Hot!High Voltage Power Supply

Page: < 1234 Showing page 4 of 4
Author
Ioannis
New Member
  • Total Posts : 29
  • Reward points : 0
  • Joined: 2010/09/09 09:14:54
  • Location: Greece
  • Status: offline
Re: High Voltage Power Supply 2018/06/26 03:12:16 (permalink)
0
Thanks Paul. I can post now.  The user name is Ioannis not Ioannis? For some strange reason I could send you a PM but not able to post!
 
I do not know what has changed on the forum but for a long time I was blocked.
 
Anyway, with your great help I have a good working High voltage Linear power supply. Unfortunately with no short circuit protection...
 
Either of the attached circuits puts MOSFET in a cutoff situation even with no load current. I can suspect  NPN may have the base-collector junction bias so than makes Vgs about 0,7 volts, but my multimeter does not confirm this.
 
If, in either circuit, I disconnect the collector of the PNP or NPN transistor, the circuits works as expected.
 
Any suggestion welcome.
post edited by Ioannis - 2018/06/26 03:19:19

Attached Image(s)

#61
PStechPaul
Super Member
  • Total Posts : 2023
  • Reward points : 0
  • Joined: 2006/06/27 16:11:32
  • Location: Cockeysville, MD, USA
  • Status: offline
Re: High Voltage Power Supply 2018/06/26 12:35:56 (permalink)
0
The NPN transistor will clamp the Vgs of the P-channel MOSFET to the forward drop of the B-C junction, or 0.7V, so it will not work. The circuit using the PNP looks like it should work:
 

However, it is possible that IC1 cannot turn the MOSFET Q1 on. With a 5V supply, it might only be capable of 3.5 volts output, so the maximum voltage on R4 will be about 2.8 volts, or 600 uA. That is only 6 volts on the 10k resistor R1, so only 6 volts on the gate. That might not be enough. Also make sure T1 and the other (unlabeled) BJT are rated at least 300 Vce. You can use a much lower value for R4 so the drive current can be high enough to turn Q1 fully on.
 
You should add a resistor, about 1K, in series with the base of the PNP transistor. And then you could add another resistor, something like 200-500k, from the base to the drain of Q1 to achieve some fold-back current limiting.

 
#62
Ioannis
New Member
  • Total Posts : 29
  • Reward points : 0
  • Joined: 2010/09/09 09:14:54
  • Location: Greece
  • Status: offline
Re: High Voltage Power Supply 2018/06/26 13:09:45 (permalink)
+1 (1)
Indeed, 5V is not enough and the supply of the Op-Amp is 12Volts.
 
The +5V was a left over... sorry about that.
 
The T1 is rated at 450Vdc, an APT27 but not the PNP. Maybe a BC557 or the like. This is floating, right?
 
Lowering R4 seems it makes the circuit unstable. Current through T1 was calculated to be max 2.5mA
 
Foldback was something I would love to do also. Will do that after I have it working OK.
 
Selecting a suitable MOSFET is tricky as there are some details in the datasheets like the SOA curves that reveal a 8A transistor being able to handle .5A at 300Vdc in one case. And this at 25 C.
 
Thank you very much for the attention to this design.
Ioannis
#63
PStechPaul
Super Member
  • Total Posts : 2023
  • Reward points : 0
  • Joined: 2006/06/27 16:11:32
  • Location: Cockeysville, MD, USA
  • Status: offline
Re: High Voltage Power Supply 2018/06/26 15:54:25 (permalink)
0
There can be almost the full input DC voltage on T1 and Q1, but not on the PNP current limit transistor (or the NPN, for that matter).
 
I found that the simulation was unstable (high frequency oscillation) until I added some capacitors to speed up the feedback loop, as well as the capacitor on the op-amp. The emitter resistor R4 also helped, but it reduces the gain of the system. I did not simulate the PMOS series pass transistor, and it is probably a lot faster than the Darlington 2N6287.
 
The SOA for the MOSFET is largely based on the power dissipation. The FQP4P40 is rated 85 watts, while 350V at 500 mA is 175W. Looking closely at the chart, it is more like 250 mA at 350V, which is about 85 watts. It can withstand higher current pulses of short duration. Your 1.5 ohm current sampling resistor limit is about 450 mA, so a short circuit would cause 0.45*280 = 126 watts.
 
A "cheap and dirty" way to limit short circuit current and power dissipation might be to add a power resistor of about 200 ohms, which will drop 100 volts at 500 mA and dissipate 50 watts, so the MOSFET would only see about 75 watts. A thermistor on the heat sink could be used to shut down (or throttle) the output in the case of extended overload. A 10k thermistor across R1 might do the job.
 
I am considering a PIC-based power supply where it will monitor the current through the series pass element as well as the voltage, and limiting power dissipation accordingly. It could also monitor temperature of the heat sink. Perhaps a PIC16F1764, which includes an op-amp as well as other useful peripherals.

