I want to place several mosfets in parallel to handle more current, but am confused about the Gate resistors and something refered to as a "balance" resistor. From what I've read, this balance resistor should be between 1.0 and 0.1 OHM and intended to protect the MOSFET's in the array, or "balance" the array for differences in the RDSON values that might cause one (or more) of the FET's to consume more of the load than the others leading to thermal catastrophic destruction of the entire array.
In addition, I'm not sure how to calculate the correct GATE RESISTANCE. I'm driving each gate with op-amps, so I am not clear why I even need a gate ressitor. I'm also not sure how to calculate the BALANCE resistor nor even where to connect this BALANCE resistor parallelly to each fet in the array.
If the RDSON value is, say, 0.036 ohms, then a 10,000 ohm resistor in parallel would maintain 0.036 ohms. However, how the heck do you attach a 10,000ohm resistor in parallel with an RDSON condition? In addition, this BALANCE resistor has always been mentioned in values of 1.0 to 0.1 ohms.
As always - THANK YOU EVERYONE fro your support.
The "balence" or "BALLAST" resistor goes in series with the source. If you place, say 50 milliohms or 100 milliohms in series with each FET in this way, variations (part to part) in the 36 milliohm Rds(on) will be a smaller percentage of the total series resistance per branch, and current hogging will be less of an issue.
The gates of power Mosfets tend to have a LOT of capacitance, so you might add gate series resistance to keep your Op-Amp from going unstable due to the capacitive load. Keep the series gate resistor as small as you can get away with, however, because the gate capacitance will form a lowpass filter with this resistor.
You can also count on the fact that the FET "on" resistance (Rds) will increase as it's temperature rises so parallel FET will "self balance". So as an example, if you have three FETs in parallel, the one that conducts the most current will also be the one that runs the hottest. As a result, it's Rds(on) resistance will rise and the other two FETs will conduct more current and thus balance the current load.
Mos fets have a positve temperature coeff so they tend to share current. However gate threashold varies from device to device. If you sort parts by threashold V with perhaps 25mv bins that will help a great deal.
Hi Laser Dude,
Depends on your application. Doing a RF push pull driver into a magnetic? Uneaven switching (treashold voltage/Rdson) will put a DC bias in the core and worse case the core saturates and fets go up in smoke.
Data sheets are fine part specs vary device to device. Doing Rdson measuremts of sub 5 ohm parts should use 4 wire ohm meter and may need to pulse current 1-20 amps to get a good reading.
Threashold voltage measurement use a DAC to drive the gate to a test current few ma or where ever you like. Then can match up parts with similar swiching performance.
Would take extra hardware but, you could measure average drain current and change each gate drive pulse width to servo the currents to be the abouth the same? New CPUs have extra pins perhaps set one PWM/Timer for 50% the other PWM to 45% to better share the current for different Rdson or PCB layout resistance etc...? Also threashold voltage changes with temperature I don't know how well the it tracks part to part.
Controlling large currents with MOSFETs is something I took a great interest in many years ago when power MOSFETs first became available. They seemed from the data sheet to be so powerful but as I observed in using them, they are very easy to kill. This prompted me to learn as much about them as possible. The body of knowledge that is available has increased tremendously over the years and understand how to make your MOSFETs perform reliably takes some time to understand. So I am presenting some of the important issues you need to consider in properly driving a MOSFET.
The great thing about MOSFETS is that the due share the load very well without the need to any balast resistor and if you need to parallel multiple devices you certainly don't what to add resistance into the path. This however must be qualified with using them as switches ie fully turned on or fully turned off. Driving the gate of the devices however does present some issues that can best be resolved by using a gate driver circuit. One device may turn on as 4V and another of the same part number may turn on at 4.5V however this does not mean that either of them is fully turned on. MOSFETs are often used are switches but they also have a linear region. At the minimum turn-on voltage the transistor starts to conduct as a low level and the level increases at the gate voltage increases, at some point the device will reach nearly complete turn-on. At this point the resistance will be very close to it's minimum value and may be low enough for your purpose if you are not switching large currents. By raising the gate voltage a bit more you get to the point where the resistance is at it's absolute minimum also know as it's saturation point. Also note that the required gate voltage will vary with the temperature of the device and the current flowing through it. For this reason when switching large currents you should drive the gate beyond where it seems to fully turn on. Due to the very low resistance of these devices they are capable of handling large currents, low current testing won't reveal much information about how they will perform under large loads. Applying 12V to a gate of most devices will ensure complete turn on, even with variances in the devices.
There are a several good reasons for using a gate resistor when dealing with large currents.
1. It is often nessacary to control the turn-on and turn-off rate to prevent large current spikes in the system. While this results in more time in the linear region thus generating more heat it can however save components from destruction due to large voltage spike that happen when you try to change current going though an inductive device too quickly.
2. Due to the large capacitance of high current devices between the gate and the drain current can flow from the drain through the gate capacitance and into your driver circuit, and kill low power driving circuits. This can be resolved with slowing down the rate of change, slower rise and fall times, and slowing down how often you switch. Put a gate resistor in place will slow down the rise and fall times and will also limit the current that can flow through the gate capacitance.
3. When dealing with large currents you in a system the fact that conductors have some resistance can cause some problems that you may not have accounted for. On of the biggest issues is that the source of the transistor may be at a different potential during portions of the switch cycle than you would expect from the circuit diagram. This can result in the transistor turning on at high resistance when it should be turned off. To prevent this you need to ensure that the driver circuit pulls the gate voltage down to the source potential strongly enough to keep it from turning back on. The best way to ensure this is using a drive circuit than is capable of 1 or more amperes sources or sinking current. The down side is the current that can flow into the drive may be exceeded due to the gate or drain capacitance. However with a minimal value gate resistor you can ensure that the gate can still be switch rapidly enough for your needs but also keeping the maximum current low enough to prevent destroying the driver circuit.
