MOSFET Tutorial: Practical NMOS and PMOS Circuits

The first test

Let's close the NMOS circuit and see the load turn on and off before any theory

Required parts

• ESP32 or ESP32-S3
• AO3400A MOSFET on SOT-23 conversion board or test PCB
• Twelve-volt LED strip or low-risk resistive load
• 12V source with current limiter
• 220 Ohm resistance for the gate path
• 100 kilo ohm resistance between Gate and Source
• multimeter

Connections, step by step

1. Connect the positive of the 12V source to the positive of the load.
2. Connect the negative charge to the Drain pin of the MOSFET.
3. Connect the Source pin of the MOSFET to the ground of the 12V source.
4. Connect the ESP32 ground to the same ground as the 12V source.
5. Connect GPIO18 to Gate through 220 ohm resistor.
6. Place a 100 kOhm resistor between Gate and Source.
7. Set the source current limiter to a low value first and then connect the power supply.

Simple command to test on and off

#include "driver/gpio.h"
#include "freertos/FreeRTOS.h"
#include "freertos/task.h"

#define MOSFET_GATE GPIO_NUM_18

void app_main(void)
{
    gpio_reset_pin(MOSFET_GATE);
    gpio_set_direction(MOSFET_GATE, GPIO_MODE_OUTPUT);
    gpio_set_level(MOSFET_GATE, 0);

    while (1) {
        gpio_set_level(MOSFET_GATE, 1);  // Load on
        vTaskDelay(pdMS_TO_TICKS(2000));

        gpio_set_level(MOSFET_GATE, 0);  // Load off
        vTaskDelay(pdMS_TO_TICKS(2000));
    }
}

What should we see?

Now let's extract the concept from the circuit

What exactly does Gate, Drain, Source and VGS mean in this circuit?

Why did we put a resistance of 100 kilo ohms?

When the ESP32 is powered on or reset, the GPIO pin may be input for a few moments with no known status. Gate MOSFET can hold residual charge like a small capacitor. A 100 kOhm resistor pulls the Gate to the Source so that the MOSFET remains off by default.

Why did we put 220 Ohm resistance of Gate series?

When changing state, the GPIO must charge or discharge the gate capacitance. Series resistance limits the instantaneous current and helps reduce oscillation and noise. For this test, 220 ohms is a good value, but in fast PWM, the exact value should be checked with the gate load, frequency and required speed.

Read the datasheet only for real decisions

Why was the AO3400A chosen for this example?

We are not going to keep all the datasheets. We only ask questions that are necessary for the 12V circuit and the 3.3 V GPIO.

P = I² × R
P = 1² × 0.048
P = 0.048 W

P = 2² × 0.048 = 0.192 W

Let's make the problem one step more difficult

Instead of just turning on and off, change the intensity of the light with PWM

Now we keep the same circuit and control the GPIO with PWM. The MOSFET turns the load on and off at a rate of several thousand times per second. The human eye sees the average light; So as the Duty Cycle changes, the brightness changes.

#define PWM_FREQUENCY_HZ 5000
#define PWM_MAX_DUTY     1023

static void pwm_set_percent(uint32_t percent)
{
    if (percent > 100) {
        percent = 100;
    }

    uint32_t duty = (PWM_MAX_DUTY * percent) / 100;
    ledc_set_duty(LEDC_LOW_SPEED_MODE, LEDC_CHANNEL_0, duty);
    ledc_update_duty(LEDC_LOW_SPEED_MODE, LEDC_CHANNEL_0);
}

Which new datasheet parameter becomes important here?

On slow on and off, gate capacitance wasn't nearly as big of an issue. In PWM, the Qg or Gate Charge parameter becomes important. The higher the Qg, the more charge the GPIO or driver has to move per cycle, and the longer the MOSFET transits through the half-on region.

A real error in projects

What changes if our load is a pump, motor, relay or solenoid valve?

An inductive load stores energy when current flows. When the MOSFET suddenly turns off, this energy tries to keep the current flowing and can raise the drain voltage very high. For this reason, we put a diode parallel to the load.

Here we realize that VDS equal to 30V alone is not enough guarantee. For 12 V inductive load, diode, TVS, short circuits and checking waveform with oscilloscope are important.

Problem number 2

Sometimes we don't want to disconnect the load; We have to stop feeding positively

Suppose a 5V sensor or module is connected to ESP32 and its data lines must have a common ground. If we cut power from the ground side with the NMOS, the signal paths may inadvertently feed the module through the data pins. A better way is to disconnect the positive 5V. This is where PMOS comes in.

Why didn't we connect the Gate directly to the ESP32?

Source MOSFET is on 5V. For the PMOS to turn off, the Gate must reach approximately the same 5V; But the ESP32 GPIO outputs a maximum of 3.3V. If the Gate is directly connected to the GPIO, in the High state, VGS will be equal to 3.3 minus 5, that is, about negative 1.7 volts, and the MOSFET may not turn off completely. So a small NPN transistor pulls the gate down and the pull-up resistor pulls it back to the source when off.

Step by step connections

1. Source Connect the AO3401A MOSFET to the positive 5V.
2. Connect the drain to the positive of the load.
3. Common the ESP32's negative load and ground.
4. Place a 100 kOhm resistor between Gate and Source so that the default PMOS is off.
5. Connect the Gate to the NPN collector through a 1 kOhm resistor.
6. Connect the NPN emitter to ground.
7. Connect GPIO19 to the NPN base through a 4.7 kOhm resistor.
8. Place a 100 kOhm resistor between the base and emitter of the NPN so that the path remains off by default when the GPIO is floating.

The 100 kohm Gate-Source resistance is for slow connection and disconnection of the power supply; This is not a fast PWM driver circuit. Test behavior during reset, load inrush current and feed back from data lines separately. No physical test of this circuit is claimed in this article.

static void load_power_set(bool enabled)
{
    // In this circuit, GPIO=1 turns on the NPN, which then turns on the PMOS.
    gpio_set_level(GPIO_NUM_19, enabled ? 1 : 0);
}

Second datasheet, just for this circuit

Let's review AO3401A with practice questions

Loss calculation for 800 mA load

P = I² × R
P = 0.8² × 0.060
P ≈ 0.038 W

Summary based on application

When to choose NMOS and when to choose PMOS?

practical exercise

This lesson is over when you measure the numbers

1. In NMOS circuit, measure VGS in off and on state.
2. In the ON state, measure VDS and calculate the approximate resistance of the path with the relation R≈VDS/I.
3. Record the load current at 25, 50, 75 and 100% duties.
4. After five minutes of operation, check the temperature of the MOSFET and the temperature of the copper around it.
5. Turn off the source, deliberately match the Drain and Source to the pinout map, and check the direction of the body diode in the Diode Test mode.
6. Run the PMOS circuit and measure VGS in two GPIO modes zero and one.
7. Disconnect and reconnect the positive load wire and check that the load ground remains common.

troubleshooting

If the circuit does not work, proceed with this procedure

1. Check common ground. In NMOS circuit, ESP32 ground and load source must be common.
2. Match the actual pinout of the part with the datasheet. Do not guess the SOT-23 pin order from the appearance.
3. Measure VGS, not just gate voltage to ground.
4. Gate should not float. Check Pull-down for NMOS and Pull-up to Source for PMOS.
5. The source has not entered the flow limiter.
6. Measure the on state VDS. A high value means that the MOSFET is not completely turned on, the part is bad, or the connection is wrong.
7. For inductive load, check the stray diode and its direction.
8. In PMOS, don't get the logic circuit wrong. Gate going low relative to Source causes it to turn on.