Can I use the same resistors for current shunt AND transistor balancing? - Page 1
Forum section: electronics / projects

author: Powermax
author_rank: Regular Contributor
timestamp: December 31, 2016, 09:10:35 pm
content: In my pursuit for a LPSU design, to get more HFe gain, higher current capability, decrease Vce dropout, improve thermals, etc. I would use 2 (or more) pass transistors. This requires load balancing resistors, so I thought I'd get clever and use these balancing resistors to also measure current for my current limiting capability! See pictures and explanation below.Details:The first picture below is a simplified diagram of how I currently have current feedback implemented. I generate a fixed (user adjustable) voltage across a 500R resistor via a current sink and an error amp (currently LM358) drives output pass transistor(s). Because the op amp drives the output to ensure the inputs are the same, the output current can be precisely regulated. It maintains a precise voltage differential across the shunt by means of the pass transistor.I don't want to take the naive approach with multiple resistances in the path of the current flow from the unregulated supply to the output, for obvious reasons. I could instead just measure the Vdrop on one of these balancing resistors, but that assumes that the current through those resistors is identical, which is not the case. So instead, what if I expand a bit on the first schematic, leading to the 2nd schematic... Would that work?I want to understand the mathematical analysis behind it to prove it does work. There are a bunch of unknowns I have and so I don't really know where to start with an analysis.MORE details!I am asking this because I plan to use this in a in my very own linear power supply design. The current design uses 2 NPN style Sziklai pair's made with MJE2955's and 2N4401's. I choose this for it's high gain and low dropout. (The highest dropout is 2V with a single pass element vs 5V of when I tried a discrete darlington configuration.) This means my supply voltage can be something simple like a 9V-0V-9V transformer, which after rectification can output 24V average under some load (haven't selected a transformer yet, and haven't yet bother to calculate the peak to peak ripple either.)

author: Kleinstein
author_rank: Super Contributor
timestamp: December 31, 2016, 10:13:00 pm
content: Yes one can use the current balancing resistors as shunts too. The only requirement is that the resistors are the same value (within accepted tolerance). The two (or more) resistors to combine the signals from the different shunts give an average voltage. The average voltage is proportional to the sum of the currents no matter how it is distributed.The downside is that you need two stable resistors instead of just 1. The typical cheap lows ohms resistors usually have quite some temperature coefficient and are thus not that well suited as a shunt, but very well OK for current sharing (here a positive TC is even an advantage). Also the drop on the current sharing resistors is usually in the 200-500 mV range - this means quite some power for an accurate shunt in the amps range. So if higher accuracy is wanted I would still consider a separate shunt.Using Sziklai pair's can give good results, but local stability can be challenging.

author: Powermax
author_rank: Regular Contributor
timestamp: December 31, 2016, 10:22:24 pm
content: Quote from: Kleinstein on December 31, 2016, 10:13:00 pmI have a whole bunch of these really old 0.25 ohm Dale wirewound resistors, they are wound around a hollow ceramic "noodle" and rated 1%. Do you think these would suffice? I guess I could try to measure the resistance while heating them up with a lighter?Quote from: Kleinstein on December 31, 2016, 10:13:00 pmI found that out pretty quick. However I found the same problem with a discrete darlington configuration. My supply is only stable with at least 6uF of output capacitance. (a few .47uF MLCC's and a 4.7uF electrolytic) Especially in current limiting mode BC My Sziklai pair(s) are equivalent to an NPN pass transistor with a low Z output (same reason PNP output transistors are less stable for voltage regulators I guess).Quote from: Kleinstein on December 31, 2016, 10:13:00 pmEven when I add that current sink to the 2 resistors performing the averaging? I guess one way to analyze it would be the superposition therom and replacing the current sink the equivelent open circuit?? Not exactly sure. Took ECE2630 at UVa (first ECE class) so I should know this stuff lol!I found that out pretty quick. However I found the same problem with a discrete darlington configuration. My supply is only stable with at least 6uF of output capacitance. (a few 047uF MLCC's and a 4.7uF electrolytic) Especially in current limiting mode BC My Sziklai pair(s) are equivalent to an NPN pass transistor with a low Z output (same reason PNP output transistors are less stable for voltage regulators I guess).

author: Kleinstein
author_rank: Super Contributor
timestamp: December 31, 2016, 10:49:48 pm
content: The superposition principle is a good way to see that combining two shunts is Ok.Stability has two components: one is the local one around the Sziklai pair and the other is the global one of the regulating OP.There are also two stages of stability: stable with a limited load impedance or stable at any passive load.I don't know the Dale resistors - they may work, but could also show quite some effect of self heating. If you have enough of them, you could check a bridge circuit: one resistor against 4 resistors as 2 in parallel 2 in series. Ideally this would be a 1:1 divider - but at higher current self heating would be stronger with the single resistor. This is a little more quantitative than just the lighter.