---
title: "Is the FET with the lowest Rds(on) the best one?"
topic: "FET choice"
answer: "Only with a fast gate driver. In moteus-c1’s 48 V, 10 A half bridge at 30 kHz, the 1.2 mΩ IRFS7530 loses 0.77 W and the 15 mΩ TPN19008QM 1.98 W with 1 A of gate current. At 60 mA they lose 9.08 W and 2.41 W."
url: "https://nearfieldnotes.com/notes/fet-choice/"
site: "Near Field Notes"
author: "Michael Walker"
note: 20
family: "Power"
published: "2026-11-05"
revised: "2026-11-05"
revision: 1
video: "https://nearfieldnotes.com/videos/fet-choice/"
---

Note 20 · [Power](https://nearfieldnotes.com/families/power/) · Rev 1 · [Watch the video · 0:19](https://nearfieldnotes.com/videos/fet-choice/)

FET choice

# Is the FET with the lowest Rds(on) the best one?

**Short answer:** Only with a fast gate driver. In moteus-c1’s 48 V, 10 A half bridge at 30 kHz, the 1.2 mΩ IRFS7530 loses 0.77 W and the 15 mΩ TPN19008QM 1.98 W with 1 A of gate current. At 60 mA they lose 9.08 W and 2.41 W.

The figure drives both FETs from the same gate driver and steps its current down from 1 A to 60 mA. The IRFS7530’s 236 nC gate takes longer to fill at every step, and its turn-on heat climbs from 13.0 µJ to 188 µJ.

The TPN19008QM holds 15 times less charge. Even at 60 mA it is half on for only 48 ns, and its leg’s loss barely moves.

**Note 20’s opening figure: One driver, two FETs.** Gate current from 1 A down to 60 mA

![Note 20’s opening figure: One driver, two FETs](https://nearfieldnotes.com/notes/fet-choice/fig-opening.png)

*What the figure shows:* Two FETs turned on by the same DRV8323 gate driver in moteus-c1's half bridge, 48 V and 10 A, as its gate current steps from 1 A down to 60 mA. For each FET, the heat in it as it turns on, and below, the leg's whole loss at 30 kHz. 1 A: IRFS7530 13.0 µJ a turn-on, 0.77 W. TPN19008QM 0.98 µJ, 1.98 W. 570 mA: IRFS7530 21.1 µJ a turn-on, 1.15 W. TPN19008QM 1.44 µJ, 2.00 W. 330 mA: IRFS7530 35.3 µJ a turn-on, 1.82 W. TPN19008QM 2.14 µJ, 2.02 W. 260 mA: IRFS7530 44.3 µJ a turn-on, 2.26 W. TPN19008QM 2.60 µJ, 2.04 W. 120 mA: IRFS7530 94.5 µJ a turn-on, 4.66 W. TPN19008QM 5.14 µJ, 2.17 W. 60 mA: IRFS7530 188 µJ a turn-on, 9.08 W. TPN19008QM 9.91 µJ, 2.41 W.

Heat in each FET as it turns on, from the gate command, at 48 V and 10 A. The faint lines are the other gate currents. Below, the leg’s whole loss at 30 kHz: darker for its on-resistance, lighter for switching.

**The numbers**

*Turn-on heat beyond plain conduction, and the leg’s loss at 30 kHz. 48 V, 10 A.*

| Gate current | IRFS7530, a turn-on | IRFS7530, leg loss | TPN19008QM, a turn-on | TPN19008QM, leg loss |
|---|---|---|---|---|
| 1 A | 13.0 µJ | 0.77 W | 0.98 µJ | 1.98 W |
| 570 mA | 21.1 µJ | 1.15 W | 1.44 µJ | 2.00 W |
| 330 mA | 35.3 µJ | 1.82 W | 2.14 µJ | 2.02 W |
| 260 mA | 44.3 µJ | 2.26 W | 2.60 µJ | 2.04 W |
| 120 mA | 94.5 µJ | 4.66 W | 5.14 µJ | 2.17 W |
| 60 mA | 188 µJ | 9.08 W | 9.91 µJ | 2.41 W |

## The half bridge and its gate driver

*One phase of moteus-c1*

A motor driver connects each motor wire to the supply or to ground through two FETs, one above the other. Switching between the two thousands of times a second sets the motor’s current.

