High side drive in a full bridge( or Half) converter

[ifrythings]

Well I'm about to kill google

I'm trying to learn about high side driving in a full bridge(or half bridge) converter that uses mosfets as the switches and drive transformers for the drive and isolation.

Google has betrayed me anyways enough about google.

Does anyone have a nice document or site explaining this with a diagram (the best way I learn, Pictures ) or posibly a few shematics to study.

Oh, and one more request, what the heck is a RESONANCE MODE SWITCHING REGULATOR? I have an IC (CXA8038) that was used in a big power supply that was half bridge from what I could see on the layout of the board.

Anyways all help and info would be extremly appreciated

Thanks

David

[luka]

See (3.5.8 Base and Gate Drive Transformers) in Marty Brown - Power Supply Cookbook (2nd ed.)(2001) book, there you will find everything you need.

[ifrythings]

Hmm. I don't think any local books stores are going to have that in stock You have the ISBN #?

Do you know of any sites or documents I could look at while wait for the book?

Thanks

David

[luka]

hm... ISBN 0-7506-7329-X is I think.
http://www.powells.com/biblio?isbn=075067329x

[batee]

This is one of the most elusive subjects I've ever researched. Some pieces of info I've found that helped me understand this issue the best are:

TI App Note on Driving FETs called slup169.pdf
http://www.ti.com/litv/pdf/slup169

IRF AN-978 Gate Drive Comparison
http://www.irf.com/technical-info/appnotes/an-978.pdf

I tried the approach of using the IRF driver ICs (at a cost of $7 each plus boards and support components, plus $100s of FETs they destroyed) and they just weren't working. It took me 6 months to figure out why: They only work within a narrow PWM duty cycle. Otherwise they can't keep up. The IRF appnote talks about providing a continuous gate drive with these ICs, but it's complicated and I just didn't trust them after all the FETs they destroyed.

I tried pulse transformers. They're heavy, they take lots of support components to implement properly, they're susceptible to stray magnetic fields turning on your FETs, and they usually only work for the designed PWM frequency. Fast turn-off requires additional passives and semiconductors, and it just took too much space. Again, it doesn't work for continuous gate drive (PWM duty cycle = 100%).

Then I found some transistor /diode circuit that was supposed to generate an isolated power supply from the high side line. It looked really complicated and I didn't use it, but it gave me the idea to use four isolated DC-DC converters to turn on each of the FETS in my H-bridge. C&D makes some ultracheap ones that are just tiny (1/4 cu in) for $8.

I was working with high voltage DC link, and still needed isolation for the turn-on circuitry and then needed a FET driver IC to provide the high current necessary for fast turn-on of large FETs and IGBTs. I've always heard that Optos were too slow and not well suited for this purpose, but it turns out that Agilent / Avago has some awesome optocouplers specifically for this purpose, such as the HCNW3120, and they even have FET drive circuitry built-in.

Circuit description: A 12VDC power supply powers all four converters. The 15V output of the isolated converters in turn power the output circuitry of the Agilent Optos. The Optocoupler inputs (LEDS) are powered directly from a PIC or anything with Open Collector outputs and 15mA capability. There's a gate resistor to tailor switch-on current/time in parallel with an "Anti-parallel" diode to vastly increase turn-off time. My FET driver uses four of these circuits to drive an H bridge. Only two were really necessary, but I opted for symmetry over cost to reduce the timing issues that will surely be encountered when building your own FET gate drive. It works great for H- bridges, half-bridges, it also becomes an LV/HV BLDC motor controller, HV lighting controller, all sorts of things.

If you're considering this, draw out an H bridge and make sure that you understand all of the five separate ground planes that a circuit like this really involves, and that you keep them separate!

Most of the app notes I've read came to the conclusion that FET driver ICs were the way to go, and that isolated power supply gate drive was either slow or expensive. With all the money I've wasted on FETs, gate drivers, boards and research time, I think they're totally wrong. My circuit works for 0-100% duty cycle, for DC Link voltages of 1000-3500VDC (depending on which DC-DC converter and Opto you buy).

The other day I was browsing the Powerex site and found a development board meant to power their FETs and IPMs. They list a schematic that was almost identical to what I describe above (they also include desat detection), so I think I'm on track here.

If others have information on this subject, please let me know.

Thanks!

Bryan A. Thompson
bryan@batee.com

[batee]

Schematic

Board w/H-Bridge

Bryan A. Thompson
bryan@batee.com

[Eva]

I have used both the IR gate drivers (IR2112, IR2113, IR2010) and high speed optocouplers, either basic ones like 6N137 and HCPL2630, and the ones with integrated gate drive from Avago (formerly Hewlett Packard) like HCPL3020 and HCPL3120. IR gate drivers obviously require the high side cell to be powered in some way, either by actively limiting duty cycle to 1% or 99% to ensure permanent switching and bootstrapping, or by providing an external floating power supply, which must feature very good common-mode filtering in order to handle fast switching transients. Optocouplers can be powered in the exact same two ways but they tend to draw 10 to 50 times more current than IR gate drivers, thus making bootstrapping more difficult.

Optocouplers provide isolated drive for free, which sometimes is a great advantage, however, they are only suitable for moderate or low speed switching applications. That's because their main pitfall is false triggering in high speed switching circuits because even the best ones are rated at just 10V/ns or 15V/ns guaranteed common-mode inmunity. Another disadvantage of HCPL optos with built-in gate driver is the pulse width distortion, specified to be +/-350ns typically, and the turn on/off delays also in the 300ns range, thus imposing quite long dead times to account for part to part mismatching. IR ICs tend to specify delay matching within +/-30ns allowing for much tighter dead times.

On the other hand, IR gate drivers are rated at 50V/ns thus allowing for reliable operation in high voltage high speed switching circuits. Also, I'm not aware of any reliability problem with these ICs if you know how to use them, for example, my last class D amplifier contains a half bridge powered with +408V and driven with a IR2113 and can produce over +/-40A peak output current (that the magnetics can't handle, so it's actively limited to 30A). I have never yet managed to blow any IR2113 or MOSFET/IGBT with that IC. The modulator is designed to keep always switching and avoid bootstrap capacitor discharge. Furthermore, turn-off voltage slopes approach 30V/ns at higher loads, something that even the best optocouplers can't handle.

[luka]

Eva

Why will u intend to use this D-amp for? Sound to me like so high power motor driver, or is it?

My IR2110 didn't give me any problems at all.