H bridge Rectifier
in [Design]
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From: john1987 on 15 Jul 2010 18:30 Hi, Why is it necessary to drive four MOSFET using HIP4081A since the transistor can be turned ON and OFF using just a micro. Thanks John
From: George Jefferson on 15 Jul 2010 19:14 "john1987" <conphiloso(a)hotmail.com> wrote in message news:52a199d7-fcc8-46d7-ad99-5cd7fa6afb19(a)z10g2000yqb.googlegroups.com... > Hi, > > Why is it necessary to drive four MOSFET using HIP4081A since the > transistor can be turned ON and OFF using just a micro. > Two reasons. A micro may not have enough current capacity to be able to drive the gates of the mosfets fast enough(the gate has capacitance). A micro may not be able to generate the high side voltage for the gates. Note how the HIP can drive high side mofsfets with gates that float possibly up to 80V. If you don't understand why a uC can't handle his case then you'll need to go back and learn what it really means to "drive"/"turn on" a mosfet(it's more complicated than "just applying a voltage"). Also notice how it says it can provide 2.5A drive current. Even though we like to think of the gates of mosfets as infinite resistance we sometimes forget there can be significant capacitance(specially for power mosfets). So what we have is a low pass filter/RC circuit. There is a time constant involved. If we want to transistion the mosfet from on to off we have charge the cap up through the resistance. This takes time. Our switching frequency is dependent upon how long we have to wait for the cap to charge. If we switch to fast our mosfet never really turns on or off and ends up acting as a resistor and probably overheating(the cap ends up never being able to fully charge or discharge and the gate ends up with an approximate constant voltage). Also an H-Bridge driver tends to have a lot of extra circuitry tailored towards what we generally use H-Bridges for. While in some cases you can use uC you can't use them in most cases.
From: Jamie on 15 Jul 2010 19:30 john1987 wrote: > Hi, > > Why is it necessary to drive four MOSFET using HIP4081A since the > transistor can be turned ON and OFF using just a micro. > > Thanks > > John Because most likely two of them are being used in a high side manner and you need more voltage than you have to turn them on.. The driver has a means to up the voltage and even isolate it, most likely.. You must remember that the gate voltage must exceed the source with N channels and be lower for P channels etc.. If you were to use a pair of P channels on the high side, you could directly switch them via the Uc.. Some drivers have a DC-to DC converter and some a basic charge pump and use the voltage that sits on the source past through this system to increase the gate voltage above the source voltage. Otherwise, the the high side don't turn on. Jamie..
From: john1987 on 18 Jul 2010 13:31 Hi, I have following questions about the HP4081A 1. How does this bootstrap works in this chip and why is it so important? How does this chip basically function? How is it switching the transistors? I am using the transistor IRF 520. I am trying to actuate an inductive load and needs 100volts and 2A current. I am thinking of monitoring the voltage across the load and will try to keep it constant using a microcontroller doing pulse width modulation. But I want to do it using some kind of control algorithm like PID or something else (Please recommend) and I have not implemented any control algorithm before. So, will appreciate any advice! http://pcbheaven.com/datasheet/irf520.pdf 2. I am planning to drive the four N Channel device IRF520 with this chip and was wondering that how can I calculate the time the chip will take to charge up the input capacitance of the transistor and how fast it can switch the transistor? The link of the circuit diagram is as blow http://www.robotpower.com/downloads/HIP4081A-datasheet.pdf thanks John
From: Grant on 18 Jul 2010 18:12
On Sun, 18 Jul 2010 10:31:50 -0700 (PDT), john1987 <conphiloso(a)hotmail.com> wrote: >Hi, > >I have following questions about the HP4081A > >1. How does this bootstrap works in this chip and why is it so >important? How does this chip basically function? Huh? H bridge with four N-channel devices requires gate drive to go above the +ve rail to turn on the top FETs. > How does this chip basically function? How is it switching >the transistors? I am using the transistor IRF 520. I am trying to >actuate an inductive load and needs 100volts and 2A current. You need a bigger chip for that, this one is 80V @ 2.5A, too small The data sheet tells you hoe it works, look for some application notes too. > I am >thinking of monitoring the voltage across the load and will try to >keep it constant using a microcontroller doing pulse width modulation. >But I want to do it using some kind of control algorithm like PID or >something else (Please recommend) and I have not implemented any >control algorithm before. So, will appreciate any advice! Well, you got some reading to do, I've seen PID algorithms described on the 'net, not implemented one in software yet. Depends what the controlled whatsit is, feedback dynamics in the controlled system. Assuming you're doing a motor drive, depends on what feedback element, if you need acceleration curves, torque limit and/or control, lots of stuff. > >http://pcbheaven.com/datasheet/irf520.pdf > >2. I am planning to drive the four N Channel device IRF520 with this >chip and was wondering that how can I calculate the time the chip will >take to charge up the input capacitance of the transistor and how fast >it can switch the transistor? The link of the circuit diagram is as >blow > >http://www.robotpower.com/downloads/HIP4081A-datasheet.pdf You got a fair bit of reading to do then, and you need to be able to translate datasheet numbers to the answers you ask for. There's some good app. notes out there describe how the gate voltage vs current is affected by the drain voltage movement as the FET turns off or on. But I filed then under F for filed, not quickly find one to quote from :) The chip looks like it's only the H-driver, you need to choose a controller too. I'm using PIC chips, some of them have app. notes explain the issues you seeking answers for, built in PWM registers -- but I'm not looking at doing motor control at the moment, so not closely followed what info is there, seen it in passing. Grant. |