On moteus-c1 the supply is 48 V, and the switching runs at 30 kHz, its firmware’s default. This page takes 10 A out of one leg.

A gate driver turns each FET on by pushing current into its gate, and off by pulling it out. The DRV8323 sets that current in 16 steps from 10 mA to 1 A. It powers up at 1 A, and moteus-c1’s firmware turns it down to 60 mA to turn on and 20 mA to turn off.

**Fig. 20: One leg and its driver.**

![Fig. 20: One leg and its driver](https://nearfieldnotes.com/notes/fet-choice/fig-20.png)

*What the figure shows:* Figure 20: the circuit. A 48 V rail feeds the high-side FET Q1 over the low-side FET Q2, each with its body diode. Their middle, the switch node, sends 10 A to the motor. The DRV8323 gate driver drives Q1's gate from GH, with SH back from Q1's source, and Q2's gate from GL. A capacitor C holds the rail to ground. The high side's gate loop is marked: out of the driver on GH to Q1's gate and back from its source on SH.

Each FET has its body diode drawn beside it. The shaded loop is the high-side FET’s gate current, out of GH and back on SH.

**The numbers**

| Part | What it is |
|---|---|
| Q1, Q2 | Both IRFS7530 or both TPN19008QM, n-channel |
| Driver | DRV8323, 60 mA to 1 A into a gate, twice that out of it |
| Supply | 48 V |
| Motor | 10 A out of the switch node |

Two FETs suit this leg. VESC 4.12 uses the IRFS7530, a 60 V part with 13 times less resistance. moteus-c1 uses the 80 V TPN19008QM.

[Low resistance](https://nearfieldnotes.com/glossary/#g-rds-on) takes a big die, and a big die has a big gate. The IRFS7530 holds 236 nC of [gate charge](https://nearfieldnotes.com/glossary/#g-gate-charge) against 16 nC, and its package takes 14 times the board.

The rest of the page puts each pair in the same leg in turn, with the same driver.

**Fig. 20b: The two FETs, to scale.**

![Fig. 20b: The two FETs, to scale](https://nearfieldnotes.com/notes/fet-choice/fig-20b.png)

*What the figure shows:* Figure 20b: the two FETs rendered at one scale. The IRFS7530 in a D2PAK-7, 10.1 by 15.25 mm over its seven leads and drain tab, and 4.45 mm tall, with 1.2 mΩ and 236 nC of gate charge. The TPN19008QM in a TSON Advance, 3.3 mm square and 0.85 mm tall, with 15 mΩ and 16 nC, shown right way up and turned over to show its drain pad.

Drawn from each datasheet’s package drawing at one scale, so they compare at true size. The TPN19008QM is shown twice, the second turned over. Its drain pad, like the D2PAK’s tab, carries the drain into the board. Resistance and gate charge are typical, at 25 °C with 10 V on the gate.

**The numbers**

*Sizes from each datasheet’s package drawing. Datasheet typical values at 25 °C and 10 V on the gate.*

| Part | Package | Size | Drain | Rds(on), typical | Gate charge | Rated | Board |
|---|---|---|---|---|---|---|---|
| IRFS7530 | D2PAK-7 | 10.1 × 15.25 × 4.45 mm | Tab | 1.15 mΩ | 236 nC | 60 V | VESC 4.12 |
| TPN19008QM | TSON Advance | 3.3 × 3.3 × 0.85 mm | Pad underneath | 14.7 mΩ | 16 nC | 80 V | moteus-c1 |

## Two ways to lose power

*48 V, 10 A, 30 kHz*

A FET loses power in two ways. While it is on, the motor’s current heats its resistance:

`P_R = I^2 R`

where P_R: loss in the on-resistance (W); I: motor current (A); R: one FET’s on-resistance at 100 °C (Ω).

