Soft-Start and Soft-Finish for P-Channel MOSFETs

I needed a Soft-Start circuit for a pair of P-Channel MOSFETs. I searched the web and found little to none of these. The ones I did find were used as one-shot deals for turning lights on slowly and softly. I needed one for modifying a waveform at 31 kHz. Since I could find no acceptable circuits for my needs, I decided to venture down this path and invent one of my own. I started out with the idea of using capacitors, diodes, and resistors, much like those used for N-Channel MOSFETs.

After few tries using capacitors, I found that they were causing more problems than they were solving. I was using the very great soft-start circuit published on the Texas Instruments website in their E2E support forums called Effects of Gate RC Soft Start. The author of the paper did not identify himself; all I have is the link to the pdf included here. This soft-start is for N-Channel MOSFETs, and I use it in my circuits today. The one illustrated at the end of the paper on page 5 is a well thought out circuit and it works great. In his circuit he used a PNP transistor to export the charge from the capacitor into ground, instead of back into the gate-drive signal, which is what most passive snubber-type circuits do. However, at over 6 kHz, the PNP burns up (gets untouchably hot). So I switched out the PNP BJT for a P-Channel MOSFET, and now it works great.

The external capacitor was causing my gate-driving signal to move to the right, like adding time into the signal. So I got rid of the external capacitor. That left me with diodes and resistors, both of which ended up in the final circuit, and I had 1 capacitor left, the MOSFET gate. The diode needed to be in the circuit to prevent the gate-driver IC's totem-pole from draining the gate of the P-Channel MOSFET. The soft-start does not work without this diode. The gate/capacitor needed to drain through the resistors, and not the IC's totem-pole, to get the soft-start effect.

The soft-finish effect was achieved by putting exactly the same set of resistors on the Drain of the MOSFET, a 5 kΩ variable resistor along with a 330 Ω backstop resistor (the backstop keeps the variable resistor from burning up when you get too low in Ω with the variable resistor).

The effects of the soft-start and soft-finish are different, but they do stop the ringing in the gate-drive signals from sudden-on and sudden-off transitions in the switching of the MOSFET.

These circuits are pretty easy to build, as they should be. They are simple but elegant devices to shape the waveform of the output signal from a MOSFET drain, and the input signal going into the gate.

1. 12 VDC power supply
2. Oscilloscope
3. P-Channel MOSFET - 1X P-Channel MOSFET needed for this project. I used IRF9540Ns (Vds -100V; TO-220 package; one of my favorite P-Channel MOSFETs) for this development work. But any P-Channel MOSFET should work: IRF9640 (Vds -200V; TO-220 package), IRFP9240 (Vds -200V; TO-247AC package). Available on both Amazon and eBay.
4. Potentiometer - 2X 5 kΩ needed for this project. I used these from Amazon.
5. Resistors - 2X 330 Ω, 1X 47 Ω (and 1X 4.7 kΩ if you want to put a power-ON LED on the breadboard). I bought my resistors from both Amazon and eBay, For a ½ W resistor at 12 VDC, the minimum Ω is 288 Ω. So I used 330 Ω as I didn't have any other Ohm-age between 288 and 330 Ω. If you want to use less Ω, you need a 1 or 2 W resistor. I use the formula (R = V² / W) to tell me what resistor to use.
6. Diodes - I use UF4007 quite a lot. I think the UF came from "Ultra Fast". These are good up to 1000V.
7. Solderless breadboard
8. Wire (I use 22 AWG)

To make signal to put through your MOSFET gate, you are going to need some kind oscillating signal, pulsed DC. I recommend my Instructable A Way-Better Variable-Frequency PWM Using Two TLC555 Timers (or One TLC556). Add a gate-drive (MC34152P) to the output signal from the 2nd timer. Connect the gate-drive to this circuit through the diode.

Before handling your MOSFET, ground yourself. I use this copper foil attached to Earth-ground and I put my bare foot on it before handling any MOSFETs or ICs. It prevents electrostatic discharge (ESD) from destroying your sensitive electronic circuits.

Here we have the necessary components laid out:

1. 2X 330 Ω resistors
2. 1X 47 Ω resistor
3. 2X 5 kΩ variable resistors (potentiometers)
4. 1X diode
5. 1X P-Channel MOSFET
6. 1X small connector block (sometimes called a terminal block)

The photo contains 1X large connector block for TO-247 size MOSFETs as an illustration. I use these connector blocks to keep from destroying my breadboards. The pins on the MOSFETs are too large for the solderless breadboards. The smaller connector blocks need to be mounted diagonally. It is easy to misjudge which pin column goes to what pin of the MOSFET. But once you get one correct, the other two are immediately adjacent to it.

If you leave the full length of the component lead wires, you will bend them at some point and short out everything on your breadboard. I recommend cutting them down to 6 mm. I use a tagboard template I cut from a box. I cut down the 4 resistors. The diode was already cut and I don't cut the potentiometers nor the MOSFET.

In order to fit the MOSFET into the connector block, I bend ~6 mm of the lead away from the front of the MOSFET. Then I twist each outer lead (Gate and Source) outward a tiny amount, maybe 5°. This helps the MOSFET fit easily into the connector block.

