MC34063 Switching Regulator - please help with step-down design

[dennisfry]

Is anybody familar with the design of the MC34063 1.5A, Step-Up/Down/Inverting Switching Regulator.

References:
https://www.electro-tech-online.com/custompdfs/2008/11/MC34063A.pdf
https://www.electro-tech-online.com/custompdfs/2008/11/AN920-DPDF.pdf

I am using solar panels and 20A solar charger for charging of batteries, which is connected to a 12v-240v invertors to power lighting & appliances I require.
Being an electronics hobbiest, im designing a few handy circuits direct from the 12v battery output to power my projects.

I need a 12v to 5v @ 890mA step-down using the MC34063. I'm not familar with some of the values required for the MC34063 design. Here are my values:

Voltage Input (MAX): 13v DC
Voltage Input (MIN): 11v DC
Voltage Output: 5v DC @ 890 mA

MC34063 Values I have used for calculations:

I_OUT = 890 mA
Freq = 33 khz (the typical value used in the spec MC34063)
V_Ripple = 25 mV (p-p) (0.5% * Voltage_Output) used in similar 12v-5v designs
V_F = 0.8 (as per Step-Down example in AD920/D)
V_Sat = 0.8 (as per Step-Down example in AD920/D)

With the above values, here are the component values I get:

CT = 640pF
L = 47uH
RSC = 0.13 ohm
CO = 269 uF

I have calulated the following 2 possible values for R1 and R2, giving 5v:

R1 = 1k, R2 = 3k ie. 1.25*(1+3/1) = 5 OR
R1 = 1.2k; R2 = 3k6 ie. 1.25*(1+3.6/1.2) = 5

Where; R1 is connected to ground to Pin 5 (comparitor input).
See Figure 10 on the MC34063 spec for the component placement.

I would appreciate anybody that has had any design work with the MC34063, to check my component values.
Im not sure if the Freq, V_RIPPLE, V_F, V_SAT values used are correct or not.

I have noticed using the above figures, the IPK_SWITCH calculation resolves to 1780mA, which is above the 1500mA maximum?? Or is this referring to the maximum output (ie. I_OUT) at 1500mA?

 

[Oblong]

You just have to add an external P FET like this.

**broken link removed**

**broken link removed**

I never standardized values or checked for component stresses but this is the general idea. This is for 60 kHz.Vi= 11 to 13V Vo=5V 890mA.

You can get the models for the controller on bassos page or Onsemis.

http://pagesperso-orange.fr/cbasso/Spice.htm

http://www.onsemi.com/PowerSolutions/supportDoc.do?type=models&category=167

 

[mvs sarma]

You just have to add an external P FET like this.
...........
I never standardized values or checked for component stresses but this is the general idea. This is for 60 kHz.Vi= 11 to 13V Vo=5V 890mA.

You can get the models for the controller on bassos page or Onsemis.
.........

Try to use this calculator
**broken link removed**

however there is a limit for the frequency. Better to try around 30KHz.

 

[dennisfry]

Mc34063

Thanks for that....

I dont think I can source the NTR0202PLT1 here in australia. Are there any common equiv FETs that could be used?

Thanks in advance....

 

[mvs sarma]

Thanks for that....

I dont think I can source the NTR0202PLT1 here in australia. Are there any common equiv FETs that could be used?

Thanks in advance....

Try IRF9Z34

 

[Oblong]

The mosfet in the schematic I posted would fry.They don't cook in spice, it was just the first PFET that came to mind when putting it together. In the real world you need something like a FDT458P.

http://www.fairchildsemi.com/ds/FD/FDT458P.pdf

or

irlms5703

http://www.irf.com/product-info/datasheets/data/irlms5703.pdf

These are both readily available I get them from Farnell (Newark) all the time.

for a SHOTTKY you could try maybe 20BQ030PbF.Again this is available at farnell, and it's cheap.

 http://www.vishay.com/docs/94157/9415720b.pdf

 

[brk]

Oblong,
I tried the circuit you suggested. I want a input of 12 to 30 V converter, outputting 10V at 500 mA. I tried various values, but load regulation is bad. Have used the following values against you referenced ciruit.
R1-4K7, R2-2K2, R3- 4E7,
R sense-0.3E, Rupper -3K6, Rlower -540E,
C1 - 100uF/50V, C3 - 470 uF/25V, Ct -1470 Pf
U2 - MOSFET IRF9450, Lp - 36uH, Dfw - 5819, D1 -5819,

The short circuit current is increasing and if I try to change the voltage regulation is becoming bad.
Can you suggest a remidy?
Thanks. - brk