`= (10 A)² × 1.45 mΩ = 0.15 W`

That is the IRFS7530. The TPN19008QM’s 19.1 mΩ loses 1.91 W, 13 times more. R is each FET’s resistance at 100 °C, and one FET or the other always carries the current, so this is the whole leg’s.

Every cycle also costs a fixed energy, while a FET is half on and the current changes hands between the two:

`P_sw = E f`

where P_sw: switching loss (W); E: energy both FETs lose in one cycle, beyond their resistance (J); f: pWM frequency (Hz).

`= 21.0 µJ × 30 kHz = 0.63 W`

E counts the whole cycle, with both edges, the dead time and the [diode’s recovery](https://nearfieldnotes.com/glossary/#g-body-diode). The 13.0 µJ turn-on in the opening figure is one part of the 21.0 µJ.

That is the IRFS7530 with 1 A of gate current. The TPN19008QM loses 2.43 µJ a cycle, so 0.07 W. At 60 mA the big FET’s switching costs 8.93 W.

**Fig. 20c: Where the loss goes.**

![Fig. 20c: Where the loss goes](https://nearfieldnotes.com/notes/fet-choice/fig-20c.png)

*What the figure shows:* Figure 20c: the leg's loss at 30 kHz, split into the on-resistance's share and switching's. IRFS7530 at 1 A: 0.15 W in its on-resistance and 0.63 W switching, 0.77 W. TPN19008QM at 1 A: 1.91 W in its on-resistance and 0.07 W switching, 1.98 W. IRFS7530 at 60 mA: 0.15 W in its on-resistance and 8.93 W switching, 9.08 W. TPN19008QM at 60 mA: 1.91 W in its on-resistance and 0.50 W switching, 2.41 W.

Each FET in the same leg at 30 kHz, with 1 A and with 60 mA of gate current, twice that to turn off. Darker is the on-resistance’s share, lighter is switching.

**The numbers**

*At 48 V, 10 A and 30 kHz, on-resistance at 100 °C.*

| FET, gate current | On-resistance | Switching | Leg loss |
|---|---|---|---|
| IRFS7530, 1 A | 0.15 W | 0.63 W | **0.77 W** |
| TPN19008QM, 1 A | 1.91 W | 0.07 W | **1.98 W** |
| IRFS7530, 60 mA | 0.15 W | 8.93 W | **9.08 W** |
| TPN19008QM, 60 mA | 1.91 W | 0.50 W | **2.41 W** |

## The catch is the gate

*Both FETs turning on at 60 mA*

Switching loss comes from the time a FET spends half on, and the gate sets that time. The driver fills the gate at a set current, so a gate that holds more charge takes longer:

`t_g = Q_g/I_g`

where t_g: time to fill the whole gate (s); Q_g: gate charge (C); I_g: gate current (A).

`= (236 nC)/(60 mA) = 3.9 µs`

The TPN19008QM’s 16 nC fills in 0.27 µs.

**Fig. 20d: Turning on at 60 mA.**

![Fig. 20d: Turning on at 60 mA](https://nearfieldnotes.com/notes/fet-choice/fig-20d.png)

*What the figure shows:* Figure 20d: both FETs turning on with 60 mA of gate current, from the gate command. The TPN19008QM's voltage falls and its current rises within 119 ns, half on for 48 ns. The IRFS7530 stays off until 886 ns, then carries the full 10 A while its voltage falls slowly, half on for 792 ns, heating at up to 538 W.

The high-side FET from its gate command, at 48 V and 10 A. Half on runs from 0.5 A through the FET until the voltage across it is down to 4.8 V.

