AN2929 datasheet(10/32 Pages) STMICROELECTRONICS

10 / 32 page

Testing the board

AN2929

10/32

Doc ID 15333 Rev 2

2.2

Precision of the regulation and output voltage ripple

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

Table 4). The output voltage is practically not affected by the line condition. The VDD voltage

was also measured to verify that it is inside the operating range of the device.

In a dual-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 on the turn ratio but also, approximately, from the output current ratio (output current of

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

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

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

voltage exceeds its operating range, an external clamp was used (D6, R19). See schematic

in Figure 2.

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

measured and the results are given in Table 5. The board is provided with an LC filter for

cleaner output voltage. The high-frequency voltage ripple across capacitor C9 (VOUT_FLY),

which is the output capacitor of the flyback converter before the LC filter, was also measured

to verify the effectiveness of the LC filter and to provide complete results.

Table 4.

Output voltage and VDD line-load regulation

VINAC (V)

No load

Half load

Full load

VOUT (V)

VDD (v)

VOUT (V)

VDD (V)

VOUT (V)

VDD (V)

5.05

9.79

5.05

18.9

5.04

19.8

115

5.05

9.71

5.05

18.9

5.04

19.7

230

5.05

9.37

5.05

18.8

5.04

19.6

265

5.05

9.22

5.05

18.8

5.04

19.6

Table 5.

Output voltage ripple

VINAC (VRMS)

Half load

Full load

VOUT (mV)

VOUT_FLY (mV)

VOUT (mV)

VOUT_FLY (mV)

275

172

115

275

201

230

273

194

265

272

195

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