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LAN_403

artemonster

Tue Jun 26 2007, 08:03AM

Registered Member #791 Joined: Sun May 20 2007, 02:02PM
Location: Riga,Latvia
Posts: 5

Hello. I have some questions,that i couldn't find answers in the interentet.
So,first of all:
In full-bridge what's the difference in driving an upper side or lower side?
Why we need to use a separate power unit for the upper side driver?
What's the difference in upper-side driver and low-side driver?
Is staccato/interrupter is essential in (DR)SSTC?
How can i manage\control dead-time?
Why we need only 2 low-side drivers for half-bridge? Or 1 low,1 high side?
Thanks.

uzzors2k

Tue Jun 26 2007, 11:15AM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

The low side is the name for the switches at ground potential, these are the "bottom" switches. The high side is the floating switches, which are "on top". They're floating because they're grounded through another switch, and direct grounding is impossible. Because they are ungrounded, and turning them on requires a positive voltage relative to the source/emitter you need a special driver. Transformers are often used for this. A low side driver can drive the switch directly if the driver is grounded to the full-bridge ground.

An interrupter is necessary in a DRSSTC to limit the current through the IGBT to "safe" levels. Even when interrupted they are usually running way over spec. Dead-time is controlled by the interrupter, which is often just a 555.

A half-bridge only has two switches, so of course it only needs one high and one low side driver. Two capacitors act as the other switches.

artemonster

Tue Jun 26 2007, 11:41AM

Registered Member #791 Joined: Sun May 20 2007, 02:02PM
Location: Riga,Latvia
Posts: 5

Thanks for the useful information. But what's the difference in high and low side drivers? Only inverting,and non-inverting logics? So to open in full bridge 1 High-side switch and low-side we need two different signals? And why we can drive half-bridge with 2 low-side drivers? So just 1 is inverting,other is normal...then output goes on GDT,and then just on half-bridge..hm) a bit strange. maybe you know some tutorials on power electronics? Cause i cant find any useful information about it((

Marko

Tue Jun 26 2007, 12:16PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Thanks for the useful information. But what's the difference in high and low side drivers? Only inverting,and non-inverting logics? So to open in full bridge 1 High-side switch and low-side we need two different signals? And why we can drive half-bridge with 2 low-side drivers? So just 1 is inverting,other is normal...then output goes on GDT,and then just on half-bridge..hm) a bit strange. maybe you know some tutorials on power electronics? Cause i cant find any useful information about it((

The inverting and non-inverting drivers we use with gate drive transformers are ALL low side. We actually rarely use high side drivers at all. GDT is what provides isolation there.

High side drivers are used where we need to avoid GDT for some reason (cost, frequency) and directly drive high side switch. IR2112 is example of an IC containing both low and high side drive. Check out the pics at pages 1 and 4:

They are *not* really directly powered from main low voltage supply, but are powered by charge pump.
There is a capacitor wich acts as a ''power supply'' to the driver. The negative pole of the capacitor is connected, instead of ground, to source of the high switch.

The supply voltage is brought to capacitor trough a diode wich is necessary to block the high voltage while high switch is on.

The driver uses the half-cycle where high switch is off and low switch is ON. Cap is now grounded and can charge from low voltage supply trough the diode.

After the high switch goes on the entire driver is elevated to half of bridge's supply voltage, and control circuits must be able to withstand this.

In mostcases it's acheived by two high voltage transistors inside the IC itself.

TheMerovingian

Tue Jun 26 2007, 01:47PM

Registered Member #14 Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383

I used a similar ic (IR2181) for my diagonal halfbridge coilgun since the low operating frequency was totally incompatible with a GDT. It worked very good

artemonster

Tue Jun 26 2007, 02:31PM

Registered Member #791 Joined: Sun May 20 2007, 02:02PM
Location: Riga,Latvia
Posts: 5

Thanks. Hm. But for driving a full bridge with 2 low-side drivers,that are connected to GDT. What's better? 2 GDT's? or all 5 windings on 1 ferrite core? or it's essential just to look at oscilloscope? :))))

Sulaiman

Tue Jun 26 2007, 05:34PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I use 1 gdt to drive a full-bridge, doesn't seem to be a problem,
in fact it seems the logical/simplest/most reliable option.

I've no idea why some use two gdt - anyone?

...

Tue Jun 26 2007, 07:37PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

You can use 2 gdt's, or even 4, to reduce the load on your gate driver(s), or to allow for a 'dead time' when none of the switches are on. This is usefull for switches that are slow to turn off, or if you want to audiomodulate the coil.

thedatastream

Tue Jun 26 2007, 08:40PM

Registered Member #505 Joined: Sun Nov 19 2006, 06:42PM
Location: Yorkshire!
Posts: 329

*toots*

for general GDT info

Why you need to treat high side FETs differently to low side FETs

As for the two vs one GDT, theres no good answer. It's whatever works best for you.

James

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