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diff --git a/_log/mosfet-switches.md b/_log/mosfet-switches.md index 5e17849..4fc1d31 100644 --- a/_log/mosfet-switches.md +++ b/_log/mosfet-switches.md @@ -1,108 +1,80 @@ --- -title: On the use of MOSFETs as electronic switches +title: High-side MOSFET switching 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. +Needed low-power switching for the [fingerprint door +lock](../fpm-door-lock-lp/). Servo and FPM draw high quiescent current--had to +cut power electronically during sleep. MOSFETs can do this. -## 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 +Schematics belong to <a href="https://electronics.stackexchange.com/users/292884/simon-fitch" class="external" target="_blank" rel="noopener noreferrer">Simon Fitch</a>. -## Preamble +## Problem with simple low-side switching -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). +Typical approach: GPIO → gate of N-channel MOSFET on low side, pull-down +resistor between gate and drain. Works if MCU and load don't share common +ground. Doesn't work when they do (like controlling a component powered by the +same MCU). -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. +Issue: source potential = gate potential - threshold voltage. Example: 3.3V +gate - 1.5V threshold → 1.8V at load--not nearly enough for a servo. Raising +the gate potential above source is not always practical. Solution: high-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. +M1 is P-channel (high-side), M2 is N-channel (level converter). MCU output low +→ M2 off → R1 pulls M1 gate to +6V → servo off. MCU output high → M2 conducts → +M1 gate drops to 0V → servo on. -## N-channel high-side switch +Note: IRF9540 in the schematic doesn't work. V<sub>GS</sub> (-10V) for +RDS<sub>on</sub> too much for 3.3V ATmega328P to drive. NDP6020P is the only +suitable through-hole MOSFET I could find. -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: +## N-channel high-side switch  -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. +Less common but works if you have voltage high enough to drive the gate. Both +M1 and M2 are N-channel. MCU low → M2 off → M1 gate rises above threshold → +servo on. MCU high → M2 on → M1 gate drops → servo off. R2 prevents +high-impedance power-up from switching 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. +M2 needed in both topologies for level conversion (0V ↔ +6V or +9V). Carries +<1mA. Gate-source threshold must be lower than MCU supply. Common choices: +2N7000, 2N7002, BSS138. -The D1 flyback diodes used in the two topologies safeguard the MOSFET from -voltage spikes caused by inductive loads such as servos. +Note: D1 flyback diodes protect MOSFETs from voltage spikes caused by inductive +loads (servos, relays). ## 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 +Simpler, cheaper, more available. Q2 conducts when MCU outputs high. Q2 +amplifies Q1's base current. Unlike MOSFETs (voltage-driven), BJTs are +current-driven. R3 and R4 must be calculated for 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. +class="external" target="_blank" rel="noopener noreferrer">Guide on BJT +switches</a>. + +## Which topology? -## Which topology to choose? +MOSFETs preferred in professional work—more efficient when on. Harder to drive +at 3.3V due to V<sub>GS</sub> requirements for full saturation (low +R<sub>DS(on)</sub>). -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: Lower on-resistance, cheaper, more efficient than P-channel. Harder +to drive high-side (gate must be above source—requires extra circuitry like +MOSFET drivers). -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. +Used P-channel high-side for the door lock redesign. Simpler to drive from 3.3V +MCU, no driver needed. ## Further reading @@ -112,12 +84,12 @@ potential. This often requires extra circuitry such as MOSFET drivers. - <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/" + - <a href="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" + - <a href="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" + - <a href="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> |