Help with identifiying what these passive components do in this electronic load - Page 1
Forum section: electronics / projects

author: teslinius
author_rank: Regular Contributor
timestamp: October 17, 2021, 09:09:08 pm
content: I'm not sure what certain components do in these circuits. Like the 1nF capacitor across drain and source of the mosfets (i've read somewhere that it's not a good idea to place extra capacitance on the mosfet except in special circumstances), the 1k resistors are probably to limit the current if anything goes short circuit and the other 1nF and 220k resistor to remove oscillations? in load-example.jpg pictureThe other picture is a schematic of a hp 6060 electronic load, as i can see each mosfet has it's own current measuring circuit, i'm not sure what the diode is for, why are the capacitors rated for 100V 22pF? and is the 100k resistor a pulldown for the mosfet?

author: ledtester
author_rank: Super Contributor
timestamp: October 18, 2021, 04:34:35 am
content: Quote from: teslinius on October 17, 2021, 09:09:08 pmThe second schematic looks a lot like the "150W 10A Electronic Load" kit you can find on aliexpress/ebay -- see attached pics. There's been discussions about it on this forum, e.g.:-https://www.eevblog.com/forum/projects/upgraded-lm324-based-150w-72v-10a-electronic-load/msg1506610/#msg1506610-https://www.eevblog.com/forum/beginners/my-lm324-electronic-load-is-oscillating/-https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/msg3075876/#msg3075876There's also been a lot of discussion here on designing electronics loads, e.g.:-https://www.eevblog.com/forum/projects/yet-another-diy-electronic-load/QuoteDiode -- it looks like it is connected to the TURNON signal. If the TURNON signal is a low enough voltage it would prevent the op amps from controlling the MOSFETs. If the signal is a high voltage the diode allows the op amp to retain control of the MOSFET.22 pF cap -- all small pF caps are rated for high voltage, so that's not a surprise. What probably matters here is the 22 pF. Looks like it's part of the feedback compensation network to prevent oscillations.100K -- looks like a pull-down to make sure the MOSFET doesn't accidentally turn on on power-up.

author: teslinius
author_rank: Regular Contributor
timestamp: October 19, 2021, 10:47:15 pm
content: Thanks a lot, wiwll have a look at the links, i have another question if you maybe know, at the bottom of the hp6060 schematic there is input protection circuit and i'm not sure what kind of circuit it is and what do these components exactly do.I'm assuming the c37 capacitor is a decoupling capacitor, not sure why the varistor has a diode wired in series, and then the combination of resistors and caps, is this some kind of snubber network maybe? to supress transient voltage?

author: Vovk_Z
author_rank: Super Contributor
timestamp: October 20, 2021, 04:56:01 pm
content: Quote from: teslinius on October 19, 2021, 10:47:15 pmIt looks like a overvoltage protection, which can work only one time.

author: MathWizard
author_rank: Super Contributor
timestamp: October 21, 2021, 11:26:34 am
content: How much different would the HP6060 act, if instead of driving each mosfet with it's own op-amp, they used a single op-amp, driving a BJT (or 2) for each mosfet gate ??I gather that mosfet have pF or nF or gate capacitance, so is it all about overcoming that with really fast, high slew rates, and high in/out-rush gate currents from the drivers? And so they go with so many op-amp's ? And for preciser control ?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 21, 2021, 11:50:41 am
content: Drive strength doesn't really matter very much in analog applications; a few mA gate current is often adequate, maybe a few 10s of mA at full slew rate, which op-amps are more than capable of. The actual gate voltage swing is small, fractional volts, going to maximum when saturated.When saturated (low Vds, Iout = Vds / Rds(on)), the op-amp attempts to raise Vgs in a vain attempt to maintain output current. In this case, Vgs(on) should be intentionally limited, so that a slow recovery can be minimized, as when load current/voltage goes back up again, it has to drop from Vgs(on) to a bit above Vgs(th).Individual op-amps are required to maintain balance. In general, Vgs(th) varies between individual parts, so they cannot all be tied to the same gate voltage. Balancing can be enforced by using large source degeneration resistors, at the expense of greatly increasing Rds(on), i.e. saturation voltage at given load current -- typically by a couple volts. If this is acceptable, then yeah, it can be done. A buffer stage might be needed for, say, four or more transistors in parallel in this way, or if higher bandwidth is demanded.Note that a buffer circuit like a BJT emitter follower and diode, isn't acceptable here, like it is for switching gate drivers. A class A/AB follower is required, generally needing more components (such as bias diodes and complementary emitter followers), or much more bias current (a single emitter follower with load resistor, has to sink (as quiescent current) basically as much current as it can source).Tim