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mosfet questions

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shortbus=

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1. is there a difference in the amount of heat dissipation between a high side and low side mosfet? with all parameters on both being the same.


2.can you control the allowed current through a mosfet by changing the gate voltage? (i know it would change the Rds, but does it change that much?)

thank you, cary
 

dknguyen

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1. By all things do you mean both specs, gate drive circuitry, and switching? If so, then yes.

2. THis is what a a transistor amplifier does. But this method of control burns off excess energy as heat, rather than leaving it in the battery so it's no good for power applications.
 
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shortbus=

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1. By all things do you mean both specs, gate drive circuitry, and switching? If so, then yes.

2. THis is what a a transistor amplifier does. But this method of control burns off excess energy as heat, rather than leaving it in the battery so it's no good for power applications.
thank you dknguyen.

1. so which has the lower heat dissipation?

2. every thing that i have found that will control the amperage in a circuit produces heat! is there a way to do this that doesn't/ my circuit is run on mains so i don't really care about loosing the power but would like to figure a better way to control output amperage than the wire wound resistors that were the my original plan. any ideas?

thank you again, cary
 

crutschow

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1. The heat dissipated in a transistor is determined by the current through it and the voltage drop across the transistor at that current (P=I x V). It makes no difference whether it's high side or low side.

2. A common, efficient way to control current is pulse-width-modulation which is used to switch a transistor rapidly from full on to full off, minimizing the dissipation (see Pulse-width modulation - Wikipedia, the free encyclopedia).
 

shortbus=

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2. A common, efficient way to control current is pulse-width-modulation which is used to switch a transistor rapidly from full on to full off, minimizing the dissipation (see Pulse-width modulation - Wikipedia, the free encyclopedia).
thank you for your reply carl!

this, PWM, is basically what my project does,but i didn't realize it would also control the current! i'll try to describe my output circuit:

a variable frequency and duty cycle signal generator to switch the mosfets

input mosfet (A)----capacitor bank-----output mosfet(B)

mosfet A on-----capacitor charging-----mosfet B off

mosfet A off-----capacitor discharging----mosfet B on

repeat

so what you're saying is that by controlling my duty cycle of the mosfets it will control the current in the output? longer on time of the duty cycle equals more current output, correct?

thank you, cary
 

RCinFLA

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Capacitors don't allow their voltage to change instantaneously. Sounds like you are doing a capacitor charge pump using MOSFET's in linear region which will dissipate power.

PWM switching power converter usually use inductors to transfer energy. The MOSFET are either fully on or fully off for the PWM switching.
 

crutschow

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As RCinFLA stated, you need an inductor (or resistive load) for PWM to control the average current. To use PWM to charge a capacitor with minimum power loss you will need an inductor between the MOSFET output and the capacitor input. The inductor value is determined by how much current you want and the PWM switching frequency.

If you turn on the MOSFET for 1/2 cycle of the resonant frequency as determined by the inductor and capacitor values, then you can charge the cap to near double the input voltage.
 

shortbus=

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thank you both for the replies.

what i am doing is not a power supply, it's a spark generator for a electrical discharge machine(EDM). it has to make a small (around one thousandth -0.001") of a inch spark between a work piece and a shaped electrode to cut that shape into the work.

the EDM originally used a RC circuit to generate the spark but then they changed to a pulsed discharge type. this is what i'm trying to build. it would i guess be similar to capacitor discharge ignition on a car engine, but lower voltage and higher amperage.

here are some links

http://www.electro-tech-online.com/custompdfs/2009/09/NCACIM-113.pdf (fig. 5 on page 3 is basically what i'm talking about)

Make-it-Better.us (this is a over view of the EDM process wire and sinker, sinker is what i'm building)

i guess that i'll just go back to the power resistors to control the current!


thanks to everyone that replied, still open to more comments from you.

cary
 

weglobal

New Member
Hi
I need a help to design a battery charger from solar panel , I have a 6V 2.2W solar panel which I am connecting to a 3.6V NIMH 2500mAh battery, I want to use a MOSFET as switch so I will be monitoring the bat voltage , when rich 4.5V the MOSFET will be turned OFF , the reason also that I am using the MOSFET is to not have too much losses and also for power of thr battery not to come back to the solar once the MOSFET is OFF and solar voltage is lower than the Battery.
 
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