Improve Speed of photovoltaic based Solid State Relay

[Electro nS]

I am in the process of creating a High power Mosfet based SSR. The idea is using a photovoltaic MOSFET driver such as VO1263 (SIlabs have similar IC but it uses CMOS coupling to create the isolated power). However these type of ICs are pretty slow since they provide about 10uA of drive current at approximately 10v.

During my research on the subject I have found only one circuit without any explanation that should serve to increase the switching speed (without using external DC/DC converter). Shown in the figure below:



I am not quite sure that I understand the circuit completely and I would like your help in sizing some of the components. off-course this circuit is supposed to work in high side configuration that means Vcc (1-60v DC) is supplied at pin4 and load at pin3.

My logic:

1. D4,R6 (around 1k ?) work as charging path for the bootstrap diode C1. T1,T2 (normal BJT such as BC537- BC527)are normal push-pull driver they work at turn on Only to provide high current to charge the mosfet gate (Sourced by C1).
2. Z1 and R7(around 100 ohm ?) are used to protect the gate ( however I think they must be reversed order or there should be zener diode across C1 to keep voltage < Vgs max.
3. R4 and R5 are pull downs (they should in 1MegaOhm range)?? however they cause power loss as R4 is voltage divider with the VO1263 output impedance. (in simulation VO1263 is modeled as voltage source with Series Impedance of 500k) .
4. D3 and D5 work to keep the mosfet ON after initially being turned on by bootstrap C1 (which will lose its charge if the relay is kept on for sometime).
5. The PV mosfet driver is not providing enough current for BJTs , So I tried using Darlington instead . The circuit worked after using 2 diodes instead of D5 and 2 diodes instead of D3 (4 diodes in total) donot fully understand why ?

Is my analysis correct ? any suggestion on the resistor value range ? any comments on the circuit is welcomed since I have not found any white paper , appnote or a development board that does something similar to get me started.

 

[dick_freebird]

I would look at separating the power isolation
from the signal isolation. The low current of the
small "solar cell" is the limitation (incorporating
also, the limits of LED power and overall
efficiency).

If (say) you left DC power to the LED on all the
time but used it to charge a capacitor bank,
and used some separate opto path or "split"
the opto path by LC, CR filters such that a
modulation could be superimposed on the
base light, and added a MOSFET gate driver
driven by the detector for said modulation,
you would get fast turnon / turnoff. Of course
rep rate will still be hostage to basic current-
throw in the opto power path, and you'll pay
for all the "off" interval with LED current that
after the cap bank is charged, contributes
nothing but heat.

 

[Easy peasy]

The ckt uses power from the collector ckt in the off state to feed to the gate - thus "bootstrapping" the volts from the photo-electric cell.

another way is to wait until the photo-electric cell has charged up a 47nF cap to 12V say, and then apply this cap to the gate-source quickly for a fast turn on - thus the first turn on would be delayed by the time necessary to charge the 47nF cap.

Turning off quickly then needs a few more bits to avoid discharging the 47nF cap when the power from the photo-cell "PC" goes away - in this way you can "top up" the 47nF cap when the ON command is there, but not discharge it very much at all when the power from the PC goes away - thus a good hysteretic ckt needed and a diode or two,

happy designing ...

--- Updated Feb 27, 2021 ---

p.s. in the ckt shown C1 needs a zener across it if the D-S off volts are higher than about 10V else the xtors and the PC will be blown up - as will C1.

 

[KlausST]

Hi,

I can think if a solution with two photovoltaic mosfet drivers.
* One just used as "power supply",
* the other for switching.

Klaus

 

[Easy peasy]

it would be simpler to have a 500kHz isolating gate drive Tx, 500kHz at +/- 15V square wave for ON ( rectified and applied ) - normal pnp turn off when turn on power removed - this has been done successfully many times - likely cheaper too ...

 

[Electro nS]

it would be simpler to have a 500kHz isolating gate drive Tx, 500kHz at +/- 15V square wave for ON ( rectified and applied ) - normal pnp turn off when turn on power removed - this has been done successfully many times - likely cheaper too ...

by 500khz gate driver , you mean using a timer to generate 500khz and a pulse transformer to isolate and move the power to mosfet ?