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 Post subject: Re: High Voltage Regulator Design Issue
PostPosted: Jul Fri 30, 2021 8:11 am 
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Try re-doing the modelling with a darlington output transistor with a 220R resistor, between the regulator output and the base of the transistor. If there is a capacitor directly on the regulator's output, keep it to the minimum value.


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 Post subject: Re: High Voltage Regulator Design Issue
PostPosted: Aug Mon 02, 2021 2:41 am 
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Not sure if this would work here, but here's what I did to regulate 120Vdc to 105Vdc with the load being under 100mA.

I took a standard three terminal voltage regulator and put zeners in series with the ground terminal and ground. The zener voltage plus regulator voltage equaled the 105Vdc I needed. Think I had used a 7812 which meant I needed about a 93Vdc drop for the zeners.

Seems the issue is the 33uF cap.

That capacitor isn't needed with a voltage regulator. A 1uF capacitor would do, however I do understand you wanting to keep it original so it can be used with other power supplies which means keeping the 33uF cap.

EDIT:

In your case what I proposed wouldn't work given the higher voltage difference between the unregulated input and regulated output.

Now if you could get the voltage to be less than 40 volts difference between the unregulated input and regulated output it would work.

Also you might try a choke between the rectifier and first filter cap as that would reduce the unregulated DC voltage provided there is room on the chassis for a choke and the supply isn't already configured that way.

Once tried to go solid state regulation for my Collins R-390 as I wanted to get away from the hard to find pass tubes. Tried a couple designs and never got any to survive so I ditched the idea and went with an older Lambda external regulated supply set to 180Vdc.


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 Post subject: Re: High Voltage Regulator Design Issue
PostPosted: Aug Mon 02, 2021 5:27 am 
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Greetings to Tube Radio and the Forum:

Although what you propose would be viable in many circumstances, the prior constraints on this design are the cause of my difficulties. The 33uF capacitor under discussion is not in the power supply, but in the radio it is designed to power. Unless I render that radio unfit to use with any standard Hallicrafters power supply, then I must allow for the 33 uF cap in the design.

Likewise, a choke would solve many of the difficulties... and the power supply originally had two of them. Unfortunately, I had to eliminate one of them in order to obtain space to mount the heat sink for the pass transistor.

...And so it goes.... the fact that I am working inside a fifty year old design with both electrical and mechanical constraints is what is making the project difficult.

I have a working model of the supply that seems to function properly with circuit simulator software. However, I have not yet figured out how to provide current limiting for it. Because of this, I have not posted the schematic in its present form.

I had another idea this evening and I will pursue that and see where it takes me. Hopefully, I will be able to post a working schematic soon.

Thanks again to all who responded. Please keep those cards and letters coming! :)

Regards,

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 Post subject: Re: High Voltage Regulator Design Issue
PostPosted: Aug Mon 02, 2021 8:37 am 
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Jim,

One thing you could consider, if you don't have any luck making the current design reliable, is to dump the regulator chip you are using and go for a simple 2 transistor design, where the output is taken from the collector of a PNP, the reason this is better for an HV supply is that there are no issues with the collector to base junction getting zenered, unlike the case with the emitter follower where the E-B junction is problematic.

With this design (once quite popular), it needs a method to start it, to attain some initial base current for the NPN, there are various methods using a start capacitor, but the easiest way is to just shunt 20% or so of the load current in a pass resistor Rb. Also in this circuit, for a high voltage version it pays to use 5W Zeners (1N53xx family) and split them into 3 or 4 or 5 series devices.

The design is intrinsically short circuit protected, because if that happens, the NPN device is depleted of base current.


Attachments:
Reg2.jpg
Reg2.jpg [ 51.77 KiB | Viewed 752 times ]
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 Post subject: Re: High Voltage Regulator Design Issue
PostPosted: Aug Mon 02, 2021 8:52 am 
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Greetings to ACORNVALVE and the Forum:

Great minds think alike. I just this evening decided to dump the LR8N and do it the old fashioned way. With 2% zeners, I will be close enough without adjustment. The new circuit also lends itself to foldback current limiting which is included.

The circuit below has been simulated with the correct loads and with a 1 ohm short on its output. I just squeak by under the dissipation limit for the pass transistor (Q4) which is rated at 100 watts. That's for the short circuit condition. For normal operation, it is dissipating around 10 watts. Now, I just have to order the zeners and build it.

