CS1601
5.2 Startup vs. Normal Operation Mode
5.4 Output Power and PFC Boost Inductor
The CS1601 has two discrete operation modes: startup and
normal. Startup mode will be activated when Vlink is less than
90% of nominal value, VO(startup) and remains active until Vlink
reaches 100% of nominal value, as shown in Figure 15.
Startup mode is activated during initial system power-up. Any
Vlink drop to less than VO(startup), such as a load change, can
cause the system to enter startup mode until Vlink is brought
back into regulation.
In normal operating mode, the nominal output power is
estimated by the following equation.
V
V
link – inmin 2
V
---------------------------------------------------------
= inmin
2
[Eq.1]
P
o
2 fmax LB Vlink
where:
Po
rated output power of the system
efficiency of the boost converter (estimated as 100%
by the PFC algorithm)
V
[V]
link
Vin(min) minimum RMS line voltage measured after the
rectifier and EMI filter. Vin(min) is equal to 90Vrms or
108Vrms depending on the AC Line Voltage
operating range.
100%
90%
Normal
Mode
Normal
Mode
Vlink
nominal PFC output voltage; Vlink = 400V when
Vin(min) = 90 Vrms or Vlink = 460 V when
Vin(min) = 108Vrms
fmax
maximum switching frequency; for the CS1601
fmax = 70kHz and the CS1601H fmax = 100kHz
t[ms]
Figure 15. Startup and Normal Modes
LB
boost inductor specified by rated power requirement
Startup mode is defined as a surge of current delivering
maximum power to the output regardless of the load. During
every active switch cycle, the 'ON' time is calculated to drive a
constant peak current over the entire line cycle. However, the
'OFF' time is calculated based on the DCM/CCM boundary
equation.
margin factor to guarantee rated output power (Po)
against boost inductor tolerances.
Equation 1 is provided for explanation purposes only. Using
substituted required design values for Vlink and fmax gives the
following equation:
2
460V – 108V 2
2 70kHz LB 460V
------------------------------------------------------------
P
= 108V
[Eq.2]
5.3 Burst Mode
o
Burst mode is utilized to improve system efficiency when the
system output power (Po) is <5% of nominal. Burst mode is
implemented by intermittently disabling the PFC over a full
half-line period under light-load conditions, as shown in
Figure 16.
Changing the value for the Vlink voltage is not recommended.
Solving Equation 2 for the PFC boost inductor LB gives the
following equation:
2
460V – 108V 2
------------------------------------------------------------
[Eq.3]
LB = 108V
Po
[W]
2 70kHz P 460V
o
If a value of the boost inductor other than that obtained from
Equation 3 above is used, the total output power capability as
well as the minimum input voltage threshold will differ
according to Equation 2. Note that if the input voltage drops
below 108Vrms and the inductance value is <LB, the link
voltage Vlink will drop below 460V and fall out of regulation.
Burst Threshold
Burst Mode
Active
t [ms]
Vin
[V]
PFC
Vin
FET
Vgs
Disable
L < LB
L = LB
t [ms]
L > LB
Figure 16. Burst Modes
108
305
VAC(rms)
Figure 17. Relative Effects of Varying Boost Inductance
10
DS931PP6
CIRRUS [ CIRRUS LOGIC ]
