Boost converter - Max1771 8.4vdc to 260vdc

Hi,

Am using MAXIM 1771 step up converter. Existing POT core was heated (30 min running time). Kindly suggest any manufactured based POT core.

POT Core specification for your reference:

Primary Inductance - 3mH
Primary Resistance - 0.4E
Sec Inductance - 170mH
Sec Resistance - 20E
Primary wire size - 27swg
Sec wire size - 42swg
primary turns - 32 turns
Secondary turns - 230 turns
Switching freq - upto 200khz
Input voltage - 7.4 to 8.4vdc
Output voltage - 260v
Ferrite POT core - 18x11

Thanks.,

The inductance numbers suggest that you have designed a transformer without air gap which can't work for a fly-back converter.

I don't apply to calculate the transformer for you, but the design might work after adding a 0.1 to 0.2mm spacer between the core halves.

And the output current is.....? The wire diameter is.......?

You could be experiencing significant copper losses.

what core material do you use?...it might not be suitable for the sw frequency

Thanks for your reply.
Primary wire size - 27swg
Secondary wire size - 42swg

ferrite potcore with bobbin - 18x11
Output voltage - 260v. How to measure Output current? When am connect a ammeter, the values are not shown(zero).

You are repeating only info already given in the initial post but don't answer to the important questions, e.g. regarding air gap or core material.

I understand that the output current question is asking about the designed inverter output current. Another question is which output voltage has been achieved in your test and which load was connected (if any).

Output current - 75mA
Im using 3(qty) acrlyic washers , 0.5mm thickness.

The inductances are far too high for the stated input voltage and frequency, you need to re-design from 1st principles:

P = 0.5 L Ipk^2 freq, dIpk/dt = V/L, => dI = t . V /L

L ~ Uo Np^2 Ae / Lg (air gap), Bmax = 80mT = V .dt / Np Ae ( Ae = centre cross section of core), Uo = permeability of air/ free space.

From the output power required (divided by 0.75 for eff losses ) and desired freq you can determine I^2 L, work out the peak current you can draw from your source, e.g. 1 amp

this gives you L, now you can determine air gap and Np^2 (knowing Ae in m^2), for a given air gap, say 1mm, you can determine Np.

Check that V . dt / Np. Ae gives less than 80mT for 200kHz, knowing Np you scale for Ns

Calc wire lengths for core bobbin diamter, calc R of each winding, knowing the power, and therefore current, calc I^2.R to see if wires will be too hot

Repeat for larger core if wires too hot.

- - - Updated - - -

I'll start you off, for 260VDC and 75mA, this is 19.5 watts, lets say ckt eff will be 80%, thus need 24.4 watts at the input.

for 7.4V in min, the average current will be 3.3 amps, if we build a fully DCM flyback at 50% duty cycle max (to help us with the o/p diode and RFI noise) then the peak input current will be 4 x 3.3 = 13.2A,

At 200kHz, the max ON time is 2.5uS, thus 7.4 volt ( assuming 10 milli ohm fet = 132mV lost across fet) allows a max inductance of (V/L=di/dt) 1.4uH for the current to build up to 13.2 amps in 2.5uS

For a Bmax of 80mT (AC peak flux swing of 40mT at 200kHz) we see that 0.08 = V.dt / Np.Ae, Ae = 47mm^2 (no centre hole) for this pot core, giving Np = 0.09 turns

This is obviously too small - so we set the pri Turns to FIVE turns.

To get the same L, 1.4uH, we need a suitably large gap, from the formulae in the panel above we see than the gap will be 1.05mm (in practice a little larger - due to energy stored in core itself).

and now the Bmax will be, 78.7mT, keeping core losses low.

Ns will be 5 x 260/7.4 = 175.6 say 178 turns to allow for the output diode, the flyback volts on the pri side fets will be 7.4 x 2 plus leakage spike...

wind half the sec, then the pri, then the rest of the sec to lower the leakage and help with efficiency, happy building ...

the output diode will need to be ultra fast recovery (Trr < 35nS) and 600v rated.

Thank you so much.

In my application, Output voltage is varied based on Output voltage and input switching frequency.

pnsuresh said:

Output current - 75mA
Im using 3(qty) acrlyic washers , 0.5mm thickness.

Click to expand...

Output voltage - 260v to 100vdc