P-Channel MOSFETs, the Best Choice for High-Side Switching
Comparison between P-channel and N-channel MOSFETs in an application
The source voltage of an N-channel MOSFET when used as a high side switch will be at a higher potential
with respect to ground. Thus, to drive an N-channel MOSFET an isolated gate driver or a pulse transformer
must be used. The driver requires an additional power supply whilst the transformer can sometimes produce
incorrect conditions. However, this is not the case with P-channel. It is easy to drive a P-channel high side
switch with a very simple level shifter circuit. Doing this simplifies the circuit and often reduces the overall cost.
However, the point to be noted here is that it is not possible to achieve the same RDS(on) performance for a
P-channel MOSFET as for an N-channel with the same chip size. As the mobility of the carriers in an
N-channel is approximately 2 to 3 times higher than that of a P-channel, for the same RDS(on) value, the
P-channel chip must be 2 to 3 times the size of the N-channel. Because of the larger chip size, the
P-channel device will have a lower thermal resistance and a higher current rating but its dynamic
performance will be affected proportionally by the chip size.
So, in a low frequency application where the conduction losses are prominent, a P-channel MOSFET should
have a comparable RDS(on) to that of an N-channel. In this case, the P-channel MOSFET chip area will be
larger than that of the N-channel.
Also, in high frequency applications where the switching losses are dominant, a P-channel MOSFET should
have similar total gate charges to that of an N-channel. In this case, a P-channel MOSFET has a similar chip
size but a lower current rating than that of an N-channel.
Thus a suitable P-channel MOSFET must be carefully selected taking into consideration the appropriate
RDS(on) and gate charge.
WEE Technology Company Limited
Manufacturer of Surface Mount (SMD) and Through Hole (DIP) Diodes & Rectifiers
Website: www.weetcl.com, www.weediode.com, www.smddip.com, www.wdiode.com
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