DC-Dc SSR vs High side Mosfet

Thread Starter

shortbus

Joined Sep 30, 2009
7,695
Can a DC-DC SSR be used in place of a a high side mosfet switch in a circuit? Which one is capable of switching faster, SSR or high side mosfet? I know if the parts would need to be bought the mosfet route would be more cost effective, but I already have the SSR, from another project. The load will be a capacitor bank instead of an inductive or resistive load.

The SSR is like this one - http://www.ebay.com/itm/Solid-State-Relay-SSR-DC-DC-40A-3-32VDC-5-220VDC/250793127324?_trksid=p2060778.c100276.m3476&_trkparms=aid=222007&algo=SIC.MBE&ao=1&asc=20140725133649&meid=198d0f4ee707439c8b5f940fbc09e389&pid=100276&rk=1&rkt=4&sd=170961637557
 

AnalogKid

Joined Aug 1, 2013
8,465
Yes. Many DC-output SSR's use a power MOSFET for the switching element. If the input is fully isolated from the output, then it should work for you..

ak
 

tracecom

Joined Apr 16, 2010
3,944
Here is the datasheet for FOTEK SSRs; the DC-DC units are a couple of pages in. Note that the switching speed is listed as 1ms. I would guess that an SSR would be slower than a directly gated MOSFET due to the delay introduced by the optocoupler in the SSR.
 

David Knight

Joined Aug 4, 2015
26
1ms switching speed is really slow for a dc-dc converter. a high-side MOSFET can switch in a few nanoseconds with a good gate driver.

An SSR used in a dc-dc converter would have to operate at very low frequency, which would require large inductors and capacitors. It seems like this would be an expensive and bulky solution.

Note that when charging a capacitor bank, it's important to use current feedback to limit the current to a level that your power source, wires, and circuitry can handle. Without a current limiting mechanism, expect to let out the magic smoke.
 

Sensacell

Joined Jun 19, 2012
2,567
SSR's are not design optimized for speed, they are more than 10 X slower than a typical switching FET.

They are optimized for simple floating drive, high voltage and current.
 

Roderick Young

Joined Feb 22, 2015
408
I agree with the others. Quoting the specs in the listing, "On-off Time: ≤ 10mS". When the units quoted are mS, that's a sure sign that the switching time is not in microseconds. For a switching converter, you will generally want an turn off time of just a few microseconds at most, and more likely, a fraction of a microsecond.

The good news is that for the price of that solid state relay, you can buy a regular mosfet, and drive it with a pulse transformer, or capacitively coupled, or by some other conventional means, and still have a few coins left over.
 

Thread Starter

shortbus

Joined Sep 30, 2009
7,695
From all the stuff on the web, the old style DC-DC SSR were speed limited, to ~200Hz. The newer ones are/seem to be able to do up to 20kHz for motor PWM drives.
 

Thread Starter

shortbus

Joined Sep 30, 2009
7,695
You correct it's not for a DC-DC converter. It's for my ongoing electrical discharge machine. Was just thinking out loud. Thought if they, the SSR, would work, it would eliminate - 2 mosfets, a few resistors, a diode 2 caps, and a gate driver IC needed for a high side switch. Don't know how the DC-DC converter came into this?
 

Thread Starter

shortbus

Joined Sep 30, 2009
7,695
After spending time reading about DC-DC SSR's, it seems the one that fits my requirements is only good to 950Hz when used with PWM. thanks for all the responses.
 
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