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Push Pull SMPS for getting multiple isolated supplies

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chinuhark

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I want to make a Push Pull type DC-DC converter with multiple outputs for driving the gates of an inverter and also some control circuits. While bootstrap method is popular, it has it's limitations. All the people at our college use a large no of isolated power supplies to do this and I want to make the Power supply part compact and cheap.
I am thinking of using an off the shelf 12V,1A SMPS supply for the input primary voltage. There will be about 10 isolated secondaries with atleast 600V insulation level between them.
I will be using an ETD39 core at 40kHz switching frequency and 3Volts per turn. Thus the no of turns are as follows:
Primary: 4+4 turns centre tapped (1Amp)

Secondary: 1) 6 isolated secondary windings of 7 turns each (350mA)
2) 4 isolated secondary windings of 4 turns each (350mA)

I was thinking of using IC78xx Linear regulator ICs at each output to get the desired regulated voltage level. The input will be a constant 50% duty cycle 12V p-p square wave at 40kHz and since I am using Linear regulators, there is no need to work on a closed loop regulation system (which I cannot imagine to be easy or even possible due to the presence of multiple secondaries with varying load levels on each).

1) Does everything seem OK and would it work?
2) Is this the best way of doing it?
3) I am thinking of using a simple atmega 8 to get the pulses for the MOSFETs which will have constant duty cycle and dead time. Should it be OK?
 

KlausST

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Hi,

ETD39? how much power do you need to drive inverters?

I´d rather use MURATA 786 series pulse transformers.

A single 50.0% duty cycle full bridge mode stage to drive the primary winding of all transformers is enough.
On secondary side you just need a schottky diode and a capacitor for each output.

Klaus
 

BradtheRad

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Do you plan to connect or disconnect heavy loads from the secondaries? This will affect output voltage at the other secondaries, even if it is for a moment. It is an issue you need to factor into your calculations for voltage levels and how to regulate them.
 

chinuhark

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Hi,

ETD39? how much power do you need to drive inverters?

I´d rather use MURATA 786 series pulse transformers.

A single 50.0% duty cycle full bridge mode stage to drive the primary winding of all transformers is enough.
On secondary side you just need a schottky diode and a capacitor for each output.

Klaus
This is the first time I am studying SMPS technology and I am sorry as after reading your post I did the necessary research and understood that the ETD39 is in fact a monster for doing what I want to. Your idea sounds fine but I want 1 single small and cheap power supply for the entire board. Secondly a full bridge increases the cost (I will need 2 IR2110, 4 MOSFETs etc) and making my own supply is economical only if it is cheaper than 6 discrete off the shelf supplies.
Basically I want a power supply for the complete VFD board. It will provide power to the dsPIC, all the gate drivers (TLP250s), comparators on the power side for SC protection, capacitor precharge circuit etc. The current requirements for all these loads should be quite small. But most of them need to be isolated from each other.
What do you think would be the right size for something like this?
 

KlausST

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Hi,

There are several full bridge ICs. No need for driver and fets.
About size: It depends on number of output voltages, the voltages and the currents...

Klaus
 

chinuhark

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I will use off the shelf SMPS to get down to regulated 12V DC. This will be converted to 12V, 40kHz square wave and fed to the primary (centre tapped, push pull).
I am aiming for 10 Nos of secondaries.
Details:
Primary: 12V
Secondaries:A) 15V (6Nos) (150mA maybe??? for gate driving)
B) 9V (3Nos) 300mA (For things like opamps, comparators etc)
C) 6V (1Nos) 300mA (For dsPIC33)
I am looking at E20 core...
 

Orson Cart

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Usually a flyback is employed for these types of aux power supplies...., but a fixed push pull can work, you need to avoid the chokes in the o/p rectifying ckts...
 

chinuhark

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Chokes in the output ckts...You mean the L in the LC filter right? As it it I was planning on using a capacitor alone, say 47uF or above and another 47uF+0.1uF parallel right near the driver IC.
 

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