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 Post subject: High Voltage Regulator Design Issue
PostPosted: Jul Fri 23, 2021 9:30 am 
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Greetings to the Forum:

I'm having a bit of trouble with a regulated high voltage power supply that I am building. Basically, it works well and does what it is supposed to do.... but it does not cope with transient loads.

I first energized the supply with a variac.... all well and good. It ramps up and regulates without difficulty. I was able to determine that the range of line voltages one was likely to encounter would neither create too much dissipation for the heat sink system to handle nor drop out of regulation. I was just about to declare it finished when I thought I'd better try it with transients just to be sure. I plugged it directly into the wall and pulled the plug several times. No problems. I then remembered that the load it was driving had 30 uF of capacitance across its input. I soldered a 33 uF / 450 volt cap across my test load and repeated the transient test. The first time, fine. The second time completely shorted the pass transistor and took out the regulator chip along with it.

Needless to say, I need to remedy this problem. At first, I thought it was simply the current peaking at more than the 1 ampere maximum Ic of the TIP50 pass transistor I have been using. I therefore began to look into higher current pass transistors. Unfortunately, the high current pass transistors have insufficient current gain; I settled on a TIP152 Darlington device. (The LR8N regulator that I am using can source only 10 mA or so.) However, the Vce rating for the TIP152 (400 volts) is less than that of the TIP 50 (450 volts). This is not an issue in normal service because the Vce is only about 60 volts. However, it MIGHT be an issue in start-up, because until the LR8N regulator begins to supply base drive to the pass transistor, its emitter will be at zero volts, while its collector could be as high as 430 volts or more due to an unloaded power supply plus ripple.

My question is: Any input as to the cause of the failure? If it was caused by an over-current condition, then I can (hopefully) correct it by going to the TIP152 which has a 10 amp Ic rating. If it was an over-voltage condition, then I need to think of something else.... perhaps a Zener across the pass transistor. Of course, that brings up the timing question of whether or not the Zener will avalanche and conduct before the junction blows up.

Perhaps the condition can be remedied by connecting a resistor across the pass transistor.

I am open to all suggestions as to the probable cause and an appropriate remedy.

The schematic of the system is supplied below in pdf form.

Attachment:
PS-500 Regulator Schematic end to end.pdf [103.02 KiB]
Downloaded 86 times


Thanks,

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 Post subject: Re: High Voltage Regulator Design Issue
PostPosted: Jul Fri 23, 2021 11:10 am 
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From what you have described it is unlikely an issue related to over current. Voltage transients are causing breakdown of your pass transistor. It requires a significantly higher voltage rated Darlington.

The ideal darlington is one designed for H deflection output stages with a very high breakdown voltage and high current rating, something like this will do it, also has an internal base-emitter protection diode:

https://www.mouser.com/datasheet/2/389/ ... 204660.pdf

In addition, I would get rid of the 1N4007 (there are many substandard versions of this on the market now). I would use a sinterglass BY448 or BY228 instead with superior ratings. But with the suggested transistor, it has an internal diode so you could omit it if you like for this one.


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 Post subject: Re: High Voltage Regulator Design Issue
PostPosted: Jul Fri 23, 2021 3:02 pm 
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Hi I think the problem may be an over-voltage between the base and emitter of the TIP50 causing it to break down, there is no protection for over voltage. a zener diode <5volts will control any transients. the max base to emitter is 5 volts .
Rich


Last edited by Rich berg on Jul Sat 24, 2021 4:24 pm, edited 1 time in total.

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 Post subject: Re: High Voltage Regulator Design Issue
PostPosted: Jul Fri 23, 2021 6:11 pm 
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This is a non-trivial problem. You might want to take advantage of the quantity discount when purchasing the components at risk of failure.

The linked circuit has no explicit output current limit. Adding a current limiter, preferably with foldback, may save your bacon.

Bipolar transistors are generally rated for pretty low currents at high voltage.
Attachment:
Screenshot from 2021-07-23 10-16-15.png
Screenshot from 2021-07-23 10-16-15.png [ 40.88 KiB | Viewed 1324 times ]

An IGBT or MOSFET might be a more rugged choice for the pass transistor.

If you need a low output impedance, the pass transistor drive circuit can be located upstream of the regulator IC.

