SSR for DC – notes

Getting Started

I have been thinking about high-current DC solid-state-relays (SSR) for a while for solar systems.  There don’t seem to be any available that suit and are reasonably priced.  I’m thinking 24V DC nominal, 50A continuous, on-board heat-sinking, thermal protection, standard off-the-shelf parts and electrical isolation between control and load.

Mechanical relays are expensive and tricky for high current DC and use a lot of power even when not carrying much load.  But the electrical isolation makes them easy to use.  Frequent operation damages the contacts resulting in faults and relay replacement.

So I have recently spent some time working on a high-current DC solid state relay design – including some bread-board experimenting.  A quick trip to Jaycar and I had a few heatsink and terminal samples.  A key requirement is to use only readily available off-the-shelf components.  With power electronics all sorts of problems compound, current, construction, heat-sinking, enclosure, parts availability, cost etc.  Mechanical design becomes a big part of the effort.

AC vs DC

AC SSRs typically use triacs which are not as efficient as power-mosfets.  Efficiency (power loss) with higher voltage AC is not usually a major concern and heat-sinking is the installers problem.  For solar systems and some DC applications better efficiency is desirable.  Power-mosfets are more efficient but depending on the layout can be polarity sensitive.  It is possible to make a DC bi-directional SSR, but it is less efficient.

DC SSR design comment

Electrical isolation between the control signal and load makes system design much easier and forgiving in the case of a wiring glitch.  Driving power-mosfets and providing gate power is more complex than in triac based AC SSRs.  Power to drive the mosfet gate must be transferred or provided on the load side.  There are some special purpose drivers available, but they are a bit expensive and almost certain to be difficult to get when needed.

One power-mosfet can switch a DC load or supply.  If electrically isolated and self powered it can switch the high-side or low-side.  Lower cost units may typically only be suitable for low-side switching.  Using two power-mosfets allows control of bi-directional current flow, as with a mechanical relay.  But this introduces and additional device and power loss.  Parallel connecting multiple power-mosfets can improve efficiency reducing power loss.  This also improves reliability and reduces required heat-sinking.

For long term reliability, temperature control is essential.  Heatsinking can be big and passive or smaller and fan assisted.  Small and passive is no good for more than a few watts.  Big and passive means large aluminium metalwork and more time spent in construction; generally quite expensive.  By using smaller on-board heatsinks and a small temperature controlled fan, the unit is smaller, easier to construct and cheaper.  The fan should only run when needed and the unit switches off if too hot, which may occur if the fan fails.

To stay in line with several other modules and to keep the cost down a simple circuit board module will do.  This can be mounted into an enclosure or control panel using stand-offs and provides screw terminals for all connections.

Design Specs – so far

A module-board of about 75x50mm x 40mm high.  On-board heat-sinking with controlled fan.  Micro-controller.  Four power-mosfets.  Can be wired for uni-directional higher efficiency or bi-directional load current.  Low current control signal.  Requires a 12-24V DC supply on the control side for the fan and micro-controller.