Applications Note AN36
Issue1 - JULY 2002
MOSFET Power Transistors
The main difference arising in the active
clamping circuit in Figure 10 is the need for a
diode in series with the active clamp Zener
diode. This prevents the Zener being forward
biased and thus detracting from the gate
drive when the MOSFET is turned on. In the
active clamp circuit there is also no need for
any drain to source negative voltage
protection as this will be achieved by the
inherent body diode of the MOSFET.
Power MOSFET devices are becoming
increasingly popular and as regard the
ZXBM200x provide a very good method of
driving the motor phase windings. Figures 9
and 10 illustrate the methodology of the
driver circuit.
When using MOSFET devices it is a
compromise between the Voltage rating and
the RDS(on) of the MOSFET with a higher
voltage rating producing a higher RDS(on).
The aim therefore should always be to use the
lowest voltage rating to suit the application
as this will give the lowest full-speed RDS(on)
related dissipation. The disadvantage is that
there will be a slightly higher dissipation
when using active clamping as this will be
working harder to maintain a lower voltage
when the fan is running at near to full speed.
The latest generation of N channel trench
MOSFETS from Zetex provide suitable
solutions with the ZXMN6A11G and the
ZXMN10A11G, 60V and 100V devices
respectively, having been used in the
applications described.
Figure 9
MOSFET Output Drive Stage
Reverse Supply protection
An increasingly common requirement for
modern systems is for components and
modules to have a degree of reverse supply
protection. This is particularly so for
Automotive applications. To achieve this a
supply blocking diode is required in the Fan
electronics.
For the best performance from the
ZXBM200x device it is best to provide
separate diodes for the device and the
winding supplies. This is illustrated in the full
applications circuit in Figure 17. The main
reason for this is that having blocked the
reverse path to the power supply for the
switching currents, high voltages can be
experienced at the winding high end. This
energy manifests itself as excessive voltages
and these are required to be kept away from
the device supply.
Figure 10
MOSFET Output Drive Stage Using Active Clamping.
The components in Figures 9 & 10 have a very
similar topography to that of the Bipolar drive
with again two variants, one with the direct
Zener clamping and the second having active
clamping.
AN 36 - 8
ZETEX [ ZETEX SEMICONDUCTORS ]