It may be difficult to make the regulator both simple and reliable.

Ted


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 Post subject: Re: High Voltage Regulator Design Issue
PostPosted: Jul Fri 23, 2021 7:27 pm 
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Power MOS also has Safe Operating Area limitations.

http://www.irf.com/product-info/datashe ... irf450.pdf

On initially, the 33 uF cap looks like a dead short for some microseconds on start-up.

If you are not burning out 1N4007s, don't bother to change them. BY448 actually has a 40 Amp surge current. https://www.vishay.com/docs/88543/by448gp.pdf That's a little better than 1N4007 and its voltage rating is higher, but you aren't zapping diodes.

I guess if it were my issue, I would use some kind of 'soft start' on the base drive to slow the rise of current in the pass transistor. I see some designs have a small electrolytic across the base drive output of the LR8.

More discussion: https://antiqueradios.com/forums/viewto ... 9&t=385104

Rich


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 Post subject: Re: High Voltage Regulator Design Issue
PostPosted: Jul Fri 23, 2021 10:22 pm 
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Delete double post.


Last edited by ACORNVALVE on Jul Sat 24, 2021 5:44 am, edited 1 time in total.

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 Post subject: Re: High Voltage Regulator Design Issue
PostPosted: Jul Fri 23, 2021 10:30 pm 
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ACORNVALVE wrote:
Rich berg wrote:
Hi I think the problem may be an over-voltage between the base and emitter of the TIP50 causing it to break down, there is no protection for over voltage. a zener diode <5volts will control any transients. the max base to emitter is 5 volts .
Rich


Yes I agree with this too which is why I suggested the better device that contains its own B-E protection diode, and also it makes the circuit simpler as the 1N4007 can be omitted because the transistor also has a C-E diode.

So just sometimes, if the right part is chosen, you can get a circuit with less parts, that is more reliable, this is one of those cases.

One interesting thing is that when people see a circuit with BJT's, playing up or not, there are often remarks about using mosfets instead. There is no advantage at all using a mosfet over a Darlington in this circuit. In high range voltage circuits where you need a follower operating in a non saturated state, the BJT is superior, especially the type of transistor I recommended.Most mosfets would require a G-S protection zener unless it is built in. Mosfets come into their own in lower voltage higher current circuits where you require a very low Rds on resistance. Mosfets have significantly higher input capacitance and this degrades the transient response unless very low source impedance gate drivers are used. Ideally though, in a regulator with an emitter follower pass transistor, the output should be sampled from the emitter, not the base drive where the 2200 resistor currently connects, this way, the output is more stable with varying loads. If the load is not variable it hardly matters.

I cannot see a definite requirement for this design to have a soft start.

In the interests of RF suppression I would place a 47R resistor in series with the base, and possibly a 0.01uF cap from collector to base if there were troubles in this area. You might think that a circuit like this, the output device would be intrinsically stable at high frequencies, being an emitter follower with a voltage gain of just under 1, however, with parasitic inductance in the collector, just with wiring, depending on the transistor, I have seen this burst into RF oscillations in some supplies when a high frequency capable Darlington is used.

Both the TIP 50 BJT and TIP152 Darlington have insufficient voltage ratings for the task, there is not nearly enough headroom to allow for transients. I would suggest to Jim.T : get the Darlington I suggested (or one with similar ratings), repeat the experiments and report the results.


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 Post subject: Re: High Voltage Regulator Design Issue
PostPosted: Jul Sat 24, 2021 3:52 am 
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Greetings to the Forum:

Thank You to all who took the time to respond. I liked ACORNVALVE's approach, so I went looking for a BU808DFI device as suggested. Unfortunately, the device is no longer manufactured and neither Digikey nor Mouser has any stock. There is an article on the web about how to replace one using a conventional high voltage bipolar transistor and building the rest of the Darlington network as a "hangy" on the outside of the replacement transistor. This is for the original application; repairing vintage TV sets. The article also warns that the real devices are very hard to find and that the counterfeits found on eBay and such places should not be trusted.

Accordingly, I went back to Digikey and looked through their search engine, with current gain and Vce above 600 volts as requirements. I found the KSC5603DTU which might do the job, although the current gain is marginal, as it is not a Darlington. (There seem to be no Darlingtons in current production with the required breakdown voltage).

