LTSpice- circuit design

Thread Starter

Burak123

Joined Aug 19, 2021
10
I just started learning circuit design. I can't claim to be good at this. There are some parts in the circuit that I need to change and I couldn't get over it. I need your help. I really want to learn how it's done.

75v dc should enter the vin (input of my integrated circuit). I had to calculate the required ac voltage and use it as a power supply(ı calculated 53v AC). I should also get 5v at the output of my circuit. IC model , LT3758. No matter what I do, everything seems totally wrong. I need your advice. :(

Wherever I want to display the voltage,I see wavy forms :(
 

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eetech00

Joined Jun 8, 2013
2,593
I just started learning circuit design. I can't claim to be good at this. There are some parts in the circuit that I need to change and I couldn't get over it. I need your help. I really want to learn how it's done.

75v dc should enter the vin (input of my integrated circuit). I had to calculate the required ac voltage and use it as a power supply(ı calculated 53v AC). I should also get 5v at the output of my circuit. IC model , LT3758. No matter what I do, everything seems totally wrong. I need your advice. :(

Wherever I want to display the voltage,I see wavy forms :(
A peak voltage should be specified in the sine voltage source for an AC output:

.param VRMS =53
.param Pk = VRMS*1.414

Then use this in the voltage source:

SINE(0 {Pk} 50)
 

Thread Starter

Burak123

Joined Aug 19, 2021
10
I just started learning circuit design. I can't claim to be good at this. There are some parts in the circuit that I need to change and I couldn't get over it. I need your help. I really want to learn how it's done.

75v dc should enter the vin (input of my integrated circuit). I had to calculate the required ac voltage and use it as a power supply(ı calculated 53v AC). I should also get 5v at the output of my circuit. IC model , LT3758. No matter what I do, everything seems totally wrong. I need your advice. :(

Wherever I want to display the voltage,I see wavy forms :(
Can you draw the circuit I'm looking for please? You need to use The Full Wave Bridge Rectifier, then we will add our capacitors and provide dc voltage input to our integrated circuit. In short, I need someone to help me by editing the circuit structure I've drawn without disturbing it.
 

eetech00

Joined Jun 8, 2013
2,593
Can you draw the circuit I'm looking for please? You need to use The Full Wave Bridge Rectifier, then we will add our capacitors and provide dc voltage input to our integrated circuit. In short, I need someone to help me by editing the circuit structure I've drawn without disturbing it.
See circled areas below

1629395024264.png
 

Thread Starter

Burak123

Joined Aug 19, 2021
10
Sir, first of all, thank you for this ss. I understand what you mean. But this is not my only problem. The rest of my circuit seems pretty much wrong from start to finish. And I'm having a hard time fixing it. Is it possible for you to check my circuit completely and correct my mistakes? For example, are the capacitors (c5,c7) values appropriate? Are the voltages on the pins of my LT3758 integrated circuit correct? You can measure and look at the voltage at many different points. For example, look at the LT3758's Vin pin. Even Vout in my circuit is wrong. When it asks for 5V, it turns out 5.4V. I am reading 2.2A while my output should be 2A. Unfortunately, I do not have the ability to read datasheets. You need to examine whether the technical values of the LT3758 and the circuit I built are suitable. I started to learn something for hobby purposes. I watched my brother, but not being able to do it makes me sad. And I just want to add LT3758 Vin voltage simulation... I don't understand why it looks like weird, I don't understand why it doesn't go steady at about 53V.
 

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crutschow

Joined Mar 14, 2008
27,893
Below is the simulation modified to give 75V at the input, and R2 and R3 selected to give 5V at the output (the FBX voltage is 1.6V when regulated).

1629398742004.png
 

Thread Starter

Burak123

Joined Aug 19, 2021
10
That's an ability you need to develop if you are going to design circuits.
Otherwise you are just spinning your wheels.
Thank you, sir.
Below is the simulation modified to give 75V at the input, and R2 and R3 selected to give 5V at the output (the FBX voltage is 1.6V when regulated).
I want to ask a question. When we passed 220v ac through the full wave rectification circuit, it was turning into 311v dc. (This was one of the basic concepts we were taught)

220v ac = 311v dc

Let's look at this circuit now. I want 75V dc voltage to be input to Vin pin of my integrated circuit. For this, how many volts ac voltage should our mains electricity be?

x Vac = 75vdc
x=53v ac???
x=75/√2=53

But when I connect 53v ac to the input, the value I measured after my c5 and c7 capacitors never shows 75v dc. It shows voltage close to 50 volts. What is the reason of this? Even though full wave rectification is installed in our circuit, is this an error or deficiency in LTSpice?

I have one more question, is there any mistake in choosing 330 microfarads for my c5 and c7 capacitors? Should I choose lower? (I'm from Europe, I think the electrical components vary according to the countries)

Please let me know if there is something I am wrong.
View attachment 246024
 

crutschow

Joined Mar 14, 2008
27,893
But when I connect 53v ac to the input, the value I measured after my c5 and c7 capacitors never shows 75v dc. It shows voltage close to 50 volts. What is the reason of this? Even though full wave rectification is installed in our circuit, is this an error or deficiency in LTSpice?
No.
As eetech00 noted in post #3, LTspice AC voltages are specified as peak, not RMS.

Thus if you want to use ACrms voltages in LTspice you enter them as shown below (for an example RMS voltage of 53V):
Note that the curly brackets are required.