Attachment:
Regulator All Discrete Darlington V6.JPG
Regulator All Discrete Darlington V6.JPG [ 231.8 KiB | Viewed 752 times ]


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 Post subject: Re: High Voltage Regulator Design Issue
PostPosted: Aug Mon 02, 2021 11:35 am 
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Given how the one choke is connected you might be able to eliminate the 4.7uF cap and 60 ohm resistor and the B+ won't be quite as high and may make it better for the regulator circuit as it won't have to drop as much voltage unless the higher B+ was accounted for in the design and it wouldn't work with a lower B+ or the B+ would be too low without the 4.7uF cap.

The rectifier section has me quite confused.


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 Post subject: Re: High Voltage Regulator Design Issue
PostPosted: Aug Mon 02, 2021 1:25 pm 
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Greetings to Tube Radio and the Forum:

The input filter capacitor was chosen to allow sufficient headroom for the regulator to work and not cause excessive B+ under no load conditions. The 60 ohm resistor does not really exist.... it is the winding resistance of the choke.

The remainder of the circuit is a standard approach for ham radio transceivers. High B+ for the PA stage is around 800 volts which is developed from a bridge rectifier circuit. The transformer center tap represents a full-wave center tap topology with the diodes reversed, making the center tap the B+ point. This is therefore about 1/2 of what the bridge rectifier puts out. This is the source of the low voltage B+ for the signal level stages in the transceiver. R20 represents a pair of equalizing and bleeder resistors across the high voltage section. C7 is actually two capacitors in series, each bridged with a resistor. R19 represents the load placed on the high voltage power supply in transmit.

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 Post subject: Re: High Voltage Regulator Design Issue
PostPosted: Aug Mon 02, 2021 3:18 pm 
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Arrgh, After 3 pages, I ask? Can the primary side of the power supply be the regulator and take a "signal" from the load side what to do? Certainly would take the high voltage high current devices out of the picture... Chas

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 Post subject: Re: High Voltage Regulator Design Issue
PostPosted: Aug Mon 02, 2021 4:53 pm 
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Oh ok.


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 Post subject: Re: High Voltage Regulator Design Issue
PostPosted: Aug Mon 02, 2021 5:34 pm 
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The choke of the LC lowpass filter preceding the voltage regulator is large (1H). It can store a significant amount of energy that needs to be taken care of when the regulator's pass transistor turns off (because the LR8 driving it shuts down). That's an important aspect that can be easily overlooked and cannot be ignored in any kind of power supply design.

As you already know, the large choke L201 acts like a current source. Its current cannot change instantaneously. When the pass transistor turns off, the choke's steady-state current continues to flow through the filter's capacitor C204A and the parallel damping resistor R208. Energy is transferred between L201 and C204A until it dies out because of the resistive loss (L201's resistance R201 and R208). This results in a damped oscillation at the filter's output. Depending on the value of C204A (and R201 and R208), the voltage overshoot can be very large. With 1uF for C204A (assuming C204 was removed and replaced with a 1uF capacitor), this overshoot is over 115V. When added to the supply's input voltage, it can kill LR8 on turn-off. It's the turning-off of the regulator that potentially injures its active components.

Restoring C204A to its original value 30uF reduces the overshoot to less than 40V. Another way to reduce the overshoot is to decrease the value of R208 to make the LC filter critically damped. The most effective way to eliminate overshoot, regardless of C204A capacitance and R208 resistance, is to provide a fast-recovery wheeling diode D1 across L201. The overshoot is reduced to D1's forward voltage. There's nothing new here. It's exactly the same technique used for protecting a transistor driving a mechanical relay.

A secondary over-voltage protection for LR8 could be a zener diode or a TVS.


Attachments:
File comment: LC filter with wheeling diode
HV_Supply_1.jpg
HV_Supply_1.jpg [ 86.61 KiB | Viewed 726 times ]
File comment: Unclamped output voltage of LC filter
HVSup_Vin_no_clamp.jpg
HVSup_Vin_no_clamp.jpg [ 91.29 KiB | Viewed 726 times ]
File comment: Clamped output voltage of LC filter
HVSup_Vin_clamped.jpg
HVSup_Vin_clamped.jpg [ 110.49 KiB | Viewed 726 times ]

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 Post subject: Re: High Voltage Regulator Design Issue
PostPosted: Aug Mon 02, 2021 8:51 pm 
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bb.odin: Good point about damping the stored energy in the choke when there is one involved in the circuit.

