IRFZ44N vs IRLZ44N: same pins, different gates

Hold the part with the printed face towards you and the leads pointing down. From the left: gate, drain, source. Both data sheets show this order, and the IRLZ44N sheet's outline page also lists the tab as lead 4, drain. The IRFZ44N has the same body and the same arrangement; a meter on continuity reads zero between the tab and the middle lead of either part.

That matters as soon as you bolt it down. If the tab touches a grounded heatsink, or a heatsink shared with another transistor, the drain is shorted to it. Put an insulating pad under the tab and an insulating shoulder washer under the screw head, as in Figure 1, then check with a meter that the tab and the heatsink are open.

The 49 A and 47 A ratings, and the 94 W and 110 W dissipation figures, assume the case is held at 25 °C. No real heatsink does that. In a real build the heat decides the current, as the worked examples below show. The IRLZ44N's 4 V row is also measured at 21 A, not 25 A.

The threshold is the gate voltage at which the part passes just 250 µA. An IRFZ44N with a threshold of 3 V is barely on at 3 V and not reliably on at 5 V. The number to read is the RDS(on) row, because each row names the gate voltage it was measured at. The IRFZ44N has one such row, at 10 V. The IRLZ44N has three, at 10 V, 5 V and 4 V. That is what "logic level" buys you.

We estimate heat as I² × RDS(on) × 1.5. The 1.5 is our own margin for a hot die, because on-resistance rises as the junction warms; it is the same margin our MOSFET guide for Arduino and ESP32 uses. We take 40 °C as the room temperature (a hot room, our assumption) and keep the junction at or under 125 °C, well below the 175 °C rating.

1. Arduino Uno pin (5 V) switching a 12 V, 5 A LED strip

IRLZ44N at 5 V: 5² × 0.025 Ω × 1.5 = 0.94 W. With no heatsink, 0.94 W × 62 °C/W = 58 °C above the room, so about 98 °C at the junction. That is inside our limit, but the tab will be too hot to touch; a small clip-on heatsink is worth adding.

IRFZ44N at 5 V: the data sheet gives no resistance at 5 V, so there is nothing to calculate. Some will work on the bench and some won't, and you can't tell which from the part number.

2. A 12 V gate driver switching a 10 A DC motor

IRFZ44N at 10 V: 10² × 0.0175 Ω × 1.5 = 2.6 W. With no heatsink that is 2.6 W × 62 °C/W = 163 °C above the room, past the 175 °C rating, so it needs a heatsink. The budget is (125 − 40) ÷ 2.6 W = 32.4 °C/W in total. Take off 1.5 °C/W junction to case and 0.5 °C/W case to sink (greased, from the data sheet), and the heatsink itself must be 30 °C/W or better. With a 30 °C/W heatsink the junction settles near 124 °C.

IRLZ44N at 10 V in the same circuit: 10² × 0.022 Ω × 1.5 = 3.3 W, so the budget is 25.8 °C/W and the heatsink must be 23 °C/W or better. It works, but here the IRFZ44N is the better part. An insulating pad adds its own resistance on top of the 0.5 °C/W, so read its data sheet and leave some margin.

3. Gate pulled up to an 18 V supply through a resistor

Some simple circuits pull the gate up to the supply rail and use a transistor to pull it low. With an 18 V rail the IRFZ44N is inside its ±20 V gate limit. The IRLZ44N is not: 18 V is over its ±16 V maximum, and going past an absolute maximum rating can destroy the part. If an IRLZ44N goes into a circuit like this, add a 12 V zener from gate to source (Figure 6). With a 10 kΩ pull-up the zener carries (18 − 12) V ÷ 10 kΩ = 0.6 mA while the MOSFET is on, so a small 500 mW zener is plenty.

4. An ESP32 pin (3.3 V)

Neither data sheet guarantees anything at 3.3 V. The IRLZ44N's lowest specified point is 4 V. Use a part that is rated at 2.5 V or 3.3 V on the gate (our MOSFET guide uses the AO3400A), or drive the gate through a gate driver or a level shifter.

• Read the marking with a torch before you fit it. IRFZ44N and IRLZ44N differ by one letter, they sit in the same drawer, and the wrong one in a 5 V circuit may even work on the bench.
• Measure drain to source with the load running. A volt or more at a few amps means the gate is not driven hard enough, whatever the threshold says.
• Keep the 10 kΩ from gate to source. A floating gate during reset can leave the part half on, and half on is where it gets hottest.
• Check the tab before power goes on: meter between the heatsink and the tab, and you want open circuit unless the heatsink is meant to be at drain voltage.
• Count the gate voltage at its highest, not its typical value. A 12 V lead-acid rail can sit near 14.4 V on charge, which is still under 16 V, but an 18 V rail is not.

If a microcontroller pin drives the gate directly at 5 V, buy the IRLZ44N. If a gate driver or a 10 to 15 V rail drives it, the IRFZ44N has the lower resistance and the more tolerant gate. If you only want one kind in the drawer, the IRLZ44N covers both cases as long as you keep its gate under 16 V. From a 3.3 V board, buy neither; see the MOSFET guide and the gate drive lesson.

For the question of which MOSFET suits your board at all, including the IRF520 module and the AO3400A for 3.3 V, read choosing a MOSFET for Arduino and ESP32. Small-signal transistors are on the parts shelf, and the LED strip in example 1 is built step by step in the 12 V LED strip system.