HV (400V-600V) to LV (16V at ~20 mA) constant current source circuit - Page 1
Forum section: electronics / projects

author: eerocketman
author_rank: Contributor
timestamp: August 01, 2017, 10:02:34 pm
content: Hi guys I've designed a constant current source circuit with some components that were available to me. What do you think of my design? I have also tested it at up to 550V it seems to work . Any suggestions for improvements?It is not in in the image but the HV is fused at 500 mA.Please excuse the horrible hand writing. *Also ignore the parallel PNPs there is no need for any in parallel*

author: duak
author_rank: Super Contributor
timestamp: August 02, 2017, 11:52:24 pm
content: Is this circuit supposed to maintain 20 mA into the zener diode over 400 to 600 V in? I don't believe it will.Below about 516 V in, R5//R6 will limit the current to (Vin - 16 V)/ 25K.Q1 will try to limit the current in R7 to be roughly 0.6 V / 20 R = 30 mA. Since R5//R6 limits the current to a lesser value, Q1 won't turn on and Q2 will essentially be saturated and its VCE will be a volt or less. Since it's dropping such a small voltage, it's not really doing anything. Note that the 0.6 V VBE value is a rough rule of thumb and is a function of temperature, collector current plus a few other factors.I don't believe the MPQ2907 transistor CE breakdown is much over 50 - 60 V. If for some reason, Q2 VCE rises much above this, it will probably avalanche, overheat and fail short circuited. This could happen for two reasons: the device temperature rises enough to allow Q1 to turn on to divert enough base current from Q2 and allow it to develop a high VCE. This will also happen if R1 - R4 increase in value or fail open circuit. Q2 will then develop a greater VCE and possibly fail.At 600 V in, R1 - R4 will each dissipate about 225 mW. Ostensibly, this is within their rating of 250 mW but if the ambient temperature is too high, the resistors ought to be derated to reduce the chance of failure.Cheers,

author: MK
author_rank: Regular Contributor
timestamp: August 03, 2017, 01:03:48 am
content: It would be easier to start with a high voltage depletion mode mosfet. The transistors chosen are much too likely to fail due to voltage transients.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 03, 2017, 03:04:08 am
content: A current mirror is a current source with respect to the output voltage, but when programmed with a resistor, it is not a current source with respect to the supply voltage.>600V PNP are very rare, if they can be found at all. Using lower voltage transistors will merely result in avalanche breakdown, in which case it's the resistors doing all the work (the transistors may also fail shorted, putting even more work on the resistors -- efficient, right?).Tim

author: Rbastler
author_rank: Frequent Contributor
timestamp: August 03, 2017, 06:39:32 am
content: You could just use a mosfet linear regulator.But wy do you want to Make 16V out of 400-600?Sent from my A0001 using Tapatalk

author: Circlotron
author_rank: Super Contributor
timestamp: August 03, 2017, 06:44:02 am
content: This might do the trick with a suitable high voltage mosfet.https://www.eevblog.com/forum/projects/high-performance-current-sink-using-cascoded-lm317-and-mosfet/

author: sibeen
author_rank: Frequent Contributor
timestamp: August 03, 2017, 06:50:21 am
content: Just to be a pain in the arse, 600 volts is considered Low Voltage and 16 volts is Extra Low Voltage.Being an absolute pedant about it, but in the fields I work never call something HV if you can get away with it

author: eerocketman
author_rank: Contributor
timestamp: August 03, 2017, 11:53:38 pm
content: Thank you for that advice! I've look into those and found the circuit in the image attached. This seems to be a very good solution for my problem!

author: floobydust
author_rank: Super Contributor
timestamp: August 04, 2017, 02:31:44 am
content: Don't forget 600V at 16mA is around 9W dissipation in the transistor. That's a fair bit of heat.I've used depletion-mode MOSFETS for this kind of thing, one resistor with a protection diode.IXYS hasIXTH10N100D21,000V 10A TO-264Microship (Supertex) DN2450N5-G 400V 0.5A TO-220.IXYS depletion-mode mosfets