**The numbers**

*Half on: from 0.5 A through it until the voltage is down to 4.8 V.*

| FET | Current starts | Half on | Peak heat | Turn-on heat |
|---|---|---|---|---|
| IRFS7530 | 886 ns | 792 ns | 538 W | 188 µJ |
| TPN19008QM | 71 ns | 48 ns | 561 W | 9.91 µJ |

Switching loss

### Half on for 792 ns, or 48 ns

**Field marks.** At 60 mA the IRFS7530 waits 886 ns before any current flows. Then it carries the full 10 A for 792 ns while its voltage falls, heating at up to 538 W. The textbook sketch, `E_on ≈ ½ V I t_half`, gives ½ × 48 V × 10 A × 792 ns = 190 µJ, and the turn-on in Fig. 20d loses 188 µJ. The TPN19008QM is through in 48 ns.

**Look-alike.** The whole gate’s fill time. The FET heats only while its voltage falls, and that lasts as long as the gate-drain charge takes to fill: `t_gd = Q_gd / I_g = 73 nC / 60 mA = 1.2 µs`. Fig. 20d has the IRFS7530 half on for 792 ns.

**Range.** At 1 A the IRFS7530 is half on for 46 ns and loses 13.0 µJ turning on, near the small FET’s 9.91 µJ at 60 mA.

**Habitat.** Drives that turn their gate current down to tame [ringing](https://nearfieldnotes.com/glossary/#g-ringing). moteus-c1 turns its gates on at 60 mA, where an IRFS7530 would stay half on 16.5 times as long as its own TPN19008QM.

## Which one wins

*Every DRV8323 step, 10 A at 30 kHz*

Add the two losses for the leg’s whole loss, and do it at every step the driver offers:

`P = I^2 R + E f`

where P: leg loss, both FETs together (W); I: motor current (A); R: one FET’s on-resistance at 100 °C (Ω); E: energy both FETs lose in one cycle, beyond their resistance (J); f: pWM frequency (Hz).

`= 0.15 W + 298 µJ × 30 kHz = 9.08 W`

That is the IRFS7530 at 60 mA, the left end of its line in Fig. 20e.

**Fig. 20e: Leg loss against gate current.**

![Fig. 20e: Leg loss against gate current](https://nearfieldnotes.com/notes/fet-choice/fig-20e.png)

*What the figure shows:* Figure 20e: each leg's loss at 30 kHz against the DRV8323's gate current, 48 V and 10 A. The IRFS7530 falls from 9.08 W at 60 mA to 0.77 W at 1 A. The TPN19008QM stays near 2 W, from 2.41 W to 1.98 W. They cross near 290 mA.

Each dot is one of the DRV8323’s steps, with twice that current to turn off. 48 V, 10 A, 30 kHz.

**The numbers**

*Leg loss at 30 kHz, 48 V, 10 A. Turn-off current is twice the turn-on current.*

| Gate current | IRFS7530 | TPN19008QM |
|---|---|---|
| 60 mA | 9.08 W | 2.41 W |
| 80 mA | 6.88 W | 2.29 W |
| 120 mA | 4.66 W | 2.17 W |
| 140 mA | 4.02 W | 2.14 W |
| 170 mA | 3.36 W | 2.10 W |
| 190 mA | 3.02 W | 2.08 W |
| 260 mA | 2.26 W | 2.04 W |
| 330 mA | 1.82 W | 2.02 W |
| 370 mA | 1.64 W | 2.02 W |
| 440 mA | 1.42 W | 2.01 W |
| 570 mA | 1.15 W | 2.00 W |
| 680 mA | 0.97 W | 1.99 W |
| 820 mA | 0.86 W | 1.98 W |
| 1 A | 0.77 W | 1.98 W |

FET choice

### The low-resistance FET wins above 290 mA

**Field marks.** With 1 A of gate current the IRFS7530 loses 0.77 W and the TPN19008QM 1.98 W. At 60 mA, 9.08 W and 2.41 W. The lines cross at 290 mA.

**Look-alike.** Pick the lowest Rds(on). Part searches sort by it, and in this leg it is the right pick only above 290 mA of gate current.

