Hello again, my second question to you all

I have a circuit with dual voltage, 44..50(spikes)volts with 1..10amps and 5.1v with about 110..300mA, It's working but the two voltage regulators (TexasI TL783 44->20v, and ST LM317T 20v->5.1) with large heatsinks dissipate quite a lot of heat, below the maximum still!

They require a lot of components and a lot of pcb space, so I'm searching for a improved solution on components and on being cheaper!

I was thinking of pwm, a voltage divider up to 20mA with zener regulation to supply voltage to 555, with a p-channel Mosfet fast switching the 44v, but some of the 5.1v components work at 20..40khz, so here are my questions:

Is it possible with 555's using a cheap p-channel Mosfet like the IRF9640(Rdson of .5 wich is a lot but I can't find less on common models) to make 5.1v without messing up with the frequency of the components ?

Is there anyway of minimizing the ripple of the fast switching 44v ?

The schematic is only for helping (diodes on lm317t are schotky 1n5819), battery is for isis simulation only..

Thks

 

I'm still trying to figure out your requirements.

You appear to have some sort of power source. Is this also the source of the voltage spikes that cause the voltage to surge from 44v to perhaps 50v? What is your actual input voltage range?

In your schematic, you show L2 (I'm assuming a lamp) rated for 50v.
Is this the point where you actually need 44v with somewhere between 1A and 10A load current? Or what is the actual voltage and current range that you require?

You also require 5.1v out at ~100mA to ~300mA.

What is the nature of your loads?

(I see that you are using a TIP122, a couple of 22v Zeners and a resistor as a voltage clamp - the TIP122 would burn up if the voltage were not limited elsewhere)

 

Thank you for the reply.

Power Source MeanWell 48v DC 14.6amps, self-regulated to 44volts, placed about 4 meters, wich at imperial units I think is about 13feet, and that's why I use my own clamping!

The clamping is for taking care of some spikes from 8 diodes connected to two L6203 chips

The 5.1 volts are for 6 optocoupler 6n137, 2 hex inverters (TTL), 1 l297, a couple of components like resistors, transistors, logic gates, and a Relay that turns that lamp(wich represents the two l6203) the whole schematic is quite big!

Any idea about replacing the two voltage regulators by pwm? considering the two question above ?

 

See the attached for a switching buck regulator schematic and simulation using a Linear Technology LT1676 and supporting components.

R1, C1 and C2 dampen oscillations on the output.
R2 and R3 set the output to ~5v.
L1 is a 100uH Coiltronix CTX100-2-52, rated for 2A peak, and has a serial resistance of 40m Ohms. Increasing L1 to 220uH would reduce the output ripple, but would also increase the overshoot on startup.
D1 is a Schottky diode rated for 100v.
C3 filters the output from the inductor.
Efficiency is better than 83% with a 300mA load. Efficiency decreases with a lower load current.

You won't get anywhere near that efficiency with a 555 and PWM of some sort. The circuit would be a lot more complex, and would not have as good voltage regulation.

 

Thank you, I did not know that IC, it has a very detailed datasheet, and look the ideal thing, but after looking all the functions, I can't really calcule the Dissipated Power for Vin = 50v and Vsw = 5 volts, can you help?

 

Ahh, did you look at my simulation?
I performed an efficiency calculation; power dissipation for each component is shown; I used an input of 44v with spikes up to 55v every 100uS for 10uS.

Isn't that close enough?

[eta]
I attached the LTSpice schematic in case you'd like to experiment with it.
Linear Technology's LTSpice is available for free.
Google: "LTSpice download"

 

uupss.. I went straight for the datasheet, but I've notice now, 183mW, I really need to learn spice models, and proteus does not have LT1676 for simulation.. I'm going forward with it , thank you !

 

OK, keep in mind that the parts I used are pretty specific.
D1 is a PDS5100; you'll need to use that or a better one to obtain same or better efficiency.
Vr (reverse voltage) needs to be 80v or more, and the Vf should be very low. The original diode used in the Linear Technology supplied example took a big efficiency hit due to the lesser-spec'ed diode they were using. I also used an inductor rated for nearly 7x your stated maximum output current to minimize the losses; both core and copper.

 

I found all the specific components here at Portugal, some of them are out of stock so for now I'll be using the TL783 and the LM317.

The complete board is posted here, I'm looking for mystakes or sugestions

http://forum.allaboutcircuits.com/showthread.php?p=377616#post377616

thank you