AN1539 STMicroelectronics, AN1539 Datasheet - Page 2

• Manufacturer Part Number: AN1539
• Description: VIPOWER: LOW COST UNIVERSAL INPUT SMPS FOR DIGITAL SET-TOP BOX BASED ON VIPER50
• Manufacturer: STMicroelectronics

AN1539 - APPLICATION NOTE
The regulation is performed on the lowest voltage output, i.e. 2.5V, and cross regulation is achieved by
means of a simple circuit and optimized transformer coupling. All the outputs are directly connected to
the load, except for the 3.3V. In fact, this output is provided using a standard linear voltage regulator on
the 5V output in order to assure high accuracy.
The input EMI filter consists in a Pi-filter for both differential and common mode emissions. In order to
improve the efficiency, a Transil clamper is used instead of a standard RCD clamper that would dissipate
power at any operating condition.
The VIPer makes power supply design easier considering start-up, sensing and no-load issues,
improving the overall efficiency and simplifying the circuit. The short circuit protection is provided with
hiccup mode for every output with the exception of 2.5V and 5V outputs that need a simple auxiliary
circuit. A leading edge blanking circuit (Q1, C16, R14) that masks voltage spikes at turn off is also used
to improve short circuit behavior. A zener diode can be connected on COMP pin in order to clamp the
voltage on such a pin and thus limiting the maximum output power. However an input 5*20 fuse is used
to protect the system against catastrophic failures. The input section also has an NTC to limit the inrush
current of the bulk capacitor during the start-up of the power supply.
The switching frequency is set by R3 and C4 according to the diagram given in the datasheet. C3 is the
VIPer supply capacitor connected on V
giving extra protection against lighting and surge spikes.
Moreover, the VIPer has a built-in burst mode circuit that allows cycle skipping under low load condition,
improving stand-by performance.
Table 1: SMPS specifications
1.1 TRANSFORMER CONSIDERATIONS
In multi-output isolated converters the transformer is of primary concern since output voltage regulation
and cross regulation are based on winding coupling.
In the considered application the Flyback transformer has 7 windings, since one winding is dedicated to
supply the VIPer, as listed in Table 2. Winding arrangement is shown in Figure 1, while transformer pin-
2/15
Input voltage
Output power
Outputs
Out
Out
Out
Out
Out
Out
Switching frequency
1
2
3
4
5
6
DD
pin and an RC network (R4, C6) is also connected on V
85-265 V
16W
6
2.5V at 800mA, P
3.3V at 600mA, P
5V at 350mA, P
12V at 100mA, P
22V at 400mA, P
30V at 10mA, P
70 kHz
ac
3
6
=1.75W, 2%
=0.06W, 10%
4
5
1
2
=1.2W, 5%
=8.8W, 5%
=2W, 2%
=1.98W, 2%
DD