ON Semiconductor NCP1608BOOSTGEVB, Руководство пользователя

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NCP1608BOOSTGEVB
http://onsemi.com
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Figure 18. Block Diagram of the Propagation Delay Components
Ct
+
−
PWM
Control
Driver
Ct
R
CT
V
DD
I
charge
DRV
V
Control
Ct
(offset)
V
Ct(off)
DRV
R
DRV
R
sense
V
gate
I
switch
There is a delay (t
delay
) from when V
Ct(off)
is reached to
when the MOSFET completely turns off. t
delay
is caused by
the propagation delay of the PWM comparator (t
PWM
) and
the time for the gate voltage of the MOSFET to decrease to
zero (t
gate
). The delays are illustrated in Figure 19.
Figure 19. Turn Off Propagation Delays
Ct
t
PWM
t
gate
t
delay
V
Ct(off)
V
gate
I
switch
The total delay is calculated using Equation 25:
t
delay
+ t
PWM
) t
gate
(eq. 25)
t
delay
increases the effective on time of the MOSFET.
If a resistor (R
CT
) is connected in series with the Ct
capacitor, then the total on time reduction is calculated using
Equation 26:
D t
on
+ Ct @
D V
RCT
D I
RCT
+ Ct @ R
CT
(eq. 26)
The value of R
CT
to compensate for the propagation delay
is calculated using Equation 27:
R
CT
+
t
delay
Ct
(eq. 27)
The NCP1608 datasheet specifies the maximum t
PWM
as
130 ns. t
gate
is a dependent on the gate charge of the
MOSFET and R
DRV
. For this demo board, the gate delay is
measured as 230 ns.
R
CT
+
360 n
1 n
+ 360 W
A value of R
CT
= 365 W compensates for the propagation
delays. Figure 20 shows the decrease of THD at Vac
HL
and
low output power by compensating for the propagation delay.
R
CT
= 0 W
25 30 35 40 45 50
0
10
20
30
40
50
THD (%)
Figure 20. Low Output Power THD Reduction with
R
CT
(V
in
= 265 Vac 50 Hz, R
CTUP
= open, Ct = 1 nF)
P
out
(W)
R
CT
= 365 W
DRV Pulse Skipping Begins
Both THD reduction techniques can be combined to
decrease the THD for the entire output power range.
Figure 21 shows the decreased THD at the maximum input
voltage
across the output power range by decreasing zero
crossing distortion and by compensating for the propagation
delay.
R
CTUP
= open
R
CT
= 0 W Ct = 1 nF
25 35 45 55 85 95
0
10
20
30
40
50
THD (%)
Figure 21. THD Reduction with R
CTUP
and R
CT
(V
in
= 265 Vac 50 Hz)
P
out
(W)
65 75
DRV Pulse Skipping Begins
R
CTUP
= 1.5 M
W
R
CT
= 365 W Ct = 1.22 nF