The author encountered a problem with a microcontroller system in which the +5-V supply voltage did not decay to 0 V sufficiently quickly after being switched off. A certain residual voltage remained, and it declined only very slowly. As a result, certain system components could not perform a clean reset if the power was quickly switched on again. To remedy this problem, a very simple circuit was used to discharge the +5-V supply. It consists of two resistors and a type Si9945 dual MOSFET from Vishay Siliconix (www.vishay.com/mosfets).
A Discharge Circuit Schematic Circuit Diagram designed to rapidly discharge the residual voltage on the +5V supply line can be described as follows:
- Dual MOSFET (Si9945): A dual MOSFET IC is used for efficient discharge.
- Resistor 1 (R1): Connected between the gate and source of one MOSFET.
- Resistor 2 (R2): Connected between the gate and source of the other MOSFET.
- +5V Supply Line: The supply voltage that needs to be discharged.
- Ground (GND): The reference ground point.
The circuit utilizes a dual MOSFET IC (such as Si9945 from Vishay Siliconix) to rapidly discharge the residual voltage on the +5V supply line. The MOSFETs are controlled by resistors R1 and R2, and they act as a low-resistance path to quickly drain the remaining charge on the supply line.
- Dual MOSFET (Si9945): The Si9945 is a dual N-channel MOSFET IC. One MOSFET in the IC can be used to create a low-impedance path between the +5V supply and ground.
- Resistor 1 (R1) and Resistor 2 (R2): These resistors are used to control the gate voltage of the MOSFETs. Proper resistor values are chosen based on the MOSFET’s gate-source voltage requirements to ensure the MOSFETs are in the saturation region when activated.
MOSFET Switching Mechanism and Discharge Circuit
These MOSFETs activate completely when the gate voltage falls within the range of +1 V to +3 V. MOSFET T2 plays a crucial role by linking discharge resistor R2 from the +5-V supply line to the ground when its gate voltage surpasses the threshold level. Upon shutting down the +5-V supply, the first MOSFET (T1) loses its connection to pull-up resistor R1, causing the standby voltage to reach the gate of T2 through R1.
Standby Voltage Requirement and Compact Construction
This setup mandates the standby voltage to persist as long as it takes to discharge the +5-V supply, even during system shutdown. R2’s sizing prevents exceeding the continuous power limit of a type 1206 SMD resistor, rated at 0.25 W. Adjusting component values might be necessary for diverse applications. The circuit’s design offers compactness, facilitated by the dual MOSFET housed in an SO8 SMD package. Alternatively, standard individual FETs like the BS170 can be used, allowing flexibility in construction methods.
Introduction to MOSFET Technology
The metal–oxide–semiconductor field-effect transistor, commonly known as MOSFET, is a vital type of field-effect transistor created through controlled silicon oxidation. It features an insulated gate whose voltage regulates the device’s conductivity.
Ubiquitous Use of MOSFETs
MOSFETs represent the prevailing transistor type in contemporary electronics. Their primary function lies in regulating conductivity. Determining the flow of electricity between the source and drain terminals based on the voltage applied to the gate terminal.