AN2063 Datasheet, PDF (6/15 Pages) STMicroelectronics – The new regulation on the power supply standby consumption for the battery charger

Charger application
3
Charger application
AN2063
3.1
Schematic general description
As the total input power dissipation at no-load condition of this solution is less than 0.1 W,
the power losses of each component have to be saved. Below the major approaches
adopted in this evaluation board.
3.2
Solutions for energy saving
1. Losses of the VIPer12A-E controller.
The power losses of the VIPer12A-E control part can be calculated by below formula:
P_viper = V_dd * I_dd1
where:
- V_dd is the supply voltage of the VIPer12A-E control part (range: 9 V-38 V)
- I_dd1 is the operation current of the VIPer12A-E control part (typical value: 4.5 mA)
V_dd is set by considering two operative conditions; if the VIPer12A-E power has to be saved,
the V_dd value has to be lowered and, at the same time the V_dd has to be higher than 10 V,
which is the required normal operation value of the VIPer12A-E (with 1 V margin).
V_dd value (10 V) fixes the suitable turn ratio between secondary and auxiliary winding.
2. Optimized voltage source for optocoupler.
In a flyback topology, the voltage source of the optocoupler primary side is normally
connected to the V_dd of the IC directly, but in this board, in order to save energy, another
winding is inserted in the transformer to supply the optocoupler; the voltage supplied by this
winding is lower than the V_dd value (typical value 3 V).
3. Snubber circuit configuration.
An RCD clamp is a popular cheap solution, however it dissipates power even at no-load
condition, in fact there is a reflected voltage across the clamp resistor all the time. The
power losses on the resistor can be calculated as follows:
P_R = _R--^V---_m-^R---_i2-_n- + ¹²-- ⋅ L_LK ⋅ I_l2im ⋅ f_sw
where V_R is the reflected voltage; R_min is the resistor value; L_LK is the leakage inductance;
I_lim is the peak current limitation value of the VIPer12A-E and f_sw is the switching frequency.
As at no-load condition, the energy ½*L_LK*I_lim*f_sw can be neglected, then the RCD losses
could be considered as:
P_R=V_R*V_R/R_min
in this case with V_R=70 V, R_min=82 kΩ, P_R is around 60 mW.
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