DC-DC Converter with a 50 Hz transformer. Viable?

[hadigan]

Hi guys, need help with some fundamentals.

I need to implement a DC-DC converter for battery "fast charging" application. The topology that will be used is the Full Bridge Phase Shifted with Diode rectifiers.

Some important details:

Input Voltage: 200 Vdc
Output Voltage: 15 Vdc
Output Current: 50 Amps minimum

All the recent journals that related to DC-DC converter use very high switching frequency (~100kHz) for the transistor switching signals, which requires high frequency transformer.

The problem is, the only transformers I can get that can support 50A minimum current is the 50 Hz transformer, which requires my switching signals to be 50 Hz. My questions are:

1) Is it academically incorrect to use 50 Hz switching frequency for my dc/dc converter?
2) Is there other advantages of using high frequency transformer other than its small size?

That is all, thanks in advance. Suggestions are most welcomed

 

[chuckey]

1, no
2, Smaller smoothing capacitors and higher switching losses, lower transformer core losses.
Frank

 

[mtwieg]

In very high power applications (like MW and above) I've seen laminated steel chokes and transformers used in SMPS, but the switching frequency is still in the KHz range. But for things like battery charging it's far from optimal.

 

[KlausST]

Hi,

It is about 750W of output power. Either you yuse the 100kHz tfansformer with about 250g of weight, or a 50Hz transformer with 10kg of weight.

This is not really calculated, but i want to say they differ in size and weight...

Klaus

 

[BradtheRad]

Here is a simulation of what it sounds like you want to do.

I assume you will chop the 200 VDC through an H-bridge, and feed square waves to the step-down transformer?

It is important that the transformer put out more than 15V, if you want 'fast-charging'. More like 16 or 17V. Remember you lose upwards of 2V due to diode drops.

Fast-charging can push a 12V battery up to 15 V. A ratio of 11.6:1 seems to work well. (This is with square waves. The spec may be different with sinewaves.)

Charge rate is high when the battery is down to 10.5V.
Charge rate tapers off as the battery rises to 15V.

 

[hadigan]

Thanks for the inputs!

@chuckey: Noted!

@mtwieg: Pardon me, what do you meant by "far from optimal"? 50 Hz is a bad idea?

@Klausst: Correct, my charger will be a static one hence weight won't be an issue

@Bradtherad: Nice one thanks a lot! yup very much similar to the one you simulated. Noted on the transformer ratio, diode voltage drops, charging voltage, and the charge rate variation.

More comments/suggestions?

 

[bill66656]

50Hz ： A far larger output capacitor is needed, which will also benefit as giving very fast transient response and less ripple. But it may have more severe EMI to other apparatus as with more switching harmonics. Soft switching technique can help in this facet.