Simplify + Improve my Discrete OverVoltage Protection Circuit? - Page 1
Forum section: electronics / beginners

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: June 19, 2025, 06:14:49 pm
content: Falstad CircuitCircuit Goals:Decrease BOM + Simplify Circuit + Improve Speed (optional). Keep the 4.5v-80v input requirement.Current Problems:- The circuit sim becomes very buggy when the op-amp is changed from the LM741 to the LM324/LM358- The circuit sim does not work when the op-amp is only supplied via +10v & gnd (oddly requires a negative voltage?). I'd love to avoid the use of a negative voltage- Can the ?D-Latch? (not even sure if this is the correct name) be simplified, such as by using discrete components, while still maintaining the fast-responding latch?- The push-pull FET gate driver can likely be improved (for speed + performance), but I have no idea howWindow Comparator Circuit Explained:R1= Sets the OVP trigger (higher resistance = Lowers voltage of OVP trigger)R2= Sets the UVP trigger (higher resistance = Increases voltage of UVP trigger)5.6v Zener diodes= Protect the op-amp inputs?D-Latch? Circuit Explained:When OVP or UVP is triggered, the op-amps pull their output LOW. This causes the latch circuit output to be latched HIGH. An SR-latch was considered but would likely cause more problems during "invalid" conditions.Switch= Used to reset the circuit, when UVP or OVP is triggered (but fixed)R3= Used so that a low-cost push-button can be used to "reset" the circuitPush-Pull FET Gate Driver + Output Circuit Explained:R4= Limits current output from the ?D-Latch?R5= Limits current applied to FET GatesR6= Ensures that the FET is normally kept in a non-conductive state15v Z1 Zener Diode= Protects the FET GateD1= Protects circuit from the voltage-spike that would occur from the FET's quickly turning off (becoming non-conductive)

author: PGPG
author_rank: Frequent Contributor
timestamp: June 20, 2025, 07:05:42 am
content: OpAms should not have their outputs connected together. It is the reason of you seeing it 'buggy'.Look through digital IC serie 1G (single gate - there are for example D-latches in STO23-5 pins).Replacing Zener diodes with two switching diodes (like BAV99) can have smaller capacitance (but it can depend on exact Zeners you used) so giving less delay.If source (input - up to 80V) has low resistance than protecting directly at input can add less delay, but have to withstand stronger pulses.I don't understand idea of your output circuit. Using 2 transistors is justified when you want to switch unknown polarity signal and then their gates should be driven regarding their Source pins.For known polarity single MOS is enough.

author: MariuszD
author_rank: Regular Contributor
timestamp: June 20, 2025, 10:12:03 am
content: Output circuit doesn't work.To turn on an NMOSFET, you need 10V more on the gate than on the source.Replace opamps with comparators. Your circuit compares rather than amplifies. With OC output wired-AND is possible

author: Zero999
author_rank: Super Contributor
timestamp: June 20, 2025, 11:01:26 am
content: The window comparator circuit needs comparator ICs, with open collector/drain outputs & a pull-up resistor. Look up window comparator LM393 for more information.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: June 20, 2025, 02:41:57 pm
content: Thanks for the suggestion to change the op-amp to an actual comparator, that helped!Quote from: PGPG on June 20, 2025, 07:05:42 amInitially I was actually experimenting with standard "D-Latches", but I could never figure out how to get it to work in myFalstad Circuit? I'm not sure if I'm doing something wrong or what, but I could never get them to actually "latch"? I've only just recently began learning about basic digital logic. I also did try to experiment with Falstad's built-in T flip-flop, JK Flip Flop, D Flip-flop, and it doesn't seem like any of them could replace my ?D-Latch? Circuit.Quote from: PGPG on June 20, 2025, 07:05:42 amThe circuit will indeed be used for low resistance, high power loads (1A or greater), hence not placing the zener diode on the 80v input. Additionally, I thought of using the 2 transistors as it was shown in various "hot swap controller" I.C datasheets (page 1 for example 1, and page 12 forexample 2), but I guess this may actually be problematic?

author: PGPG
author_rank: Frequent Contributor
timestamp: June 20, 2025, 11:31:42 pm
content: Quote from: LooseJunkHater on June 20, 2025, 02:41:57 pmNotice that in both examples you shown the controller IC was connected to MOS gates and also to their sources while in your circuit sources were left floating.This is exactly what I was saying about:Quote from: PGPG on June 20, 2025, 07:05:42 amI have never been using hot-swap controller, but I suppose it has to assume VIN>VOUT and VOUT>VIN as possible situations. In your case it probably is not true.

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: June 22, 2025, 08:56:30 am
content: I have built several over Voltage (and over current) protection boards recently using an LTC4368 and a single MOSFET (DMT6002, 100A rated). The LTC4368 will also protect against reversed input, but in many cases you don't need that and can then use a single MOSFET for reduced heating under high current loads.The data sheet is at:https://www.analog.com/en/products/ltc4368.htmlIf you need output surge protection, ie if you are switching on a load which has large capacitors and surges well above the current trip level, you can add a CR circuit to delay the current trip, it goes on the sense line (1.5K in series with the sense line, 100nF between sense and Vout on the IC will hold off for approx 1mS).There is an LTSpice model of the 4368 the data sheet circuit as a demo ready to play with in LTSpice.