APW7098
Application Information (Cont.)
Input Capacitor Selection (Cont.)
Phigh-side = IOUT 2(1+ TC)(RDS(ON))D + (0.5)( IOUT)(V )( tSW)FSW
IN
Plow-side = IOUT 2(1+ TC)(RDS(ON))(1-D)
RMS current of the bulk input capacitor is roughly calcu-
lated as the following equation :
where
I
is the load current
IOUT
OUT
IRMS =
´
2D×(1- 2D)
2
TC is the temperature dependency of RDS(ON)
FSW is the switching frequency
For a through hole design, several electrolytic capacitors
may be needed. For surface mount design, solid tan-
talum capacitors can be used, but caution must be exer-
cised with regard to the capacitor surge current rating.
tSW is the switching interval
D is the duty cycle
Note that both MOSFETs have conduction losses while
the high-side MOSFET includes an additional transi-
tion loss. The switching interval, tSW, is the function of
the reverse transfer capacitance CRSS. The (1+TC) term is
a factor in the temperature dependency of the RDS(ON) and
can be extracted from the “RDS(ON) vs. Temperature” curve
of the power MOSFET.
MOSFETSelection
The APW7098 requires two N-Channel power MOSFETs
on each phase. These should be selected based upon
RDS(ON), gate supply requirements, and thermal manage-
ment requirements.
In high-current applications, the MOSFET power
dissipation, package selection, and heatsink are the domi-
nant design factors. The power dissipation includes two
loss components, conduction loss, and switching loss.
The conduction losses are the largest component of
power dissipation for both the high-side and the low-
side MOSFETs. These losses are distributed between
the two MOSFETs according to duty factor (see the equa-
tions below). Only the high-side MOSFET has switching
losses since the low-side MOSFETs body diode or an
external Schottky rectifier across the lower MOSFET
clamps the switching node before the synchronous rec-
tifier turns on. These equations assume linear voltage-
current transitions and do not adequately model power
loss due the reverse-recovery of the low-side MOSFET
body diode. The gate-charge losses are dissipated by
the APW7098 and don’t heat the MOSFETs. However,
large gate-charge increases the switching interval, tSW
which increases the high-side MOSFET switching
losses. Ensure that all MOSFETs are within their maxi-
mum junction temperature at high ambient temperature
by calculating the temperature rise according to package
thermal-resistance specifications. A separate heatsink
may be necessary depending upon MOSFET power,
package type, ambient temperature and air flow.
Layout Consideration
In any high switching frequency converter, a correct layout
is important to ensure proper operation of the regulator.
With power devices switching at higher frequency, the
resulting current transient will cause voltage spike across
the interconnecting impedance and parasitic circuit
elements. As an example, consider the turn-off transition
of the PWM MOSFET. Before turn-off condition, the
MOSFET is carrying the full load current. During turn-off,
current stops flowing in the MOSFET and is freewheeling
by the low side MOSFET and parasitic diode. Any parasitic
inductance of the circuit generates a large voltage spike
during the switching interval. In general, using short and
wide printed circuit traces should minimize interconnect-
ing impedances and the magnitude of voltage spike.
Besides, signal and power grounds are to be kept sepa-
rating and finally combined using ground plane construc-
tion or single point grounding. The best tie-point between
the signal ground and the power ground is at the nega-
tive side of the output capacitor on each channel, where
there is less noise. Noisy traces beneath the IC are not
recommended. Figure 10. illustrates the layout, with bold
lines indicating high current paths; these traces must be
short and wide. Components along the bold lines should
be placed lose together. Below is a checklist for your
layout:
For the high-side and low-side MOSFETs, the losses are
approximately given by the following equations:
Copyright ã ANPEC Electronics Corp.
24
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Rev. A.6 - Oct., 2009
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