One thing about using the PNP pass transistor in the circuit I suggested, it can be more difficult to find PNP's with high range voltage ratings than NPN's. Therefore, one trick is, if the power supply's pre-regulator output is isolated from ground, or can be isolated from it, is to reverse the polarity of both transistors, so to make the pass transistor an NPN, the comparator transistor a PNP, and reverse the direction of the Zeners. Then the circuit is a Negative voltage regulator. Also, one convenience, the pass transistor's collector ( regulated negative output) connects to ground and no insulator is required for the transistor body which can be screwed directly to the heat sink so it is electrically and thermally better.


Regulating the negative side of a high voltage supply was suggested in Horowitz & Hill's book.


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 Post subject: Re: High Voltage Regulator Design Issue
PostPosted: Aug Mon 02, 2021 9:03 pm 
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Chas wrote:
Arrgh, After 3 pages, I ask? Can the primary side of the power supply be the regulator and take a "signal" from the load side what to do? Certainly would take the high voltage high current devices out of the picture... Chas


Normally the primary side of the supply is a transformer primary winding and operating on line power. Therefore to regulate the primary requires a switchmode power supply design which is much more complex and in addition has a number of safety issues. One one method could be to buy an existing SMPS supply and re-wind one of the windings on the the transformer for a higher voltage. Often these ferrite cored transformers are more difficult to get apart than laminated iron core types, and the SMPS would generate more RFI than the analog supply.

Edit: taking the idea further though, these days smps's are very cheap & compact , especially in the <100W range, so one method could be to get a line to 48V supply, and wind a step up auto-transformer, and add additional rectifier/chokes filter caps, on that and connect it across the smps's secondary winding.


Last edited by ACORNVALVE on Aug Mon 02, 2021 10:16 pm, edited 1 time in total.

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 Post subject: Re: High Voltage Regulator Design Issue
PostPosted: Aug Mon 02, 2021 9:51 pm 
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You can use your original circuit with the LR8 by adding a current limiter. First. the reason you were smoking the TIP50's was that you were far exceeding the Safe Operating Area. I simulated the original circuit in LTspice (using a behavioral model for the LR8 by Tony Casey found on the LTspice users group).
Attachment:
Original PS-500 Ckt.jpg
Original PS-500 Ckt.jpg [ 111.38 KiB | Viewed 695 times ]

Below is a plot of the TIP emitter current versus the collector-emitter voltage at startup.
Attachment:
Original PS-500 Ckt TIP50 IE vs VCE.jpg
Original PS-500 Ckt TIP50 IE vs VCE.jpg [ 135.66 KiB | Viewed 695 times ]

In this plot the startup begins at the right end with Vce=327v and Ie=563mA, definitely enough to kill the TIP50. The plot continues to the left as the 33uF cap charges, finally winding up at Vce=58v at Ie=196mA.
The basic current limiter can made by adding six parts as shown in the dashed box below.
Attachment:
I-Limit PS-500 Ckt .jpg
I-Limit PS-500 Ckt .jpg [ 264.7 KiB | Viewed 695 times ]

While another TIP50 might work in this circuit, the IRF830 N-Channel MOSFET was used because it requires no gate current to fully turn it on and Q2, the current sense transistor, isn't required to conduct much current when limiting. Below are the startup waveforms for this circuit.
Attachment:
I-Limit PS-500 Ckt  Startup Waves 1.jpg
I-Limit PS-500 Ckt Startup Waves 1.jpg [ 172.32 KiB | Viewed 695 times ]

Here is the plot of the TIP emitter current versus the collector-emitter voltage at startup for this circuit.
Attachment:
I-Limit PS-500 Ckt TIP50 IE vs VCE.jpg
I-Limit PS-500 Ckt TIP50 IE vs VCE.jpg [ 134.91 KiB | Viewed 695 times ]

However, Q2 is also the only drawback to this circuit. It's negative base-emitter tempco causes the limiting current to drop with rising ambient temperature as seen below.
Attachment:
I-Limit PS-500 Ckt I-Limit vs Temp .jpg
I-Limit PS-500 Ckt I-Limit vs Temp .jpg [ 189.59 KiB | Viewed 695 times ]

The peak current at startup drops from ~244mA at 25°C to ~220mA at 65°C. If this is acceptable, then the six additional parts will do the trick.
John

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 Post subject: Re: High Voltage Regulator Design Issue
PostPosted: Aug Tue 03, 2021 2:42 am 
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Greetings, All!

