isolated MOSFET gate driver problem

[drillz]

Dear Engineers,

I am currently making an offline flyback SMPS. It uses two MOSFETs in the primary circuit and so I need an isolated gate drive.

Here is my investigative attempt thus far. Unfortunately, the output is only 0 to 7V instead of the required 0 to 16V.

I would be very grateful if any reader could help with ideas to get 0 to 16V on the secondary side. The 1:1 pulse transformer is a part from Maplins.

By the way, the path to each of the totem-pole transistors has been made so that the propagation delay is the same. (-so they don’t end ever end up momentarily ON at the same time).
-However, the bottom transistor in the totem-pole does in fact get rather hot!

This is for LED displays for a marquee company.

 

[benta]

Why do you need isolated drive? Isn't high-voltage drive enough?
Anyway, don't try to do this yourself, it's too much effort and not worth it. Look for ready-made high- and low-side drivers. Here's an example:

http://www.onsemi.com/PowerSolutions/product.do?id=NCP5106APG

Benta.

 

[LionelHutz]

It seems to me the NPN transistor is not turning fully on. Which means you need more base current. But then the driving transistor may also be limiting the current so it may need to be driven harder.

Even if the signals reach the bases of both transistor at the same time it does not mean the on transistor won't take longer to turn off than the off transistor will take to turn on. If you scope the power supply current you will still likely still find some nice shoot through current.

If you want to use one drive signal then I would swap the NPN and PNP transistors and tie the bases together. Put a resistor to Vcc and a driver NPN transistor to ground and see what it does. Otherwise, I would program the PIC to provide 2 drive outputs with some delay between them.

 

[itsmoked]

Also you have a 1:1 transformer. You are feeding it 7V on the primary and getting 7V on the secondary. Why would you expect more? Voltage multipliers are weak at best. Use a different ratio.

But more importantly do this the modern way with OTS parts not jungle logic.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[benta]

"jungle logic" :-D :-D :-D

 

[electricuwe]

If you real want to go that discrete approach and and you can't get a different transformer, add an inverter and an additional totem pole stage to build an H-bridge.

But I would just do so if I really would be trapped somewhere in the jungle with only my part kit from my hobbyiest days 25 years ago.

 

[DHambley]

It looks like your bottom NPN won't turn off untinl it's base charge is swept out. This could be what's making it hot. However, I would just use a gate driver IC like a MIC4422 or a TC4420 or something similar and get rid of all that circuitry.

The C-T-C isolation with a diode looks a little odd but, I'm assuming you did the math and the transformer doesn't have magnetizing current problems. These kind of circuits have problems unless your flyback input voltage and your load don't swing over a wide range. If they do, the gate won't always be 7V as shown.

You might not need that last 1N4148. The MOSFET gate can go below zero.