Problems with switching MOSFET for boost converter

Question

I am supposed to build a boost converter that steps-up a 1.5 V input to 5 V (I only have 1.5 V as a power source). I have designed the circuit but the only problem I face right now is that I can't turn the MOSFET on with 1.5 V peak-max. I am using a B012N03LX3 MOSFET as my switch.

This is my schematic connection (assume the diode is ideal and assume that the NE555 can operate with 1.5 V which the simulation shows it can)

I checked the datasheet and I know that the V_gs(th) must be at least 1 V to turn the MOSFET on.

The NE555 has the output of this 10 kHz with 0.7 duty cycle.

But the voltage output at Vload is:

So from this I am pretty sure that the MOSFET did not get sufficient voltage at V_gs(th) to turn on. But due to the fact that I only have 1.5 V as voltage supply I am not sure what correction I need to make to my circuit to make sure that the MOSFET is able to turn on properly (with only 1.5 V as a voltage source).

BTW I think the output should look like this if the MOSFET is able to turn on properly:

So any idea on what I should change in my circuit to make it work?

Answer 1

Also I check the datasheet and I know that the Vgs(th) must be at least 1V to turn the MOSFET on

No, you are misreading the data sheet. What it says is that, in order to get 250 uA drain current, some FETs will only require 1 volt, but some will need 2.2 volts.

You are using 1.5 volts as a supply voltage, so you would have to get quite lucky in picking your FET to get one which will work in your circuit. It's even worse than you think since you want much more than 250 uA, which means you need an even higher gate voltage.

Answer 2

Vgs(th) it's just to start driving, to really serve as a switch you need VGS = 4.5 V (see rds(on))

You must try other circuits as follow

boost converter 1.5 volts input 5 volts output 200 mA, boost converter 1.5 volts input 5 volts output 10 mA, boost converter 1.5 volts input 5 volts output 500 mA, boost converter 1.5 volts input 5 volts output 300 mA

Answer 3

Question: "MOSFET did not get sufficient voltage at Vgs(th) to turn on..what correction"
Answer: Since there is an oscillator, make a charge pump (voltage double or triple). Then switch that voltage on the FET gate.

One thing you may try, just to see it on the simulator is:
Shift the operating point, by adding two resistors and a capacitor
1.5V--Ra--(X)--Rb--GND, (X)--FET gate, (X)--Cx--555 output.

Answer 4

If you ever manage to get it started, add oring diodes from 1.5vdc/output to vcc. Keep in mind this will make your start-up challenges more difficult but your gate drive will be much stronger once its operational. Remember to use shottky diodes.

Edit: you could use a transiator (add a series resistor with the base). You only need .7v to drive a 2222a.