Second Stage Joule Thief Circuits

[Groundloop]

Albert,

I will test you JT when it arrives. I have a small problem with the feed back circuit.
My P-Type mosfet will not switch on when asked to do so. I'm looking into a solution
and will try a power PNP transistor instead. The pcb will not need any updating at all.

Alex.

[gadgetmall]

Albert,

I will test you JT when it arrives. I have a small problem with the feed back circuit.
My P-Type mosfet will not switch on when asked to do so. I'm looking into a solution
and will try a power PNP transistor instead. The pcb will not need any updating at all.

Alex.

Ok this sounds like either a bad part or maybe n type mosfet ? If your signal is there to turn it on the logic says either wrong turn on volts or bad part ?
According to this the gate volts on Pmosfet must be lower to turn it on ?

Single-type MOSFET switch

This analog switch uses a four-terminal simple MOSFET of either P or N type. In the case of an N-type switch, the body is connected to the most negative supply (usually GND) and the gate is used as the switch control. Whenever the gate voltage exceeds the source voltage by at least a threshold voltage, the MOSFET conducts. The higher the voltage, the more the MOSFET can conduct. An N-MOS switch passes all voltages less than (Vgateâ€"Vtn). When the switch is conducting, it typically operates in the linear (or Ohmic) mode of operation, since the source and drain voltages will typically be nearly equal.

In the case of a P-MOS, the body is connected to the most positive voltage, and the gate is brought to a lower potential to turn the switch on. The P-MOS switch passes all voltages higher than (Vgate+|Vtp|). Threshold voltage (Vtp) is typically negative in the case of P-MOS.

A P-MOS switch will have about three times the resistance of an N-MOS device of equal dimensions because electrons have about three times the mobility of holes in silicon.

[mscoffman]

Albert,

I will test you JT when it arrives. I have a small problem with the feed back circuit.
My P-Type mosfet will not switch on when asked to do so. I'm looking into a solution
and will try a power PNP transistor instead. The pcb will not need any updating at all.

Alex.

I haven't evaluated all the details here but N - MosFETs operated on 5Volts want
to be driven, on it's gate, above +5 volts to turn fully on. Maxim/Dallas has voltage
doubler interface chips to do this for FETS operated at low voltage. MOSFet are a lot
more efficient and impedance controlled than bipolar transistors...So i would
recommend MosFets here if at all possible. I don't know about P type Mosfets
so can not guarantee them.

:S:MarkSCoffman

[Groundloop]

@mscoffman,

I have no problem with the N-Type mosfet.
It is the P-Type mosfet that wont turn on.

The problem is probably that the "ground" of the mosfet is not ground but
is floating above ground as the bcap is charging. When I put null
volt on the gate then there is not enough "negative" voltage to
switch the mosfet on.

Alex.

[Groundloop]

All,

I have found a solution to the feed back problem.
The solution was to use a power NPN transistor.

Attached is all the design files.

My design goals was like this:

Circuit should use minimum current from AA battery.
My circuit uses less than 0,7mA.

Circuit should have a automatically feed back that could charge the AA battery from the BCAP.
My circuit will automatically feed back current to AA battery when AA battery voltage
has dropped to 1,3 Volt (adjustable) AND BCAP has a charge voltage higher than the AA battery.
The base current to the NPN transistor is not wasted and will also charge the AA battery.
The maximum charge current is limited by the transistor HFE (20).
The base current are (5/270)/2 = 0,009 Amp. so max. feed back charge current is 0,185 Amp.
This is a safe charge current for a NiMeH AA battery. (/2 is because PWM control with 50% duty.)

Circuit should have a automatically discharge from BCAP to load.
My circuit will automatically discharge the BCAP to load when the BCAP has reached approx. 2,5 volt.
The discharge will automatically stop when the BCAP reach approx. 1,5 volt. This threshold voltage
was selected bacause we always want the BCAP voltage to be higher than the AA voltage so that
we can get a feed back charge to AA battery when needed. The maximum discharge current is only
limited to the pcb tracks ability to conduct current AND the max. N-mosfet current ability. This is
estimated to be approx. 40 ampere pulsed with 50% duty cycle.

This circuit will prove without any doubht if a JT or boost converter is over unity or not by simply
let the circuit run and see if the AA battery and BCAP will discharge or charge.

@Gadgetmall

I will snail mail the circuit and spare pcbs over to you on Monday.

Alex.