• Input control voltage: 3V to 30V DC
• Output load voltage: 3V to 30V DC
• Load current: Up to 3A (inductive load like solenoid or small motor)
• No separate power supply for the control/driver circuit (self-powered from input side)
• Should be use 4 screw terminal only, 2 for input & 2 for output.
• Compact and budget-friendly design
• No heatsink allowed, so low heat generation is essential
• OPTO isolated
• Any transistor is suitable

 

What I get from that requirement is that the load current must be coming from the four-terminal relay.

THAT makes no sense at all.
For a switching device that controls an externally powered circuit, "CRYDOM" and other companies sell such a product.

 

What I understood is that only the input side doesn’t require a separate supply.
Is my understanding correct?

 

Both side doesn't require a separate supply....

 

Here is my circuit, and it is working fine. I have a doubt regarding how to avoid using a heatsink. I’ve planned to use the BD911 transistor. If I want to eliminate the heatsink, how should I choose an alternative component, or can I use the PCB pad as a heatsink? I can only use D²PAK or TO-220 package components."

 

You can't dissipate 90W without a heatsink. The PCB will not be large enough to act as a heatsink.

90W will likely need a cooling fan as well.

Your constraints feel arbitrary—like a homework assignment..

 

Here is a link to 50 to 100V 5 to10A solid state relays. digikey
Screw terminals increase the cost greatly.
7A 100V screw terminals. 7A 100V

Should be use 4 screw terminal only,

There are some low-cost versions that could be mounted on a PCB with screw terminals.

If you are going to drive inductive loads you might want to put a 50V overvoltage device across the output.

Click on the price and sort by price (at digikey) There are many good choices for under $5.00. I have used most of those.

 

Here is my circuit, and it is working fine. I have a doubt regarding how to avoid using a heatsink. I’ve planned to use the BD911 transistor. If I want to eliminate the heatsink, how should I choose an alternative component, or can I use the PCB pad as a heatsink? I can only use D²PAK or TO-220 package components."

View attachment 351592

The circuit you show might be ok for your use. But its not a SSR.
A SSR will conduct voltage/current thru its normally closed "contacts" even without power to the "coil".

 

Here is my circuit, and it is working fine.

Where is the output load and power supply connected?

 

hi @Vignesh vsg

Is this a College assignment project.?

Moderation.

 

Both side doesn't require a separate supply....

Right, that will work.

Please draw a typical application circuit for your device, because what you say is impossible.

 

You can't dissipate 90W without a heatsink. The PCB will not be large enough to act as a heatsink.

90W will likely need a cooling fan as well.

Your constraints feel arbitrary—like a homework assignment..

If I'm right , I have to dissipate only ~ 1.8 watt (0.7 V x 3 A).

 

Here is a link to 50 to 100V 5 to10A solid state relays. digikey
Screw terminals increase the cost greatly.
7A 100V screw terminals. 7A 100V

View attachment 351593
There are some low-cost versions that could be mounted on a PCB with screw terminals.

If you are going to drive inductive loads you might want to put a 50V overvoltage device across the output.

Click on the price and sort by price (at digikey) There are many good choices for under $5.00. I have used most of those.

Sorry , Actually I'm trying to design that type of SSR

 

hi @Vignesh vsg

Is this a College assignment project.?

Moderation.

 

Sorry , Actually I'm trying to design that type of SSR

Then you need an opto coupler that has a photovoltaic output, to control a MOSFET gate without any additional output power supply.

 

https://analogcircuitdesign.com/solid-state-relay/

 

Actually I'm trying to design that type of SSR

Don't reinvent the wheel.

Because you are switching DC, remove the bottom MOSFET.
LED makes light. A photovoltaic solar cell takes light and makes voltage.
The voltage turns on the MOSFET.

Here is an example: