We were very curious to check what components were chosen for the power section of this power supply and also how they were set together, i.e. the design used. We were willing to see if the components could really deliver the power announced by HEC.
From all the specs provided on the databook of each component, we are more interested on the maximum continuous current parameter, given in ampères or amps for short. To find the maximum theoretical power capacity of the component in watts we need just to use the formula P = V x I, where P is power in watts, V is the voltage in volts and I is the current in ampères.
We also need to know under which temperature the component manufacturer measured the component maximum current (this piece of information is also found on the component databook). The higher the temperature, the lower current semiconductors can deliver. Currents given at temperatures lower than 50º C are no good, as temperatures below that don’t reflect the power supply real working conditions.
Keep in mind that this doesn’t mean that the power supply will deliver the maximum current rated for each component as the maximum power the power supply can deliver depends on other components used – like the transformer, coils, the PCB layout, the wire gauge and even the width of the printed circuit board traces – not only on the specs of the main components we are going to analyze.
For a better understanding of what we are talking here, please read our Anatomy of Switching Power Supplies tutorial.
This power supply uses one GBU806 rectifying bridge on its primary stage, which can deliver up to 8 A (rated at 100º C). No heatsink was used to cool down this component. This is more than adequate rating for a 480 W power supply. The reason why is that at 115 V this unit would be able to pull up to 920 W from the power grid; assuming 80% efficiency, the bridge would allow this unit to deliver up to 736 W without burning this component. Of course we are only talking about this component and the real limit will depend on all other components from the power supply.
On the switching section two 2SK2749 power MOSFET transistors are used under a modified single-transistor forward configuration. Usually this configuration uses only one transistor, but on this power supply two transistors were connected in parallel in order to double the maximum current.
Each one has a maximum rated current of 21 A at 25º C in pulsating mode, which is the mode used, as the PWM circuit feeds these transistors with a square waveform, or 7 A at 25º C in continuous mode.
If you pay close attention on Figure 11 you will see a small copper plate between the transistors and the aluminum heatsink, put there to improve heat dissipation.
click to enlarge
Figure 11: MOSFET transistors used on the primary.
