
CS5160
http://onsemi.com
14
Schottky Diode for Synchronous MOSFET
A Schottky diode may be placed in parallel with the
synchronous MOSFET to conduct the inductor current upon
turn off of the switching MOSFET to improve efficiency.
For a design operating at 200 kHz or so, the low non–overlap
time combined with Schottky forward recovery time may
make the benefits of this device not worth the additional
expense (see Figure 8, channel 2). The power dissipation in
the synchronous MOSFET due to body diode conduction
can be estimated by the following equation:
Power
+ VBD
ILOAD
conduction time
switching frequency
Where VBD = the forward drop of the MOSFET body
diode. For the CS5160 demonstration board as shown in
Figure 8;
Power
+ 1.6 V
13 A
100 ns
233 kHz
+ 0.48 W
This is only 1.3% of the 36.4 W being delivered to the
load.
Input and Output Capacitors
These components must be selected and placed carefully
to yield optimal results. Capacitors should be chosen to
provide acceptable ripple on the input supply lines and
regulator output voltage. Key specifications for input
capacitors are their ripple rating, while ESR is important for
output capacitors. For best transient response, a combination
of low value/high frequency and bulk capacitors placed
close to the load will be required.
Output Inductor
The inductor should be selected based on its inductance,
current capability, and DC resistance. Increasing the
inductor value will decrease output voltage ripple, but
degrade transient response.
THERMAL MANAGEMENT
Thermal Considerations for Power
MOSFETs and Diodes
In order to maintain good reliability, the junction
temperature of the semiconductor components should be
kept to a maximum of 150
°C or lower. The thermal
impedance (junction to ambient) required to meet this
requirement can be calculated as follows:
Thermal Impedance
+
TJUNCTION(MAX) * TAMBIENT
Power
A heatsink may be added to TO–220 components to
reduce their thermal impedance. A number of PC board
layout techniques such as thermal vias and additional copper
foil area can be used to improve the power handling
capability of surface mount components.
EMI Management
As a consequence of large currents being turned on and off
at high frequency, switching regulators generate noise as a
consequence of their normal operation. When designing for
compliance
with
EMI/EMC
regulations,
additional
components may be added to reduce noise emissions. These
components are not required for regulator operation and
experimental results may allow them to be eliminated. The
input filter inductor may not be required because bulk filter
and bypass capacitors, as well as other loads located on the
board will tend to reduce regulator di/dt effects on the circuit
board and input power supply. Placement of the power
component to minimize routing distance will also help to
reduce emissions.
Figure 21. Filter Components
1000 pF
33
2.0
H
Figure 22. Input Filter
1200 pF
× 3.0/16 V
2.0
H
+