• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

replacing an ssr with a mosfet

Status
Not open for further replies.

Marvelous Tshuma

New Member
hi

May anyone assist with the technicalities of replacing an ssr with an equivalent mosfet. I had designed a board switching a 12V, 9A load using solid state relays . The assembly is quite expensive and thus have resorted to using logic level mosfets without much very little or no compromise to the perfomance of the circuit. The ssr im replacing is the cmx60d10 and my choice of mosfet is PSMN4R3-30BL. In both cases the input switching voltage is 3.3v from a micro-controller.

What are the things I need to look out for in this replacement exercise?
 

Attachments

MrAl

Well-Known Member
Most Helpful Member
Hi,

A solid state relay passes current in both directions but can also switch current in either direction. That circuit will ALWAYS pass current in one direction because of the internal diode and so it will only turn off in one direction. Note the missing diode in your schematic that would show this internal diode.

I dont remember the circuit to do this though, but i think a search on the web should show something. It would be two mosfets in series i think along with a drive circuit.
 

MrAl

Well-Known Member
Most Helpful Member
Hi again,

Oh ok that is different, but then that is not really an SSR either.
 

Marvelous Tshuma

New Member
Hi

The schematic I uploaded is a replacement for the SSR. That there is a logic level n-channel mosfet. I did not include the driver circuit because the mosfet can be driven by the 3.3V. Any suggestions on the improvement ? Do I need a driver for isolation purposes? Is it necessary to have a driver in that schematic?
 

MrAl

Well-Known Member
Most Helpful Member
Hi

The schematic I uploaded is a replacement for the SSR. That there is a logic level n-channel mosfet. I did not include the driver circuit because the mosfet can be driven by the 3.3V. Any suggestions on the improvement ? Do I need a driver for isolation purposes? Is it necessary to have a driver in that schematic?
Hi,

It really depends on what you are using it for. Maybe you can state that too.

Also, the 10k will turn the mosfet off slowly if the output pin of the microcontroller goes to high impedance to shut off. If it goes low instead, that should be ok although 120 may be a little low for a microcontroller pin.
 

Marvelous Tshuma

New Member
Hi,



Also, the 10k will turn the mosfet off slowly if the output pin of the microcontroller goes to high impedance to shut off. If it goes low instead, that should be ok although 120 may be a little low for a microcontroller pin.
The 10k resistor is a pull-down resistor to prevent the gate from floating during logic 0 phase and or when the microcontroller pin is disconnected accidentally.
 

MrAl

Well-Known Member
Most Helpful Member
Hi,

That sounds good then as long as the drive current from the uC and through the 120 ohm resistor switches the mosfet fast enough for your application. If it has to switch with higher frequency then that may not be enough, but for a once in a while switch it's ok. The power loss during the switching transients becomes the main issue. If the mosfet heats up too much it burns out.
 

Marvelous Tshuma

New Member
Hi,

It switches on once after every 45 seconds for 15 seconds. Do you think heating up may be a problem? Since it's SMT, I thought maybe I can include thermal vias with bottom side cooling on my pcb to help curb the heat problem. Your thoughts on that?
 

MrAl

Well-Known Member
Most Helpful Member
Hi,

Yeah that does not sound bad at all. Since it does not switch much it probably wont heat up. Heating usually becomes a problem with higher frequencies. The ultimate proof is in the testing. If it does get hot, add more drive current.

Typical gate resistors are around 10 ohms but that's for switching frequencies like 10kHz, 20kHz, etc. So i doubt this will heat up with such infrequent switching.
 
Last edited:

tomizett

Active Member
Personally, I think I'd add Schottkey clamp diodes from the microprocessor pin (ie, the junction of R9 and R10) to the Vcc and Gnd of the micro. These will prevent the micro pin from being driven outside its supplies by charge coupled through the drain-gate capacitance of the FET if there should be a sudden change of voltage on the drain. The high-side diode will also prevent the micro from seeing 12V if the FETs gate oxide should break down or become shorted for any other reason (like a liquid spillage) - with a diode in place, you should just burn out R9 and not kill the micro.

Probably overkill, but worth considdering - might save you some grief.
 

Marvelous Tshuma

New Member
Hi,

Yeah that does not sound bad at all. Since it does not switch much it probably wont heat up. Heating usually becomes a problem with higher frequencies. The ultimate proof is in the testing. If it does get hot, add more drive current.

Typical gate resistors are around 10 ohms but that's for switching frequencies like 10kHz, 20kHz, etc. So i doubt this will heat up with such infrequency switching.
Thank you so much for your insight and assistance. Will keep you posted during and after the testing!!!once again, thank you so much
 

MrAl

Well-Known Member
Most Helpful Member
Personally, I think I'd add Schottkey clamp diodes from the microprocessor pin (ie, the junction of R9 and R10) to the Vcc and Gnd of the micro. These will prevent the micro pin from being driven outside its supplies by charge coupled through the drain-gate capacitance of the FET if there should be a sudden change of voltage on the drain. The high-side diode will also prevent the micro from seeing 12V if the FETs gate oxide should break down or become shorted for any other reason (like a liquid spillage) - with a diode in place, you should just burn out R9 and not kill the micro.

Probably overkill, but worth considdering - might save you some grief.
Hi Tom,

Yes interesting idea.
The uC chips have built in clamp diodes but depending on the make and model the diodes are rated for more or less current. The two i happen to know of are the PIC chips and the Atmel chips. Most of the PIC chips i have worked with in the past have a 20ma rating while the Atmel chips had only about 1ma rating. Good quality external Schottkey diodes would add to the protection, although to ensure they can take over the role of the internal diodes completely i think i would use two external resistors with that. One resistor between the gate and the two externals, and one resistor between the two externals and the uC pin. That way the external diodes get the most of the current if there is a problem during some circuit condition.
 

MrAl

Well-Known Member
Most Helpful Member
Thank you so much for your insight and assistance. Will keep you posted during and after the testing!!!once again, thank you so much
Hi again,

You're welcome, and good luck with your project.

Also, a little typo, "infrequency", ha ha, meant to type "infrequent" of course. Corrected original post.
 

tomizett

Active Member
One resistor between the gate and the two externals, and one resistor between the two externals and the uC pin
Agree with that comment - that would be the most robust approach.

Because I do repair for a living, I always think in terms of what will happen when something inevitably goes wrong. Anything that protects a microcontroller (which is generally programmed and therefore can't be replaced) makes life a lot easier for people like me!
 
Status
Not open for further replies.

Latest threads

EE World Online Articles

Loading
Top