Here is the data sheet:

Attachment:
ksc5603d datasheet.pdf [313.93 KiB]
Downloaded 30 times


I will order some of these and try them. The LR8N regulator would be looking at the load directly should the Zener fire, but it has internal current limiting, so presumably that will protect it.

I will work up a revised schematic and post it as soon as I can. I would appreciate comments on the proposed changes. For Rich: There is already a 1 uF electrolytic on the output of the LR8N regulator... I'm not sure whether a bigger cap would help without also introducing some R into the circuit. I hope the Emitter-Base Zener will do the job.

Thanks Again to all who responded.

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 Post subject: Re: High Voltage Regulator Design Issue
PostPosted: Jul Sat 24, 2021 4:41 am 
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Greetings to the Forum:

Please find attached the revised end-to-end schematic of the regulator circuit. Comments welcome and appreciated. I think I will go ahead and order the parts as Digikey has only seven of the transistors in stock. They aren't that much money and hopefully this will do the trick. If not, I will order some parts as suggested.

Attachment:
PS-500 Regulator Schematic end to end V2.pdf [105.48 KiB]
Downloaded 64 times


Edit: Wrong designation for Vebo in drawing; corrected.

Thanks,

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 Post subject: Re: High Voltage Regulator Design Issue
PostPosted: Jul Sat 24, 2021 5:07 am 
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Greetings to Ted and the Forum:

Ted (Usually Lurking) wrote:
You might want to take advantage of the quantity discount when purchasing the components at risk of failure.


I will certainly do this; I have already ordered five of the transistors and five more of the LR8N regulator chips. However, I do need to fix this issue. It will be OK to try different things here on the bench, but the modification is for the power supply of a Hallicrafters Cyclone transceiver that I wish to sell, so when it leaves here, it has to be bullet-proof.

Thanks,

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 Post subject: Re: High Voltage Regulator Design Issue
PostPosted: Jul Sat 24, 2021 6:10 am 
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The BU808 Darlinton is all over ebay. However, most are in China, Europe (Germany ) and the UK. Few in the USA for some reason.

Another option if you get stuck, it easy to get single BJT's with these voltage ratings and you could configure that with a driver transistor to make the Darlington.


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 Post subject: Re: High Voltage Regulator Design Issue
PostPosted: Jul Sat 24, 2021 6:29 am 
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ACORNVALVE wrote:
The BU808 Darlinton is all over ebay. However, most are in China, Europe (Germany ) and the UK. Few in the USA for some reason.

Another option if you get stuck, it easy to get single BJT's with these voltage ratings and you could configure that with a driver transistor to make the Darlington.


The site I perused about TV repair for sets using the BU808 said that there were a large number of counterfeits out there and not to trust them. They even gave two methods for differentiating between counterfeits and genuine parts. The emitter-base ohm meter reading for the genuine article is around 300 ohms give or take about 30 ohms. The reading for the counterfeit part is only about 40 ohms. Also, the counterfeit parts have the flat stamped leads, while the genuine article has round wire leads. Given that I won't know what I have until it is in my hand, I decided not to risk that. I have been burned before by counterfeit parts.

I will try the high voltage NPN transistor that I mentioned. Its current gain should be just barely adequate depending on how much current the LR8N can source. The minimum output current for the LR8N to pass QC is 10 mA which will not be enough. The maximum current is 20 mA, which with a current gain of 15 in the pass transistor should get me the 200 mA that I need.... with not much head room. I am trying to avoid upping the parts count as the circuit will be built on a PC board hanging off the rear of the heat sink. If I make the board much larger, it will obscure the speaker connection in the power supply which has to be undone in order to get the supply out of its case. The board is pretty crowded now.

If I can't get the gain I need, I will have to build the Darlington circuit.... but that means I have to find another high voltage transistor... preferably with a TO-92 size or similar. Just in case, I better start looking.

Thanks,

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 Post subject: Re: High Voltage Regulator Design Issue
PostPosted: Jul Sat 24, 2021 10:39 am 
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Greetings to the Forum:

I have completed massaging my board layout to squeeze in another transistor and associated components. I haven't placed the resistors yet, but they should be easy to do.

