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Adam Munich

Fri Oct 01 2010, 03:54AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

So I'm itching to play with those superflybacks I have, so I'm going to build 2 ZVS drivers that will take some heavy abuse.

Here's what I plan on ordering. Speak now if anything seems astray.
4x IRFP260
10x 2A 400V 75ns diodes
10x 1N4742A 12v zeners

Jameco didn't have any high power 470Î© resistors, so I'm going to get 4 20Î© 5watt ones, and 12 150Î© 5 watt ones. 150+150+150+20 = 470

I already have some ferrites, 10k resistors, heat sinks, and some vintage 600V rolled foil .68uf caps at home. Power source will be either some car batteries, my 2 100Ah gel cells, or a mix of the two.

Total cost including postage = $28.5 (not too bad for 2 beefy zvs's)

So, any comments? Please speak before I press place order.

Myke

Fri Oct 01 2010, 04:12AM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

For higher input voltages, you would want a higher value gate resistor. Otherwise, you will probably run into parasitic oscillations which are no fun.

Adam Munich

Fri Oct 01 2010, 05:02AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

How much higher?

I just noticed those resistors were .15Î© ohms anyway. Since jameco doesn't seem to have a large choice of resistors I'm going to get everything from allied. I may just get a bunch of hundred ohm resistors and add them as I see fit.

Edit; this is what I bought.

Adam Munich

Sun Oct 03 2010, 09:56PM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

Alright allied fails hard; my order has been pending for 3 days. I'm going to get the silicon at jameco and the resistors somewhere else. (I used a gift card so allied can't charge it if jameco does first)

I was thinking of a dual potentiometer for the gate resistors, but it's hard to find 2W ones that I can afford. Do these resistors really need to be that hefty? They're only driving the mosfet's gate.

GhostNull

Sun Oct 03 2010, 10:03PM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

With a pot, it is always easy to accidentally turn it and kill your MOSFETs. It might be an idea to put a resistor in series with the pot to prevent gate resistance from going too low. Also in this case a dual pot would be best

Adam Munich

Sun Oct 03 2010, 10:22PM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

That's what I planned on doing. Putting a 500 ohm resistor in series with a 500 ohm pot. But does it really need to be 2 watts?

Arcstarter

Mon Oct 04 2010, 12:00AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Grenadier wrote ...

That's what I planned on doing. Putting a 500 ohm resistor in series with a 500 ohm pot. But does it really need to be 2 watts?

The gate resistor pulls the gate up to the voltage supply. When the mosfets are turned off, the supply is shorted to ground through the resistor. That is why they dissipate a fair bit of power.

The larger the resistance, the slower the turn on, which means more switching losses, but less current through the resistor. The smaller the resistance, the more current will flow, but a faster turn on. I once made one of these with 2 1kohm .25 watt resistors in parallel for each resistor, and they never got very hot.

Another thing you can do is use a separate 12v supply for the gate pull-up, thus meaning less voltage drop across the pull-up resistors. I have done it with no problems, and it only needs to be something like a 500ma supply.

Adam Munich

Mon Oct 04 2010, 12:42AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

Arcstarter wrote ...

The gate resistor pulls the gate up to the voltage supply. When the mosfets are turned off, the supply is shorted to ground through the resistor. That is why they dissipate a fair bit of power

I see now... In that case a theoretical 48V supply would burn out half watt pots nearly instantly. I suppose I'm stuck with the whitebox wirewounds. Maybe I could design a switch to select a resistance from 470 to 1K.

Adam Munich

Mon Oct 04 2010, 04:35AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

I came up with a solution that will both allow the resistance to be variable, and to allow me to buy everything from jameco without breaking the bank. DIP switches! I ordered 100 .5W 4.7k resistors and 4 8 pole DIP switches. Setting the switches changes the resistance. Let me explain with a picture.

1286166956 2893 FT97679 New Bitmap Image

GhostNull

Mon Oct 04 2010, 09:28AM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

Careful =S
DIP switchs don't have a great power rating

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