varistor
From this Reference Design, I have the following inquiry:
Usually in three phase applications I see varistors connected between phase to phase, or between phase to ground. In the reference design, the common point of the varistors RV1…RV4 is not connected anywhere. I would expect it to be ground, but they intentionally left it floating. How are varistors protecting in this case?
If the circuit is already isolated then connect the varistors to the local electronic ground. The varistors should be rated 50% higher voltage than the highest signal voltage. If grounding to earth was needed for your AC power, that would normally be done at your circuit-breaker panel.
Here is an attempt at a dynamic solution (everything below is some mock-up, tinkered values):
V1 is the reference voltage. E1 serves as a feedback error amplifier that ultimately acts as a source for the load, too. At its negative input, is what would be the variation of the temperature as a function of the load current: Vx is the current sensor, F1 takes this, attenuates it by some value (here 100x), and then feeds it to a simple lowpass RC (can add a ladder afterwards for finer control), whose values make up the thermal constant you're interested in.
Note that I used Rpar=20 and ic=25. These two values represent the temperature at "rest" (I(F1)*Rpar) and the initial temperature (ic). This is what comes up:
If you look at the separate, red V(temp), it starts at a value and decreases, that's because the effect of the imposed initial condition is higher than the "rest" voltage, and, since the load sees little current drawn, the temperature slowly decreases to the equilibrum. The black voltage, V(Vin), starts a bit lower due to the initial warm conditions, but then rises towards the value given by reference-rest.
Then the current ramps up, temperature goes up, and here's the not-so-nice part: the plateau should exponentially rise as a 1-exp(-x), more than it's in the simulation, but not ad infinitum, thats why an RC ladder would have been nicer since you can add series RC, too.
Then the current drops, the temperature slowly decreases, to reach equilibrum. All this time, the voltage drops or rises according to the temperature variation.
Now, I realize this is probably less than a bare-bones show, but it's a start to show how to use the temperature in a feedback loop, seeing as you already have a VCCS in there, which can be put to good use. If you use primitives such as V+F for current sensor, and VCCS or VCVS plus RC elements, your simulation will fly, so the inclusion of such a circuit will barely hinder the rest of the schematic.
Best Answer
Varistors, in this case MOV's, are installed based on modes of protection. 10 mode protection for 3-phase devices covers phase to neutral (3), phase to ground (3), Phase to phase (3), and neutral to ground (1). This is for stand-alone surge suppressors which can get expensive if parallel MOV's are used, so devices with built-in MOV's are not likely to protect all possible modes.
Ac input is 3-phase plus neutral for the power supply shown.
The AC input voltage is very high, so 460 VAC MOV's are used but not directly phase to phase. They share a common node which has a 460 VAC MOV to neutral. The MOV's are in series so their soft clamp voltage is 460 * 2 * 1.5, or about 1,380 VAC. This is plenty of safe headroom yet will clamp on input voltage spikes.
Neutral is the lowest voltage connection, and normally neutral is Earth grounded at the service entrance panel. The MOV's clamp to neutral (in series) so any AC input over 1,380 volts is at least soft-clamped. This supply uses 3 phases in case 1 or 2 phases fail. The isolated output is only 12 VDC at 125 mA, so 1 working phase will keep the power supply working.
Notice the isolated grounds, and that there is NO Earth ground on the AC side. The very low voltage on neutral (<= 10 VAC) makes it a virtual ground. The ground symbol refers to a common ground for all parts on the AC side of the power supply. This will have 1/2 the DC supply voltage on it as a negative value (compared to Earth ground), so touching any part of the AC supply while power is on could give a bad shock.
NOTE: The voltage on neutral is dynamic but ranges from effectively zero VAC to normally not over 10 VAC. Some surge protection devices will issue a alarm if neutral is greater then 30 VAC above Earth ground. Neutral is never to be high enough to be a shock hazard. In some cases double neutral wires help lower the neutral voltage at point of use.