# Transformer , Mosfet selection for 800 Watt Push Pull converter and

Hi,
I need help in the selection of Ferrtie core tranforerm for 800 Watt push pull transformer.

The specs are

Input Voltage:

VDC min 10.5 V
VDC max 21 V

Out put volatge:

180 VDC ( output 800 watt when input voltage is from 10.5 to 14 VDC)

180VDC ( output 350 watt when input voltage is 15 to 21VDC).

Switching frequency 50 KHz .

I want to know the material of core . Can is use PQ 35/35, or PQ 40/40 core for this power level? I am using Kg criteria for the selection of core there is apparent power factor. I need explanation for it.

Is it possible to use two small cores one for each ,primary winding leg?

What should be the rating of power mosfets?

Regards

#### FvM

##### Super Moderator
Staff member
Your previous thread https://www.edaboard.com/threads/push-pull-converter-closed-loop-issue.393459 manifested big overvoltage problems with you two-transistor push-pull configuration due to leakage inductance. You should better consider a half- or full bridge push-pull topology. It would allow ,e.g. 40V transistors for 21 V input.

The design tools mentioned in your previous thread give about 1000 W for PQ40/40. I dont know what Kg is, where do you see it?
Using two cores is possible, but not with separated push-pull windings. Expect core saturation due to DC magnetization!

points: 2

I found Core geometry Kg parameter in this document.

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#### FvM

##### Super Moderator
Staff member
Core geometry coefficient kg is introduced in McLymann, Transformer and Inductor Design Handbook. Review the explanation and find out if it's useful for you. There are other calculation methods without referring to Kg.

points: 2

I want to implemet short circuit protection at primary side. For 120A current level what is simple solution?

Regards

##### Super Moderator
Staff member
I want to implemet short circuit protection at primary side. For 120A current level what is simple solution?
A fuse is a simple method. Prices range from a few dollars to $100. Walmart carries metal strip links rated 120A for automotive installations. 4 for$7.99 clearance sale.

The home backup power market has fuses for DC at high Ampere levels. Expect high prices because it's still not a high volume market.

Mains fuses are for AC, over \$50. I don't know for sure that they're suitable for your purpose.

points: 2

I want to measure the primary side current by using shunt and op-amp type circuit. In case of overload and short circuit the PWM ic should be turned off.

Can i use shunt and op-amp circuit?

Regards

#### Easy peasy

current transformer and peak current mode control ...

At first I tried to use UC3846 pwm controller. But due to instability in control loop. I moved to TL494.

I cannot use Current tranforerm due to cost and size limitations.

Regards

#### KlausST

##### Super Moderator
Staff member
Hi,

For sure you can use shunt and Opamp ...if you desgn it correctly.

But a 150A shunt may be big and expensive, too.

And usually the PWM controller should be able to limit the current ....if there is a failure, are you sure it still can react on your "overcurrent signal"?

And it needs some extra circuitry: delay circuit, or some latching circuit, soft start...
Is your overcurrent detection on an AC signal or a DC signal?

Klaus

points: 2

I want to sense current at primary side. The over current signal should turn off the TL494. I have a latching circuit . Can I use MCP6022 type opamp in differential configuration for current sensing? Is there any CMRR issue in differential amplifier while sensing current at low side ?

Regards

##### Super Moderator
Staff member
150 Amperes seems large enough that a Hall effect sensor should be able to detect it, if you were to place it near the right component, or perhaps a few loops of wire.
--- Updated ---

By wrapping wire around a transformer, you create an additional winding which can generate a small voltage. It's not calibrated so you'd need to run tests to find what amplitude it puts out when the transformer carries 150A.

points: 2

I have multiple secondary windings in PushPull transformer.

One secondary provides 180VDC @ 800 Watts.

The other secondary is used as an auxiliary power supply and it's voltage is 18 VDC.

Is it possible to sense the feedback from this auxiliary supply ( 18 V ) and regulate the main secondary volatge ( 180V) . There is no tight restriction on regulation. I can have 10% accuracy.

Regards