High voltage pnp's (250 volts)

With an oscilloscope you can easily tell where the problem is. If the leading edge is slow, the PNP isn't getting turned on hard enough. If the PNP isn't fully on, d.c. drive. If the falling edge is a ramp, you're not turning it off hard enough.

It's not the transistor that's giving you trouble. It's the drive.

Cheers, James Arthur

The FZT957's poor performance turns out to be caused by the device itself. Looking through it's datasheet again, I saw the, previously unseen, typical rise & fall time spec: 109ns and 2500ns...

Hul

Hul Tytus wrote:

Hul, an amp at 250V implies quite a bit of power. I trust you're not creating an inefficient design, saving 50 cents in parts cost during manufacture, but costing the user $3 in extra electricity used over its operating lifetime?

--
 Thanks, 
    - Win

But those times are a result of the manufacturer's test setup, not the way we use the devices in real life. Those test fixtures often drive the device with 0..10V through a weenie resistor, with no speed-up cap.

In this case, the t.off is spec'd at i.b=-50mA, that's why it's so slow. You should be pulling hundreds of mA out of the base to turn the FZT957 off *hard*.

You're going to find, looking around, that the FZT957 is not some poorly-designed geometry that's 1/10th as good as it should be :-). It's a low-saturation device optimized for switching power supplies.

But you'll find that making efficient 250V x 500ns x 1A pulses with a bipolar transistor -- or even a FET -- takes a bit of doing.

Cheers, James Arthur

Well, that is an issue I have heard before for sure

You need to be competitive, so always doing designs with lowest possible loss is not an option

Cheers

Klaus

It's one thing to be competitive, it's another to be eeking out every last cent for profit. Klaus, you have a history of designing for low manufacturing cost, but also for high efficiency, higher than most of us manage when we're trying.

--
 Thanks, 
    - Win

Phil - placed an order today for the mje15035. Thanks for the suggestion.

Hul

Phil Allis> =====================

I have always thought of the MJE15034 and MJE15035 transistors as linear amplifier parts, rather than switching transistors. It'll be interesting to see how they work out for you. Let us know.

--
 Thanks, 
    - Win

Winfield Hill Nit Picks Me Again:

===================================

** Same goes for most of the parts suggested here.

** Which is totally irrelevant to the doubt raised.

With near constant Hfe of 200 up to 1A, typical Ft of 80Mhz plus Vce sat of 0.2V at 1A falling to and 50mV at 100mA - you never know, it might just work out OK ?

Want another fight over mS v ms ?

Can you spell the word " pedant " aloud please ?

..... Phil

Correct, many power supplies or motor drives can be made with a higher efficiency, just spending a little more time

Problem about it is that the competitors, often the Chinese manufacturers, only think about minimum cost, and that is what the customer looks for first. Trying to explain full lifetime cost is a battle lost forhand in most cases

Some years ago the adapter supplies would need to comply to level 6 (>90%) efficiency. All the big players placed their efficiency close to that mark to squeeze last cost out of the product.

Even today many regions allow Level 5 power supplies, which only has about 88% efficiency. In this case US regulations demand level 6, which is more strict than level 5 (maybe Trump sees this and pulls it back to level 5)

Graph of converter efficiencies:

Cheers

Klaus