PWM RGB LED strip with IRLZ44N

Question

So this is part 3 (or 4 depending how you look at it) in a series of questions I've had around PWMing an RGB LED strip from an ESP8266 (check my question history if you desire).

I apparently keep misunderstanding/misreading/misinterpreting the MOSFET datasheets. I believe my previous mistake (with the IRFZ44N) was interpreting the "max" \$V_{GS(th)}\$ as the maximum I should give it vs the maximum it will require to fully turn on (please correct me if I'm wrong though).

This time around, I found the IRLZ44N that has a max \$R_{GS(th)}\$ of \$2V\$, great! Ordered a few of those and got a very similar result, damn!

This is my schematic:

TP1 (11.4 V coming from the power supply):

TP2 (3.6 V PWM signal going to the gate of the MOSFET):

TP3 (I was (again) expecting 11.4 V max, 0 V min here):

My guess this time is that the \$R_{DS(on)}\$ being specified at \$4~\mathrm{V}\$ is the issue? So I assume that the MOSFET is fully on (due to the \$V_{GS(th)}\$ max value being exceeded), but has too high of a resistance, hence it being dim?

Is that right? Even though I've said that, I'm not sure it sounds correct and I could do with a bit of guidance please.

I'm also aware that I likely don't know the right questions to ask (hence me being back here for the 3rd time), so if you can see what I'm getting at, even though I may not have asked it directly, please can you provide the question and answer?

Thanks!

Answer 1

Threshold voltage (VGS(th)) is the gate to source voltage at which the conductive channel between the source and drain is just beginning to form. The datasheet specifies it vaguely as between 1.0 and 2.0 V since you aren't really going to design around it - the datasheet states that 250 microamps of IDS is the threshold.

VGS(max) is the absolute maximum voltage you can apply to the gate, referenced to the source, before damage occurs. For the IRLZ44, this is +/- 10 V.

In between, you have the transfer characteristics given in fig. 1 for 25 C and fig. 2 for 175 C. From the curve, it looks like you should be getting the behavior you expect but there might be some interaction with the internal circuitry of the LED strip or the circuit is connected differently than the schematic.

I would also add a gate resistor between the 10k pulldowns and the gate terminals.

Answer 2

The threshold voltage is just that, a threshold. It is like entering a house , if you stop on the threshold you are still not in the house. \$V_{GSmax}\$ is the maximum value at which the threshold occurs. But it is still the threshold. \$R_{DS}\$ is still pretty high.

I use the following chart, from the data sheet, to choose a \$V_{GS}\$ value that I consider ON for PWM purpose.

The flat region where \$V_{GS}\$ barely changes is called the Miller region. In this region \$V_{DS}\$ is dropping requiring the gate-drain capacitance to charge from the gate perspective. This steals current from the gate slowing the voltage increase.

Once \$V_{DS}\$ is zero \$V_{GS}\$ can increase to lower \$R_{DSon}\$.

The gate pulses must rise quickly above this level for good performance. This is significantly higher than the threshold voltage.

Although the x-axis is in coulombs, this effect can be seen by monitoring the gate voltage (zoom in on the rising edge). It is particularly noticeable when switching inductive loads.