Linear Voltage Regulator Design

[audioguru2]

Hi nearly old man Staigen,
You are absolutely correct! I was way off with my selection of high-value "turn-off" resistors for the output transistors in my "fixed" circuit. I have now chosen lower value resistors based on what is inside a TIP120 darlington.

I used a 1k resistor in series with the collector of the 2N3906 to limit its collector current and dissipation if the project's output is shorted. It will still provide enough drive to the output transistors to blow a fuse if the output is shorted.

I think the circuit can deliver 30V at 1.5A with a 34V supply to protect the 36V max-rated opamp.

It would be fun to try to stop this extremely high gain circuit from oscillating. ;D


Hi Alun and Cyw,
I don't think a single 2N3055 or power Mosfet can dissipate more than about 60W even on a pretty big heatsink. Therefore the max current is only 2A at low output voltages. That's why I proposed using 2 output transistors in our 0V-30V modified project for 3A output, and using 3 output transistors on our modified for 5A project.

View attachment 37601

 

[Alun]

Hi Alun and Cyw,
I don't think a single 2N3055 or power Mosfet can dissipate more than about 60W even on a pretty big heatsink.

That's why I'd use a large MOSFET like the IRF9131 or IRF9531 they can safely dissipate 75W and the dropout voltage will be only 0.9V at 3A, while yours will be at least 3.6V ;D

 

[audioguru2]

Hi Alun,
Those little Mosfets can't dissipate 75W safely, in practise. They are rated for 75W if their case is held somehow at 25 degrees C. You can't find a heatsink that is infinitely large to do the job of perfect cooling, so your heatsink will get hot and you must follow the power derating curve. You might be able to allow a dissipation of 50W if you use a big heatsink plus a fan. With a supply of 31V, the max current would be only 50/31 = 1.6A.

A 115W-rated 2N3055 would be able to dissipate 60W with a big heatsink and without a fan. Since they have a metal case, their chip is allowed to go to 200 degrees C vs only 150 degrees C for the plastic case.

I like 300W-rated Mosfets that will do a good job, but they are expensive. Actually, I haven't seen P-channel 300W Mosfets, only N-channel. ;D

 

[Alun]

Just parallel them then - it's also far easier to do this with MOSFETS than BJTs.

 

[audioguru2]

Yeah, When you parallel Mosfets then the hottest one increases its resistance so it allows the cooler one to carry more of the current. ;D

 

[Alun]

Here's an idea to make the current limit programable.

U3 is a differential amplifier with a gain of R4/R3, this measures the volagate across

 

[cyw1984]

Here's an idea to make the current limit programable.

U3 is a differential amplifier with a gain of R4/R3, this measures the volagate across

 

[cyw1984]

Hi nearly old man Staigen,
You are absolutely correct! I was way off with my selection of high-value "turn-off" resistors for the output transistors in my "fixed" circuit. I have now chosen lower value resistors based on what is inside a TIP120 darlington.

I used a 1k resistor in series with the collector of the 2N3906 to limit its collector current and dissipation if the project's output is shorted. It will still provide enough drive to the output transistors to blow a fuse if the output is shorted.

I think the circuit can deliver 30V at 1.5A with a 34V supply to protect the 36V max-rated opamp.

It would be fun to try to stop this extremely high gain circuit from oscillating. ;D


Hi Alun and Cyw,
I don't think a single 2N3055 or power Mosfet can dissipate more than about 60W even on a pretty big heatsink. Therefore the max current is only 2A at low output voltages. That's why I proposed using 2 output transistors in our 0V-30V modified project for 3A output, and using 3 output transistors on our modified for 5A project.

Thank you for your help....

Can the cirucit deliver 30V at 2A??

On the other hand, I would like to use LM358 for testing, Is it OK??

any op-amp is prefectly on this cirucit??
As I know CA3140 is 4.5MHz, BiMOS Operational Amplifier with MOSFET

 

[Alun]

Which U1, U2, U3, diode and P-channel MOSEFT is prefect in this circuit???Thanks

The MC34071 will be fine for U1, but U2 needs to swing down to below 0.6V and U3 needs to swing as low as possible, I was going to suggest the the CA3130 but it's only rated to 16V, perhaps audioguru can think of a suitable op-amp.

