Over Voltage, Reverse Voltage Protection circuit optimizations?  - Page 1
Forum section: electronics / projects

author: littleboot
author_rank: Regular Contributor
timestamp: October 24, 2022, 11:12:55 am
content: Hello,I have designed an overvoltage and reverse voltage protection circuit that disconnects the load from the source if an over voltage is detection or if a reverse voltage is detected.I have the feeling this circuit could be optimized, or may be implemented differently. So I have created this topic to ask for any recommendations and to share this circuit. *note I plan to use other part numbers for the transistors and fets, the parts selected were available in TINA-SPICE.Explanation of how the circuit operates.The circuit is divided in 3 modules:• OV-detect• invert signal• load switchCircuit specifications:Should function up to +/-24VLoad disconnect if Vin>3.25VDisconnects load if Vin<0VNormal operating voltage: 12VOV-detectThe Over voltage detection is implemented using a TL431The TL431 is configured in voltage comparator mode and the PNP transistor is used to create a near rail to rail output (it also inverts the signal). This is required because the TL431 low voltage is ~1.8V and will turn on a logic level NMOS Fet.The diode D1 is used to protect the PNP transistor when a reverse voltage is connected (Emitter base breakdown voltage).Invert signalJust a Nmost with a pull-up to invert the signal of the OV-detect circuit.Load switchBack to back P-most fets form a bi-directional switch that can disconnect the source form the load.Other infoThe zener diodes in the circuit are used to protect the mosfet gates from overvoltage.The capacitors are used to slow down the turn on/off of the switch: soft start (inrush limit).

author: ledtester
author_rank: Super Contributor
timestamp: October 24, 2022, 11:49:03 am
content: Does Z3 need a resistor to protect it from excessive current draw when T1 conducts?Update: In these TI app notes the '431 is driving the N-MOSFET directly:AN-1533 Over Voltage Protection Circuit for Automotive Load Dumphttps://www.ti.com/lit/an/snva190b/snva190b.pdfSNVA681A–Load Dump and Cranking Protection for Automotive Backlight LED Power Supplyhttps://www.ti.com/lit/an/snva681a/snva681a.pdf

author: littleboot
author_rank: Regular Contributor
timestamp: October 25, 2022, 12:38:46 pm
content: Hi ledtester,Thank you for your reply,"Does Z3 need a resistor to protect it from excessive current draw when T1 conducts?" Oepsie yes I forgot.I was aware of the AN-1533 app-note and used it as reference for the first circuit I build, however I used different parts and that is why this circuit did not work for me.I was not aware the low voltage of the 431 and I incorrectly assumed it was close to 0V at the time.The low voltage Vka(low) of the 431 (I used a AZ431 - diodes inc) was around ~1.8V when measured.In this first circuit I re-used a AP2302B N-mos with a Vgs(th) of just 0.55V, so it was allways on.I think I will change it to an other fet with a higher Vgs(th) limit to so simplify the circuit and make it work properly.AN-1533 uses the ZVN4106FTA NMOS with a Vgs(th min) of 1.3V in combination with the:LMV431AIMF uses a 1.24V reference so the output Vka low ~1V. This will workSNVA681A uses the TL431 in combination with IRF840 which has a Vgs(th min) of 2V, and is a very big THT part.the TL431 has a low voltage of ~2V.It seems this is not a very good part choice, and might not work.This appnote was a good read and helped me alotUsing the TL431 for Undervoltage and Overvoltage Detectionhttps://www.ti.com/lit/an/slva987a/slva987a.pdf?ts=1666614347749