AN3011 datasheet(11/37 Pages) STMICROELECTRONICS

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AN3011

Testing the board

Doc ID 16043 Rev 1

11/37

2.2

Precision of the regulation and output voltage ripple

The output voltage of the board was measured in different line and load conditions. The

results are given in Table 4. The output voltage is practically not affected by the line

condition and only slightly affected by load condition (a difference of 10 mV between max

and minimum VOUT, see Table 4). The VDD voltage was also measured.

In a two-output flyback converter, when just one output is regulated, the unregulated output

does not rigorously respect the turn ratio. The unregulated output voltage value depends not

only by the turn ratio but also, approximately, from the output currents ratio (output current at

the regulated output divided by output current of the unregulated output).

As confirmed from the results reported in Table 4, the VDD voltage (unregulated auxiliary

output) increases as the load on the regulated output increases. In order to avoid the VDD

voltage exceeding the VIPer27 operating range, an external clamp was used (D6, R19, see

schematic).

The ripple at the switching frequency superimposed at the output voltage was also

measured. The board is provided with an LC filter for cleaner output voltage. The high

frequency voltage ripple across capacitors C9 and C14 (VOUT_FLY), that is the output

capacitors of the flyback converter before the LC filter (see schematic in Figure 2), was also

measured to verify the effectiveness of the LC filter.

The waveforms of the two voltages (VOUT and VOUT_FLY) are reported in Figure 11 and

. The output voltage ripple when the converter input voltage is 115 VAC is shown in

Figure 11

, and the output voltage ripple when the converter input voltage is 230 VAC is

shown in Figure 12.

Table 4.

Output voltage and VDD line-load regulation

VINAC (V)

Full load

Half load

No load

VOUT (V)

VDD (v)

VOUT (V)

VDD (V)

VOUT (V)

VDD (V)

5.073

21.1

5.078

20.00

5.083

9.98

115

5.073

20.98

5.078

20.02

5.083

9.83

230

5.073

20.94

5.077

20.08

5.083

9.30

265

5.073

20.98

5.077

20.04

5.083

9.17

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