Computing stuff tied to the physical world

Digital opto switching

In Hardware on Oct 12, 2012 at 00:01

While on the subject of optocouplers, there’s another type besides “analog” ones and “digital” ones (which include a comparator), and that’s the opto-relay. Again with several kilovolts of isolation.

The Avago ASSR-1611 is an interesting one, for example, because it uses MOSFETs:

Screen Shot 2012 10 07 at 14 11 47

Basically, it lets you switch up to 60V at a few amps. To see how it performs, while I still have that linear ramp circuit up and running, I hooked it up – as a big mess-o-wires:

DSC 4173

It’s getting pretty crowded in there. The ASSR-1611 is on the left. Here’s its schematic:

Screen Shot 2012 10 07 at 14 07 07

The interesting bit is that there are diodes in there, so it can deal with alternating current when hooked up differently, i.e. by using pins 4 and 6 instead of 5 and 6.

First thing to notice, is that this thing behaves in a strange way when switched at 1000 Hz:

SCR43

It “sort of” triggers … slowly (keep in mind that turning on translates to shorting the output to ground, as before). And then it decides to turn off again very quickly. For some reason this repeats about 10 times per second.

To slow the triangle wave rate way down, I used a 10 µF capacitor instead of 0.1 µF:

SCR44

Aha! Much better. At about 0.7 mA (purple voltage over 1 kΩ = 0.7V), this solid state relay switches on, and once the current drops back to almost 0, it switches off. Note how these readings match the specs nicely: turning on, i.e. the blue line dropping to 0, takes a few ms, whereas turning off is virtually instant.

At a few Euro each, these chips are not really cheaper than mechanical relays, but when you only need to switch a few dozen Volt at a few Amps, then this solution still has the benefit that it switches far more cleanly – with no arcing or mechanical wear. And it’s totally quiet, of course…

  1. It also needs less power than an electromechanical relay and no free running diode on the input side.

    BTW, those diodes between source and drain are not put in there intentionally. They are parasitic and unavoidable, and they are the reason why two MOS-FETs in anti-serial configuration are needed for switching AC.

    If there was a way to avoid those diodes, AC could be switched with a single MOS-FET.

  2. What about going deeper into this particular SSR? It’s not triac-based, but uses this complimentary MOSFET pair. What are properties of such pair? Is it really good at passing AC? Switching? Are there MOSFET-based SSRs for mains AC voltage? Reasonably priced? If not, is it possible to make one from discrete components? Finally, is it possible to achieve sub-microamp leakage current with such setup? Sorry for so many questions ;-)

    • It is not a complementary pair in the usual sense (one N and one P semiconductor with similar parameters), but two identical FETs connected in series with reverse polarity.

    • Yup, I meant complementary in that “loose” sense – each MOSFET complements each other to pass positive or negative half-wave of AC.

    • Actually, they complement each other in blocking the half waves that the parasitic diode of the other MOS-FET would let pass through, even when it is off. When the transistors are on, both half waves pass through both transistors (between pins 4 and 6).

  3. Very neat little “chip”.

    My brain still has great difficulty looking at a little package like that and coming to terms with it being able to handle 5 amps!

    Honestly, I’m not that old that the low internal resistance of a MOSFET is a new concept to me!

    BTW, whilst you’re on the subject of opto-isolation, there’s one I’ve played with quite a lot. The MOC3020. An opto-triac. I use them in my camera flash triggers. They don’t handle huge currents, but boy can they handle high voltages.

    http://www.flickr.com/photos/33952917@N08/4566541603

  4. The relaxation oscillation in the first ‘scope picture takes some explaining. Recalling the basic MOSFET characteristic of tiny gate leakage but considerable capacitance, the blue trace shows a pumping effect. Not enough charge is injected into the gate to reach the threshold Vp until ~6th cycle – then the MOSFET comes slowly into conduction like a decreasing variable resistor. Note the flat “pauses” where the gate charge is constant, hence constant Rds, waiting for the next injection.

    After ~20 cycles of the drive waveform, the MOSFET is firmly ‘ON’ and still receiving charge injections – so why the switch off ~80mS later?

    The mechanism is in that cryptic box on the schematic “Turn-off Circuit”. There is a tricky problem to solve. Easy to control the charge injection by the photon stream, but how to take the charge back out again? Without some active technique this is equivalent to a “sticky” switch. Loading the gate with a simple resistive discharge path to source is not effective – it trades sensitivity for turn-off time.

    Circuits to do this well are often proprietary, only an opaque box or approximate equivalant schematic gets published. One general form though is shown by Vishay.

    Here is the rough logic – sense that drive current is still present by sniffing across the 2MΩ resistor, holding the PMOS JFET ‘OFF’. When the current drops below some threshold, the JFET turns ‘ON’, draining the gate charge and opening the main output path. The JFET drain-source potential collapses, turning it off ready for another cycle.

    And there is the key – the true turn off circuit has some delay built in to avoid premature triggering from noise (hence the spec sheet value of 0.1ms turnoff time – the scope trace shows the power MOSFET itself is turning off much faster than that). The trigger control circuit is confused by the fast sawtooth drive and takes many cycles to decide to turn off. Finally the gate charge is drained and it’s back to a new cycle…

    …. phew, all from trying to drive way faster than the chip designer’s intent

  5. Thank you MartinJ!

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