Whew! That's a lot of things to reply to. I hope that I will cover all the bases... if I missed something, please let me know.

First, I realized finally that the LR8N regulator was placing severe constraints on an already difficult design. The TIP50 device was also marginal as it was necessary to use a 5% film capacitor as the first cap in the Pi net filter with a value selected to limit the unloaded voltage to a value below the limits of the TIP50. For the same reason, the TIP50 was shunted with a 1.5 K resistor.

The extremely narrow current window for the LR8N was the other problem. Below 500 uAmps of load current, the device is unstable. Above about 1.5 mA or so in this circuit, the device dissipation is exceeded. The LR8N had to go.

If you look at the latest circuit I posted, it uses all discrete 800 volt NPN transistors. Q4 (the pass half of the Darlington) is rated at 100 watts. I am simulating the circuit in MicroCap-12. I have run numerous transient analyses and everything stays within bounds under normal loads and with a short circuit on the output. The current limiting circuit I have is marginal, which I will discuss later when I ask for additional help.

Here are the specific answers to the questions as best I understand them:

For Chas: The power supply is a triple voltage power supply providing high B+ to the transmitter PA stages, low B+ (the circuit I am trying to regulate) for other stages and filament power for the whole radio. It is not possible to regulate the primary.

For bb.odin: The choke energy when the pass transistor turns off is not a problem.... this is not a switcher. The pass transistor never turns off. C204A, L202 and C204B no longer exist. They have been removed from the power supply chassis to make room for the heat sink. I appreciate your efforts concerning protection for the LR8N, but as mentioned above it has been abandoned due to the severe constraints it puts on the design. Further, there is a board area problem; please see my comments to John below.

For ACORNVALVE: The input to the regulator cannot be isolated from ground.... it is a bridge supply that provides high B+ to the PA stages as well as low B+ to the rest of the radio. Attempting to regulate the negative side would cause the PA B+ to be reduced, preventing the radio from making specified power.

For John: The TIP50 and the LR8N have been abandoned because there is no getting around the fact that they are just marginal for the application. Regarding the protection circuit you engineered.... I appreciate the effort that went into that, but unfortunately, I have mechanical constraints to deal with as well as electrical ones. I doubt that I have sufficient room on the maximum size PC board that I can fit to accommodate all of the components in the basic regulator circuit plus your added components.

I have not yet expanded the board to the maximum possible dimensions, but I intend to do so as soon as possible. I will be sweating to squeeze my stack of zeners and a 7 watt resistor on the board.... if I can't I may have to abandon the entire project.

One thing further... My current limiting circuit is not entirely satisfactory. It does the job, but it is operating in fold-back mode and the pass transistor is dissipating 96 watts or so with a 1 ohm short on the output. I would like to have a circuit that shorts the input to the regulator to ground with an SCR or similar and blows the input fuse. (I should point out that there is a 1 amp fuse on the input to the regulator and a 1/2 amp fuse on its output, together with a following zener string designed to blow the output fuse and protect the radio if the pass transistor shorts. I have eliminated these things from the posted schematic for simplicity, but I suppose I had better add them back in so everyone knows exactly what the circuit looks like.) I will post the revised schematic as soon as I can edit it.

Thanks again for all of the skull sweat.

Regards,

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 Post subject: Re: High Voltage Regulator Design Issue
PostPosted: Aug Tue 03, 2021 3:28 am 
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Greetings, All!

Here is the revised circuit.... I think this is accurate and correct.