Unscrew the connector block screws and insert the MOSFET leads. Screw down the screws. We will insert this into the breadboard as the last component.

I am using the IRFP9540, which has a TO-220 package, in this Instructable. If you are going to use a larger MOSFET, like the IRFP9240 in the larger TO-247 package, I show the example of the larger connector block and how it fits into the breadboard.

Your components are now all prepared. We will go to the next step.

First thing (I sometimes forget to do this) to do is place two wires at one end of your breadboard connecting your upper power rails to your lower power rails.

Begin placing your components into the breadboard. The 330 Ω resistors connect into the (-) rail (ground rail). Each connects to the left pin of the 3-pin potentiometer. The gate resistor will cross the channel. The banded side (cathode) of the diode will connect in the upper end of the gate resistor. Later your oscillating signal will come in through the anode (unbanded end) side of this diode.

Now place the MOSFET diagonally on the bread board. Make sure the gate pin, the left pin, is in the same column as the gate resistor.

Now we are going to place the wires into the board.

There are only 5 wires internal to this circuit. The first two wires to place, connect pins 1 and 2 of each potentiometer. These wires turn the potentiometer into a variable resistor.

Next put a wire from the (+) rail to the Source of the MOSFET, pin 3, the right side pin. The path of power in a P-Channel MOSFET is from Source to Drain, exact opposite of an N-Channel MOSFET.

Next put a wire from the junction of the banded end of the diode and the gate resistor to the right pin of the left variable resistor. This wire drains the MOSFET's internal gate capacitor through the variable resistor then the fixed resistor on the left side of the circuit.

Next put a wire from the Drain of the MOSFET (middle pin) to pin 3 (right side) of the right side variable resistor. This is also where signal and power will come out the of the circuit, this third column, the right side of the right side variable resistor.

This part is optional. It is not part of this circuit officially. I like to have a power-ON LED on all of my boards. In the last photo, there is a wire from the (+) rail to the anode of the LED (longer wire). The cathode connects to a 4.7 kΩ resistor and the far end of the resistor plugs into the ground or (-) rail. The order of resistor and LED does not matter, if you put the LED first to the (+) rail or the resistor. It works either way.

Put a +12 VDC wire into the red (+) rail, and ground wire into the blue (-) rail.

The last connection you will make is to put a oscillating signal into the anode (unbanded) side of the UF4007 diode, preferably at 31 kHz. If you go to different Hz, you will need to adjust your resistors. I believe if you go to higher Hz, you will need lower resistance. If you go to lower Hz, you will need higher resistance. You will need to experiment with it.

Next, we will measure our results and see what the variable resistors do.

Connect an oscilloscope probe to breadboard column where pin 3 (right side) of the right side variable resistor is plugged. This also the MOSFET's Drain output. This is where you will be able to observe the modified waveform from the soft-start and soft-finish modifications.

Photo 1 of this section is the starting point with no resistance on either of the variable resistors. Photos 2-4 are from turning the left side variable resistor. The blue trace peaks in photos 2-4 show the effects. The yellow trace peaks are from the gate-drive IC (MC34152P; dual gate drive with hysteresis, Schmitt Trigger with totem pole). The signal going into the gate-drive IC comes from my circuit A Way-Better Variable-Frequency PWM Using Two TLC555 Timers (or One TLC556), also noted in the Supplies section of this Instructable.

Photos 5-6 show the effects of turning the right side variable resistor. The effect can be even more extreme than shown here in the photos. See the video in the link below, where I twist the right side variable resistor to its fullest.

Notice in the photos that the blue trace is inverted from the yellow trace. That is the nature of P-Channel MOSFETs (and PNP BJT transistors). When the gate-signal is high, the power flowing through the MOSFET is low. The exact opposite is true for N-Channel MOSFETs, the output is 0° in-phase with input signal.

I was able to create both a soft-start as well as a soft finish for my P-Channel MOSFETs. These are already included in my bigger project on which I am working. I hope others will be able to make use of this invention.

Thank you for your interest. If you like the circuit and you find it useful, give me an Instructables Heart (Favorite). If you find any errors, or I left something out, send me a comment so I can address it.

Today is 28-APR-2025. Since I first submitted this Instructable, I have added two additional components (see new circuit diagram). These are two new SiC diodes, connected to ground.

The first diode is connected between the MOSFET Gate and gate-resistor, and it goes to ground. The second diode is connected to the output wire and ground. These are very fast Schottky diodes that respond in less than 10 ns. They are overkill but they are the fastest diode I had in my inventory. They can block 1200 V and are SiC diodes (D10S120). I was seeing a ~2 V spike in the input and output signals that I wanted to try to dampen. So I tried a simple set of diode clamps, and they worked spectacularly well. If you are going to use these, be sure to connect the cathode side (pin 1) to the higher voltage wire, your 12 VDC supply wire, and the anode (pin 2), to the lower voltage wire, in this case, ground.

With this update, I included a new circuit diagram, 3 photos, and a 24 second video of the update.

In the first photo, the two TO-220 packaged diodes can be seen on the right. I also exchanged a larger TO-247 IRFP9240 in place of the previous TO-220 IRF9540.