4. When a device that is handing large power levels fails, the reaction can be very violent. It is very possible that they device could short between the drain and the gate and cause a large current to flow into your driver circuit. Limiting the current via the gate resistor could keep the damage to fewer components than if the current is not controlled at all. This is called fault tolerant design and is very important when dealing with high voltages or high currents.
From the information above it would seem that simply using a single resistor between the gates and the driver should work, and in some cases it does however there is a good reason for separate resistors. Using the shortest possible path between the gate resistor and the gate of the device will result in less stray capacitance from surrounding circuits. With a separate resistor for each gate the individual resistance can be higher thus providing more protection for each transistor from issues mention above. You may also consider using a zener diode from the gate to the source of the transistor. This will protect the transistor from the easiest way of killing a MOSFET which is to the exceed the gate breakdown voltage. When switching large currents ground bounce can cause the potential at the source of the transistor to not match the ground voltage of the driver circuit. This could result in a much larger voltage from gate to source than intended. The zener diode protects the gate from breakdown and the gate resistor protects the zener from conducting excessive current.
As jbuske mention driving a inductive circuit using a push pull configuration could have some addition problems due to vaiants in turn on voltage. One way of deal with this is to purposely add a capacitor to the gate that is much higher than the internal capacitance and using a small value gate resistor. This ensures the different in gate capacitance between the devices is smaller but also makes it more difficult to switch the devices fast enough. Another method of dealing with this is to insert a resistor into the source circuit which has two effects. One effect is it forces some balance in current sharing but more importantly if reduces the voltage between the source and gate of the device that turns on earlier thus lowering the drive which will balance the gate drive.
As a last note don't trust the first page of most data sheets as they tend to show the good and not the bad numbers. Many MOSFETS show that they are capable on very large currents in 100's of amperes. However the pins on the devices are typically limited to 75 amperes. I usually limit continuous current through a single devices to 20-30 amperes as a rule of thumb. One of the reasons for this is heat. The great resistance value quoted is usually as room temperature but as the device heats up the resistance increases. One of the reasons I love power MOSFETs is that they can be so efficient and can be packed densely in a circuit if you can keep the heat down. By paralleling transistors you can control large currents with little heating of each transistor and using the nature heat dissipation of each package to avoid the need to heatsinks in many cases. So using two transistors to control 20 amperes may not make much sense when each transistor can easily handle 50 amperes but it could keep power losses low enough to simply use the PC board as a heat sink. Note that the faster you turn the transistors on and off the more time is spend in the linear region and harder it is turn fully drive each transistor to full on. To make things worse when you drive a transistor to it's saturation point it takes more time to turn if off.
As an example, I have a circuit that I built to prevent excess current flow in case of a short circuit for trailer lights. The normal current was less than 10 amperes but the allowed current was 20 amperes, which allows for the surge currrent that light bulbs experience. I designed the circuit to be used with higher current levels up 50 amperes. It also was designed to handle current flowing into and out of a battery. They difference between the circuit when using it at lower current levels was just how many transistors I populated the board with. When using it at a higher current level I used 8 mounted back to back 4 across while only using 2 for lower current levels. The driver circuit for the transistors was a simple 1K resistor pulled down by a open collector comparator. Because this circuit is either on all the time or turns off for several seconds when the current is exceeded a very simple drive circuit was possible. I did use a single 100 ohm gate resistor in this case to ensure that a short in the transistors would not fry the comparator. In my testing using 8 transistors and no heat sinking I was able to handle 30 amperes of current flow with barely enough temperature rise to notice using my finger.
Don't know if you are driving lasers for range finding i.e. Very high currents for nano seconds. Operating many devices in || reduces each average current and increases switching times as the miller feed back is deduced.
Parts like the IXYS IXRFD631 are designed for vey high speed switching of 30 amps with rise and fall times of less than 5ns and on time down to 8ns. Directed energy, EO-Devices and others like Dr. Heller Elektronic KG offer such devices and or drivers.
Or a avalanche transistor like Zetex ZTX413 can be used generate short high current pulses.
With any high speed high current SHORT leads and traces are a must.
You want to look for devices with the lowest gate capacitance and charge required to switch. Super low Rdson devices are generally slower may not be what you want. As the faster device may run cooler as it speeds less time in the linear region.
Low cost devices like 2n6660 can switch in less than 7ns may be an option for your application.
Regarding gate resistance 1-50 ohms slows the switching time and can keep the device from oscillating. PCB layout and transmission lines must be used for the high speeds and currents.
If you don't blow up a few devices you are not having fun. Mfg speed a great deal to seal the part to keep the smoke in.
A "secret" to successful paralleling of MOSFETs in linear mode it to keep things symmetrical, otherwise some of the FETs may oscillate. Since you're driving many MOSFETs from an op amp the turn-on time even in switch mode (FETs on hard) is long enough for one or more to build up a destructive oscillation. So, don't daisy chain the gate drive from FET to FET, run the gate drive traces from the op amp in the center of the MOSFET array and place a 100 ohm series resistor right at each MOSFET gate. Reducing the driver (op amp in this case) output impedance can help. A driver such as the LT1010 can help. I learned all of this the hard way in more than one design.
In switch mode, series source resistors are often not needed for balancing. In linear mode they can be important as the often cited positive temperature coefficient function may not be strong enough to ensure adequate balancing. In this case the source resistor value should be chosen to produce at least a 1 volt drop at maximum operating current. The required voltage drop depends on the threshold voltage range for the FETs, the transconductance range, the heatsinking thermal impedance, and the maximum die temperature the hottest FET can have. So, there's not a one-size-fits-all answer. Iterative work on paper will yield a good range of resistor values.