 

[audioguru2]

Hi Alun,
It is difficult to select U3 for your circuit.
It needs inputs that operate near the positive supply, and an output to ground. Cmos opamps do this, but have limited supply voltage rating.
If its 10k input resistors were increased so that they have 1.8V or more voltage drop then an MC34071 opamp could be used if you can tolerate its output going only as low as about 0.1V. ;D

 

[cyw1984]

audioguru old man,

I have tied your circuit....It has some problem......
It use 15K resistor series as DAC input....
The Vref is not between 0-10V.voltage drop on the resistor

Also....the output is exactly the same of Vin
What is the cause???

I used the LM358 to test (Vin 36V and Gnd it)

 

[Staigen1]

Hi there again

Maybee the LM358 blow, its only good for 32 Volts, and you put 36 Volts on it!

Hey cyw1984, cant you read? The datasheet clearly says its max voltage is 32 Volts!

And Audioguru, why didn't you included some type of voltage regulation or something for the poor little IC? A zener and a resistor and a cap will do that!

//Staigen

 

[audioguru2]

Hi Staigen,
Yeah, the LM358 is blown-up. It is supposed to have a gain of 3.0.

I didn't regulate the supply for the opamp because the MC34071 that I recommended and most other modern opamps have a + power supply rejection ratio of about 82dB at the rectifier ripple frequency. So 1V of ripple on the supply will cause only 0.1mV of ripple at its output. ;D

 

[Staigen1]

Hi Audioguru

I didn't regulate the supply for the opamp because the MC34071 that I recommended

Ok, i missed that!

//Staigen

 

[cyw1984]

thanks again

I get a problem here...
If my Vin 30 or 32V..can the output achieve 30V???
how to change the circuit...because I want to common the VCC and GND to supply voltage....

And what values of filter is suitable???
thanks

 

[audioguru2]

If my Vin 30 or 32V..can the output achieve 30V?

Of course not. The max output voltage will be 3V to 4V lower than its supply voltage.

how to change the circuit...because I want to common the VCC and GND to supply voltage?

I don't know what you mean.

And what values of filter is suitable?

You'll have to try different values until its transient response isn't too slow, and that it doesn't oscillate.

 

[cyw1984]

Of course not. The max output voltage will be 3V to 4V lower than its supply voltage.


I don't know what you mean.


You'll have to try different values until its transient response isn't too slow, and that it doesn't oscillate.

I want to common the Opamp VCC and GND to the Vin and GND
how to do that- -sorry fot my poor english

 

[ante1]

Hi Guys,

How about the MAX420 it might work here! :

 

[audioguru2]

I want to common the Opamp VCC and GND to the Vin and GND. How to do that?

The MC34071 opamp (or MC34072 dual or MC34074 quad) that I recommend has a max supply voltage of 44V. Its inputs can work fine down to ground. Its output can swing almost to ground.
Just connect its supply pins to Vin and ground. ;D

Hi Ante,
The MAX420 is a chopper stabilized opamp that is very slow. I have never used a thingy like that. I am amazed that its offset voltage is only a few uV. I wouldn't use it here.

 

[Alun]

I get a problem here...
If my Vin 30 or 32V..can the output achieve 30V???
how to change the circuit...because I want to common the VCC and GND to supply voltage....

With your circuit?
No.

But with my MOSFET circuit, yes but is depends on the on resistance of the MOSFET and Rsence.
Vdropout  = current drawn * (Rsence + RMOSFET)
For example if you want 3A @ 30V and the MOSFET's on reistance is 0.4ohms and Rsence is 0.1ohms:
Vdropout = 3 * (0.1 + 0.4) = 1.5V
So the input voltage must be at least 31.5V for the output to be 30V when 3A is drawn.

Hi Alun,
It is difficult to select U3 for your circuit.
It needs inputs that operate near the positive supply, and an output to ground. Cmos opamps do this, but have limited supply voltage rating.
If its 10k input resistors were increased so that they have 1.8V or more voltage drop then an MC34071 opamp could be used if you can tolerate its output going only as low as about 0.1V. ;D

Then (I know it's a pain) but build a little negitive power supply for it, I don't know why I didn't think of this before. ;D