This power supply uses one of the most unusual configurations we’ve ever seen on its secondary. We decided to draw a simplified schematics from the secondary so you can better understand the configuration used on this power supply. Read our Anatomy of Switching Power Supplies tutorial to compare the configuration used on this power supply with the configuration normally used. In the name of simplification we didn’t draw the controlling circuit of the MOSFET transistors and that is why we left their gates unconnected.
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Figure 11: Secondary from Rocketfish 700 W.
The +12 V output is produced by three Schottky rectifier packs. Two STPS30150CW are in charge of the rectification, while one STPS4045CW is in charge of the freewheeling diodes. This is a very unusual design, as usually power supplies use the same number of rectifying and freewheeling diodes and also they are usually identical. Here we have four diodes for the direct rectification and two diodes for freewheeling part.
Because of this asymmetrical design, we have to consider the part with lower current limit in our calculation. This would be the freewheeling path, which has two 20 A diodes in parallel. The maximum theoretical current the +12 V line can deliver is given by the formula I / (1 - D), where D is the duty cycle used and I is the maximum current supported by the diode (which in this case is made by two 20 A diodes in parallel, as mentioned). Just as an exercise, we can assume a typical duty cycle of 30%. This would give us a maximum theoretical current of 57 A or 686 W for the +12 V output. The maximum current this line can really deliver will depend on other components, in particular the coil used.
The +5 V output is rectified using a synchronous topology, while the +3.3 V is rectified using a partial synchronous topology, where only the freewheeling diode was replaced by a power MOSFET transistor: the rectification diode wasn't replaced like in a full synchronous topology. The transistors used are IRL7833 power MOSFET transistor, which can handle 110 A at 100º C each and the +3.3 V output is rectified through an SBL4040PT Schottky rectifier (40 A at 100º C, 20 A per internal diode).
As an exercise we can try to calculate the maximum theoretical current/power also for the +5 V and +3.3 V outputs. For the +5 V output the current limit would be of 157 A with 786 W maximum power and for the +3.3 V output the current limit would be of 29 A with 94 W maximum power. In both cases we are assuming a 30% duty cycle.
The use of this partial synchronous design is really intriguing, as this design in theory offers a higher efficiency (the reason why is that MOSFET transistors usually offer a lower voltage drop compared to Schottky rectifiers, i.e., less wasted power), but at the same time Huntkey used regular BJT transistors on the switching section. Go figure.
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Figure 12: +12 V negative rectifier, transistor and +3.3 V rectifier.
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Figure 13: Transistors and +12 V rectifiers.
For the protection circuit instead of using a monitoring integrated circuit this power supply uses a discrete protection circuit, i.e., the manufacturer created their own protection circuit instead of using an off-the-shelf integrated circuit. For this circuit three quad-comparators integrated circuits (AS339) are used. These integrated circuits are located on a small printed circuit board located on the secondary. Because of the use of a customized circuit we couldn’t check exactly what protections this power supply really had (well, we could if we spent a lot of time analyzing this circuit). We could clearly see the over current protection (OCP) circuit, as we will explain in the next page.
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Figure 14: Protection circuit.
All electrolytic capacitors are Taiwanese, Teapo, KSC, Fcon
The active PFC electrolytic capacitors are rated at 85º C (and manufactured by Teapo, a Taiwanese company), while the electrolytic capacitors from the secondary are rated at 105º C and coming from several vendors (Teapo, Fcon and KSC).
