Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
What limits the current through the mosfets there are no resistors and mosfets resistance is .015 ohms, 1" diameter LC coil 8 turns 1.5" long, choke 8mh, Frequency is 74 KHz, 240 ohm gate resistors?
The maximum current is determined by the saturation current of the inductor.
When the saturation current is reached, the voltage that turns on the one ON transistor will start to fall, turning off that transistor and turning on the other transistor.
It's a sell oscillating converter that depends upon the inductor saturation current for operation.
Online calculator says the 8 turn coil is 1 uh. The coil has a center tap, a 4 turn coil is .25 uh.
The circuit calls for a 1 uh coil but with a center tap only 1/2 the coil is being use on each mosfet. When it comes to calculating the effective reactance to 74KHz does the circuit see the coil as 4 turn or 8 turns?
I am trying to calculate current limiting for this circuit.
The maximum current is determined by the saturation current of the inductor.
When the saturation current is reached, the voltage that turns on the one ON transistor will start to fall, turning off that transistor and turning on the other transistor.
It's a sell oscillating converter that depends upon the inductor saturation current for operation.
That runs with exactly what Carl covered. As a load, like a steel pipe is placed in the inductor the inductance will change as will the saturation current. Shown at idle, no load, the 15 V supply looks to be running 2.71 Amps. The article also mentions:
Choke (inductor L2)
This limits the power to your induction heater. If yours is not oscillating, then you may need more inductance to prevent voltage drop in your PSU. You will need to experiment with how much inductance you need. Better to have too much, than too little as this will only limit the power of the heater. Too little may mean it wont work at all.
The maximum current is determined by the saturation current of the inductor.
When the saturation current is reached, the voltage that turns on the one ON transistor will start to fall, turning off that transistor and turning on the other transistor.
It's a sell oscillating converter that depends upon the inductor saturation current for operation.
Gary
Curmudgeon Elektroniker is basically right, probably L2 plays most important role in limiting the current. When a coil is saturated it turns short circuit (0 ohm), you cannot saturate a coil that has no core.
Gary,
When you start it one mosfet is on and it keeps the other one off with the forward conducting diode. As the current builds up (the coils don't have to saturate) the mosfet that is on its drain voltage goes up due to the high current it allows the other mosfet to come on. The frequency is determent by the tank circuit.
Gary,
To the best of my knowledge the only way you can change the power supply current is by using mosfets with higher on resistance, then you will get less power out of it. The oscillations happen only because of the high current, and this is the biggest disadvantage of this very pretty oscillator.
Because I'm not certain I suggest that you don't accept my advice before somebody else will agree with me. crutschow is a good reference.
I am trying to limit how HOT the mosfets get. I read online the higher the mosfet resistance the hotter they get because they act like resistors. The lower the mosfet resistance the more power output they produce and the cooler they run. My experements have proven that to be true.
I also learned by changing the gate resistor I can lower the current or increase the current. I can probably use a variable resistor on the gate then use the meter to dial in the current I want on each mosfet so it never produces more BTUs than can be transfered and cooled by the heat sink.
T x V/L = 36,500 x 15/.25 = 36,500. x 3.75 = 135,000.
I hope the math is correct.
T= mosfet on time. I am guessing this is 1/2 the frequency of 73KHz = 36.5 KHz
V = volts = 15vdc
L = inductance of the LC coil = .25 uh for 4 turns.
Changing the gate resistance will affect how fast the mosfet turns on and off. Remember that the gate of a mosfet looks like a capacitor to the source. (note 1) A larger resistance slows the charge and discharge time. The drain-source resistance of the mosfet goes from almost infinite resistance when off, down to x milliohms when on. Depending on the mosfet, it is in it's linear mode from about 2 to 5 volts gate-source. While it is potentially possible to limit the total system current by making use of this characteristic, it is not very practical, as that is also the region of operation when a mosfet is most prone to failure. (note 2)
Good mosfet power switching circuits are designed to force the gate voltage from off to on and back again as fast as possible.
Notes:
1) There are other parasitic capacitances in mosfets, particularly one from gate to drain whose effect is to try to turn the mosfet off at the very moment it is turning on. Google "miller effect in mosfet"
2) The other common failure modes are excess voltage at drain or gate.
This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
By continuing to use this site, you are consenting to our use of cookies.
