are not working or used in the same way. Their operation principles are opposite:As I understand it thus far, inductors and capacitors are used in much the same way. You push a current through them one way, they store energy, you ground one end and the stored energy comes out as current the other way.
So why couldn't I just replace the charge pump capacitors in my doubler with inductors plus any other required alterations?
I would still like to know how to do a -12V to -18V conversion.
I don't know which literature you have been reviewing, but -12 to -18 refers clearly to a negative voltage boost (or step-up) converter.I have been searching for the wrong 'string' for starters. Should have been 'buck a negative voltage' rather than 'boost a negative voltage'.
Similar to a transformer where you convert a lot of current with little voltage to a lot of voltage with little current.Both are storing energy but they are not working or used in the same way. Their operation principles are opposite:
Vinductor = dI/dt
Icapacitor = dV/dt
According to different operation principle, capacitive and inductive converters are implemented in quite different circuits.
I don't know which literature you have been reviewing, but -12 to -18 refers clearly to a negative voltage boost (or step-up) converter.
I think, there are two aspects of the question:
- in principle, you can design the said a -12 to -18 converter by flipping the polarity of all components in a +12 to +18 converter. In so far there's no basic problem.
- in practice you won't find dedicated negative boost switcher ICs. You can possibly implement the converter based on switcher ICs for positive input voltage, or switch mode controllers and external transistors. That's just daily engineer's work.
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