MOSFET, HEXFET,IGBT,ChargePump etc...

[iambrus]

Hi there, Could any one explain me what MOSFET, HEXFET,IGBT,Current feedback Amplier and ChargePump-s are and how they can be used in UPS or power Supplies. I am interested in 220V AC 50Hz True sine wave.<p>Please, be prepared for very stupid questions later on.

[Chris Smith]

FETS and family are basically well designed Heavy amperage switches. When you charge up a current and release it acts like a spring, over extending past the original point or voltage, thus an increase can be had. Using coils, chokes, or caps, you can step up a voltage very nicely and the Fet is simply the heavy amperage switch to do it. <p>Start with the common voltage multiplier and its theory of operation for a beginning or lesson, and then study these charge pumps because are similar in operation but much more efficient.<p> The other advantage of the Fet family is their “low on resistance” of less than one ohm, and the gate or base doesn’t use an actual current to open or close it but merely the presence of a voltage. They also handle high speeds as well, making they the ideal switch in the transistor family.

[russlk]

I don't know the detail difference between MOSFET, HESFET, and IGBT, I use them interchangebly. The IGBT has higher voltage ratings but switches slower.
You will have to research info on current feedback controllers, Texas Instruments has a lot of info. A Google search for charge pump should turn up info, Maxim makes a good charge pump look up MAX1681.<p>[ December 12, 2003: Message edited by: Russ Kincaid ]</p>

[rosborne]

I think IGBT (Insulated Gate Bipolar Junction Transistor) is like a Darlington connectected transistor pair except the first transistor is a FET of some flavor and the second transistor is usually a big BJT. I saw one at my last contracting gig that probably weighed about 10 lbs. The heatsink was similarly enormous. I think the basic idea is voltage controlled/activated current switch.
Charge pumps are used to help fire MOSFETs by cranking up the gate voltage. MOSFETs are subject to a phenomenon know as pinch off voltage. The charge accumulated on the gate of a MOSFET can be insufficient to open the source to drain current path and also tends to accumulate in an uneven matter resulting in say greater charge at the source side of the gate and lesser charge at the drain side which causes the channel to be open at one side and pinched off at the other. The solution is to apply a larger amount of charge to the gate and in effect blow the channel wide open for current flow. The problem is that maybe we are using 5V TTL technology and the only way to open the channel all the way is to have 10V on the gate. Imagine a switching network with a five volt source and two (or more) caps as loads. At time 0 the five volt source in connected to C1 and charges it to a voltage of 5V then a switch is opened disconnecting C1 from the load and another is closed connecting the load to C2 until C2 is charged then switches change in the network until C1 and C2 are now in series and become the load of a MOSFET gate. The gate effectively has 10V, blows the channel open and our MOSFET no longer suffers from pinchoff. Charge pump is here "to pump *clap* you up."
Can't help you with HEXFETs, but my guess is that its just a different process for making field effect transistors from the regular vanilla metal oxide silicon (the MOS) way.<p>-Rick

[rshayes]

Field Effect Transistors (FETs) are transistors where the output current passes through a thin channel between a source connection and a drain connection. The amount of current flow is controlled by the electric field from a gate electrode. The gate electrode must be very close to the channel and the channel must be very thin. If the gate is isolated from the channel by a very thin insulating layer, then the device is considered a MOSFET. If the isolation is a back biased diode, the device is considered a junction FET (JFET).<p>MOSFET originally stood for Metal-Oxide-Silicon Field Effect Transistor. It is still used as a generic term, even though the gate may be polycrystalline silicon instead of metal, the insulator may be silicon nitride instead of silicon oxide, and the semiconductor may be gallium arsenide instead of silicon.<p>In a FET, the current flows in a thin sheet of semiconductor material. The resistance of this sheet is minimized by making it as wide as possible. The length of the channel is determined by the breakdown voltage of the device, and its depth is limited by the applied gate voltage. HEXFET was a term originally used by International Rectifier (and it may be a trademark) to describe a geometry that lowered the channel resistance by paralleling tens of thousands of hexagonal shaped transistors on the same die. A large fraction of the power FETs made use a similar geometry.<p>As rosborne stated, the Insulated Gate Bipolar Junction Transistor is a combination of a MOSFET and a bipolar transistor. A bipolar transistor can carry much more current than a field effect transistor for a given die size, since the cross sectional area of the device can be very large. Current flows through an area, rather than through a thin strip. Power MOSfets are usually used for currents up to tens of amps. Above this point, the die becomes too large to be economically manufactured and the package becomes too bulky. Bipolra devices can carry more current, but they require a substantial amount of base current, especially in high voltage devices. If the collector current is hundreds of amps, the base current will be tens of amps. In the IGBJT, the base current is supplied by a MOSFET connected between the collector and base. This reduces the drive current required to practically nothing.The combined device is slower than a MOSFET, since base charge must still be removed to turn the device off, but not as slow as a normal bipolar transistor, since the collector junction never enters saturation, which minimizes the amount of charge which must be removed to turn the transistor off.

