MC44603ADWR2 View Datasheet(PDF) - ON Semiconductor
Part Name
Description
MFG CO.
'MC44603ADWR2' PDF : 22 Pages View PDF
MC44603A
RICL
AC Line
Vin
+
Rstartup
VCC
MC44603A
Clamping
Network
+
RS
Snubber
Figure 40. Power Losses in a Classical
Flyback Structure
In a classical flyback (as depicted in Figure 40), the
standby losses mainly consist of the energy waste due to:
− the startup resistor Rstartup
→ Pstartup
− the consumption of the IC and the power
− switch control
→ Pcontrol
− the inrush current limitation resistor RICL → PICL
− the switching losses in the power switch → PSW
− the snubber and clamping network
→ PSN−CLN
Pstartup is nearly constant and is equal to:
ǒ(Vin–VCC)2ńRstartupǓ
PICL only depends on the current drawn from the mains.
Losses can be considered constant. This waste of energy
decreases when the standby losses are reduced.
Pcontrol increases when the oscillator frequency is
increased (each switching requires some energy to turn on
the power switch).
PSW and PSN−CLN are proportional to the switching
frequency.
Consequently, standby losses can be minimized by
decreasing the switching frequency as much as possible.
The MC44603A was designed to operate at a standby
frequency lower than the normal working one.
• Standby Power Calculations with MC44603A
During a switching period, the energy drawn by the
transformer during the on−time to be transferred to the
output during the off−time, is equal to:
E
+
1
2
x
L
x
Ipk2
where:
− L is the transformer primary inductor,
− lpk is the inductor peak current.
Input power is labelled Pin:
Pin + 0.5 x L x Ipk2 x fS
where fS is the normal working switching frequency.
Also,
Ipk
+
VCS
RS
where RS is the resistor used to measure the power switch
current.
Thus, the input power is proportional to VCS2 (VCS being
the internal current sense comparator input).
That is why the standby detection is performed by creating
a VCS threshold. An internal current source (0.4 x Iref) sets
the threshold level by connecting a resistor to Pin 12.
As depicted in Figure 41, the standby comparator
noninverting input voltage is typically equal to (3.0 x VCS +
VF) while the inverter input value is (VR P Stby + VF).
Vref Vref
Vref Vref
0.4 Iref
0.6 Iref 0.8 Iref
RP Stby
12
01
1
CStby
0.25
IF Stby
0
Oscillator
Discharge
Current
Vref
0.2 Iref
13
ERAmpOut
IDischarge/2
IDischarge
2R C. S. Comparator
Current Mirror X2
1R
Figure 41. Standby
The VCS threshold level is typically equal to
[(VR P Stby)/3] and if the corresponding power threshold is
labelled PthL:
ǒ Ǔ PthL + 0.5 x L x
VR P Stby
3.0 RS
2
x fS
And as:
VR P Stby + RP Stby x 0.4 x Iref
+ RR
P
Stby x
0.4 x
Vref
Rref
Ǹ RP Stby
+
10.6 x RS x Rref
Vref
x
PthL
L x fS
Thus, when the power drawn by the converter decreases,
VCS decreases and when VCS becomes lower than [VCS−th
x (VR P Stby)/3], the standby mode is activated. This results in
an oscillator discharge current reduction in order to increase
the oscillator period and to diminish the switching
frequency. As it is represented in Figure 41, the (0.8 x Iref)
current source is disconnected and is replaced by a lower
value one (0.25 x IF Stby).
Where: IF Stby = Vref/RF Stby
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