ST AN1818 Application note
Vref
AN1818
Reducing the Total No-Load Power Consumption of
Battery Chargers and Adapter Applications
1 Introduction
This paper describes how to reduce the power consumption, under no-load conditions, of battery
chargers and adapters by using STMi croelect ronics’s TSM famil y of seconda ry-side devices . This fam ily
of devices provides accurate voltage and current regulation, while incurring very low consumption at noload conditions. In fact, owing to these innovative, integrated devices, the total power consumption for the
entire system at no-load conditions can be reduced down to nearly 100mW. With the arrival of new power
consumption regulations, this capability is increasingly sought after.
The most innovative of S T’s devices are the TSM 101x family. These a re highly integrated solutions for
SMPS applications requiring CV and CC mode, integrating one voltage reference and two operational
amplifiers. The voltage reference combined with one operational amplifier makes them ideal voltage
controllers. The other operational am plifier, combined with the integrated voltage reference and a few
external resistors, can be used as a current limiter.
These products family are designed for use in battery chargers with a constant voltage and a limited
output current and in adapters.They can be used in every type of application requiring 0.5% and 1%
voltage reference precision.
2 Power dissipation under no-load conditions
In a typical system for battery charger and adapter applica tions, different factors contribute to the total
power dissipation under no-load conditions. However, in b road t erm s, the total power dissipation can be
divided into the dissipation o wing to the secondary-side (P
primary-side (P
).
in
) and the power dissipation owi ng to the
out
Secondary-side power dissipation
This article deals with reducing the secondary-side power dissipation, so let us begin by conside ring a
typical schematic of the secondary-side of an SMPS application, shown in Figure 1.
Figure1: Typical application using CC-CV Standard in SMPS
Cvc1
optocoupler
secondary side
R2
R1
OUT+
C3
OUT-
CC-CV Standard
OP1
Rsense
OP2
Ric2
Rlimit
R3
Rvc1
Cic1
Ric1
PWM
controller
optocoupler
primary s ide
D2
Rref
C1
C2
R4
R5
D1
AN1818/100 4 Revision 1 1/6
AN1818 Power dissipation under n o-load conditions
The CC-CV (Constant Current - Constant Voltage) standard is a monolithic IC that includes one
independent op-amp and another op-amp for which the non-inverting input is wired to a 2.5V fixed voltage
reference. A good example of such a secondary-side device is ST’s TSM103W.
Normally the CC-CV voltage reference is “shunted”, meaning that the internal current generator requires
an external power supply in order to polarize and fix the voltage reference at 2.5V (V
If we assume that V
is connected to a discharge ba ttery, the resulting ch aracteristic curve, V
out
shown in Figure 2.
- I
Figure2: Characteristic V
25
25
20
20
15
15
Vout (V)
Vout (V)
10
10
out
for adapter application
out
= 2.5V).
ref
out-Iout
, is
5
5
Vout_min in CC mode
0
0
0123456
0123456
Iout (A)
Iout (A)
Vout_min in CC mode
In Figure 2, we can see that the load charges gradually, by increasing the current and the voltage in order
to reach a minimal voltage drop. This gradual increase guarantees a limited and stable current. Following
this, the voltage value increases (while the current stays constant) up to the constant voltage value.
A typical adapter application will have V
out_max
= 20V (at no-load conditions) and V
= 5V (which is
out_min
the minimal voltage necessary to have a constant current).
In order to have V
out_min
= 5V, V
= 5V. If the minimum current value to bias V
cc_min
is 1mA, this means
ref
that:
R
ref
Therefore, in order to have V
= 5V, we must fix R
out_min
Now that we have fixed the value of R
I
V
outminVref
--------------------------------------- -
I
ref
, let's consider a no-load condition where V
ref
ref
–
1mA=
5V 2.5V–
------------------------ 2.5k Ω===
ref
1mA
= 2.5kΩ.
out_max
= 20V. It
follows that:
V
–
I
ref
outVref
-----------------------------
R
ref
20 2.5–
------------------ - 7 mA===
2.5
2/6
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