Attachment:
Regulator All Discrete Darlington V8.JPG
Regulator All Discrete Darlington V8.JPG [ 274.98 KiB | Viewed 686 times ]


Thanks,

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 Post subject: Re: High Voltage Regulator Design Issue
PostPosted: Aug Tue 03, 2021 4:26 am 
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Jthorusen wrote:

For bb.odin: The choke energy when the pass transistor turns off is not a problem.... this is not a switcher. The pass transistor never turns off. C204A, L202 and C204B no longer exist. They have been removed from the power supply chassis to make room for the heat sink. I appreciate your efforts concerning protection for the LR8N, but as mentioned above it has been abandoned due to the severe constraints it puts on the design. Further, there is a board area problem; please see my comments to John below.

Regards,


I think what bb.odin was getting at is the transients that occur due to stored choke energy with on off power cycling. That is how you discovered your transistor and regulator was prone to failure.


Last edited by ACORNVALVE on Aug Tue 03, 2021 6:55 am, edited 1 time in total.

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 Post subject: Re: High Voltage Regulator Design Issue
PostPosted: Aug Tue 03, 2021 4:39 am 
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Greetings to ACORNVALVE and the Forum:

Interesting.... I hadn't fully grasped the implications of this since the failure seemed to occur at power on. However, it would explain very nicely the fact that the first power on went without difficulty. It was always the second that exhibited the failure. (Two tries only.) If we assume that the failure occurred on power down at the end of the first attempt, then it would not have been observable until the second attempt to power it up.

I will add the diode across the choke. Thanks for the additional explanation... it was very helpful.

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 Post subject: Re: High Voltage Regulator Design Issue
PostPosted: Aug Tue 03, 2021 5:17 am 
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Greetings to ACORNVALVE and the Forum:

So many beautiful theories wrecked by ugly facts. I cannot put a diode across the choke. It adds 80 volts to the filter output, doubling the dissipation of Q4 to 20 watts.

I have not figured out how to change the state of a circuit when the simulation is running in MicroCap-12; therefore I can't simulate what happens when I pull the plug. The best I can do is substitute a pulsed voltage source for the power supply. Unfortunately, MicroCap-12 is very touchy about this; about half the time, for no apparent reason, it results in a matrix singularity and the simulator refuses to run.

I will attempt it and get back to you.

Regards,

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 Post subject: Re: High Voltage Regulator Design Issue
PostPosted: Aug Tue 03, 2021 6:11 am 
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Greetings to the Forum:

The pulse test ran OK. With 1 uSecond rise and fall times, everything stays in the safe range. The pulse amplitude was 400 volts and the duration was .4 second.

Key to graph:

Blue (Node 4) = filter output voltage (Q4 collector) [0 to 675 volt scale]
Red (Node 3) = regulator output voltage (applied to load) [0 to 675 volt scale]
Magenta = dissipation of Q4 (pass transistor) in watts [0 to 100 scale]
Black = dissipation of Q5 (Darlington input transistor) in watts [0 to 10 scale]
Orange = dissipation of Q6 (current limiter) in watts [0 to 10 scale]

Attachment:
Regulator All Discrete Darlington Vtest2.JPG
Regulator All Discrete Darlington Vtest2.JPG [ 303.28 KiB | Viewed 667 times ]


Q6 seems to take the worst beating on shutdown but it does not exceed its power ratings. Interestingly, the worst transient is at startup... however this voltage remains within the 800 volt limits of the transistors in use. This phenomenon is also observed with the actual supply but its maximum voltage is around 400 volts and the steady state voltage is also lower, so the steady-state dissipation of the pass transistor (Q4) is around 10 watts. There does not seem to be any significant voltage transient at shutdown except in Q6's power... I suspect it is clamping the shutdown pulse.

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 Post subject: Re: High Voltage Regulator Design Issue
PostPosted: Aug Tue 03, 2021 7:09 am 
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Does the KSC5603DTU have any FBSOA ratings at all? I searched the data sheet without finding any. Not even any RBSOA info.

This part seems to be intended and rated for use as a high speed switch, not as a linear amplifier.

https://www.onsemi.com/pdf/datasheet/ksc5603d-d.pdf

I'm afraid that this guy is likely to die in a linear circuit where much current is involved.

As already suggested, MOS parts would solve the current gain problem with just one part. These are available with good FBSOA ratings, but cost a bit more.

The more or less fixed current limit is a big step in the right direction, but you need to add fold-back, so the current is low when the drop across the pass transistor is large. Two more resistors, on the base of Q6 will do this for the last circuit posted.

Ted


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