Difference between mosfet and SSR

Different animals altogether.
I guess you understand the MOSFET.
An SSR is a "Solid State Relay", an alternative to a mechanical relay where conditions allow. It has no gate connection and very little if any linear region, it is an on or off device that is optically coupled so it provides the same kind of isolation that a mechanical relay would offer.

Brian.

Hello, I am use to think of a MOSFET as it opens when Vgs>Vt .

Click to expand...

"Normally" one thinks of open as off, high impedance. There
are two basic types of MOSFETS, the most common enhancement
mode where as Vgs increases it turns on and its R drain to source
connection drops, to a value know as Rdson. "Enhancement" basically
means enhancing the current density in the MOSFET channel D to S.

Then there is depletion mode MOSFETs that do the opposite. As Vgs
increases they raise their Rds until they are effectively off.
"Depletion" basically means reducing the current density in the MOSFET
channel D to S.




Regards, Dana.

The SSR (if a MOSFET type at its core) surrounds the FET with isolation and control / power path to drive it neatly. Freeing the "contacts" path galvanically from the "control" domain.

Note in POST #3 one has to take into account Vgs polarity and affects on channel conductivity.
As you can see here.

Comparison table



Depletion NMOS



Enhancement NMOS




Regards, Dana.

The specific device linked in post #1 (Vishay VO14642A) uses a photovoltaic opto coupler to drive two MOSFET in anti-serial circuit.

As the opto coupler input is an IR LED, it can be characterized by a current threshold.

Most SSR's are triac based for switching AC mains

Hello , I have succseeded in using SSR to charge and discharge DC-DC output.
I needed when pulse is 0 to discharge the output and when pulse is 3.3 to chatge it to -2.4 as shown below.
Three questions:
1. how do i know the proper functionality of this SSR? how much current it needs or "gate" voltage?
In the attached table I see the caracteristics but there are LED forward switch on and LED forward only .
There 4 states and i dont know which apply to my case how do i open or close is like in a case of mosfet.


2. The charging time is much mush higher then what i have with mosfet , is there a way to speed it up?

3. what if i needed to discharge the output to 0V at pulse 3.3V (opposite behavior) how it could be done ?
Thanks.





update:
The bigger problem is that i have 40A spike.
What could be done?
Thanks.

Worst case you need 2 mA to turn on LED, and worst case the max diode V
(at 10 mA) is 1.6V, so that allows you to determine the limiting drive R to
limit max allowed current into diode. Note you have to also look at LED
Pdiss to make sure you dont over drive it.

Your spike, discharging the 1 uF to ground. How would you alter size of
spike, and what tradeoffs do you have to make ? Hint resistor.....hint slow
SSR turn on.....hint lower C......and the pros and cons of each approach.

Esoteric but put cap in H bridge as a load, that way it can be disconnected,
discharged, polarity inverted.........just a thought. But increases effective
ESR.

Speedup SSR, ignoring impact of load type answer is no as long as input
Trise is not excessively slow.


Regards, Dana.