**Range.** Over the driver’s steps the IRFS7530’s loss moves 12 times, from 0.77 W to 9.08 W. The TPN19008QM’s moves from 1.98 W to 2.41 W.

**Habitat.** VESC 4.12 drives its IRFS7530 from a DRV8302, a fixed driver rated 1.7 A, through 4.7 Ω. moteus-c1 turns its DRV8323 down to 60 mA on and 20 mA off, and uses the TPN19008QM.

## Why a driver runs slow

*The FET that is off, as the other turns on*

If a fast gate lets the big FET win, why turn a driver down? A fast edge makes the loop’s inductance ring, and the FET that is off takes the spike. [Note 4](https://nearfieldnotes.com/notes/gate-ringing/) follows it through moteus-c1’s copper.

At 1 A the TPN19008QM rings up to its breakdown, 81 V in the model, past its 80 V rating. At 60 mA it peaks at 47.8 V.

The IRFS7530’s 13.0 nF gate slows its own edge, so even at 1 A it peaks at 48.8 V. Its rating is 60 V, though, so a 48 V supply leaves it 12 V.

moteus-c1’s firmware runs 60 mA on and 20 mA off. With that setting the TPN19008QM’s leg loses 3.19 W. A pair of IRFS7530 would lose 23.9 W.

**Fig. 20f: The spike on the other FET.**

![Fig. 20f: The spike on the other FET](https://nearfieldnotes.com/notes/fet-choice/fig-20f.png)

*What the figure shows:* Figure 20f: the voltage across the low-side FET as the high side turns on. The TPN19008QM at 1 A of gate current rings up to its breakdown, 81 V in the model, past its 80 V rating. At 60 mA it rises to 47.8 V and stops. The IRFS7530 at 1 A peaks at 48.8 V. It is rated 60 V.

The voltage across the low-side FET as the high side turns on, lined up where the switch node passes 24 V. 48 V, 10 A. The dotted lines are each FET’s rating, the gray dashed line the 48 V supply. 81 V is where the model’s FET breaks down.

**The numbers**

*Lined up where the switch node passes 24 V. 48 V, 10 A. 81 V is where the model’s TPN19008QM breaks down.*

| FET, gate current | Peak | Rated |
|---|---|---|
| TPN19008QM, 1 A | 81.2 V | 80 V |
| TPN19008QM, 60 mA | 47.8 V | 80 V |
| IRFS7530, 1 A | 48.8 V | 60 V |

## Current and frequency move the line

*Where both FETs lose the same*

Fig. 20e holds for one motor current and one PWM frequency. Set the two losses equal and solve for the frequency where neither FET wins:

`f_even = (I^2 (R_2 − R_1))/(E_1 − E_2)`

where f_even: pWM frequency where both FETs lose the same (Hz); I: motor current (A); R_2: the TPN19008QM’s on-resistance at 100 °C (Ω); R_1: the IRFS7530’s on-resistance at 100 °C (Ω); E_1: energy a pair of IRFS7530 lose in one cycle (J); E_2: energy a pair of TPN19008QM lose in one cycle (J).

`= ((10 A)² × 17.6 mΩ)/(18.5 µJ) = 95 kHz`

That is 10 A with 1 A of gate current. Above that frequency the TPN19008QM loses less, and below it the IRFS7530.

**Fig. 20g: Break-even frequency.**

![Fig. 20g: Break-even frequency](https://nearfieldnotes.com/notes/fet-choice/fig-20g.png)

*What the figure shows:* Figure 20g: for each motor current, the PWM frequency at which both FETs lose the same, against the gate current, both on log scales. At 2 A: 1.3 kHz at 60 mA, rising to 12 kHz at 1 A. At 5 A: 3.2 kHz at 60 mA, rising to 45 kHz at 1 A. At 10 A: 6.3 kHz at 60 mA, rising to 95 kHz at 1 A. At 20 A: 12 kHz at 60 mA, rising to 187 kHz at 1 A. Above a line the TPN19008QM loses less, below it the IRFS7530. A dot marks 60 mA at 30 kHz.

