How to not exceed max Vgs - Page 1
Forum section: electronics / beginners

author: MaurizioT
author_rank: Newbie
timestamp: July 23, 2025, 08:17:00 pm
content: Hi!I have a problem driving a fet.Please look at image1, the maximum Vgs is 15V and the fet work well.But I have the necessity to increase the power supply to 24V therefore the fet became broken because the maximum Vgs is over.I have thinked to solve the problem as in image2, adding two zener diodes.The circuit can work or there is a better solution?

author: Zero999
author_rank: Super Contributor
timestamp: July 23, 2025, 08:34:39 pm
content: Add an emitter resistor to the common emitter stage.

author: MaurizioT
author_rank: Newbie
timestamp: July 23, 2025, 08:39:29 pm
content: Thank you for your reply!But the Vgs exceed the same, it is not enough.

author: wraper
author_rank: Unknown
timestamp: July 23, 2025, 08:49:46 pm
content: What is this circuit, what is the IC used? How to achieve this depends on how fast it needs to be driven. As of circuit with two Zener diodes, this won't work. Upper Zener makes upper transistor useless and MOSFET will slowly shut down (if at all) only due to Zener leakage current. But presence of lower Zener will make will make another ~6V zener out of reverse biased Q1 C-E junction so voltage will be limited to 18V at best.

author: Zero999
author_rank: Super Contributor
timestamp: July 23, 2025, 08:57:00 pm
content: Quote from: MaurizioT on July 23, 2025, 08:39:29 pmWhat are you talking about?All voltages are relative. Look at the plot. The gate voltage swings between 48V and 38V. The gate voltage is relative to the source voltage. The maximum potential difference between the gate and source is 10V, which is well within specification.In your case, with a 24V supply, the gate voltage should swing between 24V and 14V.I missed the fact you were using an IC to drive it, which might have an output which takes this into account. Post the part number.

author: MaurizioT
author_rank: Newbie
timestamp: July 23, 2025, 09:00:11 pm
content: The IC is the max713 but the problem is generic.However you can found the full schematic in image3.Thank you for your support.

author: langwadt
author_rank: Super Contributor
timestamp: July 23, 2025, 09:03:23 pm
content: Quote from: MaurizioT on July 23, 2025, 08:17:00 pmput a zener across R2 (assuming DRV is current limited)

author: wraper
author_rank: Unknown
timestamp: July 23, 2025, 09:10:45 pm
content: Even if you fix the issue with Vgs, MAX713 itself becomes an issue at 24V.

author: MaurizioT
author_rank: Newbie
timestamp: July 23, 2025, 09:19:07 pm
content: The diode D4 limit the current to 8 mA and the voltage is about 5V also at 24V.I meet the problem only with the fet.

author: wraper
author_rank: Unknown
timestamp: July 23, 2025, 09:23:47 pm
content: Max V+ is 20V, max voltage on DRV pin is 20V. Max voltage on Batt+ is 20V, Even if battery voltage is lower, voltage will exceed 20V if you disconnect the battery. Max FASTCHG voltage is +12V.

author: Zero999
author_rank: Super Contributor
timestamp: July 24, 2025, 10:03:24 am
content: Quote from: MaurizioT on July 23, 2025, 09:00:11 pmThe quickest way is to put a zener diode between pin 14 and Q2. Note that it will reduce the gate voltage by the zener drop, so it will stop it from working at lower voltages.

author: PGPG
author_rank: Frequent Contributor
timestamp: July 24, 2025, 10:25:55 am
content: Quote from: Zero999 on July 24, 2025, 10:03:24 amOr resistor.

author: Zero999
author_rank: Super Contributor
timestamp: July 24, 2025, 11:05:34 am
content: Quote from: PGPG on July 24, 2025, 10:25:55 amPerhaps, but I chose a zener because it also reduces the voltage seen by the IC on pin 14. I haven't read the datasheet in detail, it said something about the maxium battery voltage being 20V and a 10V zener should keep it below 20V, with a 24V supply, even if pin 14 is in the high impedance state with only a tiny leakage current.

author: PGPG
author_rank: Frequent Contributor
timestamp: July 24, 2025, 07:00:53 pm
content: Quote from: Zero999 on July 24, 2025, 11:05:34 amRight.