Nettop Platform for 2008 System Design White Paper

System Configuration

White Paper 21

4.3.3 External Adapter and DC-DC Board

Ultra small form factor system can use a DC-DC board and external adapter for the

power solution. The system size is greatly reduced by removing part of the AC-DC

converter from the system and using an external adapter.

Designing this solution, the following items need to be taken into consideration:

• The DC-DC board should meet the minimum power distribution requirements in

Table 7.

• Main Power Connector: A 20 pin main connector is recommended. A 20+4 pin

main connector is acceptable.

• Input voltage: The input voltage of the DC-DC board in the market is typically

19 V or 12 V. Comparing these two DC-DC solution, 12 V input DC-DC board

usually has no additional regulator for 12 V output. (see

Figure 5 and Figure 6)

12 V output of the adaptor passes through the DC-DC board and supply current to

the system directly. The advantage of this approach is lower cost due to lack of a

12 V regulator. The potential disadvantage is the 12 V voltage drop on the DC-DC

board and higher thermal cost in the adaptor if the system requires large 12 V

power. System designer should calculate the power cost together with the

adaptor.

• Adaptor Matching: The worst efficiency of the DC-DC should be considered for the

adaptor matching. Such as, Total power of DC-DC board is 70 W and its efficiency

at full load is ≧ 70%. The adaptor should be at least 70 W / 70% ≧ 100 W to

support the DC-DC board power output and power loss.

• Airflow: Some of the DC-DC boards require a certain amount of airflow to support

the maximum power. System designers should check the system airflow if the DC-

DC board requires this airflow for cooling.

• Cable length: DC-DC board solution is commonly chosen by small form factor or

ultra small form factor design. Since the system component density is high, long

power cables will increase the system airflow impendence significantly. System

designers should optimize the power cable length to reduce the system airflow

impendence.

Figure 5 Typical Input Voltage of the DC-DC board in the Market, for example 12 V

Adaptor

12V

DC-DC Board

DC Regulator

12V

3.3V

-12V

5VSB

System