1629433227018.png
 
Last edited:

LowQCab

Joined Nov 6, 2012
1,183
"" Unfortunately, I do not have the ability to read datasheets. ""

The Trick to learning ANY new subject is to define ALL of the terms associated with it.
You should be constantly looking-up new words,
especially Abbreviations and Acronyms.

I personally hate Abbreviations and Acronyms because I am involved in
so many diverse subjects, and every one of them has
a ton of proprietary Abbreviations and Acronyms,
it's like having to speak ~8-different Languages that all sound and look very similar.
But that's the way the World currently works.
.
.
.
 

Thread Starter

Burak123

Joined Aug 19, 2021
10
"" Unfortunately, I do not have the ability to read datasheets. ""

The Trick to learning ANY new subject is to define ALL of the terms associated with it.
You should be constantly looking-up new words,
especially Abbreviations and Acronyms.

I personally hate Abbreviations and Acronyms because I am involved in
so many diverse subjects, and every one of them has
a ton of proprietary Abbreviations and Acronyms,
it's like having to speak ~8-different Languages that all sound and look very similar.
But that's the way the World currently works.
.
.
.
Thank you for your recommendations.
My brother's profession taught him to make these circuit designs. I got curious and started researching the subject. I am not very strong in my electrical knowledge, but I really like these circuit designs.
 

Thread Starter

Burak123

Joined Aug 19, 2021
10
No.
As eetech00 noted in post #3, LTspice AC voltages are specified as peak, not RMS.

Thus if you want to use ACrms voltages in LTspice you enter them as shown below (for an example RMS voltage of 53V):
Note that the curly brackets are required.

View attachment 246054
Sir, I have two more questions. Are the values on my c5 and c7 capacitors large? How much should it be?
My second question is about the SHDN/UVLO pin, using a voltage divider, I got a voltage reading between about 6.4V-6.18V. Is this value sufficient? When I read the datasheet, I couldn't see any information about it.Ekran Resmi 2021-08-20 09.50.22.png
 

Thread Starter

Burak123

Joined Aug 19, 2021
10
hi 123,
Welcome to AAC.
This is what I see.

Post a photo-shot of what you see.
Sir, can you help me?
I have two more questions. Are the values on my c5 and c7 capacitors large? How much should it be?
My second question is about the SHDN/UVLO pin, using a voltage divider, I got a voltage reading between about 6.4V-6.18V. Is this value sufficient? When I read the datasheet, I couldn't see any information about it.
 

LowQCab

Joined Nov 6, 2012
1,183
There's plenty of info on the SHDN/UVLO-Pin, but for simulation You don't
need to figure out its hysteresis function. .............

"" For applications where the SHDN/UVLO pin is only used as a logic input,
the SHDN/UVLO pin can be connected directly to the input voltage VIN
through a 1k resistor for always-on operation. ""

As for the Power-Supply Bulk-Capacitors, they almost can't be too big.
You should do a separate simulation of your Power-Supply and change the
Load on it to see the effects of changing to larger Bulk-Capacitors.
I like adding a Choke into the mix, but that can be tricky sometimes.
Again, simulate just the Power Supply by its self,
it can't supply unlimited Current without serious Voltage-Sag and heavy-Ripple.

I like to specify the Ripple-Current rating of the Capacitors at 200+%,
you're not going to get anywhere close to 100% with just 660uf.
It's possible that in the real-World those small Capacitors would be
over-heated, and their life expectancy seriously shortened.

In addition to the Bulk-Capacitor(s), at least one of the configurations
shown in the Data-Sheet calls for
~4.7uf of Ceramic-Bypass-Caps on the Power-Input-Pin,
You can't leave this out, especially on any kind of Switching-Power-Supply.
The Bulk-Capacitors can not operate properly at super-high-Frequencies.
.
.
.
 

Thread Starter

Burak123

Joined Aug 19, 2021
10
There's plenty of info on the SHDN/UVLO-Pin, but for simulation You don't
need to figure out its hysteresis function. .............

"" For applications where the SHDN/UVLO pin is only used as a logic input,
the SHDN/UVLO pin can be connected directly to the input voltage VIN
through a 1k resistor for always-on operation. ""

As for the Power-Supply Bulk-Capacitors, they almost can't be too big.
You should do a separate simulation of your Power-Supply and change the
Load on it to see the effects of changing to larger Bulk-Capacitors.
I like adding a Choke into the mix, but that can be tricky sometimes.
Again, simulate just the Power Supply by its self,
it can't supply unlimited Current without serious Voltage-Sag and heavy-Ripple.

I like to specify the Ripple-Current rating of the Capacitors at 200+%,
you're not going to get anywhere close to 100% with just 660uf.
It's possible that in the real-World those small Capacitors would be
over-heated, and their life expectancy seriously shortened.

In addition to the Bulk-Capacitor(s), at least one of the configurations
shown in the Data-Sheet calls for
~4.7uf of Ceramic-Bypass-Caps on the Power-Input-Pin,
You can't leave this out, especially on any kind of Switching-Power-Supply.
The Bulk-Capacitors can not operate properly at super-high-Frequencies.
.
.
.
 

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Thread Starter

Burak123

Joined Aug 19, 2021
10
The Q1 MOSFET shown has a Vgs(ths) of 4V while the gate drive is just 7V, which is marginal.
Q1 should be a logic-level type [Vgs(ths)max ≤2V] so it is fully turned on by the 7V drive.

Otherwise it looks OK.
Finally can you check my snubber circuit? Are my values good? Is there anything I need to change?
 
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