The main structure is H-bridge that combines 2 N-channelMOSFETs and 2 P-channel MOSFETs. After testing several MOSFET, in the end wechoose IRFR5305 as P-channel MOSFET and IRFZ48N as N-channel MOSFET. Due tofuture using purpose to apply PWM (Pulse-Width Modulation) as DC motor controltools we choose MOSFET which have lower switch time (include delay, rise andfalling time) to allow us to set high frequency PWM.
For IRFZ48N, Switch ON and delay total take 80ns,Switch OFF with delay take 84ns. As for IRFR5305, total ON state take 90ns andOFF state take 102ns. Due to the design, the most critical action is to changethe H-bridge currect flow direction. This action closed 1 P-MOSFET and 1N-MOSFET. At the same time switch on other 2 MOSFET. Thus we take P-channelMOSFETs OFF state as longest calculation which is 102ns. We multiply it by 2,due to a full consist 2 action (change direction and change back). For safetyreason we advice to apply the period 100 times bigger value than the MOSFETdelay and turn state period, this will make the change state period to bealmost 1% of the total period. This action believes to help on maintain theMOSFET temperature in room temperature. At the same time reduce the chances ofshoot-through situation to happen. The maximum frequency to apply on PWM is 49kHz or 50 kHz.
Besides that we do consider the maximum Vds and peak currentrating. Due to motor maximum voltage is not very high but the current may shootup to very high value when it is hang thus we go for low voltage, high currentMOSFET. Beside both of the MOSFETs we using have very low Rds (on) which is 65and 14 mohm. Assume normal case which is running in 0.5A. Total voltage drop isonly (0.065+0.014)*0.5 = 0.04V. 0.04V is 0.33% loss if we use 12V source.
Diode placed across Drain and Source on every MOSFET asprotection. The Diode will be the path ways to let the back EMF current toflow. Instate of the currect pass through the MOSFETs from Drain to Sourcewhich may burn the MOSFETs. But at the first place we use normal diode and itcause many problem and keep on burn our mosfet. This is due to the recoverytime for our diode is too long compare to our mosfet. The mosfet is notfunction at all. That is why we are now using Fast Recovery Diode, UF4002. Itsturn ON time is 50ns which is faster than the MOSFET switching period. Besidethat it allow 30A currect surge less than 8.3ms, continues current 1A. This isbeyond our safety consideration.