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Transformer Suitability

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LAN_403

KLH

Wed Mar 04 2009, 08:55PM

Registered Member #1819 Joined: Thu Nov 20 2008, 04:05PM
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Posts: 137

I recently designed this circuit to convert a single-ended +4.5V battery supply into dual +4.5V and -4.5V rails. It is a standard push-pull topology operating at 30kHz, with a full-bridge rectifier output split in half.

Circuit:

The transformer is a Murata Power Solutions 78615/4C generic pulse transformer, with two 100u windings, both center-tapped. Each winding has a resistance of 0.17 ohm.

First of all, did I get the individual inductance values for the center-tapped windings right? I think that center tapping a winding will result in 1/4 the total inductance value, so a 100u winding center-tapped might yield two 25u windings.

Next, is this transformer suitable for the application? When I simulated the circuit, the reactance of the windings at 30kHz proved to be a problem by reducing the voltage output. Is the rated inductance the mutual inductance rating? Or are these transformers designed to store energy, since they are "pulse" transformers? The simulator seems to be treating the windings as energy storagte devices.

Any suggestions on the circuit or advice for the transformer would be greatly appreciated.

GeordieBoy

Thu Mar 05 2009, 12:16AM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

Those inductance values sound too low for a transformer in a push-pull DC-DC converter, particularly if it is only rated at a few watts. They are closer to what you would expect for a transformer in a flyback converter with a gapped transformer core.

It sounds like the pulse transformers might be designed for high-frequency use, or might be intended to convey a train of narrow pulses for something like thyristor firing applications.

You are also missing the buck chokes from the output side of your DC to DC converter schematic. These are important to support the output current after one MOSFET turns off before the other turns on. Without this indutance each MOSFET will also see a huge turn-on current spike as it tries to instantly re-charge the output capacitors through the transformer.

A coupled buck choke is best if you desired good cross-regulation between the +V and -V output rails.

-Richie,

KLH

Thu Mar 05 2009, 06:25PM

Registered Member #1819 Joined: Thu Nov 20 2008, 04:05PM
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Posts: 137

GeordieBoy wrote ...

You are also missing the buck chokes from the output side of your DC to DC converter schematic. These are important to support the output current after one MOSFET turns off before the other turns on. Without this indutance each MOSFET will also see a huge turn-on current spike as it tries to instantly re-charge the output capacitors through the transformer.

A coupled buck choke is best if you desired good cross-regulation between the +V and -V output rails.

-Richie,

Thanks for the suggestion. I knew I was missing something from the schematic somewhere, I just couldn't remember where it went...

A coupled inductor sounds like a good idea; I think I could even use another one of the Murata transformers, if they are designed to store energy.

From your explanation about the inductance values, does this mean that the transformer presents no reactance (except that from leakage inductance)? If I increase the inductance value in the simulation, the voltage output gets worse and falls even lower. Is it something wrong with my simulator? I might not understand some crucial concept about these push-pull transformers.

Thanks again for the help.

GeordieBoy

Thu Mar 05 2009, 06:47PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

Look up the equivalent transformer model. It consists of an ideal transformer, with the addition of leakage inductance referred to one of its windings, and magnetising inductance across one of the windings.

In a push-pull converter the current drawn by the magnetising inductance will usually be small compared to the load current, so the magnetising inductance is large. (In a flyback converter the opposite is true, and the magnetising inductance is purposely low to store lots of energy in the flyback "transformer" to be released later.)

The leakage inductance of the transformer controls the rate at which energy transfers from primary to secondary. Leakage inductance is undesirable, and should ideally be as low as possible for most converter topologies. The SLR converter is one notable exception where some leakage inductance is desirable.

Leakage inductance also causes incomplete energy transfer from primary to secondary in DC-DC converter necessitating snubbers to catch the voltage spikes produced.

Unfortunately tight coupling, and good inter-winding insulation are conflicting design requirements so there is always a compromise.

Any decent power electronics book will cover this stuff in the first few chapters.

-Richie,

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