One line for each motor current, on log scales. Above its line the TPN19008QM loses less at that current, below it the IRFS7530. The dot is 60 mA at 30 kHz, the turn-on step and PWM rate moteus-c1 uses.

**The numbers**

*PWM frequency where both FETs lose the same, at each motor current.*

| Gate current | 2 A | 5 A | 10 A | 15 A | 20 A |
|---|---|---|---|---|---|
| 60 mA | 1.3 kHz | 3.2 kHz | 6.3 kHz | 9.3 kHz | 12 kHz |
| 80 mA | 1.7 kHz | 4.2 kHz | 8.3 kHz | 12 kHz | 16 kHz |
| 120 mA | 2.5 kHz | 6.3 kHz | 12 kHz | 19 kHz | 25 kHz |
| 140 mA | 3.0 kHz | 7.3 kHz | 14 kHz | 22 kHz | 29 kHz |
| 170 mA | 3.6 kHz | 8.8 kHz | 18 kHz | 26 kHz | 35 kHz |
| 190 mA | 4.0 kHz | 9.9 kHz | 20 kHz | 29 kHz | 39 kHz |
| 260 mA | 5.3 kHz | 14 kHz | 27 kHz | 40 kHz | 53 kHz |
| 330 mA | 6.5 kHz | 17 kHz | 34 kHz | 51 kHz | 67 kHz |
| 370 mA | 7.0 kHz | 19 kHz | 38 kHz | 57 kHz | 75 kHz |
| 440 mA | 7.9 kHz | 23 kHz | 45 kHz | 67 kHz | 89 kHz |
| 570 mA | 9.2 kHz | 29 kHz | 58 kHz | 86 kHz | 114 kHz |
| 680 mA | 10 kHz | 35 kHz | 71 kHz | 106 kHz | 135 kHz |
| 820 mA | 11 kHz | 40 kHz | 82 kHz | 123 kHz | 162 kHz |
| 1 A | 12 kHz | 45 kHz | 95 kHz | 141 kHz | 187 kHz |

Break-even frequency

### More current favors less resistance

**Field marks.** Resistance loss grows with the square of the current, switching loss roughly in step with it. The line climbs with current. With 1 A of gate current it sits at 12 kHz for 2 A and 187 kHz for 20 A.

**Look-alike.** One crossover gate current for every drive. Fig. 20e’s 290 mA holds at 10 A and 30 kHz. At 30 kHz the 5 A line crosses at 590 mA, and the 20 A line at 150 mA.

**Range.** At 140 mA and below, the 15 mΩ FET loses less at 30 kHz at every current here, from 2 to 20 A.

**Habitat.** moteus-c1 turns its gates on at 60 mA and runs at 30 kHz, above every line. At 60 mA the highest, for 20 A, is 12 kHz. Its 20 mA turn-off only lowers the lines. At 10 A it moves the line from 6.3 kHz down to 2.4 kHz.

## Choosing a FET

*Two FETs, one 48 V leg*

The answer turns on how hard the driver pushes the gate and how often the leg switches. The key sorts them.

Key to FET choice

### Which FET for your drive?

1. 1a The driver turns the gate on with 140 mA or less Go to 2
2. 1b With more Go to 3
3. 2a The PWM runs at 30 kHz or faster The one with less gate charge
4. 2b Slower Go to 3
5. 3a The PWM runs above your motor current’s line in Fig. 20g The one with less gate charge
6. 3b Below it The one with lower Rds(on)

For these two FETs on a 48 V leg, at 2 to 20 A. Another pair has its own lines, from `P = I^2 R + E f`. moteus-c1 turns its gates on at 60 mA and runs at 30 kHz, so it ends at 2a. Its 20 mA turn-off widens the gap. The TPN19008QM’s leg loses 3.19 W, a pair of IRFS7530 23.9 W.

## Pick the FET yourself

*Every gate current and motor current*

Choose a gate current and a motor current. Against PWM frequency each FET’s loss starts flat at its resistance loss, then climbs as its switching energy is paid more often. Both scales are logarithmic.

