iru3038 International Rectifier Corp., iru3038 Datasheet - Page 7

• Manufacturer Part Number: iru3038
• Description: Synchronous Pwm Controller For Termination Power Supply Applications
• Manufacturer: International Rectifier Corp.

The MOSFET has the following data:
The total conduction losses will be:
The switching loss is more difficult to calculate, even
though the switching transition is well understood. The
reason is the effect of the parasitic components and
switching times during the switching procedures such
as turn-on / turnoff delays and rise and fall times. With a
linear approximation, the total switching loss can be ex-
pressed as:
The switching time waveform is shown in Figure 4.
From IRF7460 data sheet we obtain:
These values are taken under a certain condition test.
For more detail please refer to the IRF7460 data sheet.
By using equation (6), we can calculate the total switch-
ing losses.
Rev. 2.0
09/12/02
IRF7460
V
I
R
q = 1.8 for 1508C (Junction Temperature)
P
P
Where:
V
t
t
T = Switching Period
I
t
t
P
D
LOAD
r
f
r
f
90%
10%
DSS
DS(ON)
CON(TOTAL)
SW
DS(OFF)
SW(TOTAL)
V
= Rise Time
= Fall Time
= 6.9ns
= 4.3ns
= 10A @ 758C
V
GS
D S
=
= Load Current
Figure 4 - Switching time waveforms.
= 20V
t
d
V
(ON)
= 10mV @ V
= Drain to Source Voltage at off time
DS(OFF)
= 44.8mW
2
= 1.152W
3
t
r
t
+
r
T
GS
t
f
=10V
t
d
3 I
(OFF)
LOAD
---(6)
t
f
www.irf.com
Feedback Compensation
The IRU3038 is a voltage mode controller; the control
loop is a single voltage feedback path including error
amplifier and error comparator. To achieve fast transient
response and accurate output regulation, a compensa-
tion circuit is necessary. The goal of the compensation
network is to provide a closed loop transfer function with
the highest 0dB crossing frequency and adequate phase
margin (greater than 458).
The output LC filter introduces a double pole, –40dB/
decade gain slope above its corner resonant frequency,
and a total phase lag of 1808 (see Figure 5). The Reso-
nant frequency of the LC filter is expressed as follows:
Figure 5 shows gain and phase of the LC filter. Since we
already have 1808 phase shift just from the output filter,
the system risks being unstable.
The IRU3038’s error amplifier is a differential-input
transconductance amplifier. The output is available for
DC gain control or AC phase compensation.
The E/A can be compensated with or without the use of
local feedback. When operated without local feedback,
the transconductance properties of the E/A become evi-
dent and can be used to cancel one of the output filter
poles. This will be accomplished with a series RC circuit
from Comp pin to ground as shown in Figure 6.
0dB
Gain
F
LC
=
Figure 5 - Gain and phase of LC filter.
2p3
F
LC
1
-40dB/decade
Frequency
L
O
3C
O
-180
Phase
0
8
8
---(7)
IRU3038
F
LC
Frequency
7