M

#### Michael

- Jan 1, 1970

- 0

converter. It needs to go on a PCB that can be no greater than 1cm in

width and can be no greater than 1cm in height. Length is more

flexible. It will have a 5VDC input and needs to have a 1KV, 1ma

output. Regulation is not terribly important.

First I thought about just using a normal DC/DC boost converter and

using a HV fet on the output. However, it seems all HV FETs are

massive. The smallest relatively HV FET I could find was the STMicro

STN1NK80Z, which is an 800V (I could probably get away with just an

800V output - but 1KV is my goal) 250mA FET in a SOT-223 (7x6.5mm)

package. However, I noticed that RDSon is ~13 ohms. So since my boost

ratio is about 200, that'd mean I'd have about 200ma on average (if

I'm remembering my maths right) going through the FET, producing a

2.6V drop. So I don't think this FET is even going to be able to

handle my 1ma load. So I'll probably have to go up to a larger FET.

There are more FETs in the D-PAK (AKA TO-252) package available. The

Infineon SPD06N80C3T looks much better 0.9 RDSon. A quick calculation

shows my boost ratio ending up at about 208 once the RDSon is factored

in (assuming the resistance of the inductor is much smaller than

RDSon). So I'd have about 208ma going through the inductor. Now, for

choosing the inductor, I may be completely off in the way I am

calculating its value. This is the equation I came up with:

Einductor = .5LI^2 = output power * (1/ boost ratio) * (1/operating

frequency)

If that is right, if I used a 100uH Sumida CDFH6D28, I'd have to

operate it at 462KHz. I could put a couple of those guys in series, or

find a higher inductance inductor, but already 462KHz seems nearly

reasonable already. That circuit is seeming almost doable, though

driving that big HV FET at that speed will take a decent amount of

power.

However, I remember being told that boost converters with high boost

ratios do not work very well for some reason that escapes me. So then

I started thinking about transformers. However, I can't seem to find

any that are this small. Heck I can't seem to find any reasonable

sources for boost transformers (none of the big three - Digi-Key,

Mouser, and Newark) seem to have any.

So then I started thinking about PZTs (piezo transformers). PZTs are

typically very small with very high ratios - however again I can't

seem to find anybody that sells any.

With all of this in mind - anybody have any recommendations as to a

possible route for me to go? Are my calculations for the boost

converter right? I am not typically a power guy - so I could

definitely be messing some things up here.

Thanks!

-Michael