SCR I(h) question - Page 1
Forum section: electronics / projects

author: kenw232
author_rank: Contributor
timestamp: April 04, 2014, 11:29:30 pm
content: Hi, I have an SCR (VS-40TPS12APBF-ND) that will be working with. The Ih value from the datasheet is 150mA. If I wanted the SCR to switch and turn on a current that will only be 75mA max, is that possible or not then? So the Ih value is 150mA to hold it on when I turn it on via the gate, it won't last and turn right off because 75mA is too low from what I would guess. If so can I force the SCR to continue conducting 75mA by keeping the voltage on the gate? So the applied gate voltage will override the failure to reach Ih then correct?

author: w2aew
author_rank: Super Contributor
timestamp: April 05, 2014, 12:28:25 am
content: Quote from: kenw232 on April 04, 2014, 11:29:30 pmThe holding current is often spec'd as a max value to guarantee staying on. So below this value it *might* stay on, or might turn off when the gate drive is removed.

author: Marco
author_rank: Super Contributor
timestamp: April 05, 2014, 12:37:55 am
content: Why would you want to use a thyristor as a really really bad BJT?A MOSFET or IGBT with the same voltage rating for the current you want to switch costs significantly less.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 05, 2014, 01:25:47 am
content: Quote from: Marco on April 05, 2014, 12:37:55 am^^ At currents below I_h, an SCR behaves just as a crappy (slow, particularly nonlinear) BJT.Tim

author: kenw232
author_rank: Contributor
timestamp: April 05, 2014, 01:27:40 am
content: Quote from: Marco on April 05, 2014, 12:37:55 amI actually am using a MOSFET, but my load required voltage isolation when the mosfet is in the off state - it can't have any voltage at all applied to the + side of the load when off. So I want to do this:But this requires I float the gate up to 1400V which is difficult. They don't make P-Channel mosfets above 600V so I'm out of luck there too. So I thought use an SCR to block or isolate the load and switch the SCR and mosfet on/off at the same time.Any thoughts?

author: kenw232
author_rank: Contributor
timestamp: April 05, 2014, 01:30:36 am
content: Quote from: T3sl4co1l on April 05, 2014, 01:25:47 amSo there no way to force it open and thereby continue to conduct at a maximum by maintaining the voltage against the Gate?

author: Marco
author_rank: Super Contributor
timestamp: April 05, 2014, 02:05:31 am
content: Quote from: kenw232 on April 05, 2014, 01:27:40 amPut the MOSFET above the load, put a pull up resistor on the gate and pull it down with a second MOSFET? (The switch will turn on if the control circuitry isn't powered but the high voltage supply is.)

author: oldway
author_rank: Super Contributor
timestamp: April 05, 2014, 07:24:22 am
content: Yours schematics are almost wrong and will never work.Use a bipolar high voltage transistor as 2sc4913.I hope you know that you are risking your life: 1400V 75mA islethal.To see your diagram, it seems to me that you do not have a level of knowledge in electronics and electricity to work with such high voltages ...Be very careful, ask for help from someone qualified

author: kenw232
author_rank: Contributor
timestamp: April 05, 2014, 09:15:11 pm
content: I'm just trying to turn 1400V on and off. It could not be more difficult.

author: dannyf
author_rank: Super Contributor
timestamp: April 05, 2014, 09:43:50 pm
content: QuoteThat's because you made it so.It will help you greatly and potentially save you life and those around you if you just slow down a little bit and start learning basics on electronics, etc. before attempting to design something so lethal.

author: madires
author_rank: Super Contributor
timestamp: April 05, 2014, 10:21:38 pm
content: As mentioned in the other thread, please search for "high voltage high side switching". Either find a driver which is able to float at 1400V or buy a proven solution from a vendor likehttp://www.behlke.com.