[iambrus]

Thank you all for your explanation. My question would be this. Which one would you use in order to increase AC current and how? For instance I saw a SonnyBoy converter (using MOSFETS) that can supply 2000W converting 12V DC from a battery. Only it costs 3000USD. I would like to develop an extensible block like system for solar panels where the number of panels and batteries can be increased only the AC converter prices are far too high. I was thinking to buy smaller ones and increase current or what seams to me easier to build is increasing DC current but how to convert it safely to true sine wave AC without fireworks or huge transformers. I know it is not easy, so I should just drop the idea. My other question would be this. Do you know anything that could be inserted between a normal main and lets say my air-conditioning in order to reduce power consumption from the main. In order words a charge-pump or a circuitry that can manipulate AC power from the main. Thank you in advance

[iambrus]

Oh, I forgot to tell that my ultimate aim would be to sell power back to the mains and I have to be able to provide true sine wave 50HZ 220V, so 2000W would be very little to start with. Maybe there's a good idea out there only I missed it.

[Richard Bagett]

Most of the so-called power savers simply distort the current waveform to make it appear that less power is being consumed while measuring with inexpensive test equipment. The savings quickly evaporate when true-RMS equipment is used. (yes, sometimes a VERY bad motor can be helped, slightly)<p>The whole trick is to manipulate the power factor(timing difference between peak current and peak voltage) to trick the instrumentation. This is also where these free-power folks get their "power gain"<p>While we're messing up the power factor..<p>A small 4 quadrant DC motor drive can be used to put DC voltage back into the AC line, and can be had on the cheap on Ebay. (Check FINCOR and AMICON and numerous others). Will it make a pretty sine wave? NO. Will it put saleable power back into the line? YES. Simply hook the DC voltage to the motor terminals and the power terminals to the appropriate AC line, (They can be had for any standard line voltage) set the speed adjustment to zero, and the current limit to something that your DC source can handle, and there it is.<p>Once all this is working, there are ways to fix the power factor, if needed.

[iambrus]

Dear Richard, after reading your message I am chilled and I must say that I am almost ready to reconsider my little project and go on with it. It is also true that I have never ever seen a DC motor driver and I have no idea how it works. You see I am the funny type who needs to understand things. So, I have to abuse your patience and ask you if could explain a bit more in details what you mean. I also have got another question. If I wire the output of a couple AC circuits in parallel the signals will be in different phases. How can I convince them to flow together without significant losses? And than have you ever try to add DC current to an AC signal? AND Have you ever used Charge-pumps in AC circuitry? My head is spinning after reading so many datasheets, I just cannot figure out which one to use.
Thank you in advance

[russlk]

If you want to sell power back to the power company, a motor-generator set is the most straight-forward way to go. The efficiency is not great, but since the power is free, who cares? If you generate less power than you use, then your power meter will read less than normal and that is your pay. If you generate more power than you use, the power meter will run backward. Whether the power company will pay you for that has to be negotiated.<p>It is not possible to reduce the power used by your air conditioner except by reducing the amount of cold air you get. You can improve the power factor but that only helps the power company, not you because the power meter measures true power but the power company has to supply both true and imaginary power.

[iambrus]

Thank you Russ, Could you be a bit more spesific.
It is all new for me. Is there any small one, and how do they work?
Thanks