PMP20289: Clarification on the PMP20289 reference design

Hi Lenin,

Here are my response to your questions:

1. For the given spec, the value of resonant tank components is coming around Lr= 7.8 uH and Cr=26 nf. pls clarify what is the actual value of
Lr and Cr used ? In the schematic it shows 6 nos 8.2 nf connected in parallel (C111 to C116) is all these capacitors populated in the board ?

Please check test report section 1.6 for transformer detail. The primary inductance of the transformer is 67.5uH. The totall series resonant inductance is 2uH.
Yes, C111 to C116 are all populated. So the total Cr is 49.2nF.

2. In the schematic, between the main planar tranasformer primary and the resonant capacitor, another transformer is connected in series , i guess
for current sensing, which has a inductance of 40 mH. How could we connect such an high inductance value in series with the resonant tank ? kindly clarify ..

The current transformer has 1 turn on high current side and 100 turns on signal side. 40mH is the inductance on 100 turn side.

3.what is the value of output capacitor used ? In the schematic i was able to see 2 x 1 uf and a 0.1 uf cap..
The schematic i referred is PMP20289 Schematic and Block Diagram.

Most of the output capacitors are on transformer PCB windings. There are 4 output windings and each winding has 190uF capacitance on it. So total 760uF.

Hi Sheng-yang Yu1

Thanks for the response.

1. For Lr= 2 uH and Cr= 49.2nf, The resonant frequency turns out to be fr= 507 kHz. However the document mentions the resonant frequency is 350 kHz. Also in the document it is mentioned, there is a leakage inductance of 1.6 uH in the transformer, obviously this will get added to the inductor. In section 1.6, it is mentioned there is a leakage inductance of 2 uH between the transformer and the inductor... I'm still not clear what is the value of total resonant inductor Lr including all the leakage inductances..

Thanks for clarifying the other two points.

Regards
Lenin.

Lenin,

I'm sorry. You are right. It should be 1.66uH leakage inductance from the transformer and 2.05uH from external inductor. So the total series resonant inductor should be 1.66uH + 2.05uH =3.71uH.

Regards,
Sheng-Yang Yu

Hi Lenin,

1. At lower frequency, lower power closed loop LLC design, you will see higher Q and lower Lm/Lr ratio. But when the switching frequency goes higher and higher, you will have to keep Lm as large as possible to lower the circulating current to keep efficiency high. That's why the Lm is large. Also, we need low Q to have high enough gain peak at heavy load. That's why Lr is small and Cr is large and Lm/Lr ratio is large.

2. R100 and R103 is to have a discharge path for resonant capacitors when the power is off. This is to keep the initial condition of the capacitors within a desirable range.

3. Yes, if you have a bulk cap up front, you can just leave decoupling capacitor at LLC input.

4. Current sensing transformer is attached.G135036LF Rev1.pdf

Hi Lenin,

C5 and C6 were placed to minimize the power loop (i.e. from input capacitor -> top GaN FET -> resonant tank -> GND -> input capacitor) to minimize loop inductance.

C1, C2, C3 were placed to compensate the possible wire inductance from the input.

However, I never remove them to compare the difference and to see the necessity of these capacitor. It is highly depended on the layout placement. It is always good to have placeholders for these decoupling capacitors.

Regards,

Sheng-Yang Yu