Here is the Darlington schematic as implemented in the BU808 transsistor that is NLA.

Attachment:
Darlington Schematic.jpg
Darlington Schematic.jpg [ 40.52 KiB | Viewed 1270 times ]


Two problems: One is that I cannot find any small transistors that would serve as the input device for the Darlington. It looks as though I will have to go with another TO-220 transistor. I hope I can get by without any heat sink for it, as mounting another transistor on the existing heat sink would be an incredible pain.

I am open to suggestions as to what device I should use. If I try to duplicate the circuit of the BU808 exactly, then I will need to know the values of R1 and R2.... and I don't have the slightest idea of how to calculate them. Also, I will need to know what sort of diode is used across R1. I looked up SC08790 on the web and that number is either a FET or a Samsung vacuum cleaner! :D

So, any suggestions on how to create this Darlington would be helpful. I hope I don't have to do it, but if I do, then some engineering support would be very much appreciated.

Thanks,

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 Post subject: Re: High Voltage Regulator Design Issue
PostPosted: Jul Sat 24, 2021 12:01 pm 
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If you you were not against the idea of a TO-3 case Darlington transistor, you could use a MJ10024 or MJ10025.

They have these a Newark:

https://www.newark.com/solid-state/mj10 ... transistor

The data sheet :

https://www.farnell.com/datasheets/669581.pdf

They have the two internal resistors and two internal diodes, and are less than $10.

Unlikely to be fakes from Newark. And it would be "very difficult" to destroy this Darlington transistor in your application.

I agree with you about the risks of fakes with the BU808. Especially the ones coming out of the far east.

( Personally, I have a love affair with the TO-3 case, because it is thermally superior to epoxy cased devices and doesn't suffer from the delamination modes of failure seen at high temps with epoxy case parts. When the TO-3 became obsolete I was very disappointed. If you look at the thermal resistance specs of the BU808, its 2.4 Deg/Watt junction to case, for the MJ TO-3 part its 0.7 DegC/W. Also the max junction temp of power TO-3's reaches 200 Deg C, usually only 150 to 175 Deg C for epoxy cased parts, not as good)

PS: Since you require 200mA out of your supply and the LR8 regulator current limits at 20mA out (typ), a darlington is a better option than a single BJT, for the primary reason that a transistor's current gain is an uncontrolled parameter, likely it would be >10 for many specimens of a power BJT, especially with modern parts, but it is best not to assume that, and use the darlington, where you can assume no worse case MJ part will have a beta of <50, typically it is 100 or more. Worse case could be an LR8 with a 10mA current limit, combined with a single BJT with a beta <20 not good. But if you combine that LR8 with a worse case Beta = 50, MJ10025 darlington, your regulator could still support up to 500mA, and definitely 200mA.

Perhaps one other thing, make sure to put the transistor on a substantial heat sink. Although the current is low, high voltage supplies pose and equal problem with power dissipation, especially as the output voltage gets dialed down. I'm not sure how low you might adjust the output voltage down. (400-270) x 0.2A current, the power would be 26W, so ideally to keep the transistor's case temp respectable you need a decent sized heat sink, you can get these with the TO_3 holes already in them, if not easy to drill using a TO-3 mica washer as a template. If I were making your regulator I'd use a heatsink with 3 degree C per watt or less. This way you can be sure that the case temp will be well within an acceptable margin and keep you transistor thermally safe, with a mica washer and liberal thermal compound on both sides. (I have gone off sil pads as the material is too compressible and the coupling force drops over time).

Obviously if the output voltage will never be dialed down and you had a lower max voltage drop across the transistor, you can use a more compact heat sink.

Good heatsink from Newark (0.85 deg C/W), to place a TO-3 transistor on, and cheap too < $10:

https://www.newark.com/abl-heatsinks/34 ... p/07WX4343

With this sink you could dial the supply output down to 200V and 0.2A load (about 40W dissipation if the input was 400V) and it would still be ok.

If you did go down this road with the TO-3, mica washer and heat sink, your power supply will have a somewhat retro look to it, but it will be "tough as old boots" and practically unbreakable.

