Quasi-resonant CoolSET design guide
ICE2QRxx65/80x
Design Guide
13
8 August 2011
If the output load is still low, the feedback signal decreases as the PWM section is operating. When feedback
signal reaches the low threshold V
BL
(3.0V), the internal bias is reset again and the PWM section is disabled
until next time regultaion signal increases beyond the V
BH
threshold. During the active burst mode the
feedback signal is changing like a saw tooth between 3.0V and 3.6V shown in Figure 7.
4.4.3
Leaving Active Burst Mode
The feedback voltage immediately increases if there is a high load jump. This is observed by one comparator.
As the current limit is 34% during active burst mode a certain load is needed so that feedback voltage can
exceed VLB (4.5V). After leaving active burst mode, maximum current can now be provided to stabilize V
O
. In
addition, the up/down counter will be set to 1 immediately after leaving active burst mode. This is helpful to
decrease the output voltage undershoot.
4.5
Current sense
The PWM comparator inside the CoolSET has two inputs: one from current sense pin and the other from
feedback voltage. Before being sent to the PWM comparator, there is an offset and operational gain on
current sense voltage. In normal operation, the relationship between feedback voltage and maximum current
sense voltage is determined by equation (8).
PWM
pk
CS
PWM
FB
V
V
G
V
_
(8)
where G
PWM
=3.3 and V
PWM
=0.7
The absolute maximum current sense voltage, V
CS_PK
is 1V. Therefore, the current sense resistor can be
chosen according to the maximum required peak current in the transformer as shown in (9).
p
pk
CS
I
R
_
/
1
(9)
The design procedure of quasi-resonant flyback transformer is shown in [2]. In addition, a leading edge
blanking (LEB) is already built inside the current sense pin. The typical value of leading edge blanking time is
330ns, which can be thought as a minimum on time.
4.6
Feedback
Inside the CoolSET, the feedback (FB) pin is connected to the 5V voltage source through a pull-up resistor
R
FB
. Outside the CoolSET, this pin is connected to the collector of opto-coupler. Normally, a ceramic
capacitor C
FB
, 1nF for example, can be put between this pin and ground for smooting the signal.
Feedback voltage will be used for a few functions as following:
It determines the maximum current sense voltage, equivalent to the transformer peak current.
It determines the ZC counter value according to load condition
4.7
Zero crossing
The circuit components connected to zero crossing (ZC) pin include resistors R
ZC1
and R
ZC2
and capacitor
C
ZC
. The values of three components shall be chosen so that the three functions combined to this pin will
perform as designed.
At first, the ratio between R
ZC1
and R
ZC2
is chosen first to set the trigger level of output overvoltage protection.
Assuming the protection level of output voltage is V
O_OVP
, the turns of auxiliary winding is Na and the turns of
secondary output winding is Ns, the ratio is calculated as
a
O
S
ZCOVP
ZC
ZC
ZC
N
V
N
V
R
R
R
2
1
2
(10)
In (10), V
ZCOVP
is the trigger level of output overvoltage protection which can be found in product datasheet.
