Operation
The mini-z use a surface mount MOSFET or FETs in a SO-8 packaging. Two identical fets are needed to control the motor. Each fet chip contains a N-channel and a P-channel switch. The reason for both N channel and P channel is because a MOSFET works differently than a light switch. You can put a light switch before or after the lamp, and it will still work. For mosfets, if you want to put a switch in front of the motor (positive side), you need a P-channel mosfet. Use a N-channel mosfet for the negative side.
Each mosfet contains three connections: gate, source and drain. The gate as the name implies controls the switch. Positive voltage on the gate turns on the switch. No voltage turns it off. To get no voltage, connect the gate to ground with a resistor so all stray currents in the MOSFET travels to ground in a slow orderly fashion throught the resistor. The source is where is connected to the battery. The drain connects to the motor. The source and drain are the power connection, and need a nice solder joint. Gate is just signal to turn on and off the mosfet. As long as you have connection, is ok.
The reason for having two chips each with a p and n channel mosfet is so the can can go in reverse. One set of p and n channel mosfet controls the forward speed. The other set of n and p channel mosfet controls reverse. Presumably, the computer on the car turns on one set of mosfet or the other. If both gets turned on at the same time then there would be a short. Same reason if you solder one of the source connection where there is battery voltage to one of the gate, that mosfet is permanently turned on, and will create a short when it is not suppose to get turned on.
Note: Dot on chip
is on the right side on both chips.
Picture of board shows flow of power thru the MOSFET chip on the mini-z board. Signal wire instructs each of the two individual MOSFETs on the chip to turn on. The power wire connects to the battery. By turning on the the right P and N channel MOSFET on the chip, the car can go forwar or reverse.
Performance
Above is from the data sheet for the 3010 mosfet that is used on the latest mini-z. Kyosho claims it is 30% faster than the old 3004 mosfet. The important parameter to watch for is the on state resistance. It gives the resistance at two voltages, 4.5V and 10V. Ignore the negative sign on the voltage. It just tells you this is used on the positive side of the motor. The max resistance is 130 mOhms or .13 Ohms. Remember, this is only for the P channel as stated in the upper left hand corner of the sheet. Another page in the data sheet shows the N channel resistance to be .045 Ohms. Since both P and N channel runs the motor, you have to add up the resistance which comes out to be .175 Ohms.
The maximum continuous current is show also in the data sheet under "Drain
current". P channel is 5 amps and N channel is 6 amps. You have to take
the lower of the two since that is the limiting factor. However, remember that
the current numbers are not realistic in that heat is what really kills the
mosfet. Remember power through any electronic device is I^2*R. I is current
and R is resistance. That is the power that the mosfet has to dissipate and
stay cool enough to have a long life. It comes back to having a lower resistance
allow you to run higher current doesn't matter what the spec say.
As seen in the above chart, 4562 is tied with SP8M4 as the best MOSFET available. One of that is equivalent to at least a stack of 6 stock 3004 or stack of 4 3010 found in the latest upgraded mini-z chassis. Note that the chart above does not include the PNRacing AN0113 MOSFET since there is no information available for that. Some people claim it is better than 4562. Base on resistance above, here is the estimated real continuous current rating of the FETs without heatsink. The 4562 is the tried and true MOSFET. The SP8M4 is a new one. It should be just as good.
Again 4562 is the best choice.
Stacking FETs
The graph shown above has a maximum of 6 FETs stacked together. In practice, 3 is the practical maximum. There are reports that after a stack of 3 FETs, operation becomes eratic. There may be two reasons why. The gate on a FET acts like a capacitor. The analogy is the gate being a cup. It takes some electricity to fill the cup before the FET turns on. If you have 6 FETs, then you have to fill 6 cups with electricity before all 6 FETs turn on. The FET driver on the board may not be able to supply that much current fast enough to turn on the FETs at the switching frequency of the speed control. Therefore, the FETs never turn on fully. In this scenario, at maximum throttle, there would be no issue since the FET is turned on full time. Second possible reason is resonance in the FET gate circuit. Since each FET acts like a capacitor, and the wires leading up the FET acts like an inductor, some resonance may be set up which cause the FETs to turn on and off by themselves at very high speed. When you parallel FETs, the right way to do it is to tie a resistor from the FET driver to each FET. In practice, there is rarely a need to stack more than 3 FETs, and nobody use a resistor to connect to each FET. There seens not be a proble with less than 3 FETs.
Reference:
Continuous power dissipation of typical SO-8 packaging is 1 watt. Base on Fairchild application note 1032. For conservative continuous running, divide the above current chart by 2.
Data Sheets:
International Rectifier - IRF7389
Fairchild Semiconductor - FDS8958