Mica washers, with thermal grease, have about 0.35 DegC/W thermal resistance. The transistor junction to case for MJ transistor is 0.7 Deg/W and the Newark heat sink 0.85 deg C watt, meaning the total temp rise of the transistor junction above ambient is 1.9/ Dec watt, so in this condition with 40W dissipation the junction will reach about 76 deg C above ambient. Assuming ambient of 30 deg C, it is inside the power de-rating graph on the data sheet. Also you don't want to exceed the 200V drop and 0.2A current of the safe operating area graph, and its just fractionally over the edge of that. So at 0.2A load at least, ideally the voltage would not be dialed down such that the voltage across the transistor's C-E exceeded about 190V.


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 Post subject: Re: High Voltage Regulator Design Issue
PostPosted: Jul Sun 25, 2021 2:30 am 
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Greetings to ACORNVALVE and the Forum:

I pretty much reached the same conclusion that you did with regards to the current gain issue. I do not want to have to go the "part selected in test" routine; I would like it to just work.

To answer a couple of your questions, the designed output of the supply is fixed (with an adjustment capability) at 270 volts. I have carefully selected the input capacitor to the Pi network filter in the power supply to deliver around 330 volts under the 200 mA load. The maximum voltage across the pass transistor in normal service (steady state) is less than 100 volts. (By the way, the load is variable; it is around 170 mA on receive and around 200 mA on transmit.) The calculated dissipation is around 9 to 11 watts depending. Before I blew up the device, I ran extended trials for temperature. The heat sink got up to about 120 degrees F and the device seemed to be at about 130 degrees F on its tab. An infrared thermometer gun captured some temps at around 200 degrees F at the package surface, but I am not sure how reliable those temps are.

The problem arises from the heat sink I am using. I have limited real-estate on the power supply chassis, so I ordered a heat sink to fit. It is about 2" high and 6" long. It consists of a flat plate with fins. It does have mounting holes in it to allow it to be mounted by one edge. There are no other provisions on the thing for mounting any device. I found that the hardware necessary for mounting a TO-220 transistor would not fit between the fins, so I had to mill away a space in the finned area big enough to accommodate the washer stack and nut for the TO-220 mounting kit. The catch is, I don't have a milling machine. I ordered some milling cutters and a cross-slide drill press vise and managed to clear the spot I needed for the TO-220 hardware. I didn't get the depth quite right, so the aluminum base in that area is .01" to .015" thinner than it should be.

One picture is worth a thousand words, so I have included two photos below:

Attachment:
PS-500 Top View.JPG
PS-500 Top View.JPG [ 812.87 KiB | Viewed 1219 times ]


Attachment:
PS-500 Heat Sink Detail.JPG
PS-500 Heat Sink Detail.JPG [ 735.98 KiB | Viewed 1219 times ]


This technique is very crude and I do not believe that my machining skills and tools are up to fabricating a TO-3 mounting area.

The information on how to fabricate a BU808 from other devices is given here:

https://obsoletetellyemuseum.blogspot.c ... -scan.html

I have started modifying my circuit board design to accommodate the Darlington circuitry. I have values for the resistors thanks to the above web site, but I do not have wattage values as yet. I will take a look at the circuit with my voltage numbers and see if I can figure that out. I do not have any idea of what to use for an input transistor (I am calling it Q1). I suppose I could use a pair of the KSC5603 devices, but Digikey had only 7 in stock and I ordered 5 of them. I don't want to run into another case of unobtanium devices.

Any way, I calculated the dissipation of the first transistor and it is about 2.5 watts.... which means that I will have to bite the bullet, dismount the heat sink and try to mill another spot for a second TO-220. I was hoping to avoid that, but I suppose it would be better to have them at the same temperature anyway.

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 Post subject: Re: High Voltage Regulator Design Issue
PostPosted: Jul Sun 25, 2021 3:23 am 
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Your plan should work .

With that sort of heatsink, rather than using a nut etc, is better just to drill an undersized hole, in the place between the fins and tap it with a coarse thread like 4-40 UNC, and screw the transistor down, you won't have to worry about any milling and you can use the same insulators.

You can do the same thing for a TO-3 mounting too, and align the holes for the B & E pins between the fins and its just a matter of running the B-E wires around to the finned side of the heat sink. It still works if you have to tap into the base of the fins, or you can roughly mill those off if they get in the path of the screws.

