Application Information: continued
input RMS current IIN(RMS). CIN discharges during the on-
Step 6: Power MOSFETs
FET Basics
time.
The discharge current is given by:
The use of the MOSFET as a power switch is propelled by
two reasons: 1) Its very high input impedance and 2) Its
very fast switching times. The electrical characteristics of a
MOSFET are considered to be those of a perfect switch.
Control and drive circuitry power is therefore reduced.
Because the input impedance is so high, it is voltage driv-
en. The input of the MOSFET acts as if it were a small
capacitor, which the driving circuit must charge at turn on.
ICINDISRMS
=
(IL(PEAK)2 + (IL(PEAK) ´ IL(VALLEY)) + IL(VALLEY2) ´ D = 10.2A.
3
The lower the drive impedance, the higher the rate of rise
of VGS, and the faster the turn- on time. Power dissipation
in the switching MOSFET consists of 1) conduction losses,
2) leakage losses, 3) turn-on switching losses, 4) turn-off
switching losses, and 5) gate-transitions losses. The latter
three losses are proportional to frequency. For the conduct-
ing power dissipation rms values of current and resistance
are used for true power calculations.
The fast switching speed of the MOSFET makes it indis-
pensable for high-frequency power supply applications.
Not only are switching power losses minimized, but the
maximum usable switching frequency is considerably
higher. Switching time is independent of temperature.
Also, at higher frequencies, the use of smaller and lighter
components (transformer, filter choke, filter capacitor)
reduces overall component cost while using less space for
more efficient packaging at lower weight.
CIN charges during the off-time, the average current
through the capacitor over one switching cycle is zero:
D
ICIN(CH) = ICIN(DIS)
´
,
1-D
0.4
ICIN(CH) = 10.2A ´
= 6.8A.
(1-0.4)
So the total Input RMS current is:
ICIN(RMS) = (ICIN(DIS)2 ´ D) +(ICIN(CH)2 ´ (1-D)),
2
2
The MOSFET has purely capacitive input impedance. No
DC current is required. It is important to keep in mind the
drain current of the FET has a negative temperature coeffi-
cient. Increase in temperature causes higher on-resistance
and greater leakage current.
For switching circuits, VDS(ON) should be low to minimize
power dissipation at a given ID, and VGS should be high to
accomplish this. MOSFET switching times are determined
by device capacitances, stray capacitances, and the
impedance of the gate drive circuit. Thus the gate driving
circuit must have high momentary peak current sourcing
and sinking capability for switching the MOSFET. The
input capacitance, output capacitance and reverse-transfer
capacitance also increase with increased device current
rating.
ICIN(RMS)
=
.
(10.2 ´ 0.4) + (6.8 (´ 0.6)) = 8.3A
The number of input capacitors required is given by:
ICIN(RMS)
NCIN
=
.
IRIPPLE
For Sanyo capacitors type GX:
1200µF/10V, IRIPPLE = 1.25A.
Hence,
8.3
Two considerations complicate the task of estimating
switching times. First, since the magnitude of the input
capacitance, CISS, varies with VDS, the RC time constant
determined by the gate-drive impedance and CISS changes
during the switching cycle. Consequently, computation of
the rise time of the gate voltage by using a specific gate-
drive impedance and input capacitance yields only a rough
estimate. The second consideration is the effect of the
"Miller" capacitance, CRSS, which is referred to as Cdg in the
following discussion. For example, when a device is on,
VDS is fairly small and VGS is about 12V. Cdg is charged to
VDS(ON) - VGS, which is a negative potential if the drain is
considered the positive electrode. When the drain is "off",
Cdg is charged to quite a different potential. In this case the
voltage across Cdg is a positive value since the potential
from gate-to-source is near zero volts and VDS is essentially
the drain supply voltage. During turn-on and turn-off,
NCIN
=
= 6.6.
1.25
The number of input capacitors can be rounded off to 6.
Calculate the Input Capacitor Ripple Voltage:
VRMS = IRMS ´ Total ESR = 8.3A ´ 7.3m½ = 60mV.
Calculate the Input Capacitor Power Loss:
PCIN = IRMS2 ´ Total ESR = 0.504W.
Step 5b: VI/O Buck Regulator Input Capacitors
Repeating for the 3.3V output, we select 3 GX 1200µF/10V
capacitors.
13
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
