AP3988
Document number: DS36722 Rev. 4 - 2
June 2014
© Diodes Incorporated
A Product Line of
Diodes Incorporated
Operation Description (Cont.)
Constant Primary Peak Current
The primary i
p
(t) current is sensed by a current sense resistor R
CS
as shown in Figure 1.
The current rises up linearly at a rate of:
M
b
L
tV
dt
tdi
)(
)(
ulk
p
……… (1)
i
p
(t)
0A
See equation 2
I
PK
Figure 2. Primary Current Waveform
As illustrated in Figure 2, when the current i
p
(t) rises up to I
PK
, the switch Q1 turns off. The constant peak current is given by:
………(2)
The energy stored in the magnetizing inductance L
M
each cycle is therefore:
………(3)
So the power transferring from input to output is given by:
………(4)
Where f
SW
is the switching frequency. When the peak current I
PK
is constant, the output power depends on the switching frequency f
SW
.
Constant Voltage Operation
The AP3988 captures the auxiliary winding feedback voltage at FB pin and operates in constant-voltage (CV) mode to regulate the output voltage.
Assuming the secondary winding is master, the auxiliary winding is slave during the D1 on-time. The auxiliary voltage is given by:
d
S
AUX
AUX
VV
N
N
V
O
………(5)
Where V
d
is the diode forward drop voltage, N
AUX
is the turns of auxiliary winding, and N
S
is the turns of secondary winding.
See equation 5
V
AUX
0V
Portion of T
ons
T
ons
Figure 3. Auxiliary Voltage Waveform
The output voltage is different from the secondary voltage in a diode forward drop voltage V
d
which depends on the current. If the secondary
voltage is always detected at a constant secondary current, the difference between the output voltage and the secondary voltage will be a fixed V
d
.
The voltage detection point is portion of T
ons
after D1 is turned on. The CV loop control function of AP3988 then generates a D1 off-time to regulate
the output voltage.
