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AN1894 - APPLICATION NOTE

2.2.2 Circuit Operation

The total schematic of the power supply can be seen in Figure 22. The output current flowing to the

output capacitor C3 is discontinuous for the buck-boost converter (unlike the above described buck

converter). During primary switch ON-time the inductor current flows through the inductor to ground and

not to the output capacitor as for buck converter. As soon as the primary switch is turned off, the inductor

reverses the voltage between its terminals and current starts to flow through the forward biased D5 diode

and charges the output capacitor C3. In this switch-off phase the source terminal of the VIPer12 sees

negative level of forward biased D5 and negative output voltage (when referenced to ground). The

converter is called inverting. It is because the output voltage is negative compared to the input voltage

referred to the same common ground. By removing the jumper R5 and placement of the jumper R4 the

buck converter can be easily changed to the inverting buck-boost converter.

Figure 22: Schematic Diagram of Non Isolated Buck-Boost Converter with Negative Output Voltage

2.2.3 Buck-Boost Converter Evaluation and Measurements

The output regulation characteristics can be seen in Figure 23. It shows the variation of the output

voltage with output current at specific DC input voltage. The zener diode D6 was not connected during

the measurements.

L

D1

GL1M

1000V

1A

D2

GL1M

1000V

1A

VDD

4

FB

3

U1

VIPer12AS

N

D4

RGL34J

600V

0.5A

D5

RGL34J

600V

0.5A

D3

ZMM13

VDD

+

C3

56uF

35V

LXY

1

2

3

CON2

clamp

R3

1k

D7

LL4148

L2

1mH

200mA

LBC

+

C2

4.7uF

400V

KMG

+

C4

10uF

50V

KME

+

C1

4.7uF

400V

KMG

R4

0R

C6

220pF

90...264V~

L

N

-15V

R6

0R

L1

1.5mH 100mA

BC

Layout Hints: C4, C6 have

to be close to VIPer12A

C7

100nF

R1

10R

3W

1

2

CON1

clamp