author: David Hess
author_rank: Super Contributor
timestamp: April 06, 2014, 02:16:48 am
content: If switching speed is not an issue, they make optically isolated MOSFET drivers which simply consist of an LED on one side and a photovoltaic pile on the other side which will generate the voltage necessary to fully turn on a MOSFET. The IRF application note includes an example which exactly matches what you are trying to do.http://www.irf.com/product/_/N~1njcib#tab-tab1http://www.irf.com/product-info/datasheets/data/pvin.pdfhttp://www.irf.com/technical-info/appnotes/an-1017.pdfhttp://www.toshiba-components.com/couplers/photocoupler_photovoltaic.html

author: kenw232
author_rank: Contributor
timestamp: April 06, 2014, 02:41:18 am
content: Quote from: David Hess on April 06, 2014, 02:16:48 amThank you for this, it looks interesting. I've read a lot about high side switches, I just find it difficult to manage a second 1400V rail to float the gate.These look like they would be great too but have been discontinued.http://datasheet.eeworld.com.cn/pdf/120667_INTERSIL_MCT3D65P100F2.html

author: oldway
author_rank: Super Contributor
timestamp: April 06, 2014, 02:47:51 am
content: We don't even know if the 0V of the 1400Vdc power supply is grounded or floating.kenw232 think it's easy to interrupt 1400Vdc, but it's not.A commun switch is not enough, and may not interrupt such a voltage (arcing between contacts).Imagine what could happen:"The switch is off, that's secure, there is no voltage."but it is still with 1400Vdc, he touch something and he is dead

author: David Hess
author_rank: Super Contributor
timestamp: April 06, 2014, 03:49:55 am
content: Quote from: kenw232 on April 06, 2014, 02:41:18 amCommon alternatives include high voltage direct level shifting, inductive coupling, and capacitive coupling. That last one is probably what I would do since it in inherently failsafe (If the input signal sticks then the MOSFET would turn off.) and does not require a custom transformer. An AC signal is used across a capacitive barrier to charge the MOSFET gate. DC restorers in CRT circuits work this way and handle thousands of volts with no problems.QuoteI remember them distantly but did not know they had ever made it into production. I suspect insulated gate bipolar transistors took their wind.

author: kenw232
author_rank: Contributor
timestamp: April 06, 2014, 04:23:34 am
content: Quote from: David Hess on April 06, 2014, 02:16:48 amSo these Photovoltaic Isolators seem to be self adjusting. They drive the gate automatically by dynamically floating the gate relative to the drain voltage applied in real time I think. I mean in the quick schematic below how does the PVI know what the + rail voltage is to float the gate up to? Must be smarter then the average IC.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 06, 2014, 06:07:16 am
content: Voltages in series add.Cool, an MCT datasheet: I always thought they were unobtainium 4+ inch hockey puck devices used in power distribution only -- the only context I've seen them discussed in. I guess they really don't exist after all?Tim

author: oldway
author_rank: Super Contributor
timestamp: April 06, 2014, 07:22:10 am
content: The secure solution is to feed the load by a separated 1400Vdc power supply (it's only 105W) and to switch the low voltage (230Vac or 110Vac or 24Vdc, ...and so on) feeding this power supply.

author: dannyf
author_rank: Super Contributor
timestamp: April 06, 2014, 12:01:39 pm
content: QuoteHow fast can it charge up / down a hefty power mosfet? and what would be the power dissipation in that process?

author: David Hess
author_rank: Super Contributor
timestamp: April 06, 2014, 12:39:06 pm
content: Quote from: kenw232 on April 06, 2014, 04:23:34 amIt is not that tricky. Only the gate to source voltage of the MOSFET matters for turning it on. When the PVI is off, the gate to source voltage is zero, the MOSFET is off, the source it pulled to ground via the load, and there is 1400 volts from the drain to the source. When the PVI is on, the gate is pulled above the source by 5 to 10 volts. As the MOSFET turns on, the voltage from drain to source drops and the source voltage rises above ground but as it does so because the PVI output is floating, it brings the gate voltage along with it. The same thing would occur with any "suspended" gate drive scheme.The PVI output however is low current so the MOSFET turn-on and turn-off times will be slow on the order of 10s to 100s of milliseconds which may be a virtue. For fast transition times, some other scheme is needed which can provide higher charge and discharge currents to the gate.There actually *is* a way to do this with P-channel MOSFETs. A cascode could be used with multiple P-channel MOSFETs in series but it would still require a high voltage pull-down level shifter although *that* part could be a ground referred N-channel MOSFET.

author: mrflibble
author_rank: Super Contributor
timestamp: April 06, 2014, 12:57:31 pm
content: Quote from: oldway on April 06, 2014, 07:22:10 amThat sounds far too reasonable and safe. Why do it the sensible way when you can live life on the edge instead?https://www.eevblog.com/forum/projects/mosfet-as-a-high-side-switch/msg417640/#msg417640