I have a low power 1400 volt power supply and I'd like to add a bleeder, and have an LED light up if there's voltage present on the caps.
I was thinking about a series stack of three LND150 depletion fets and the LED, with source resistors to set the currents to about 150 uA or so. That would keep the LED brightness constant down to 10 volts maybe, and bleed linearly.
At least one of the fets will avalanche, maybe two. I think you said that this is OK.
I could also use a single 2SK4177 as the current sink, but I'd have to get some gate voltage from somewhere.
In general MOSFETs are perfectly happy in avalanche; in fact most power parts have avalanche ratings. You just have to stay within the power dissipation limit. (If you analyze a datasheet "avalanche rating" you'll see it's simply derived from junction-temp limit and thermal mass from the transient thermal impedance plots, using the inductor value, voltage and currents in question, so there's nothing special about avalanche.)
150uA at say 550 or 600 volts breakdown is 90 mW. That's fine for the wimpy LND150. Note, avalanche voltage can go up another 10% if the junction heats towards 125 or 150 deg C.
That part has a sensible power rating. If you want it's easy to use MOSFETs in series, while avoiding avalanche, with a stack of resistors. See AoE III, Figure 9.110 on page 696. The inset shows how you use one LKND150 to set the current, with additional series MOSFETs, even ordinary enhancement-mode types. The resistor stack divides the voltage for the gates. You'll need five resistors, if you limit them to 300V.
You could run the LND150 at say 300V, the 2SK4117 can have the remaining 1100V, a relaxed value for both of them.
If I let two of them avalanche, they will hog the power dissipation, but that's OK.
Again, the problem with using an enhancement fet is that it needs gate voltage from somewhere guaranteed to be available always. Deriving that from +1400 volts is a nuisance. A battery would be silly.
Here's my HV regulator. It has almost zero quiescent dissipation and fast programmable rise/fall times, about 1KV/ms up or down.
Gawd-awful schematic color choices :-( ...Jim Thompson
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| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: skypeanalog | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
It's a dual coupler, used elsewhere on the board, so why not? If the CTR is too high, I can always change the 100 ohm drive resistor. But I want a bunch of drive current.
The load is 22 nF, which I hope is enough C to bypass the output pulser stage. The customer wants to be able to change the voltage fast, up or down. The average DC load current is tiny.
I was testing some opto-fets last week for this - there was nothing in the datasheet. But an application note says they must not be subjected to transient voltages outside their rating.
Customers might connect them to a wire with inductance, or a relay coil perhaps.
I started with 100uH but they seemed to be quite happy switching a couple of Henrys at 0.5A all day. (It was supposed to be a test to destruction but I felt sorry for them so stopped there!).
I will be using a TVS anyway now but was curious how much there is to worry about, really.
How could you master the current through the opto ACPL's when the TLP191 dr ives the MOSFET VGS ? How could master the power dissipation on MOSFET and opto ACPL ?
Don't understand how it works ... may be some math would be more explicit ?
My questions was in relation with the current through the MOSFET and power dissipations (hey 1400 V !) ; TLP191 gives 7V --> the MOSFET flows current
--> Vgs becomes smaller when current is increasing through 100 Ohm resistor --> then vgs becomes smaller ... There it may have needs some math or Spic e simulation isn't it ?
This is likely intentional, because it effectively creates a current limit, which is desirable. Of course the FETs have to be able to handle this. Ultimately the 1.5kV supply will sag and shut down. The current limit would act until the HV byass capacitors are drained.
I Spice-modeled the TLP191B... it's on the Device Models & Subcircuits page of my website.
The schematic pin-out doesn't match the datasheet I have. ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: skypeanalog | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
If there's no photo current in either input loop, the 2SK4177's are OFF... though I'm not sure how well-behaved they are with an essentially floating source.
Needs some kind of shut-down means to ensure no photo-current. ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: skypeanalog | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
The 100 ohm resistors and the opto CTR limit the mosfet currents. That's not so much to protect the fets as to limit power supply dip.
One 2SK4177 can dissipate about a kilowatt for one millisecond, and around 3KW for 100 us. It could easily discharge all the energy that I have available.
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