But I think you should buy the MJ10025 from Newark, it is simpler, less components & wiring, more rugged, you will have a lower junction temperature than with the TO-220, probably your heat sink is only barely adequate from the temps you reported, but good idea to check the transistor's case's temp at full load. Do you know the degree C/Watt rating for your heatsink ?


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 Post subject: Re: High Voltage Regulator Design Issue
PostPosted: Jul Sun 25, 2021 5:08 am 
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Hi All. I do not have a way to put up a diagram but you could use a transistor tha is connected between base and emitter of pass transistor and a resistor to the ic regulator. The base of the transistor should go to the output. The emitter of the output transistor should have a small value wire wound resistor to develop a drop that corresponds to the current you wish to limit to. The idea is the collector of this transistor would pull the pass transistor base towards its emitter cutting it off. A hi gain lo power transistor would work here since its drop is only base emitter of the pass device. For example on that resistor at the emitter of the pass transistor it only has to drop .7 volt so lets say you want .5 amp limit then a 2 ohm resistor will drop enough to bring the protection transistor out of cut off and limit the output. Hope this is a goo set of guide lines for you. This idea has been used for years on power supplies from low voltage on up. Found this online.


Attachments:
regulator-voltage-emitter-follower-with-foldback-current-limit-01.png
regulator-voltage-emitter-follower-with-foldback-current-limit-01.png [ 5.24 KiB | Viewed 1205 times ]
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 Post subject: Re: High Voltage Regulator Design Issue
PostPosted: Jul Sun 25, 2021 6:24 am 
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Greetings to Audioman and the Forum:

I think I am pretty well locked into something pretty close to the design I have. I simply don't have the board real-estate to add a third device. I am squeezing a circuit board onto the end of the heat sink shown in the photos above. I can't grow the board much more or there will be interference with the case and other parts of the power supply. In addition, it is difficult to find transistors for these kinds of voltages that are still in production. Virtually all of them are TO162 or TO-220... which take up a lot of board room. The existing board size (see below) is pretty stuffed with the components surrounding two transistors and the regulator chip. I don't think I can go for three.

Attachment:
PS-500 Regulator V4 Darlington.jpg
PS-500 Regulator V4 Darlington.jpg [ 164.05 KiB | Viewed 1201 times ]



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 Post subject: Re: High Voltage Regulator Design Issue
PostPosted: Jul Sun 25, 2021 7:45 am 
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Jthorusen wrote:
... If I can't get the gain I need, I will have to build the Darlington circuit.... but that means I have to find another high voltage transistor... preferably with a TO-92 size or similar. Just in case, I better start looking.

Thanks,

How about this from Diodes Inc.: https://www.diodes.com/part/view/APT13003NZ?BackID=9771
John

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 Post subject: Re: High Voltage Regulator Design Issue
PostPosted: Jul Sun 25, 2021 3:57 pm 
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Rich berg wrote:
Hi I think the problem may be an over-voltage between the base and emitter of the TIP50 causing it to break down, there is no protection for over voltage. a zener diode <5volts will control any transients. the max base to emitter is 5 volts .


Rich nailed it. A zener diode across B & E provides some over-voltage protection during start-up with large output capacitor. However there is no output short-circuit protection as designed. When an output short circuit occurs, the high-voltage charge from the 1 uF capacitor at the base and LR8 output will destroy both the pass transistor and zener diode. The remedy is to eliminate the 1uF capacitor at the base and the reverse protection diode, across the LR8, which is no longer necessary. This elimination allows the short-circuit and thermal protection of the LR8 to work. The run-of-the-milk 1N4007 diode accross E & C is fine. You don't need a fancy diode.

The TIP50 transistor is fine. You don't really need a Darlington power transistor. With a minimum current gain of 30, it should be sufficient to provide 200mA of continuous output current. The max. current required from the LR8 is 200/30 = 6.67 mA (less than 10mA).

See this article by our late Leigh Bassett.
https://antiquewireless.org/wp-content/uploads/37-a_solid-state_filter_choke_or_field_coil_replacement.pdf
Attachment:
HV_Reg_modified.jpg
HV_Reg_modified.jpg [ 44.98 KiB | Viewed 1181 times ]

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