DN06028/D

June 2007, Rev. 0 www.onsemi.com 1

Design Note – DN06028/D

7 W, Non-Isolated Buck AC-DC Converter

Device Application Input Voltage Output Power Topology I/O Isolation

NCP1055

1 Output AC to DC

converter

108-132 Vac 7 Watts

Non-Isolated

Buck

Converter

Circuit Description

This design note explains the changes required to modify

the NCP1052 Buck Demo Board’s (AND8098/D) output

from 12V/100mA into 91V/75mA. Please read the original

NCP1052 evaluation board manual for reference

Design Considerations

NCP105X and Inductor

The first step in the design process is determining which

version of the NCP105x to use. Refer to the “NCP105x

Discontinuous Mode Flyback Design Worksheet”

. The

desired input voltages of this particular board range from

108 to 132 Vac (about 150 Vdc to 190 Vdc). The inductor

is chosen to be around 390 μH, a slightly reduced value

compared to the original NCP1052 circuit. The result from

table 1 of the Worksheet indicates that the NCP1055 at 136

kHz can be used for the desired output power of 7 W. A

different version NCP105X may be suggested by the

Worksheet by simply changing the inductor value. The

NCP1055 136 kHz version is used in this design.

Zener Diodes

The output voltage of the converter is set by the breakdown

voltage of Z1. The breakdown voltage of the diode is

calculated using the equation below:

VVoutV

−=

Available at http://www.onsemi.com/pub/Collateral/NCP1052EVB_MANUAL.PDF.

Available at http://www.onsemi.com/pub/Collateral/NCP105X_WORKSHEET.XLS

This equation comes from application note AND8098/D. A

91 V output is achieved using an 86 V Zener. The

breakdown of zener diodes can be added in series to

obtain the desired breakdown voltage. In this design two

43 V Zeners in series are used for Z1. Zener diode Z1 is

used in the original NCP1052 board to clamp the voltage

down at light or no load. However, the higher voltage of

this design requires a higher power dummy load (R2) to

maintain the output in regulation when no load is present.

It may be possible to use a lower frequency version of the

NCP1055 and reduce the power rating of R2.

Capacitors

Since the output voltage is increased to 91V, the output

capacitor (C5) must be changed. An electrolytic capacitor

is used as they are more readily available at this voltage

level. The value of C5 is reduced to reduce cost. The

value of the bulk input capacitor (C3) is increased from 10

μF to 33 μF to reduce the input voltage ripple to 3 V. C3

and C5 can be increased to reduce the output voltage

ripple, or they may be reduced to reduce cost. Capacitor

C2 is increased to increase the startup time. Finally, a

filtering capacitor (C4) is added between the feedback pin

and source pin to reduce noise into the feedback pin; this

is a very important addition to the board.

Unchanged Components

Diodes D1, D2, D3, D4 and Capacitor C1 remain the same

as the original NCP1052 board.

Other Specifications

Output

Output Voltage

91 V

Ripple

3 V

Nominal Current

75 mA

Max Current

100 mA

Min Current

8 mA

PFC (Yes/No)

Inrush Limiting / Fuse

None

Cooling Method/Supply

Orientation

Convection

DN06028/D

June 2007, Rev. 0 www.onsemi.com 2

Key Features

y Low-cost, simple design

y Relatively few changes for output voltage change

y Self-powered from high input voltage

y Frequency jittering for low electromagnetic

interference(EMI)

y Thermal and short circuit fault protection

Schematic

Figure 1: Circuit Schematic

Bill of Materials

Designator Quantity Description Value Tolerance Manufacturer Manufacturer

Part Number

U1 1 Switching Regulator 680 mA / 136 kHz N/A ON Semiconductor NCP1055ST136G

D1, D4 2 Ultrafast Diode 1 A / 600 V N/A ON Semiconductor MURS160T3

D2 1 Switching Diode 200 mA / 100 V N/A ON Semiconductor MMSD914T1

D3 1 General Diode 1 A / 600 V N/A ON Semiconductor MRA4006T1

Z1a, Z1b 2 Zener Diode 43 V 5% ON Semiconductor MMSZ5260BT1G

R1 1 Chip Resistor 2 kΩ / 250 mW 5% Vishay CRCW12062K00JNEA

R2 1 Axial Resistor 12 kΩ / 1 W 10% Ohmite OX123KE

C1 1 Ceramic Capacitor 0.22 µF / 50 V 10% Vishay VJ1206Y224KXXA

C2 1 Tantalum Capacitor 10 µF / 25 V 10% Vishay 595D106X9025B2T

C3 1 Electrolytic Capacitor 33 µF / 450 V 20% Panasonic ECA-2WM330

C4 1 Ceramic Capacitor 1000 pF / 50 V 10% Vishay VJ1206Y102KXXA

C5 1 Electrolytic Capacitor 10 µF / 250 V 20% Panasonic ECA-2EM100

L1 1 Inductor 390 µH / 450 mA 20% Coilcraft RFB0807-391L