Quote:
Originally Posted by MPaulHolmes
Basically, you run the 555 timer at around 50 to 100kHz, and that 5v signal goes to the mosfet driver. The mosfet driver makes a square wave on the primary side of the line filter above. The output side then gets a similar square wave. YOu use the zener diode to clamp the output voltage, and the 4 schottkys to rectify it. Bam!
|
The URL for those parts didn't work. But I'm guessing that it's a capacitive line filter, not a transformer. If so, you are implementing a capacitor level shifter.
That's good for a local isolated supply. But it has high capacitive coupling between the house/accessory supply and the traction supply. Which is bad for safety, has essentially no common-mode noise immunity, and won't work just when you need it the most.
The DC-DC converter is an expensive part of the system, but it's pretty hard to substitute something less expensive and get the same level of protection and performance.
You can save some money by getting a semi-regulated module. And after all, a gate driver doesn't really need 12.00V. These modules have a narrower input voltage range, typically +-10% (e.g. 10.8-13.2V) to meet spec, and non-guaranteed +-20% to work (often 9V-15V). If input voltage is a concern you can waste a little power by getting a 5V to 12V module and putting it behind a regulator. Or a 5V to 15V converter, and putting a regulator on both sides.
An alternate approach is using traction voltage to create the gate driver supply. Read the datasheet for the LNK306 switching converter to see how to do this with just a few components. But it does give up one of the features of the Cougar -- the ability to work with any traction voltage. If you want to operate at 12V (which is useful for testing), you need to build a buck-boost design, and that involves more components.
For our controller we put all of the isolation on the gate driver board, with overlapping footprints for two types of DC-DC converters. That allows us to use $5 modules (only 1W output and 1KV isolation) for most testing, or use better modules (2W, wide input voltage range, 5KV isolation) if needed.
Our gate driver board is using a Si8233 chip, which provides isolation and dual gate drivers in one package. It does an awesome job. You don't have to deal with all of the optoisolator hassle -- drive current, production variations, dimming with age and temperature.