Design an auto dicharge circuit for Capacitor using MOSFET - Page 1
Forum section: electronics / beginners

author: navin
author_rank: Newbie
timestamp: June 17, 2023, 09:56:36 am
content: i've been trying to design a discharge circuit for a capacitor placed at the output of DC-DC convertor. The discharge circuit should work such that when the supply is cut off, it should discharge the capacitor as fast as possible. it takes almost 3-4 seconds to fully discharge the capacitor in my current working board. How can we increase the discharge speed? Thank you

author: wraper
author_rank: Unknown
timestamp: June 17, 2023, 10:12:40 am
content: No schematic or explanation how you're doing it. Reducing discharge time is as simple as increasing discharge current.

author: Psi
author_rank: Super Contributor
timestamp: June 17, 2023, 10:14:47 am
content: What about a PFet and resistor across the cap.The gate has a pull down to GND to turn it on and discharge the cap through the resistor .When your system has power it pulls the gate high and holds the pfet off.You need to isolate the incoming power and make a separate signal that will go low as soon as the device is unplugged even if the main bulk caps are still charged. So you may need some extra diodes.Also want the fet to have logic level gate so it can stay on until lowest voltage possible

author: Kibabalu
author_rank: Regular Contributor
timestamp: June 17, 2023, 10:23:20 am
content: Have a look inside Horowitz/Hill The Art of Electronic - The x-Chapters. There'll you find a solution for your requirement, based on a depletion mode JFET.

author: ArdWar
author_rank: Frequent Contributor
timestamp: June 17, 2023, 10:54:22 am
content: Does your converter have a Power Good output?

author: Infraviolet
author_rank: Super Contributor
timestamp: June 17, 2023, 03:53:00 pm
content: The rather over-engineered, but probably still quite suitable, way to do this might be:..Have a small microcontroller powered from a section of power rail which is separated from the main rail by a diode, have some large caps to ground from this separated power rail. This will keep the MCU running until the extra caps discharge,even when the main power rail has gone low...Have that MCU monitor the analogue voltage at the input of the DC convertor, when that input voltage goes low enough for long enough have the MCU bring high a particular pin...This MCU pin will, via any protection circuits or transistor current buffers necesary, control a solid state equivalent of a relay between the ends of the cap you want to rapidly discharge...Put some small series resistance before or after this "relay" to limit the fast discharge current to the maximum the "relay" can handle, i this makes the discharge too slow for you then select a "realy" able to handle mroe current.

author: DavidAlfa
author_rank: Super Contributor
timestamp: June 17, 2023, 05:17:55 pm
content: Why a MOSFET?Use the correct device for the job, a bjt will work down to 0.6V, a MOSFET won't belown ~2-3V.This simple circuit will discharge the output when the voltage drops below a certain voltage, but you might use any other condition to trigger it.The zener must be selected so it turns off at working voltages plus a small margin, for example if it works at 12V, use a 10V zener.It will actuate briefly at power-on (For a few ms) until it fully loads, so don't be too heavy on the load.You might omit the power resistor by adjusting R3 so it doesn't saturate the transistor.

author: DavidAlfa
author_rank: Super Contributor
timestamp: June 17, 2023, 05:37:46 pm
content: You might add current limiting and remove the power resistor:

author: wraper
author_rank: Unknown
timestamp: June 18, 2023, 06:15:14 am
content: Quote from: DavidAlfa on June 17, 2023, 05:17:55 pmDepends on what MOSFET. You can use one that will go <1V. However you actually don't need that, you can go to 0V by using MOSFET gate capacitance or adding some small additional capacitance as MOSFET does not need current flowing through its gate to stay open.

author: Psi
author_rank: Super Contributor
timestamp: June 18, 2023, 06:30:23 am
content: An opto-fet or opto-transistor might be another option depending on your circuit and how the voltages are setup.They make pretty high current opto-fets as gate drivers.Sometimes having that isolation makes things a lot easier.