Sepic(Buck-Boost) Converter

Thread Starter

Peter523

Joined Aug 15, 2021
97
hi P,
As a test I have changed the MOSFET for a lower Vgate Vth.
Find a suitable power MOS with a lower Vgs On.

EView attachment 246117
Perfect, thank you very much. Tt seems to work reaching 8.4 V for PW of 2.1u. Do you maybe know which component values should i change to reduce voltage ripple on output?

EDIT: Solved by increasing some capacitor values
 
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ericgibbs

Joined Jan 29, 2010
19,082
hi Peter,
I hope you have noted the Gate drive voltage on the sim post #43 has been increased in order to highlight the insufficient Gate drive voltage for the original MOSFET.?

The problem is that the 5V Vgate is not switching the MOSFET hard ON.

If you can find a suitable MOS that will be switched fully On with a 5V Gate voltage, it should help with the Vout problem


As Ron points out, you have high current levels in some components.

E
ESP_ 698 Aug. 21 08.12.png

ESP_ 699 Aug. 21 08.58.png
 
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Thread Starter

Peter523

Joined Aug 15, 2021
97
hi Peter,
I hope you have noted the Gate drive voltage on the sim post #43 has been increased in order to highlight the insufficient Gate drive voltage for the original MOSFET.?

The problem is that the 5V Vgate is not switching the MOSFET hard ON.

If you can find a suitable MOS that will be switched fully On with a 5V Gate voltage, it should help with the Vout problem


As Ron points out, you have high current levels in some components.

E
View attachment 246139

View attachment 246141
Hi ericgibbs,
to be honest i didint notice that you increased the gate drive voltage. I thought that replacing transistor irf540n with irf7413 makes the cicrcuit operate even with 5V gate drive voltage. I also simulated with 5V gate drive and it run properly. That's my last simulated circuit .
Screenshot_1.png
 

Danko

Joined Nov 22, 2017
1,855
Peter523,
You can use one more, independent, self-oscillating converter
specially to provide 12V for PWM's of all your converters.
for example:
Pololu 12V Step-Up/Step-Down Voltage Regulator S18V20F12
Vin=2.9V to 32V
Vout=12V
Iout=up to 2A
The ENABLE pin can be used to reduce the quiescent current to well under 1 mA.
1629541009370.png1629541107222.png1629541183254.png
ADDED:
Hi Danko,
The problem is that the TS wants to lift the source voltage from+3V to ~+8V , with an expected load of 0.77R.
My English is stupid, so trying to say the same, using picture:
1629550627958.png
 
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Danko

Joined Nov 22, 2017
1,855
Rds(on)=0.0007Ω
Vgate=12V
P, dissipated by M1=5.1W
P on R of L1 =?
P on R of L2 =?
P on D1 =?
P on C1 =?
P on C2 =?
1629567805204.png 1629570535013.png_____
 

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ericgibbs

Joined Jan 29, 2010
19,082
hi Danko.
Nice solution.
The TS will have to level shift his 5V PWM [ MCU derived] to the 12V level required, that should not be a problem.

E
 

Ian0

Joined Aug 7, 2020
10,254
Or maybe he can use a logic-level MOSFET.
5.1W seems like a lot of dissipation for a 0.7mΩ MOSFET - is it all switching losses?
If so, probably a MOSFET with a smaller die (higher Rds(on)) would have less loss?
 

Thread Starter

Peter523

Joined Aug 15, 2021
97
hi Peter,
You must attach a Load on the output, else the Vout result will be meaningless.

Try your 0.77R
E
Sorry i may wasnt clear. The SEPIC i posted on post #47 had on its output the three DC-DC converters and it reached the desired voltage on its output (8.4V) with gate drive voltage of 5V. That's why i think problem is solved
 

Danko

Joined Nov 22, 2017
1,855
Sorry i may wasnt clear. The SEPIC i posted on post #47 had on its output the three DC-DC converters and it reached the desired voltage on its output (8.4V) with gate drive voltage of 5V. That's why i think problem is solved
Circuit #47 has 7.4 V on its OUT.
Transistor dissipates 86,7 W of power:
1629581749197.png1629581858295.png
============================================================================================================================
ADDED:
For example, how it can work:
1629592432260.png
 

Attachments

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Thread Starter

Peter523

Joined Aug 15, 2021
97
I don't have your spice file so I can't tell. Something is really wrong if this in true. The transistor should be under one watt.
I agree. Something seems to be wrong with that amaount of dissipated power. But the output voltage is the desired one. What may be the problem...
 
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