 
#64
PStechPaul
Super Member
  • Total Posts : 2023
  • Reward points : 0
  • Joined: 2006/06/27 16:11:32
  • Location: Cockeysville, MD, USA
  • Status: offline
Re: High Voltage Power Supply 2018/06/26 17:23:11 (permalink)
+1 (1)
Here is a good article on fold-back current limiting:
 
http://www.eeeguide.com/foldback-current-limiting/
 

This uses an NPN series pass element, which has the disadvantage of needing a high voltage drive to the gate. But the principles are the same.
 
I made a spreadsheet that can be used to calculate the values for the current limiting circuit I used in my simulations:

 
http://enginuitysystems.c...back_Current_Limit.xls
 
http://enginuitysystems.c...back_Current_Limit.ods

 
#65
PStechPaul
Super Member
  • Total Posts : 2023
  • Reward points : 0
  • Joined: 2006/06/27 16:11:32
  • Location: Cockeysville, MD, USA
  • Status: offline
Re: High Voltage Power Supply 2018/06/27 01:04:23 (permalink)
0
I found that I had simulated a power supply with a PMOS series pass element. Originally I needed 100 milliohms resistance in the output to work into a capacitive load, but a small speed-up capacitor in the feedback loop fixed it. I added fold-back current limiting, and it seems pretty good. Note that there is no emitter resistor on Q3, either.
 


 
#66
Ioannis
New Member
  • Total Posts : 29
  • Reward points : 0
  • Joined: 2010/09/09 09:14:54
  • Location: Greece
  • Status: offline
Re: High Voltage Power Supply 2018/06/28 02:56:58 (permalink)
0
Thank you Paul for the wealth of info. The last simulation is a great starting point for the foldback.
 
At the same time I am waiting for a more powerful MosFet to be safe!
 
Ioannis
#67
PStechPaul
Super Member
  • Total Posts : 2023
  • Reward points : 0
  • Joined: 2006/06/27 16:11:32
  • Location: Cockeysville, MD, USA
  • Status: offline
Re: High Voltage Power Supply 2018/06/28 03:11:39 (permalink)
0
I am rebuilding another old power supply, and I decided to try an all-discrete component approach. Here's a simulation:
 

http://enginuitysystems.c...p_NPN_PNP_Foldback.asc

 
#68
Ioannis
New Member
  • Total Posts : 29
  • Reward points : 0
  • Joined: 2010/09/09 09:14:54
  • Location: Greece
  • Status: offline
Re: High Voltage Power Supply 2018/07/10 13:35:45 (permalink)
0
Hi Paul and who ever is following.
 
The usual problem with linear PS is the power loss on the pass element.
 
My design also suffers a lot of heat dissipation, especially around the center of the complete output span. Around 25-30 watts and this needs a massive heat sink.
 
By chance I found a PCB of such high voltage regulator (200 volts output) that had a very small heatsink. I tried to load it but heat remained to a minimum. So, analyzing was my next step.
 
Proved to be a classic floating 723 regulator but with a clever trick.
 
I attache the rectifier part of the design. With this idea,based on the D7 zener, the amount of voltage on the pass element is kept to a minimum.
 
Enjoy!

Attached Image(s)

#69
PStechPaul
Super Member
  • Total Posts : 2023
  • Reward points : 0
  • Joined: 2006/06/27 16:11:32
  • Location: Cockeysville, MD, USA
  • Status: offline
Re: High Voltage Power Supply 2018/07/10 15:21:47 (permalink)
0
Yes, I saw a similar design for a very high power DC supply, where it used SCRs as a pre-regulator and then linear elements for final regulation.
 
I also conceived of a variable DC power supply that uses a Variac to obtain raw DC output voltage, with a regulator that uses a separate control voltage derived from the output of the Variac. The Variac itself has pretty good AC regulation, but the raw DC output after the rectifiers and filter capacitors varies greatly with load, and has a lot of ripple. The voltage across the regulating element is maintained at a much lower value than a purely linear circuit on the full DC raw voltage. I already have a power supply much like this, but without the regulator. IIRC it has meters for 0-40 VDC and 0-10 ADC. I think I will build this into it and see how it works in the real world:
 


 
#70
Ioannis
New Member
  • Total Posts : 29
  • Reward points : 0
  • Joined: 2010/09/09 09:14:54
  • Location: Greece
  • Status: offline
Re: High Voltage Power Supply 2018/07/11 00:28:07 (permalink)
0
Interesting approach too. You don't have a cap at the output. Maybe you will get lower ripple with a small cap like a 10μF or so.
#71
Page: < 1234 Showing page 4 of 4
Jump to:
© 2018 APG vNext Commercial Version 4.5