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diff --git a/_blog/mosfet-switches.md b/_blog/mosfet-switches.md new file mode 100644 index 0000000..bb3514d --- /dev/null +++ b/_blog/mosfet-switches.md @@ -0,0 +1,123 @@ +--- +title: MOSFETs as electronic switches +date: 2025-06-22 +layout: post +--- + +Recently, I needed a low-power circuit for one of my battery-operated projects. +Much of the system's power savings depended on its ability to electronically +switch off components, such as servos, that draw high levels of quiescent +currents. My search for a solution led me to MOSFETs, transistors capable of +controlling circuits operating at voltages far above their own. + +## Acknowledgments + +This article is a summary of what I learnt about using MOSFETs as switches. +I'm not an electronics engineer, and this is not an authoritative guide. The +circuits in this post must be considered within the narrow context in which +I've used them. All credits for the schematics belong to <a +href="https://electronics.stackexchange.com/users/292884/simon-fitch" +class="external" target="_blank" rel="noopener noreferrer">Simon Fitch</a>. + +## Preamble + +For a typical MOSFET-based switch, we can connect a GPIO pin of a +microcontroller to the gate of a logic-level N-channel MOSFET placed on the low +side of the load and tie the gate and the drain pins of the MOSFET with a +pull-down resistor. This would work as long as the power supplies of the +microcontroller and the load don't share a common ground. Things become more +complicated when they do (e.g., controlling power to a component driven by the +same microcontroller). + +In that scenario, the source potential visible to the load is the difference +between the gate and the threshold potentials of the MOSFET. For example, when +the gate and the threshold potentials are 3.3 V and 1.5 V, the potential the +load sees is 1.8 V. So, to use a low-side N-channel MOSFET, we need the gate +potential to be higher than the source potential, which may not always be +practical. The alternative would be a hide-side switch. + +## P-channel high-side switch + +The following schematic shows how a high-side P-channel MOSFET (M1) could +switch power to a 6 V servo driven by a 3.3 V MCU. + + + +When the microcontroller outputs low, the M2 N-channel MOSFET stops conducting. +The R1 resistor pulls the gate of the M1 P-channel MOSFET up to +6 V, switching +the servo off. When the microcontroller outputs high on the GPIO pin, M2's +source-drain connection starts conducting, causing M1's gate potential to drop +to 0 V, which switches on power to the servo. + +## N-channel high-side switch + +The P-channel high-side switch would be the typical architecture for our use +case. However, if we have access to a potential high enough to safely raise the +gate potential above the threshold such that their difference outputs the source +potential required to drive the load, we can switch on the high side using an +N-channel MOSFET: + + + +In the schematic, both M1 and M2 are N-channel MOSFETs. When the +microcontroller output is low, M2 stops conducting. This causes the M1's gate +potential to rise above the threshold, turning the servo on. Conversely, a high +output on the GPIO line switches M2 on, which lowers M1's gate potential. This +switches the servo off. The R2 pull-up resistor prevents the high impedance of +the output pins at power-up from switching the servo on. + +Both topologies require M2 to act as a level converter between circuits +containing the microcontroller and the servo, converting between 0 V and +6 V +or +9 V. M2 is a low-power signal converter carrying less than a milliamp of +current. The gate-source threshold voltage of M2 must be lower than the MCU's +supply voltage. 2N7000, 2N7002, and BSS138 are popular choices for M2. + +The D1 flyback diodes used in the two topologies safeguard the MOSFET from +voltage spikes caused by inductive loads such as servos. + +## A BJT alternative + +A Bipolar Junction Transistor (BJT) is a simpler, cheaper, and more widely +available type of transistor that can be used as a switch. + + + +In the schematic, when the MCU outputs high, Q2 starts conducting. Q2 amplifies +Q1's base current. Unlike MOSFETs, which are voltage-driven, BJTs are driven by +base current. Resistors R3 and R4 must be chosen carefully to output the +desired base currents. <a +href="https://teachmetomake.wordpress.com/how-to-use-a-transistor-as-a-switch/" +class="external" target="_blank" rel="noopener noreferrer">"How to choose a +transistor as a switch"</a> is an excellent guide on using BJTs as electronic +switches. + +## Which topology to choose? + +The professional community appears to prefer MOSFETs over BJTs. MOSFETs are +more efficient when the switch is on. However, they are more challenging to +drive, especially with a 3.3 V MCU, due to the V<sub>GS</sub> potentials +required to achieve specified R<sub>DS(on)</sub> values (i.e., to turn them on +fully). + +N-channel MOSFETs have lower on-resistance values, making them more efficient +than P-channel ones. They are also cheaper. However, they are harder to drive +on the high side as their gate potential must be higher than the source +potential. This often requires extra circuitry such as MOSFET drivers. + +## Further reading + + - <a href="https://www.embeddedrelated.com/showarticle/98.php" + class="external" target="_blank" rel="noopener noreferrer">Different MOSFET + topologies</a> + - <a href="https://www.embeddedrelated.com/showarticle/809.php" + class="external" target="_blank" rel="noopener noreferrer">How to read + MOSFET datasheets</a> + - <a src="https://teachmetomake.wordpress.com/how-to-use-a-transistor-as-a-switch/" + class="external" target="_blank" rel="noopener noreferrer">How to use a + transistor as a switch</a> + - <a src="https://forum.digikey.com/t/guide-to-selecting-and-controlling-a-mosfet-for-3-3-vdc-logic-applications/42606" + class="external" target="_blank" rel="noopener noreferrer">Guide to + selecting and controlling a MOSFET for 3.3 VDC logic applications</a> + - <a src="https://forum.digikey.com/t/driving-a-large-relay-from-a-3-3-vdc-microcontroller-using-an-npn-darlington-transistor/41751" + class="external" target="_blank" rel="noopener noreferrer">Driving a large + relay from a 3.3 VDC microcontroller using an NPN Darlington transistor</a> |