PMP20551: Changing the output voltage to 24VDC

Shaofeng Luo

Part Number: PMP20551
Other Parts Discussed in Thread: LM5046, UCC2897A, LM5022

Hi team

Customer are evaluating PMP20551, and they need to change the output to 24VDC, which will lead to the change of transformer, loop comp, feedback network. By the way, they want to increase the frequency to minimize the total size. So I'm wondering if you have any insights for this requirement. I think the key is the transformer selection, so do we have a recommendation(ratio, Lp, or even specific device) for this requirement? Expect for your reply!

over 4 years ago

0 Bernd Geck over 4 years ago

TI__Intellectual 2770 points

Hello Sir,

by twice the output voltage you got twice the output power, here 72W;
would recommend to go from a flyback to an Active Clamp Forward topology.
Better efficiency, less EMI and well suited for power range 50W to 150W.

Please check UCC2897A or LM5046, PMP4844 could be a starting point,

best regards, Bernd

0 Shaofeng Luo over 4 years ago in reply to Bernd Geck

TI__Intellectual 1540 points

Hi Bernd

Sorry for the missing information. The output power is fixed, still ~36W. Just the output voltage rises to 24V.

0 Bernd Geck over 4 years ago in reply to Shaofeng Luo

TI__Intellectual 2770 points

Hello Sir,

so just keep the primary side and use twice the windings on secondary to move from 12V to 24V;
set R17 to22k6, set R9 to 20k, set C16 to 1nF, set R12 to 100k, set R16 to 10k
Check that your new output capacitors will be rated for 35V or more.
Depending on this output capacitance set integrator capacitance C19 to reasonable loop
bandwidth and phase margin >70degs.
Depending on your desired magnetizing inductance RHPZ will be in a range 5kHz to 10kHz
go for loop bw 1kHz to 2.5kHz max.
And finally by your desired windings ratio check voltage stress across FET and rectifier to select
devices. Change RCD clamping voltage at least > 1/2 of flyback voltage to allow commutation.
Power Stage Designer might be useful here, see attachment.

Best regards, Bernd

Flyback_Shaofeng.pdf

0 Shaofeng Luo over 4 years ago in reply to Bernd Geck

TI__Intellectual 1540 points

Hi Bernd

Thank you, that's very detailed. By the way, I see a BAT54S-7-F in PMP20551 as the figure below, what's the function of these two diodes? Is it necessary?

0 Bernd Geck over 4 years ago in reply to Shaofeng Luo

TI__Intellectual 2770 points

Hello Sir,

the tiny diodes plus R12/C18 provides soft start function and prevents from large inrush currents,

best regards, Bernd

0 Shaofeng Luo over 4 years ago in reply to Bernd Geck

TI__Intellectual 1540 points

Hi, Bernd

In PMP20551, there is a aux winding for VCC, but it seems that VCC is the internal regulator output of LM5022. Is the aux winding necessary here?

0 Bernd Geck over 4 years ago in reply to Shaofeng Luo

TI__Intellectual 2770 points

Hello Sir,

indeed the controller could work just by Vin and generate Vcc
out of internal linear regulator - but for high input voltage this
is a very lossy mode of operation, let's assume 5mA driver current:

(60Vin - 7Vcc) x 5mA = 265mW

(Of course you need to add housekeeping currents for PWM etc.)

Driver losses itself depends on parasitic capacitances of your FET
and your selected switching frequency

Pdrv = Vcc x Qg x Fsw

To avoid the losses at the internal regulator Vcc is applied from
auxiliary winding, so internal linear regulator is just used to startup.
For a low voltage application I would not use aux winding, but
for high input voltage I would recommend.

Best regards, Bernd

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PMP20551: Changing the output voltage to 24VDC