Where the two lines cross is the break-even frequency of Fig. 20g.

**Note 20’s explorer: Leg loss against PWM frequency.**

![Note 20’s explorer: Leg loss against PWM frequency](https://nearfieldnotes.com/notes/fet-choice/fig-explore.png)

*What the figure shows:* Leg loss against PWM frequency for both FETs at the chosen gate current and motor current, 48 V, on log scales from 0.5 to 300 kHz. Each FET's loss is its on-resistance's loss, flat at low frequency, plus its switching energy times the frequency, which takes over as the frequency climbs. The circle is where they lose the same.

48 V, resistance at 100 °C, twice the gate current to turn off. The dots sit at 30 kHz, moteus-c1’s rate.

**The numbers**

*Leg loss at 30 kHz at each motor current: IRFS7530 / TPN19008QM.*

| Gate current | 2 A | 5 A | 10 A | 15 A | 20 A |
|---|---|---|---|---|---|
| 60 mA | 1.74 / 0.17 W | 4.43 / 0.71 W | 9.08 / 2.41 W | 13.92 / 5.10 W | 18.92 / 8.76 W |
| 80 mA | 1.32 / 0.16 W | 3.35 / 0.66 W | 6.88 / 2.29 W | 10.58 / 4.90 W | 14.41 / 8.49 W |
| 120 mA | 0.90 / 0.14 W | 2.26 / 0.60 W | 4.66 / 2.17 W | 7.17 / 4.71 W | 9.78 / 8.22 W |
| 140 mA | 0.78 / 0.13 W | 1.96 / 0.59 W | 4.02 / 2.14 W | 6.20 / 4.65 W | 8.47 / 8.14 W |
| 170 mA | 0.65 / 0.13 W | 1.63 / 0.58 W | 3.36 / 2.10 W | 5.18 / 4.60 W | 7.10 / 8.06 W |
| 190 mA | 0.59 / 0.13 W | 1.46 / 0.57 W | 3.02 / 2.08 W | 4.68 / 4.57 W | 6.42 / 8.02 W |
| 260 mA | 0.45 / 0.12 W | 1.09 / 0.55 W | 2.26 / 2.04 W | 3.52 / 4.50 W | 4.87 / 7.93 W |
| 330 mA | 0.37 / 0.12 W | 0.87 / 0.54 W | 1.82 / 2.02 W | 2.85 / 4.47 W | 3.97 / 7.88 W |
| 370 mA | 0.35 / 0.12 W | 0.79 / 0.54 W | 1.64 / 2.02 W | 2.59 / 4.45 W | 3.62 / 7.86 W |
| 440 mA | 0.31 / 0.12 W | 0.68 / 0.54 W | 1.42 / 2.01 W | 2.24 / 4.44 W | 3.15 / 7.84 W |
| 570 mA | 0.27 / 0.11 W | 0.55 / 0.53 W | 1.15 / 2.00 W | 1.83 / 4.42 W | 2.60 / 7.81 W |
| 680 mA | 0.25 / 0.11 W | 0.47 / 0.53 W | 0.97 / 1.99 W | 1.56 / 4.41 W | 2.30 / 7.79 W |
| 820 mA | 0.23 / 0.11 W | 0.42 / 0.53 W | 0.86 / 1.98 W | 1.40 / 4.40 W | 2.02 / 7.78 W |
| 1 A | 0.22 / 0.11 W | 0.38 / 0.53 W | 0.77 / 1.98 W | 1.27 / 4.39 W | 1.84 / 7.77 W |

