Buck-Boost converter low output voltage

[muwasif]

hi,
Attached is the schematic of a buck-boost DC-DC converter to convert a range of (8V to 25V) to -14V. when I run this simulation on proteus it results the output voltage of -3.63V instead of -14V. Is there any thing wrong in the schematic?
Specs are:
Vin = 8-25 VDC
Vout = -14VDC
Freq. = 100kHz
ΔI_L = 2.27A

Inductor = 22.8µH
Output Capacitor = 47µH

Please suggest the solutions.

 

[qwertyuiop415263]

You didn't include the error detector circuit.

See this


https://obrazki.elektroda.pl/97_1322146111.png

 

[ALERTLINKS]

100KHz signal should have 10V above supply voltage(20V-24V) as pmos is driven in source follower mode.

 

[muwasif]

You didn't include the error detector circuit.

See this


https://obrazki.elektroda.pl/97_1322146111.png

I'll include this error detection. I just made this simple for proof of concept. I calculated the duty ratio of PWM by using the formula
V_out/V_in = -D/(1-D)

---------- Post added at 07:40 ---------- Previous post was at 07:38 ----------

100KHz signal should have 10V above supply voltage(20V-24V) as pmos is driven in source follower mode.

Increased the 100KHz signal voltage it results 6.3V as out put. still not working

 

[FvM]

I really "love" those absurd Proteus schematics with unspecified signals coming from nowhere.

The gate voltage controlling Q1 must not only swing above the input supply voltage but also swing below cicruit ground. I can only guess, that's it's an unipolar square wave referred to ground now, which explains the observed results. It would be easy to provide a floating gate voltage referred to Q1 source in a simulation, but you'll have difficulties to implement it in a real circuit.

A NMOSFET is unsuitable for an inverting buck-boost converter, that's the simple truth. Use PMOS, as suggested.

 

[ALERTLINKS]

Error amplifier can only reduce and regulate output. First you have to generate higher voltage than required. Try incresing duty cycle.

 

[FvM]

In CCM, 50% duty cycle is roughly corresponding to Vout = -Vin. In DCM (with low load current), Vout will be even higher. So the low output voltage is surely not a duty cycle problem.

 

[ALERTLINKS]

So the low output voltage is surely not a duty cycle problem.

I agree but at least should see the effect.

Using a buck converter in an inverting
buck-boost topology