Electrical – Micro:bit LED control using P30N06LE

ledmicrobitmosfet

EDIT: My reference in this question to analogue voltages are just how my voltmeter reads and averages the PWM from the Microbit. Apologies if I'm confusing the matter with this. The project is using PWM with a varying duty cycle.

I'm trying to power a strip of 12v 5050 LED's using the circuit below. I'm changing the output of the Micro:bit between 0 and 1023 in steps of 128 to get 9 levels. At the MOSFET gate I'm getting between 0 and 3.14V in steps of 0.4V and without a load I'm getting between 0 and 12.16V at the drain in steps of 1.5V (the first step is 2V and the last step 0.95V but I assume the response just isn't quite linear). When I connect the LED strip the drain voltage drops to about 40% of what I am expecting i.e. 0 to 4.75V which makes the LED's barely visible. Can someone explain why this is please? The power supply is rated at 12v 3A so has plenty of capacity. The LED's shine at full brightness when connected directly to the power supply.

My electronics knowledge is very basic – some 30+ year old school physics and a lot of Googling so feel free to respond using "for dummies" level answers.

schematic

simulate this circuit – Schematic created using CircuitLab

Best Answer

FET's are non-linear Voltage controlled resistors except the threshold has a wide tolerance. This means there is a big tolerance for just the threshold voltage which is not enough to be used as a current switch. You need at least 2x to 3x the threshold voltage to conduct high currents where the guaranteeon yours is only at Vgs=5V and not 3V.

You made a good choice of FETs for RdsOn but not a good method of gate control.

This is why they are non-linear.

Gate to Threshold Voltage Vgs(TH) = 1 - 2 V when Vgs = Vds , @ Id = 250µA, Figure 10

Drain to Source On Resistance (Note 2) rDS(ON)=0.047 Ω I D = 30A, Vgs = 5V, Figure 9 - -

The drain-source resistance is very high (60V/25uA) at Vgs=0, then the Vgs(th)=Vt threshold may occur anywhere from 1 to 2V to ~6 kΩ avg = 1.5V/250uA since 1V/250µA = 4kΩ and 2V/250µA=8kΩ

The 2nd important parameter is RdsOn=0.047Ω @ Vgs=5V which is ~ 3x the gate threshold voltage, , Vt (aka) Vgs(th). So in between 5V and Vt (= 1 ~ 2V) it drops from 6k to 50m or nearly 5 decades. So very non-linear current vs gate voltage. AND you need at least 2*Vt to switch decent current, and 3* for lower losses.

I have superimposed some Vgs to Vt ratios on the datasheet curve to show these more clearly vs Drain current.

![enter image description here

Suggestions

To use a power FET as a dimmer there are 2 simple options;

  • PWM with Vgs >= 3V fixed frequency , variable duty cycle from a uC
  • Voltage control Current sink using a comparator with say 100mV max drop across R from Source to Gnd and control from 0 to 100mV into comparator to drive gate from a fixed voltage and pot divider or DAC output.

p.s. The blue text is a rule of thumb and the tolerance for Vt =1.5V avg +/-0.5V but as the Vgs increases the current follows these curves at 25% C and stated Vds and the guaranteed value for RdsOn is in the table at 5V.

Yet in-between it is non-linear.

a simpler choice of FET has a threshold Vt<= 1V instead of 1 to 2V which has a RdsOn <= 0.1 Ohm.

But to make raise Vgs, you can also do this.

schematic

simulate this circuit – Schematic created using CircuitLab

Another solution for your 1 to 2V Vgs(th) from 3V

schematic

simulate this circuit

As I wasn't expecting your drive to be so low V, there are logic level FETS suitable for this task as well. Ensure current rating is 2x what you need to keep cool.

  • ZXMN6A08GTA Diodes Incorporated MOSFET N-CH 60V 3.8A SOT223
  • STN4NF03L STMicroelectronics MOSFET N-CH 30V 6.5A SOT223
  • IRLR2703TRPBF Infineon Technologies MOSFET N-CH 30V 23A DPAK
  • IRL2703PBF Infineon Technologies MOSFET N-CH 30V 24A TO-220AB
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