#### Conditions

- **Board:** One leg of moteus-c1 r1.2, with the loop inductances of its copper from the field runs in Note 4. Each FET pair takes the same place in it.
- **FETs:** The IRFS7530 and the TPN19008QM, as models fitted by hand to their datasheets, within 7% of them except the small FET’s gate-drain charge, 26% high.
- **Driver:** A DRV8323 model built from its datasheet: every step from 60 mA to 1 A to turn on, twice that to turn off, with its drive time, t_DRIVE, at its longest, 4 µs, so the big FET’s gate fills at the set current. moteus-c1’s own setting, 60 mA on and 20 mA off, is two more runs.
- **Load:** 48 V, with 10 A held steady over each edge. 2 to 20 A in Fig. 20g and the explorer.
- **Resistance:** Each model’s own on-resistance in the leg at 10 A, 1.04 mΩ and 13.8 mΩ. That is below the datasheets’ typicals because the driver puts 11 V on the gate, against their 10 V. Times each datasheet’s rise from 25 to 100 °C, 1.40 and 1.38, that gives 1.45 mΩ and 19.1 mΩ.
- **Lost:** Everything both FETs dissipate over one PWM cycle beyond their resistance’s share at 25 °C: both edges, the body diode through the dead time, its recovery and the FETs’ capacitance.
- **Renders:** Blender, one camera. Outsides from each datasheet’s package drawing.
- **Solver:** ngspice: 142 runs of the whole cycle, 28 turn-ons for the opening and Fig. 20d, and 3 for Fig. 20f.

Simulated only. Nothing on this page has been measured on a board yet.

#### Limits

- The IRFS7530 sits in moteus-c1’s small copper loop. Its larger package would need a larger loop, with more inductance.
- 81 V is the small FET model’s breakdown. From 820 mA up its ring reaches it, so the real peak would be higher.
- The driver’s own power isn’t counted: 0.16 W for a pair of IRFS7530 at 30 kHz, 0.01 W for the TPN19008QM.
- Whether the DRV8323’s charge pump can keep up with the big FET’s gate charge hasn’t been checked.
- Both FETs are taken at 100 °C. Their real temperature depends on how they’re cooled.

#### Files

- fsim/site.py Both FETs at every gate current and motor current on this page
- fsim/losses.py The leg, the loss count and the 100 °C factors
- fsim/waves.py The turn-ons in the opening and Fig. 20d
- fsim/ringing.py The low-side spikes in Fig. 20f
- render/site_parts.py The renders in Fig. 20b
- gatesim/fets.py The fitted FET models, from Note 4

#### Sources

- [Infineon, IRFS7530-7PPbF datasheet](https://www.infineon.com/assets/row/public/documents/24/49/infineon-irfs7530-7p-datasheet-en.pdf)
- [Toshiba, TPN19008QM datasheet, rev. 2.0](https://toshiba.semicon-storage.com/info/docget.jsp?did=68568&prodName=TPN19008QM)
- [TI DRV8323, the gate driver](https://www.ti.com/product/DRV8323)
- [moteus-c1’s KiCad files, Apache-2.0](https://github.com/mjbots/moteus/tree/main/hw/c1)
- [moteus firmware, the gate-current setting](https://github.com/mjbots/moteus/blob/main/fw/drv8323.h)
- [moteus configuration reference, the 30 kHz default](https://github.com/mjbots/moteus/blob/main/docs/reference/configuration.md)
- [TI DRV8302 datasheet, its 1.7 A rating](https://www.ti.com/lit/ds/symlink/drv8302.pdf)
- [VESC 4.12’s FET schematic, the 4.7 Ω gate resistors, CC BY-SA 4.0](https://github.com/vedderb/bldc-hardware/blob/master/design/mosfets.sch)
- [ngspice, the circuit simulator](https://ngspice.sourceforge.io/)

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Source: https://nearfieldnotes.com/notes/fet-choice/

Cite as: Michael Walker, “FET choice”, Near Field Notes, Note 20, Rev 1, 5 Nov 2026. https://nearfieldnotes.com/notes/fet-choice/

Figures: free to reuse under CC BY 4.0 (https://nearfieldnotes.com/license/). Each one's image is linked under its name. Credit Michael Walker, Near Field Notes, and link the figure: this address plus #fig- and its number, like https://nearfieldnotes.com/notes/fet-choice/#fig-20.